Information

Photosynthesis - Light Intensity


Say I was conducting an experiment for photosynthesis. If I moved light closer to the plant, what effect would this have on the process of photosynthesis?


The rate of photosynthesis varies from plant to plant. Some plants require more light and some require less. If you move light closer to the plant, in most scenarios the rate of photosynthesis is likely to be increased. For some plants a minimal light is enough for their photosynthesis, so for those plants, moving light source closer or further will have less effect.


Photosynthesis

· Only certain wavelengths of light are used for photosynthesis.

· High temps have an effect on :

o Stomata -They close at high temps to avoid losing too much water. This slows down photosynthesis because less CO2 enters the leaf when the stomata are closed.

o Thylakoid membranes – may be damaged, reducing the rate of the light-dependent stage by reducing the number of sites available for electron transfer.

o Chloroplasts – membranes around them could be damaged, possibly causing enzymes important in the Cycle to be released into the cell. This would lessen the rate of the light-independent stage.

· Less CO2 will enter the leaf for the Calvin cycle

Saturation point = where increasing the factor after this point makes no difference because something else has become the limiting factor. A graph levels off here.

Light intensity, temperature and CO2 concentration all affect the rate of photosynthesis, so all affect the levels of GP, RuBP, and TP in the Calvin cycle.


Photosynthesis - Light Intensity - Biology

School biology notes: PHOTOSYNTHESIS - importance and factors affecting rate

Its importance and limiting and interacting factors controlling the rate of plant photosynthesis

The ideas are applied to horticultural operations e.g. a greenhouse

Doc Brown's school biology revision notes: GCSE biology, IGCSE biology, O level biology,

US grades 8, 9 and 10 school science courses or equivalent for

14-16 year old students of biology

Sub-index for this page

Green p lants and algae are producers based on the chemistry of photosynthesis and the start of most food chains and the base of subsequent food webs.

We are highly dependant on crops whether to eat directly, processed food or animal feed - so, we might not be 'green', but we ultimately depend for a lot of our food on photosynthesis!

AND, its not just life on land, all aquatic life e.g. fish, also depend, initially, on photosynthesis in plankton or algae.

A food chain is a means of transferring the energy from photosynthesis in the biomass to support many forms of life, including us!

Even the meat we eat, high in protein and fat, did depend at some point on photosynthesis, so there is no getting away from photosynthesis!

TOP OF PAGE for SUB-INDEX

What is the process of PHOTOSYNTHESIS? A simplified version of the biochemistry of photosynthesis

Plants absorb water through their roots and carbon dioxide through their leaves and covert these into carbohydrate molecules, initially in the form of glucose, the waste product is oxygen! handy for us!

The carbon dioxide in air diffuses into leaves through the stomata, water comes up from the roots via the xylem tubes, oxygen diffuses out and sugars are transported around the plant by the phloem tubes.

For more on plant structure and function including gas exchange and leaf adaptations see below and also .

Carbon dioxide into leaves and oxygen out of the leaves is an example of gas exchange system on the surface pores (stomata) of the leaves.

The biochemical process of photosynthesis takes place in the chloroplasts of plant cells in the green leaves and stems with the help of green molecules called chlorophyll.

It is the green pigment chlorophyll that absorbs the light energy to power photosynthesis.

Photosynthesis is summarised by the equation:

carbon dioxide + water == light + chlorophyll ==> glucose + oxygen

This is overall an endothermic chemical reaction, energy is taken in, i.e. sunlight energy is absorbed in the process of photosynthesis.

Photosynthesis is the process by which plants make food, initially in the form of glucose, for themselves, and for most animal life, including us too via food chains!

The plant will use some of the glucose immediately to fuel all the necessary life maintaining processes.

The plant converts some of the glucose to starch - a chemical potential energy food store for the plant and animals like us too!

Photosynthesis utilises sunlight energy to convert carbon dioxide and water into glucose (basis of food) and oxygen.

Most of the oxygen is a waste gas by-product to plants, but vital for respiration for us and other animals!

The green pigment chlorophyll is in the subcellular structures called chloroplasts, where photosynthesis takes place in green plant cells.

All of the photosynthetic chemistry facilitated by enzymes (biological catalysts).

The chemistry of photosynthesis is very complicated but it takes place in two main stages.

1. Chlorophyll absorbs a photon of light energy. This sunlight energy (visible light photons) splits water (H2O) into hydrogen ions (H + ) and oxygen (O2).

From the plants point of view, the oxygen gas is given out as a waste material.

2. The hydrogen ions combine with carbon dioxide (CO2) to form glucose molecules (C6H12O6).

The carbon dioxide diffuses in through the stomata of the guard cells - effectively pores that can open and close ie CO2 in, and oxygen O2 out in the day and O2 in at night.

In daylight the rate of photosynthesis will exceed the rate of respiration.

At night the rate of respiration will exceed that of photosynthesis.

Both processes are need to keep the plant alive.

During photosynthesis light energy is absorbed by the green chlorophyll, which is found in chloroplasts in some plant cells and algae.

Chlorophyll looks green because it absorbs in the violet-blue and orange-red regions of visible light, so plants can absorb use the energy from visible electromagnetic radiation.

Plant structure and photosynthesis - leaf structure adaptations that help!

Photosynthesis in the context of plant organs including stems, roots and leaves.

Water and minerals are absorbed from the soil through the roots and moved up through the plant by transpiration.

Wherever a plant is green, photosynthesis is taking place, at least in daylight!

One essential green molecule for photosynthesis is chlorophyll.

The broad green leaves of plants exposed to light provide a large surface area for the light absorbing sites of photosynthesis - more than the thinner stem.

The leaves are thin so the absorbed carbon dioxide has only a short distance to diffuse to photosynthesising cells.

Leaves have veins (vascular bundles) that support the leaf and transport water and minerals to the leaf and glucose away from the leaf.

Epidermal tissues are the outer layers which cover the whole plant.

The mesophyll, between two epidermis layers, is where most photosynthesis happens in the chloroplasts - it all looks green due to the green chlorophyll molecules needed for photosynthesis (they don't absorb green light).

Palisade cells in the mesophyll contains lots of chloroplasts containing chlorophyll - so palisade cells are well adapted for photosynthesis.

The palisade cells are near the top of the leaf and exposed to the most light.

'Physics note': Plants look green because the chlorophyll absorbs the blue and red wavelengths of visible light, but not the green. The green light is either reflected or transmitted so the plant tissue looks green which ever angle you view it.

The upper side of a leaf is smoother and greener - richer in chloroplasts to capture the sunlight The under side of a leaf is rougher - more 'porous' for efficient gas exchange and the veins more prominent

Xylem and phloem networks of cells, transport substances around the plant e.g. sugars like sucrose and glucose from photosynthesis, and through the roots minerals (e.g. magnesium) and water for photosynthesis.

The tissues of leaves are adapted for gas exchange.

The lower epidermis contains lots of stomata (plural of stoma, pores) which let carbon dioxide directly diffuse into the leaf for photosynthesis and oxygen to diffuse out of the leaves - the gas exchange system.

The spongy mesophyll tissue also contains air spaces that help increase the rate of diffusion of gases in and out of the leaves.

In the outer epidermis layer guard cells are adapted to open and close the pores of the stomata (stomatal pores) which allows gas exchange and water evaporation eg for photosynthesis carbon dioxide in and oxygen out.

This helps regulate transpiration and respiration and all connected with photosynthesis. See transport in plants

The epidermal tissues are covered with a waxy cuticle which helps reduce the loss of water by evaporation.

All of the above structures mentioned must be 'connected' for the 'system to function' in a healthy plant.

It should be mentioned that a large percentage of the Earth's photosynthesis occurs in oceans in phytoplankton.

For more on plant structure and function including gas exchange and leaf adaptations see also .

Leaf structure, diffusion and photosynthesis

Carbon dioxide diffuses into the leaves through the stomata and is depleted through photosynthesis.

Therefore as photosynthesis proceeds, the internal carbon dioxide concentration in the leaf is much lower than in the surrounding air, so carbon dioxide will diffuse into the leaf down this concentration gradient.

The rate of diffusion of the carbon dioxide (and any other gas) is increased by:

Increasing the surface area of the leaf - always the broadest part of any plant.

The smaller the distance the molecules have to travel as they diffuse - thin leaves with an even thinner mesophyll layer.

What does the plant do with the glucose produced by photosynthesis using sunlight energy?

Glucose provides energy and can be converted into, and help to synthesise, a wide range of molecules in plant cell chemistry (plant biochemistry). This means plants make their own food!

The glucose produced in photosynthesis may be converted into insoluble starch for storage in leaves, roots and stems.

The insoluble nature of starch makes it a very useful concentrated chemical store of energy - if it was soluble, it would dissolve and diffuse all over the place.

Starch is a natural polymer made from linking many glucose molecules together and is the main chemical energy store in a plant.

A plant can't photosynthesise at night, so it needs energy from somewhere to stay alive at night!

When needed, starch is hydrolysed (broken down) into the useful sugar glucose, so the process of starch formation is reversed.

Glucose sugar is soluble and easily transported around a plant and fuels respiration in the mitochondria of plant cells - which in turn provides the energy for all the cellular processes needed by a plant.

If a plant tried to store the soluble glucose, the cells would absorb water by osmosis, swell up and burst!

Plants need energy from sugars (from photosynthesis) to power their own life supporting systems just as we do.

Plant cells use some of the glucose produced during photosynthesis for immediate respiration - release of energy to power the cell functions and particularly at night when no light can shine on the leaves.

Plant respiration in principle is the reverse of photosynthesis.

glucose + oxygen ==> products + chemical energy (to power the plant cell chemistry)

The energy released enables the plant to convert glucose plus other elements/ions like nitrogen/nitrate into other essential useful chemical substances - some are listed below.

At night there be a net loss of glucose/starch in respiration, but in daylight the rate of photosynthesis will exceed that of respiration in a growing plant so excess glucose can be converted into starch for storage.

. noting that starch and glucose are chemical energy stores.

Glucose is consumed in plant respiration, e.g. in aerobic respiration, plants use oxygen to oxidise glucose to carbon dioxide and water.

The released chemical energy to power all the cell chemistry including the conversion of glucose into starch and making protein.

Don't forget that plants respire all the time, just like us!

Glucose can be converted into starch that can be stored in roots (e.g. potato), stems and leaves, this provides energy at night and in winter.

Starch has the advantage of being insoluble in water, so won't dissolve away unnecessarily from vital energy reserve storage areas.

It can be used when sunlight is low e.g. winter, and of course at night when photosynthesis stops completely.

Also, by being insoluble, it won't affect the water concentration in cells by osmosis.

A cell with a high concentration of glucose would swell up by water absorption interfering with its function.

The chemical energy from glucose is needed to build larger more complex molecules.

Through growth and accumulation of these larger molecules biomass is built up in plants and algae.

Biomass means the mass of living material.

The energy built up in a plant's or algal organism's biomass enters the food chain so animals can now feed on it (herbivores) and themselves be fed on by other animals (carnivores).

This is why at the start of this page it was emphasised that photosynthesising organisms are the main producers of food for most of life on Earth.

Examples of the larger molecules in the biomass of plants and algae

Glucose is used to produce fats or oils (lipids) for storage - provides sources of energy via aerobic respiration, seeds contain food stores based on oils and fats (think of cooking oil from olives or sunflower oil for margarine) and waxes.

Glucose is used to make cellulose, which makes up and strengthens the cell walls eg of the xylem and phloem and is particularly needed in larger quantities in rapidly growing plants.

Amino acids are first synthesised from glucose and nitrate ions (absorbed from soil through the roots) and other minerals before conversion to proteins for tissue cell growth and repair.

Note that to produce proteins, plants also use nitrate ions that are absorbed from the soil.

W hat factors affect the rate of photosynthesis?

The rate of photosynthesis is usually limited by three main environmental conditions - factors :

(i) Shortage of light (usually lack of sunlight) slows photosynthesis - since the greater the light intensity, the greater the rate of photosynthesis.

(ii) Low temperature, slows down the rate of photosynthesis - a general rule for all chemical reactions

A combination of both (i) and (ii) will cause very different rates between photosynthesis in winter (less sunlight time, less intense light, slower) compared to summer (more sunlight time, more intense light, faster).

At night, light is the limiting factor, in winter its usually the temperature in daylight.

If the temperature gets too high photosynthesis will slow down due to enzyme damage.

(iii) A shortage of carbon dioxide will also slow down the rate of photosynthesis but you can artificially increase it by pumping CO2 into a greenhouse structure.

If there is sufficient light and the temperature not too low, the ambient carbon dioxide concentration becomes the limiting factor.

So, three factors affecting the rate of photosynthesis that can be investigated in the laboratory - see 7 graphs later!

Graphs further down the page, separately, discuss a single limiting factor i.e. (i) to (iii) mentioned above.

(iv) However, under some circumstances the essential green pigment chlorophyll might be the limiting factor too.

Lack of chlorophyll/chloroplasts in the plant cells reduce the plant's capacity to photosynthesise.

Stressed or damaged plants may turn pale yellow or develop spots from a fungus, bacteria or virus.

Plants maybe affected by disease eg halo blight, tobacco mosaic virus, poor nutrition - lack of vital minerals.

Also (v) lack of water denatures cells and plants droop, reducing photosynthesis, and eventually die.

Any of these factors can cause damage to chloroplasts or the cell cannot make enough chlorophyll.

Therefore the plant cell capacity to absorb sunlight is reduced weakening the plants growth and development.

(v) Strictly speaking, lack of water is another factor, but that does affect the whole plant.

Light intensity, temperature and the availability of carbon dioxide interact and in practice any one of them may be the factor that limits the speed (rate) photosynthesis.

You can relate the principle of limiting factors to the economics of enhancing the following conditions in greenhouses.

You can carry out laboratory experiments to measure the rate of photosynthesis under various conditions i.e. changing any of the three factors and keeping the other two factors constant.

These experiments and graphical data analysis are discussed in detail further down the page.

Factors controlling the rate of photosynthesis - detailed discussion of typical data graphs

The limiting factor is one that controls the maximum possible rate of the photosynthesis reactions for given set of conditions.

Graph 1. Light intensity limitation

Light energy is needed for photosynthesis, so as the light intensity increases, the rate of photosynthesis chemical reactions steadily increases in a linear manner - 1st part of the graph is 'light limiting'.

More light, more molecules 'energised' to react.

BUT, at the point where the graph becomes horizontal, light is no longer the limiting factor.

However, eventually the rate levels off to become constant due to limitation of the carbon dioxide concentration (too low) or the temperature (too low) and any increase in light intensity has no further effect on the rate of photosynthesis for plant growth.

Two points to bear in mind when studying any of the graphs dealing with photosynthesis.

Since the graph line has become horizontal (flattened out, constant rate), this also means that light intensity is no longer the limiting factor - you must increase carbon dioxide concentration or temperature to increase the rate of photosynthesis - in other words you need increase some other factor.

Remember: Whenever the graph line on a photosynthesis graph becomes horizontal, a limiting factor is coming into play.

Light intensity falls to

zero at night and there is much less light in winter, so these place limits on photosynthesis.

Plants have adapted to live in shaded areas by having larger and thinner leaves to increase the number of chlorophyll molecules to absorb light (see graph 8 ).

Greenhouse design/operation and light intensity.

Lots of glass window panes to let light in.

Site the greenhouse in a non-shaded area.

At night artificial light can be supplied.

However, the light level with have its limit (either sunlight or artificial light at night), so for maximum effect you may still need a warm temperature and a fresh supply of carbon dioxide.

Graph 2. Temperature limitation

Photosynthesis chemical reactions cannot happen without the help of enzymes.

Raising the temperature gives the molecules more kinetic energy so more of them react on collision, and initially, you get the expected (exponential) increase in the speed of the photosynthesis reaction - initially an accelerating curve upwards (non-linear) with increase in temperature increasing plant growth..

However, too high a temperature is just as bad as too a low temperature (which would be too slow).

At temperatures over 40 o C enzymes involved in the process are increasingly destroyed, so photosynthesis slows down and eventually stops because the photosynthesis enzymes are destroyed.

The denaturing of the protein structure caused by the higher temperatures affects the active sites on enzymes (x-reference key and lock mechanism) and they can no longer catalyse the photosynthesis reactions.

A graph of rate of photosynthesis versus temperature rises at first (usual rate of chemical reaction factor), goes through a maximum (optimum temperature) and then falls as the enzymes are becoming increasingly denatured and eventually cease to function.

The final shape of the graph is due to the combination of the two graph trends from increasing rate of reaction versus increase denaturing, both coincident with increase in temperature.

Greenhouse design/operation and temperature

Ideally in greenhouses you would want the optimum temperature, a constant adequate supply of carbon dioxide and plenty of light - hence the use of transparent glass!

A greenhouse warms up by trapping the heat radiation from the sun - the 'greenhouse effect'.

BUT take care that the greenhouse does not get too hot eg by opening ventilation systems or putting up shades.

In cold weather, heaters might be employed in a greenhouse because the temperature may be too low for efficient photosynthesis for plant growth - but heaters increase cost of production.

If the heaters are not electric and burn a fuel like paraffin, then lots of carbon dioxide is produced - quite handy, two factors catered for at the same time!

3. CARBON DIOXIDE CONCENTRATION

Graph 3. Carbon dioxide limitation

Carbon dioxide is needed for photosynthesis, so as the carbon dioxide concentration increases, the rate of photosynthesis chemical reactions steadily increases in a linear manner - initially the reaction rate of photosynthesis is directly proportional to CO2 concentration (can be in air or water)..

However, eventually the rate levels off due to limitation of the light intensity (too low) or the temperature (can be too low or too high) no matter what the increase in the CO2 concentration.

Since the graph line has become horizontal (flattened out), this also means that carbon dioxide concentration is no longer the limiting factor - you must increase light intensity or temperature to increase the rate of photosynthesis.

You should note that the concentration of carbon dioxide in air is only

0.04%, and is often the limiting factor, especially on warm bright sunny days ..

BUT, short dull winter days (low light intensity) and low temperature (slows chemical reactions) can also be the limiting factors.

Greenhouse design/operation and carbon dioxide concentration

If the ambient temperature is warm and the plants/greenhouse in bright sunshine, the limiting factor might be the concentration of carbon dioxide in air.

You do need some ventilation or the level of carbon dioxide gas will fall if the air is not replenished as the carbon dioxide is used up by the plants.

BUT, for maximum effect you need a warm temperature, plenty of light and extra CO2 if you can supply it!

For more on photosynthesis graphs see:

How to successfully operate a commercial greenhouse!

So 3 three factors can be manipulated to increase the rate of photosynthesis and hence increase plant growth.

Summary so far to help increase crop yields

Greenhouse design/operation and the photosynthesis light intensity factor

Lots of glass window panes to let light in.

Site the greenhouse in a non-shaded area.

At night artificial light can be supplied.

However, the light level with have its limit (either sunlight or artificial light at night), so for maximum effect you may still need a warm temperature and a fresh supply of carbon dioxide.

Greenhouse design/operation and the photosynthesis temperature factor

Ideally in greenhouses you would want the optimum temperature, a constant adequate supply of carbon dioxide and plenty of light - hence the use of transparent glass!

A greenhouse warms up by trapping the heat radiation from the sun - the 'greenhouse effect'.

BUT take care that the greenhouse does not get too hot eg by opening ventilation systems or putting up shades.

In cold weather, heaters might be employed in a greenhouse because the temperature may be too low for efficient photosynthesis for plant growth - but heaters increase cost of production.

If the heaters are not electric and burn a fuel like paraffin, then lots of carbon dioxide is produced - quite handy, two factors catered for at the same time!

Greenhouse design/operation and the photosynthesis carbon dioxide level factor

If the ambient temperature is warm and the plants/greenhouse in bright sunshine, the limiting factor might be the concentration of carbon dioxide in air.

You do need some ventilation or the level of carbon dioxide gas will fall if the air is not replenished as the carbon dioxide is used up by the plants.

BUT, for maximum effect you need a warm temperature, plenty of light and extra CO2 if you can supply it!

Overview of operating a successful greenhouse - commercial or amateur grower!

A greenhouse used is to artificially create the best environment for growing plants and increase photosynthesis efficiency.

ventilation - need to keep the air fresh and ensure the carbon dioxide level doesn't fall below that in the air outside.

glass (or transparent plastic) panels - allows the transmission of visible light for photosynthesis and infrared radiation to be absorbed and raise the temperature.

carbon dioxide supply - can artificially increase CO2 available to plants to increase rate of photosynthesis.

water supply - plants need a constant supply of water, the soil or compost may get to dry for optimum plant growth and the higher temperatures in a greenhouse increase the rate of transpiration.

heater - electric to raise temperature on colder days, preferably from renewable source, if paraffin, the combustion produces CO2 so that helps increase the rate of photosynthesis.

artificial lighting - enables photosynthesis to be continuous 24/7 and independent of the weather, BUT you need periods of darkness (use a timer) to allow the plant to transport and store glucose as starch.

humidifier - if the atmosphere becomes too dry the rate of transpiration increases and plants may droop from lack of water

blinds - can be used to control the light if necessary.

thermostat - not sure if this is used in greenhouses?

Growing crops in greenhouses can significantly increase the crop yield for a given area.

Greenhouse horticulture (agricultural growing of flowers, fruit and vegetables) is an intensive farming method using various technological developments - this particularly applies to hydroponics (described on my food production page).

ideally farmers-horticulturalists want optimum yields of crops without excessive leaf or root production.

A greenhouse traps the sunlight energy raising the internal temperature to make it less of a limiting factor but heating may be required in winter.

However, the extra costs of heating, artificial lighting or adding CO2 to the air, must be off-set by selling an acceptable quality product at a sustainable market price that the consumer is prepared to pay!

You can increase the temperature and carbon dioxide levels at the same time by using a paraffin heater - one of the better uses of a fossil fuel when burned to form carbon dioxide!

In summer it might get too hot so extra shade and ventilation may be needed to create cooler conditions.

Using artificial light extends the growing period beyond normal daylight hours - but an extra cost.

You should also note that plants enclosed in a greenhouse are less susceptible to pests and diseases.

Fertilisers may be added to the soil to provide the minerals the plant need's and absorbed from the soil by the root system.

Using greenhouses enables market gardeners to produce more good crops per year and if you can control the conditions and efficiently produce a reasonable quality crop - then your business can be commercially successful.

Large scale greenhouse complexes are proving successful in using artificial growing conditions and employ light and heat controls.

More complex graphs demonstrating more than one limiting factor controlling the rate of photosynthesis

Reminder: The limiting factor is one that controls the maximum possible rate of the photosynthesis reactions for given set of conditions.

For these experiments a suitable temperature must be chosen and kept constant! (eg lab. temp. of

Graph 8 Chlorophyll as a limiting factor

Graph 8 shows the rate of photosynthesis for two plants A and B.

We have looked at the way in which light, temperature or carbon dioxide can be limiting factor.

A shortage of chlorophyll can also be the 4th limiting factor.

Assume the graph for plant A is typical of most plants which are not adapted to live in shaded areas and receive an sample of sunlight i.e. do not live in a very shaded area.

In this case the rate of photosynthesis is limited by temperature or carbon dioxide concentration in the air.

Some plants, like plant B, live in continuous shade i.e. a low level of light intensity.

These plants have adapted to these conditions by evolving to grow a higher ratio of leaves to roots compared to other plants.

The leaves are larger and thinner with a greater surface area so more chlorophyll in chloroplasts is available to absorb light, so increasing the plant's photosynthesis efficiency.

The graph for B show a faster initial rate of photosynthesis because of the higher concentration of chlorophyll, but the rate of photosynthesis levels off before that of plant A as a limiting factor comes into play.

The limiting factor might a low temperature in a shaded area,

or carbon dioxide level if there is no air movement.

Possible practical work you may have encountered - methods of measuring the rate of photosynthesis

You can investigate the need for chlorophyll for photosynthesis with variegated leaves

Taking thin slices of potato and apple and adding iodine to observe under the microscope - test for starch.

Investigating the effects of light, temperature and carbon dioxide levels (using Canadian pondweed, Cabomba, algal balls or leaf discs from brassicas) on the rate of photosynthesis.

You can use computer simulations to model the rate of photosynthesis in different conditions

You can use sensors to investigate the effect of carbon dioxide and light levels on the rate of photosynthesis and the release of oxygen.

You may have done/seen experiments on the rate of photosynthesis in which the volume of oxygen formed is measured with a gas syringe connected to a flask of sodium hydrogen carbonate solution (to supply the carbon dioxide) and Canadian pondweed immersed in it.

All experimental methods depend on measuring the rate of oxygen production as a measure of the rate of photosynthesis.

The faster the oxygen production the faster the photosynthesis.

It is assumed that the rate of oxygen production is proportional to the rate of photosynthesis.

So, how can we measure the rate of photosynthesis?

Next, methods of measuring the rate of photosynthesis

Measuring the r ate of photosynthesis - experimental method 1 measuring the volume of oxygen produced with a gas syringe

You can use this gas syringe system to measure the effects of changing temperature, light intensity and carbon dioxide level (via a sodium hydrogencarbonate solution).

Method 1. Gas syringe system

A lamp and thermostated water bath are not shown in this diagram, but they are in the apparatus diagram for method 2 .

There are several aquatic plants you can use, the most popular seems to Canadian pondweed (elodea canadensis), but this is regarded as an invasive species, so perhaps some other oxygenated aquatic plant should be used!

In this 'set-up' you measure the rate of photosynthesis by measuring the rate of oxygen production as the gas is collected in the gas syringe.

From the graph of volume of oxygen versus time you measure the initial gradient to calculate the rate of production of oxygen as a measure of the rate of photosynthesis.

The graph should be reasonably linear at first e.g. rate of photosynthesis in cm 3 /min .

You can use sodium hydrogencarbonate (NaHCO3) as source of carbon dioxide and vary its concentration to vary the carbon dioxide concentration. You can use from 0.1% to 5% of NaHCO3 ie 0.1g to 5g per 100 cm 3 of water.

With increasing concentration you should see an increase in the rate of oxygen bubbles (eg cm 3 /min), but you must keep the temperature constant eg lab. temp. 20-25 o C, and the light intensity constant by keeping the lamp (not shown in the diagram) the same distance from the flask.

The light from the laboratory itself will contribute, but the total light should be constant.

You need to use the same quantity and batch of pondweed (or other oxygenating aquatic plant).

You use the same volume of water/sodium hydrogencarbonate solution.

Using the set-up described in the diagram, at constant temperature, constant light intensity - by using same lamp at the same distance from the flask, you can investigate the effect of the concentration of carbonate/carbon dioxide on the rate of photosynthesis.

To vary temperature you need to immerse the conical flask in a water bath (not shown) of different, but carefully controlled constant temperatures.

You should be able to demonstrate a maximum

35-40 o C i.e. the rate should be significantly lower at

The concentration of NaHCO3 and the light intensity should be both kept constant.

Varying the light intensity is quite difficult, you need to position a lamp at different measured distances away from the flask, but for accurate results you must take a light meter reading by the flask in the direction of the lamp - but you can still use the basic set-up of apparatus described in method 1. above.

A lamp in position is shown in method 2. and see the discussion on the inverse square law further down the page.

This simple experiment can readily show in principle the effect of changing the three controlling factors of the rate of photosynthesis.

Problems and errors with the method

Ideally the experiments should be done in the dark, with the lamp the only source of light, not very convenient in a classroom situation but it is particularly important when varying the light intensity - I don't see how you can get accurate results for light intensity though using a light meter might just ok?

Do you swirl the flask so the NaHCO3 concentration remains reasonably constant?, but will the same leaf area be exposed to the light in the direction of the lamp?

When varying the temperature it is not easy to maintain a constant temperature - if it falls a little, you could use the average temperature, not as accurate, but better than nothing! A thermostated water bath would be ideal.

The above apparatus is typical of that used in rate of reaction experiments in chemistry.

You can use other experiment designs to look more conveniently, and hopefully more accurately at the three factors that influence the rate of photosynthesis.

Measuring the r ate of photosynthesis - experimental method 2 - timing the movement of a bubbles of gas

You can use this gas syringe system to measure the effects of changing temperature, light intensity and carbon dioxide level (via a sodium hydrogencarbonate solution).

At the end of method 2 the inverse square low of light intensity is explained.

Method2.

Method 2. Following the gas evolution from a gas bubble in a capillary tube

I've seen this sort of set-up in textbooks and on the internet and it seems ok in principle, but I have doubts about its use in practice?

In this the Canadian pondweed (elodea) is enclosed in a boiling tube and placed in a large beaker of water that acts as a simple thermostated bath to keep the temperature constant. Again a thermostated water bath would be ideal.

A lamp is positioned at suitable distances with a ruler.

The oxygen bubbles are channelled into a capillary tube.

From the rate of movement of the bubbles you get an estimate of the rate of production of oxygen as a measure of the rate of photosynthesis.

It might ok just to measure the speed of a bubble down the capillary tube, BUT what happens if it fills with oxygen gas - you won't see any movement.

The general points about investigating the three variables were described in method 1. should be no need to repeat them.

How do you measure the rate?

You can measure the speed of an air bubble by the scale,

If you used a gas syringe here you would get a mixture of gas and liquid in the syringe - not very satisfactory, liquid in the syringe might make it quite stiff in movement and difficult to measure an accurate volume of oxygen gas formed.

Further thoughts on the experimental methods described in methods 1. and 2. above for determining the rate of photosynthesis in Canadian pondweed experiment.

The 'set-up' probably the best system I can devise sitting at home in front of the computer screen!

In method 2 the pondweed tube could be enclosed in a large beaker of water that acts as a simple thermostated bath to keep the temperature constant - ideally a thermostated water bath.

The tube of pondweed is immersed in NaHCO3 solution is subjected to a lamp emitting bright white light at a specific distance from tube of pondweed.

You can again use sodium hydrogencarbonate (NaHCO3) as source of carbon dioxide and vary its concentration to vary the carbon dioxide concentration.

You can use from 0.1% to 5% of NaHCO3 ie 0.1g to 5g per 100 cm 3 of water.

(i) The oxygen bubbles are still channelled into a capillary tube but the gases and liquids allowed to freely exit from the capillary tube - no problem with liquid in the syringe which might quite stiff anyway and difficult to measure an accurate volume.

(ii) A T junction in the tubing allows the 'injection' of water into the gas stream to make bubbles of gas visible.

You need to use the same quantity and batch of pondweed (or other oxygenating aquatic plant).

You use the same volume of water/sodium hydrogencarbonate solution.

What can you measure and vary?

Measuring the rate of photosynthesis by measuring the rate of oxygen gas production in the gas syringe is more accurate but requires more time to get a set of readings to plot a graph.

Measuring the speed of the horizontal movement of the gas bubbles is quite easy via the accurate linear scale and stopwatch.

You can use quite a long uniform capillary tube to increase the sensitivity and hence accuracy of the experiment.

For each set of experimental conditions get at least three reasonably consistent readings and compute an average for the best accuracy.

The speed of bubbles in cm/s gives you a relative measure of the rate of the overall reaction of photosynthesis to produce oxygen.

With increasing concentration (of NaHCO3) you should see an increase in the rate of oxygen bubbles, but you must keep the temperature constant eg lab. temp. 20-25 o C, and the light intensity constant by keeping the lamp a fixed distance from the flask. The light from the laboratory itself will contribute, but the total light should be constant and you can use a light meter to ensure the same light intensity.

Try to use a range of concentrations eg 1% to 5% solutions (1g - 5g NaHCO3 per 100 cm 3 of water).

To vary temperature you need to immerse the boiling tube in water baths of different carefully controlled and constant temperatures - ideally using a thermostated water bath.

You should be able to get enough results eg 5 o increments from 15 o C to 50 o C to show maximum the maximum rate of photosynthesis expected to be around 35-40 o C.

The concentration of NaHCO3 and the light intensity should be both kept constant.

Varying the light intensity is quite difficult, you need to position a lamp at different measured distances away from the pondweed tube.

You can calculate the relative intensity using the inverse square law, see light intensity section on this page.

BUT, for accurate results you should take a light meter reading by the flask in the direction of the lamp (see the discussion on the inverse square law further down the page).

You must choose, and keep constant, both the temperature and sodium hydrogencarbonate concentration of appropriate values eg a 2% solution of NaHCO3 and 25 o C.

Although I think this is an improvement on method 2, its still quite difficult to get accurate results.

I think a light meter is essential for accurate results - changing the lamp distance is relevant to changing the light intensity, BUT, intensity is NOT a simple function of distance.

You need to use the same sample of pondweed, but is it always the same leaf area towards the light?

The experimental runs should not take too long as the NaHCO3/CO2 concentration is falling all the time.

Graphs of experimental data and their interpretation

Seven graphs have already been fully described on this page.

The relative intensity of the light from a fixed power is governed by an inverse square law.

When investigating the influence of light intensity on the rate of photosynthesis you must appreciate the inverse square law applied to light intensity for a fixed lamp power and light emission.

As you move the lamp further away, the light intensity falls, and so should the rate of photosynthesis.

BUT the light intensity is inversely proportional to the distance between the light source and the experiment tube squared.

From a specific light source .

relative light intensity = 1 / d 2

. the light intensity is in arbitrary units, d = distance of the lamp from experiment.

The effect of the law can be demonstrated by some simple calculations .

. treating this idea as both predictions and ideal theoretical results!

The inverse square law for relative light intensity means that the relative brightness that the plant experiences falls away quite dramatically as the lamp is move further and further from the experiment tube.

Graphs of rate of photosynthesis versus distance of the lamp from experiments such as method 3.

These graphs are plots of the theoretical data used in the table above assuming a constant light source (a lamp!).

Graph (a) shows how rapidly the light intensity decreases as you move the experiment tube/flask away from the light source, shown by the equally rapid decline in the rate of photosynthesis. This is a consequence of the inverse square law of light intensity. You can show by experiment the rate of photosynthesis is proportional to the light intensity where it is the limiting factor. The graph also shows that the relationship between rate of photosynthesis and lamp distance is not linear.

Graph (b) shows that the rate photosynthesis is not proportional to reciprocal of the lamp distance, but it is a more linear graph than (a).

Graph (c) shows (for ideal results) that the rate of photosynthesis is proportional to the reciprocal of the lamp distance squared (and the lamp light intensity is proportional to 1 / d 2 ). Therefore in graph (c) the horizontal axis could be also labelled relative light intensity, a proportional linear relationship with the rate of photosynthesis.

Some simple experiments to investigate aspects of photosynthesis

Some demonstrations of involving photosynthesis

Demonstrating the presence of starch in plant leaves

Simple experiments on starch production in plants

Experiment 1. To test for starch in leaves

A leaf, held with tongs/tweezers is 'dunked' into boiling water - to stop all its chemical reactions.

The leaf is placed in a boiling tube of alcohol and gently heated in an electric water bath - this dissolves the green chlorophyll and turns the leaf a very pale colour - no longer green!

Take care, ethanol is highly flammable - bunsen burners not recommended!

The almost white leaf is rinsed with cold water and laid out on a filter paper.

With a teat pipette, spot a few drops of iodine solution onto the leaf.

If starch is present , a blue-black colour will appear - the simple standard food test for starch molecules .

Preparation of plants and set-up for experiments 2. and 3.

You can do simple experiments with a small plants in plant pots, if necessary keep them enclosed in a bell jar.

You have to 'de-starch' the plants by leaving them in the dark for at least 48 hours.

The plant will use up its starch energy store to keep itself alive!

You can use this set-up to do a couple of simple experiments, and, finally using the starch test described above, to show what is required for photosynthesis, or indeed, if photosynthesis was taking place.

Experiment 2. To show that light is needed for photosynthesis

From your stock of 'de-starched' plants, you keep one in the dark and one into bright sunlight or artificial light.

After 24 hours you test a sample leaf from each plant for the presence of starch.

The leaf from the plant in the dark should not give the blue-black colour with iodine solution - photosynthesis had not taken place.

The leaf from the plant in the light should have produced starch from photosynthesis, and after testing should give a blue-black colour with iodine solution showing the presence of starch.

This shows light is need for photosynthesis.

The plants should not be enclosed in the bell jar, so that each plant has access to carbon dioxide in the atmosphere.

Ideally, the plants should be identical and kept at the same temperature for a fair test.

Experiment 3. To show that carbon dioxide is needed

From your stock of 'de-starched' plants, two plants are left out in daylight or artificial light.

BUT, one of the plants is put in a bell jar with a small petri dish of soda lime.

(a) This isolates one of the plants from the surrounding 'normal' atmosphere.

(b) Soda lime absorbs and chemically reacts with carbon dioxide to give a solid product - thus removing carbon dioxide from the atmosphere around the plant.

The plants are then left out for 24 hours.

After 24 hours you test a sample leaf from each plant for the presence of starch.

The leaf from the plant left out in the laboratory (not in the bell jar) with access to the atmosphere should have produced starch from photosynthesis - after testing should give a blue-black colour with iodine solution showing the presence of starch.

However, the leaf from the plant in the bell jar should not give the blue-black colour with iodine solution - showing photosynthesis to form starch had not taken place despite having access to light.

This shows carbon dioxide is need for photosynthesis.

Ideally, the plants should be identical and kept at the same temperature for a fair test.

Experiment 4. To show that carbon dioxide is involved in the gas exchange of photosynthesis

This is a simple photosynthesis gas exchange experiment using hydrogencarbonate indicator and plant leaves.

Three test tubes are set up as in the diagram and described below.

All three test tubes are exposed to the same intensity of bright light - sunlight or lamp.

A test tube is set up just containing a few cm 3 of the orange hydrogencarbonate indicator.

This acts as a control and shouldn't change from its original orange colour, since there are no plant leaves in it and it is sealed to the atmosphere, there should be just the normal background level of carbon dioxide above the indicator.

If the indicator solution becomes more acidic, it becomes yellow.

If the indicator solution becomes less acidic, it becomes red.

Now remember, carbon dioxide gas is acidic when dissolved in water.

Observations and interpretation

2. Leaves exposed to bright light

When exposed to light the leaves can photosynthesise and absorb carbon dioxide.

6H2O + 6CO2 ====> C6H12O6 + 6O2

Therefore carbon dioxide will be absorbed by the plant and the reduction of carbon dioxide means conditions are less acidic.

So the hydrogencarbonate indicator changes to red - solution less acidic.

In bright light the rate of photosynthesis will be greater than the rate of respiration.

(Oxygen will replace the carbon dioxide, but that is not detected in this experiment, but you could set up a system to collect it and test the gas with a glowing splint - which should be reignited.)

3. Leaves shaded from light

If little light can reach the surface of the leaves, then photosynthesis cannot take place efficiently.

In order for the leaves (plant) to survive they must be a switch from less photosynthesis to more respiration.

C6H12O6 + 6O2 ====> 6H2O + 6CO2

The respiring plant leaves give out carbon dioxide which makes the condition more acidic.

Therefore the hydrogencarbonate indicator turns yellow - solution more acidic.

In shade the rate of photosynthesis will be less than the rate of respiration.

Experiment 5. Simple demonstration of the effect of light on the rate of photosynthesis

You can use this simple investigation experiment to help you design more sophisticated and more accurate quantitative experiments described in method 1. gas syringe system and method 2. moving gas bubble system .

You set up a beaker filled with water or sodium hydrogen carbonate solution.

In the beaker you place an oxygenating plant like a pondweed inside an inverted filter funnel.

You fill a test tube with water and invert it, still filled with water, and place it over the exit from the filter funnel.

When you shine bright light (sunlight or lamp) on the system, you should see bubbles of oxygen gas rising and collecting in the inverted test tube.

6H2O + 6 CO2 == light ==> C6H12O6 + 6 O2

If you collect enough gas, it should ignite a glowing splint - a simple chemical test for oxygen.

You can play around with a lamp distance to increase or decrease the light intensity and note any difference in the rate of bubble formation.

You should find adding sodium hydrogencarbonate speeds up photosynthesis because it supplies more carbon dioxide - there is only small amount dissolved in tap/deionised water.

Again, you could compare water with sodium hydrogencarbonate solution at the same light intensity.

BUT, this set-up is no good for looking at temperature.

In fact the whole experiment isn't very accurate at all.

The bubbles tend to form randomly, no way of accurately measuring gas volume or the rate at which the gas is evolved, no thermostated bath to control and vary temperature.

Hence the need for method 1. gas syringe system and method 2. moving gas bubble system .

Three year old granddaughter Niamh doing a bit of garden science!

General PLANT BIOLOGY revision notes

and a section on Stem cells and uses - meristems in plants (at the end of the page!)

Section on plants on Cloning - tissue culture of plants gcse biology revision notes

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General characteristics

The study of photosynthesis began in 1771 with observations made by the English clergyman and scientist Joseph Priestley. Priestley had burned a candle in a closed container until the air within the container could no longer support combustion. He then placed a sprig of mint plant in the container and discovered that after several days the mint had produced some substance (later recognized as oxygen) that enabled the confined air to again support combustion. In 1779 the Dutch physician Jan Ingenhousz expanded upon Priestley’s work, showing that the plant had to be exposed to light if the combustible substance (i.e., oxygen) was to be restored. He also demonstrated that this process required the presence of the green tissues of the plant.

In 1782 it was demonstrated that the combustion-supporting gas (oxygen) was formed at the expense of another gas, or “fixed air,” which had been identified the year before as carbon dioxide. Gas-exchange experiments in 1804 showed that the gain in weight of a plant grown in a carefully weighed pot resulted from the uptake of carbon, which came entirely from absorbed carbon dioxide, and water taken up by plant roots the balance is oxygen, released back to the atmosphere. Almost half a century passed before the concept of chemical energy had developed sufficiently to permit the discovery (in 1845) that light energy from the sun is stored as chemical energy in products formed during photosynthesis.


Photosynthesis - Light Intensity - Biology

In photosynthesis, the energy from the sun is used to turn carbon dioxide (CO2) and water into sugar. Oxygen is a waste product.

More light can mean more photosynthesis. It doesn’t necessarily mean more though. When we think of photosynthesis as a process, we can see that there are at least three things that can limit the process: light, water, and carbon dioxide. More light won’t help if we don’t have enough water and carbon dioxide.

Actually, most places on Earth have the same amount of carbon dioxide in the atmosphere, but a plant can only get it by opening holes in its leaves. These holes are too small for you to see without a strong microscope, but they are big enough to let water vapor out of the plant. So water is an important limit on a plant. More light is actually a problem if water is scarce, because even more water will evaporate from the plant.

This is an example of how increasing one factor (sunlight) can lead to another factor (water) being limiting.

How can you look at a landscape and tell whether a lot of photosynthesis usually happens there?

So by level of light you probably mean light intensity which is something that can be measured. Light intensity is usually defined as the energy hitting an area over some time period. So in the case of a plant, a higher light intensity means more packets of light called “photons” are hitting the leaves. As you rise from low light intensity to higher light intensity, the rate of photosynthesis will increase because there is more light available to drive the reactions of photosynthesis. However, once the light intensity gets high enough, the rate won’t increase anymore because there will other factors that are limiting the rate of photosynthesis. A limiting factor could be the amount of chlorophyll molecules that are absorbing the light. At a very high intensity of light, the rate of photosynthesis would drop quickly as the light starts to damage the plant.

This is a very important aspect of photosynthesis. As you are probably aware, Photosynthesis is a chemical reaction that captures light energy and turns it into sugar. These sugars are then used by the plant as energy for any number of things. The process of photosynthesis requires three things: Light, Carbon dioxide and water. If any one of these things is in short supply, then photosynthesis cannot happen. When you increase the level of light, plants will photosynthesize more. But, if you have too much light, than the other 2 ingredients become limiting and photosynthesis can no longer increase with the level of light. When this occurs, leaves can experience sunburn damage. If you've ever seen a leaf with large dry brown sections on a living leaf, it is because that leaf experienced sunburn.

With too little light, photosynthesis cannot occur either and the plant suffers without the production of sugars. There are many complicating interactions between plants and light. I hope that you continue to investigate this as the story gets more interesting and exciting the deeper you go.

Photosynthesis needs light, but it also needs other things, and too much light can create heat and dryness that are bad for photosynthesis. For this reason plants in different environments have different structures to help them get the right amount of light. In rain forests, where there is plenty of water, trees grow very tall to reach as much light as they can. In deserts, plants use hairs or scales on their leaves to reduce the amount of light they receive to keep the light from driving the temperature up too high or causing the plants to dry out.

I am not sure what you mean by "level" of light, but I will answer your question in to ways - in terms of the intensity of light and wavelength of light.

Photosynthesis needs water, carbon dioxide, chlorophyll, light, and the right temperature. Light is an extremely important factor for the process. If there is enough water, carbon dioxide, and the temperature is right, light becomes the factor which will affect photosynthesis. Most of the time, when the intensity of light is high, you will get a a greater rate of photosynthesis. But, this rate has a limit, and once that limit is hit you can't increase the rate past that limit.

Chlorophyll is a green pigment in the chloroplast of the plant cell which absorbs the light. This mean it will absorb any wavelength of light which is not in the green spectrum of light. If you look at a spectrum from 400nm-700nm. The amount of light absorbed will increase until it reaches a peak at about 450nm ​(blue light). Then it will start decreasing and be very low (almost 0) through the 500-550nm (green light) and then it will increase again peaking at about 700nm (red and yellow light).


8.2 The Light-Dependent Reactions of Photosynthesis

How can light be used to make food? When a person turns on a lamp, electrical energy becomes light energy. Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build carbohydrate molecules (Figure 8.9). However, autotrophs only use a few specific components of sunlight.

What Is Light Energy?

The sun emits an enormous amount of electromagnetic radiation (solar energy). Humans can see only a fraction of this energy, which portion is therefore referred to as “visible light.” The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength , the distance between consecutive points of a wave. A single wave is measured from two consecutive points, such as from crest to crest or from trough to trough (Figure 8.10).

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The electromagnetic spectrum is the range of all possible frequencies of radiation (Figure 8.11). The difference between wavelengths relates to the amount of energy carried by them.

Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength (or the more stretched out it appears in the diagram), the less energy is carried. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving a heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

The electromagnetic spectrum (Figure 8.11) shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, explaining why both X-rays and UV rays can be harmful to living organisms.

Absorption of Light

Light energy initiates the process of photosynthesis when pigments absorb the light. Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an orbital electron to a populatable, excited (quantum) state. Energy levels higher than those in blue light will physically tear the molecules apart, called bleaching. So retinal pigments can only “see” (absorb) 700 nm to 400 nm light, which is therefore called visible light. For the same reasons, plants pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm plant physiologists refer to this range for plants as photosynthetically active radiation.

The visible light seen by humans as white light actually exists in a rainbow of colors. Certain objects, such as a prism or a drop of water, disperse white light to reveal the colors to the human eye. The visible light portion of the electromagnetic spectrum shows the rainbow of colors, with violet and blue having shorter wavelengths, and therefore higher energy. At the other end of the spectrum toward red, the wavelengths are longer and have lower energy (Figure 8.12).

Understanding Pigments

Different kinds of pigments exist, and each has evolved to absorb only certain wavelengths (colors) of visible light. Pigments reflect or transmit the wavelengths they cannot absorb, making them appear in the corresponding color.

Chlorophylls and carotenoids are the two major classes of photosynthetic pigments found in plants and algae each class has multiple types of pigment molecules. There are five major chlorophylls: a, b, c and d and a related molecule found in prokaryotes called bacteriochlorophyll. Chlorophyll a and chlorophyll b are found in higher plant chloroplasts and will be the focus of the following discussion.

With dozens of different forms, carotenoids are a much larger group of pigments. The carotenoids found in fruit—such as the red of tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the orange of an orange peel (β-carotene)—are used as advertisements to attract seed dispersers. In photosynthesis, carotenoids function as photosynthetic pigments that are very efficient molecules for the disposal of excess energy. When a leaf is exposed to full sun, the light-dependent reactions are required to process an enormous amount of energy if that energy is not handled properly, it can do significant damage. Therefore, many carotenoids reside in the thylakoid membrane, absorb excess energy, and safely dissipate that energy as heat.

Each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light, which is the absorption spectrum . The graph in Figure 8.13 shows the absorption spectra for chlorophyll a, chlorophyll b, and a type of carotenoid pigment called β-carotene (which absorbs blue and green light). Notice how each pigment has a distinct set of peaks and troughs, revealing a highly specific pattern of absorption. Chlorophyll a absorbs wavelengths from either end of the visible spectrum (blue and red), but not green. Because green is reflected or transmitted, chlorophyll appears green. Carotenoids absorb in the short-wavelength blue region, and reflect the longer yellow, red, and orange wavelengths.

Many photosynthetic organisms have a mixture of pigments using them, the organism can absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation (Figure 8.14).

When studying a photosynthetic organism, scientists can determine the types of pigments present by generating absorption spectra. An instrument called a spectrophotometer can differentiate which wavelengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute from it the absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify which wavelengths of light an organism can absorb. Additional methods for the identification of plant pigments include various types of chromatography that separate the pigments by their relative affinities to solid and mobile phases.

How Light-Dependent Reactions Work

The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules. The light-dependent reactions are depicted in Figure 8.15. Protein complexes and pigment molecules work together to produce NADPH and ATP.

The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a photosystem , two types of which are found embedded in the thylakoid membrane, photosystem II (PSII) and photosystem I (PSI) (Figure 8.16). The two complexes differ on the basis of what they oxidize (that is, the source of the low-energy electron supply) and what they reduce (the place to which they deliver their energized electrons).

Both photosystems have the same basic structure a number of antenna proteins to which the chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by the light-harvesting complex , which passes energy from sunlight to the reaction center it consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules as well as other pigments like carotenoids. The absorption of a single photon or distinct quantity or “packet” of light by any of the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons.

Visual Connection

What is the initial source of electrons for the chloroplast electron transport chain?

The reaction center contains a pair of chlorophyll a molecules with a special property. Those two chlorophylls can undergo oxidation upon excitation they can actually give up an electron in a process called a photoact . It is at this step in the reaction center, this step in photosynthesis, that light energy is converted into an excited electron. All of the subsequent steps involve getting that electron onto the energy carrier NADPH for delivery to the Calvin cycle where the electron is deposited onto carbon for long-term storage in the form of a carbohydrate.PSII and PSI are two major components of the photosynthetic electron transport chain , which also includes the cytochrome complex . The cytochrome complex, an enzyme composed of two protein complexes, transfers the electrons from the carrier molecule plastoquinone (Pq) to the protein plastocyanin (Pc), thus enabling both the transfer of protons across the thylakoid membrane and the transfer of electrons from PSII to PSI.

The reaction center of PSII (called P680 ) delivers its high-energy electrons, one at the time, to the primary electron acceptor , and through the electron transport chain (Pq to cytochrome complex to plastocyanine) to PSI. P680’s missing electron is replaced by extracting a low-energy electron from water thus, water is split and PSII is re-reduced after every photoact. Splitting one H2O molecule releases two electrons, two hydrogen atoms, and one atom of oxygen. Splitting two molecules is required to form one molecule of diatomic O2 gas. About 10 percent of the oxygen is used by mitochondria in the leaf to support oxidative phosphorylation. The remainder escapes to the atmosphere where it is used by aerobic organisms to support respiration.

As electrons move through the proteins that reside between PSII and PSI, they lose energy. That energy is used to move hydrogen atoms from the stromal side of the membrane to the thylakoid lumen. Those hydrogen atoms, plus the ones produced by splitting water, accumulate in the thylakoid lumen and will be used synthesize ATP in a later step. Because the electrons have lost energy prior to their arrival at PSI, they must be re-energized by PSI, hence, another photon is absorbed by the PSI antenna. That energy is relayed to the PSI reaction center (called P700 ). P700 is oxidized and sends a high-energy electron to NADP + to form NADPH. Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP + into NADPH. The two photosystems work in concert, in part, to guarantee that the production of NADPH will roughly equal the production of ATP. Other mechanisms exist to fine tune that ratio to exactly match the chloroplast’s constantly changing energy needs.

Generating an Energy Carrier: ATP

As in the intermembrane space of the mitochondria during cellular respiration, the buildup of hydrogen ions inside the thylakoid lumen creates a concentration gradient. The passive diffusion of hydrogen ions from high concentration (in the thylakoid lumen) to low concentration (in the stroma) is harnessed to create ATP, just as in the electron transport chain of cellular respiration. The ions build up energy because of diffusion and because they all have the same electrical charge, repelling each other.

To release this energy, hydrogen ions will rush through any opening, similar to water jetting through a hole in a dam. In the thylakoid, that opening is a passage through a specialized protein channel called the ATP synthase. The energy released by the hydrogen ion stream allows ATP synthase to attach a third phosphate group to ADP, which forms a molecule of ATP (Figure 8.16). The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to an area of low concentration through a semi-permeable structure.


Earlier Research Highlights

Photosynthetic organisms use several strategies to cope with vast solar flux intensity differences based on seasonal changes, time of day and weather conditions. Given too much light, the photosynthetic apparatus produces damaging radical species, while not enough light leaves the organism starved for chemical energy and stunts growth. Photosynthesis at very low light flux is particularly challenging, as the usual coping mechanism – increasing the pigment antenna to reaction center ratio by synthesizing more antenna pigment or protein complexes – requires more chemical energy to be spent, not less. This paradox has been resolved in some cyanobacteria that have developed an alternative strategy. Under low light conditions many cyanobacteria use a standard version of the D1 reaction center protein subunit (D1:1) of Photosystem II (PSII), the complex that uses sunlight to convert solar into chemical energy by making its own fuel. It does so by transferring hydrogen atoms from water to plastoquinone molecules, while producing a proton gradient (a form of stored energy) and oxygen as byproduct. However, when the cells are exposed to a stress such as even moderate light intensity, a more robust D1 isoform (D1:2) is produced and preferentially incorporated into PSII. D1:2 protects cell from the consequences of high light flux: both radical damage and aberrant photochemistry. The D1:2 isoform is so useful that eukaryotic phototrophs (all algae and higher plants on earth) only contain this version in their genomes. At low light, they must divert resources to increase their relative antenna size, which may result in survival, but severely stunts growth.

Why then have cyanobacteria maintained the seemingly inferior D1:1 isoform over billions of years of evolution and, equally enigmatic, why is it the dominant version expressed under normal light conditions? In a recent paper in The Journal of Biological Chemistry, researchers at Rutgers University and the University of California, San Diego have shown that D1:1-PSII is not only more efficient at converting solar to chemical energy at very low light intensities compared to D1:2-PSII, but also grows faster. Their research shows that D1:1-PSII extends the lifetime of the transient chemical intermediates that form the charge separated state in the reaction center, the first “electrical battery” of photosynthesis. The authors show that transgenic algal cells containing only the cyanobacterial D1:1-PSII accumulate more biomass than cells containing only cyanobacterial D1:2-PSII or the native algal D1 isoform, at very low incident light flux or in dense cultures where cells self shade. This discovery not only answers an important question in the evolution of photosynthesis, but may open the door for applications in the commercial growth of high density biofuels and agronomic crops.

Publication:
Vinyard, D. J., Gimpel, J., Ananyev, G. M., Cornejo, M. A., Golden, S. S., Mayfield, S. P., and G. C. Dismukes, “Natural variants of Photosystem II subunit D1 tune photochemical fitness to solar intensity.” Journal of Biological Chemistry, 2013, 288(8): 5451-5462.

Group Members:
Visit the G. Charles Dismukes website for information about group members.


The Effects of Light Intensity and Wavelength on the Rate of Photosynthesis

In this simulation, you will be manipulating two variables: light intensity and light wavelength. The amount of ATP produced will change depending upon the set parameters. Go to "Johnson Explorations: Photosynthesis" located at http://www.mhhe.com/biosci/genbio/biolink/j_explorations/ch09expl.htm .

Procedure: Your task is to use the simulation to determine how wavelength and intensity affect the rate of photosynthesis (and the production of ATP). Keep in mind you are dealing with two variables, so in order to determine absolutely how each factor affects photosynthesis, you must keep one variable constant while manipulating the other variable.

Lab Report must containt the following typed sections.

1. Introduction: Describe briefly the light dependent reactions and propose a hypothesis to answer the experimental question.

2. Data: Include data tables for the simulation. The tables must show clearly trends resulting from changes of intensity and changes of wavelenth. Multiple data tables would probably be best here. In Microsoft Word, use the "table" function to organize your data.

3. A graph showing how the percentage of ATP changed (Y axis) as a result of changes in wavelength and intensity (X axis). Two graphs would be best here. You may use Create A Graph to make your graphs, or use a spreadsheet or graphing program of your choice.

3. Conclusion: Use your data to answer the experimental question. Answer clearly how light wavelength affects the reaction, and how light intensity affects the reactions. Offer an explanation of the results, taking into account the principles of photosynthesis and the light reaction.


Effect of Light Intensity on Photosynthesis

A scientific paper submitted in partial fulfillment of the requirements in Botany 1 laboratory under Ms. Ivy Amor F. Lambio, 2 nd semester, 2015-2016.

This study was conducted to know the effect of varying light intensities on the rate of photosynthesis of a Hydrilla plant. The rate of photosynthesis of the plant was determined using bubble counting method. The bubble is the oxygen produced during the process. The set-up was placed in three areas with different light intensities, with darker light, low light, and strong light. The number of bubbles observed in the control is less than the observed bubbles in the low light. Also, the bubbles in the low light are less than those in the strong light. Thus, the higher the light intensity, the greater the rate of photosynthesis of a Hydrilla plant.

Photosynthesis is one of the two plant physiological processes which occur primarily in leaves (Zafaralla, 2007). It is the process in which solar energy is converted into chemical energy. According to Merriam-Webster (2016), “photosynthesis is the synthesis of chemical compounds with the aid of radiant energy and especially light.” Photosynthesis involves two sets of reactions the light reactions where chlorophyll molecules absorb solar energy to energize electrons used in ATP production (Roes, 1991). The second is the Calvin cycle reaction where CO2 is taken up and reduced to a carbohydrate that can be converted to glucose, ATP and NADPH from light reactions are also needed. Therefore light is actually one of the essential factors in the process of photosynthesis.

Light intensity is the rate at which the light spreads over a surface of a given area. Light is one of the external factors that affect the rate of photosynthesis.

The rate of photosynthesis is directly proportional to light intensity (Kent, 2000).

In this experiment, a Hydrilla sprigs, a beaker, test tube, funnel, water, box, and light source are needed. The Hydrilla sprigs was placed in a large glass funnel such that their freshly cut ends are towards the stem of the funnel. The funnel was inverted into a 500-mL beaker 3/4 filled with water. The test tube was filled with water and was carefully inverted over the funnel stem. [pic 1]

Plate 8a . A set-up to demonstrate photosynthesis by the bubble-counting method

This set-up was observed under ordinary room light condition (low light intensity). The oxygen given off by the photosynthesizing sprigs appears in the form of bubbles. A relative estimate of the process of photosynthesis was made by counting the number of bubbles coming out of the funnel regularly per minute time. Next, the set-up was illuminated with strong light (high light intensity). And lastly, the light was put off then the set-up was transferred to a darker part of the room (inside a box). Observations were made and recorded.

Table 8a. The number of oxygen bubbles per minute given off by photosynthesizing Hydrilla sprigs in high and low light intensities


Photosynthesis

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar.

Green Tree Leaves

The plant leaves are green because that color is the part of sunlight reflected by a pigment in the leaves called chlorophyll.

Photograph courtesy of Shutterstock

Most life on Earth depends on photosynthesis.The process is carried out by plants, algae, and some types of bacteria, which capture energy from sunlight to produce oxygen (O2) and chemical energy stored in glucose (a sugar). Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores.

The process

During photosynthesis, plants take in carbon dioxide (CO2) and water (H2O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Inside the plant cell are small organelles called chloroplasts, which store the energy of sunlight. Within the thylakoid membranes of the chloroplast is a light-absorbing pigment called chlorophyll, which is responsible for giving the plant its green color. During photosynthesis, chlorophyll absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.

Light-dependent reactions vs. light-independent reactions

While there are many steps behind the process of photosynthesis, it can be broken down into two major stages: light-dependent reactions and light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light-dependent reaction. The chlorophyll absorbs energy from the light waves, which is converted into chemical energy in the form of the molecules ATP and NADPH. The light-independent stage, also known as the Calvin Cycle, takes place in the stroma, the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light-independent reaction. During this stage, energy from the ATP and NADPH molecules is used to assemble carbohydrate molecules, like glucose, from carbon dioxide.

Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. C3 photosynthesis is used by the majority of plants. It involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become glucose. C4 photosynthesis, on the other hand, produces a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle. A benefit of C4 photosynthesis is that by producing higher levels of carbon, it allows plants to thrive in environments without much light or water.

The plant leaves are green because that color is the part of sunlight reflected by a pigment in the leaves called chlorophyll.


Watch the video: Effect of a Varying Light Intensity on Rate of Photosynthesis-Updated (January 2022).