PHOTOSYNTHESIS

Its importance and factors controlling the rate of plant photosynthesis

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

and perhaps some stuff for A level biology students?

INTRODUCTION

Plants and algae are producers based on the chemistry of photosynthesis and the start of most food chains and 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 eg fish, also depend, initially, on photosynthesis in eg plankton or algae. A food chain is a means of transferring the energy from photosynthesis 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!

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

    • Wherever a plant is green, photosynthesis is taking place!

    • The leaves of plants exposed to light provide a broader area than the stem for sites of photosynthesis.

    • Epidermal tissues, the outer layers which cover the whole plant,

    • 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.

      • Palisade cells in the mesophyll contain chlorophyll and are adapted for photosynthesis.

    • Xylem and phloem, which transport substances around the plant eg sugars like sucrose and glucose from photosynthesis, and through the roots minerals (eg magnesium) and water for photosynthesis.

    • 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.

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

    • It should be mentioned that a large percentage of photosynthesis occurs in oceans via phytoplankton.


  • What is the process of PHOTOSYNTHESIS

  • Photosynthesis is summarised by the equation:

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

    •  6H2O(l) + 6CO2(g) == sunlight ==> C6H12O6(aq) + 6O2(g)

    • This is overall an endothermic chemical reaction, sunlight energy is absorbed in the process.

    • Although the photosynthesis equation presented above looks like a one stage reaction, its much more complicated than that.

      • There are two main sets of reactions or 'stages' to photosynthesis.

      • (i) The light energy (photons) is used to split water into oxygen gas and hydrogen ions.

      • (ii) The hydrogen ions combine with carbon dioxide to make glucose (and water, but this is not an overall product of photosynthesis).

    • Photosynthesis is the process by which plants make food, for themselves, and for most animal life, including us too via food chains!

    • Photosynthesis utilises sunlight energy to convert carbon dioxide and water into glucose (basis of food) and oxygen (most of it a waste gas to plants, but vital for respiration for us and other animals!).

    • The green pigment chlorophyll is in structures called chloroplasts, so photosynthesis takes place in chloroplasts in green plant cells with the photosynthetic chemistry facilitated by enzymes (biological catalysts).

    • 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.

    • 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.

    • This energy is used by converting carbon dioxide (from the air) and water (from the soil) into sugars (e.g. glucose) and oxygen is released as a by-product.

    • Photosynthesis takes place in the leaves of all green plants and is the main function of leaves.

    • So carbon dioxide diffuses into leaves, water comes up from the roots via the xylem tubes, oxygen diffuses out and sugars are transported around the plant by the phloem tubes.

  • The structure of a leaf is adapted for photosynthesis e.g.
    • (a) A large surface area is provided by having broad leaves, beneath the apparently flat surface of a leaf is quite a porous layer of air spaces between the outer layers of cells - particularly on the underside of leaves - quite often the lower surface of a leaves feel rougher and 'roughness' means a more disrupted surface of a larger surface area.
    • (b) The cells contain the green pigment chlorophyll in chloroplasts to absorb light, that's why plants look so green, they contain a relatively high concentration of chlorophyll,
    • (c) The stomata facilitate gas exchange (carbon dioxide, oxygen and water vapour), stomata are tiny holes in the leaves which can open and close to let oxygen and carbon dioxide in and out (both associated with photosynthesis and respiration) and water vapour out (transpiration).
  • The rate of photosynthesis may be limited by two environmental conditions:

    • (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.

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

    • But there are other factors too.

    • (iii) 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.

    • These are three factors affect the rate of photosynthesis can be investigated in the laboratory - see seven graphs later!

      • Graphs 1. to 3. down the page discuss a single limiting factor ie (i) to (iii) mentioned above.

    • (iv) The essential green pigment chlorophyll might also be the limiting factor. 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.

      • The plant maybe affected by disease eg halo blight, tobacco mosaic virus, or poor nutrition - lack of vital minerals. Also lack of water denatures cells and plants droop 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 weakened the 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.


  • What does the plant do with the glucose produced by photosynthesis?

  • Glucose provides energy and can be converted into, and help to synthesise, a wide range of molecules in plant cell chemistry (plant biochemistry).

  • 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.

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

    • 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.

    • 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.

  • Some glucose in plants and algae is also used for ...

    • Glucose is consumed in plant respiration, plants use oxygen to oxidise glucose to carbon dioxide and water and releasing energy to power all the cell chemistry including the conversion of glucose into starch and making protein.

    • 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.

    • Glucose is used to produce fat or oil (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 converted to proteins for tissue cell growth and repair.

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


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

  • Graph 1. Light 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.

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

      • Since the graph line has become horizontal (flattened out), 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.

    • Light intensity falls to ~zero at night and there is much less light in winter, so these place limits on photosynthesis.

    • 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.

    • Light initiated reactions - effect of changing intensity


  • 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 40oC 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 and then falls as the enzymes are becoming increasingly denatured and eventually cease to function.

    • 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.

      • 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!

      • You should also note that plants enclosed in a greenhouse are less susceptible to pests and diseases and fertilisers may be added to the soil to provide the minerals the plant need's and absorbed from the soil by the root system.


  • 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.

    • 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!

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

    • Using greenhouses enables market gardeners to produce more good crops per year.

    • 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!

    • Large scale greenhouse complexes are proving successful in using artificial growing conditions.


  • Graphs demonstrating more than one limiting factor controlling the rate of photosynthesis

    • In experiments using eg Canadian pondweed, you can immerse the green weed in sodium hydrogencarbonate solution to supply the carbon dioxide (one variable) needed for photosynthesis.

    • The other two photosynthesis rate variables are temperature and light intensity.

    • Remember, one of the three variables must be kept constant for a given set of experiments involving the changing of the other two variables.

  • Graph 4. Temperature limiting

    • Graph 4. Rate of photosynthesis versus light intensity at different temperatures (2 factors)

    • Initially the graph lines are linear as the rate of photosynthesis is proportional to the light intensity (see also Graph 1).

    • However, just prior to point X on the graph, the increase in rate slows down, and finally at point X on the graphs, the rate of photosynthesis reaches a maximum irrespective of the light intensity.

    • This is shown by the graph line becoming horizontal, and the maximum rate is now dependent on the temperature ie the higher the temperature the greater the maximum rate of photosynthesis possible.

    • For these experiments a suitable concentration of CO2/NaHCO3 must be chosen and kept constant!

  • Graph 5. CO2/NaHCO3 concentration limiting

    • Graph 5. Rate of photosynthesis versus light intensity with different CO2/NaHCO3 concentrations (2 factors)

    • Initially the graph lines are linear as the rate of photosynthesis is proportional to the light intensity (see also Graph 1).

    • However, just prior to point X on the graph, the increase in rate slows down, and finally at point X on the graphs, the rate of photosynthesis reaches a maximum irrespective of the light intensity.

    • This is shown by the graph line becoming horizontal, and the maximum rate is now dependent on the concentration of the carbon dioxide (from the 1%-3% NaHCO3 solution) ie the higher the concentration the greater the maximum rate of photosynthesis possible.

    • For these experiments a suitable temperature must be chosen and kept constant! (eg lab. temp. of ~20-25oC)

  • Graph 6. Temperature limiting

    • Graph 6. Rate of photosynthesis versus CO2/NaHCO3 at different temperatures (2 factors)

    • Initially the graph lines are linear as the rate of photosynthesis is proportional to the carbon dioxide (or sodium hydrogencarbonate) concentration (see also Graph 3).

    • However, just prior to point X on the graph, the increase in rate slows down, and finally at point X on the graphs, the rate of photosynthesis reaches a maximum irrespective of the carbon dioxide concentration.

    • This is shown by the graph line becoming horizontal, and the maximum rate is now dependent on the temperature ie the higher the temperature the greater the maximum rate possible.

    • For these experiments a suitable light intensity must be chosen and kept constant!

  • Graph 7. Light intensity limiting

    • Graph 7. Rate of photosynthesis versus CO2/NaHCO3 concentrations at light intensities (2 factors)

    • Initially the graph lines are linear as the rate of photosynthesis is proportional to the carbon dioxide (or sodium hydrogencarbonate) concentration (see also Graph 3).

    • However, just prior to point X on the graph, the increase in rate slows down, and finally at point X on the graphs, the rate of photosynthesis reaches a maximum irrespective of the carbon dioxide concentration.

    • This is shown by the graph line becoming horizontal, and the maximum rate is now dependent on the light intensity ie the higher the light intensity the greater the maximum rate possible.

    • For these experiments a suitable temperature (eg 20-25oC) must be chosen and kept constant!


  • 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?

  • Methods of measuring the rate of photosynthesis

  • Rate of photosynthesis experimental method 1.

  • Method 1.

    • 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 oxygen production as the gas is collected in the gas syringe.

    • 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 cm3 of water.

      • With increasing concentration you should see an increase in the rate of oxygen bubbles (eg cm3/min), but you must keep the temperature constant eg lab. temp. 20-25oC, 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.

    • 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 ~35-40oC ie the rate should be significantly lower at ~20oC and 50oC.

      • 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, but for accurate results you must 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).

    • 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 its 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.

    • How can we measure the speed or rate of a chemical reaction?

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

  • Rate of photosynthesis experimental method 2.

  • Method 2.

    • The 'set-up'

      • 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 and collected in a syringe.

      • It might ok just to measure the speed of bubble down the capillary tube, BUT what happens if it fills with oxygen gas - you won't see any movement (See method 3 next).

      • 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 (ok) or measure the volume of gas formed, maybe?

    • Problems!

      • 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.

  • Rate of photosynthesis experimental method 3.

  • Method 3. Further thoughts on the experimental methods described in methods 1. and 2. above for determining the rate of photosynthesis in Canadian pondweed.

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

      • Again in this method the pondweed is enclosed in a large boiling tube and placed 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 cm3 of water.

      • Its essentially the same as method 2. with two important differences.

        • (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.

    • What can you measure and vary?

      • Again, in this 'set-up' you measure the rate of photosynthesis by measuring the rate of oxygen gas production but NOT collected in the gas syringe.

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

        • 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. 20-25oC, 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 cm3 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 5o increments from 15oC to 50oC to show maximum the maximum rate of photosynthesis expected to be around 35-40oC.

        • 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 last 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 25oC.

    • How do you measure the rate?

      • You can measure the speed of movement of an air/oxygen bubbles relative to the linear scale placed alongside the capillary tube eg cm/s.

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

      • You need to allow time for the rate of oxygen production to settle down to a reasonably constant value and take a minimum of three readings to give an average.

      • Every so often you can inject some water at the T junction to keep on creating gas bubbles that can be observed. If it was a stream of just gas or liquid, you couldn't make any measurements.

    • Problems!

      • Although I think this is an improvement on methods 1 and 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 / d2

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

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

      • ... treat them as theoretical results/predictions!

    • distance from lamp d 10 20 30 40 distance to the experiment in cm
      1 / d 0.1 0.05 0.033 0.025 reciprocal of distance
      d2 100 400 900 1600 distance squared
      1 / d2 0.01 0.0025 0.00111 0.000625 reciprocal of distance squared
      relative intensity 1 / d2 1.0 0.25 0.111 0.0625 arbitrary units calculated by the inverse square law equation
      relative distance x1 x 2 x 3 x 4 distance from lamp to experiment
      relative intensity as a fraction 1 1/4 1/9 1/16 decreasing with the inverse square law
          not 1/2 not 1/3 not/1/4 this is what it would be if intensity = 1 / d
      relative rate of photosynthesis (see graphs) 1.0 0.25 0.111 0.0625 assuming rate of photosynthesis is proportional to light intensity
    • 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 / d2). Therefore in graph (c) the horizontal axis could be also labelled relative light intensity, a proportional linear relationship with the rate of photosynthesis.


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


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