Factors affecting rates of Photosynthesis: Grade 9 Understanding for IGCSE Biology (part 1) 2.18 2.19

Photosynthesis is the process occurring in plants in which sunlight is trapped by chlorophyll pigments and used to power the chemical reactions involved in making food molecules such as carbohydrates from carbon dioxide and water.  Oxygen is released as a waste product of these reactions.

(I can’t write a chemical equation as I can’t find a way of writing subscript in WordPress….. Can anyone help?)







In the equation above, the carbohydrate produced is glucose, a six carbon sugar.

The reactions of photosynthesis happen in specialised mesophyll tissue in the leaf of the plant (see previous post)  Inside the palisade and spongy mesophyll cells there are thousands of tiny organelles called chloroplasts in which the reactions of photosynthesis occur.


So what environmental factors could be altered to vary the rate of photosynthesis in a plant?

Light Intensity – light provides the energy for photosynthesis and so the higher the intensity of light, the more energy the chloroplasts receive to make carbohydrates.

Light wavelength – chlorophyll pigments absorb the blue-violet and red parts of the spectrum well but cannot absorb green light.

Carbon Dioxide concentration – this is a reactant for photosynthesis so increasing the concentration makes a collision between the reactant molecule and the enzyme inside the chloroplast that bind it more likely, so the rate will go up.

Temperature – many reactions in photosynthesis are catalysed by enzymes and enzymes are very affected by temperature:  too low temperatures and the enzymes and substrate molecules move very slowly and so there are few collisions, too high temperatures and the enzymes change shape (denature) so the substrate molecules cannot fit into the active site.

NB – water availability is never a factor that can alter rates of photosynthesis even though it is a reactant molecule.  This might seem unusual until one remembers that plants that are dehydrating will close the stomata in their leaves to minimise transpiration.  Closed stomata mean that carbon dioxide cannot get into the air spaces in the leaf so this is ultimately what limits photosynthesis in a dehydrated plant.


The experimental set up above is the best way to measure rates of photosynthesis and so investigate the effect of any of the four factors listed above.  Light intensity can be varied either with a dimmer switch as above or by altering the distance between the lamp and the plant.  The heat shield is transparent to let light through but will absorb the heat from the bulb ensuring the temperature of the water stays constant.  Carbon dioxide concentration can be altered by dissolving different masses of sodium hydrogen carbonate in the water.  The wavelength of light will stay constant so long as the build remains the same.

How to measure rates of photosynthesis in this set up?

Well you could collect the gas produced over a long period of time and measure its volume with a gas syringe.  This might sound more accurate than counting bubbles but in fact it is a less reliable way as you would have to leave the set up for a long time and variables might change.  So it is fine to assume that the bubbles produced are oxygen and that every bubble is the same volume: if you do this, the rate of production of bubbles is directly proportional to the rate of photosynthesis in the Elodea plant.



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