Factors affecting photosynthesis
There are several ways of measuring the rate of photosynthesis in the lab. These include:
- the rate of Gaseous element making up about 20% of the air, which is needed by living organisms for respiration. output
- the rate of A gaseous compound of carbon and oxygen, which is a by-product of respiration, and which is needed by plants for photosynthesis. uptake
- the rate of Food belonging to the food group consisting of sugars, starch and cellulose. Carbohydrates are vital for energy in humans and are stored as fat if eaten in excess. In plants, carbohydrates are important for photosynthesis. production
These are not perfect methods as the plant will also be respiring, which will use up some oxygen and carbohydrate and increase carbon dioxide output.
Several factors can affect the rate of photosynthesis:
- light intensity
- carbon dioxide concentration
- temperature
The amount of The green chemical inside the chloroplasts of plant cells. It enables photosynthesis to take place. also affects the rate of photosynthesis:
- plants in lighting conditions unfavourable for photosynthesis may synthesise more chlorophyll, to absorb the light required
- the effects of some plant diseases affect the amount of chlorophyll, and therefore the ability of a plant to photosynthesise
Light intensity
Without enough light, a plant cannot photosynthesise very quickly - even if there is plenty of water and carbon dioxide and a suitable temperature.
Increasing the light intensity increases the rate of photosynthesis , until some other factor - a A factor which, if in short supply limits or reduces the rate of photosynthesis, eg temperature, light intensity and carbon dioxide concentration. - becomes in short supply.
Image caption, At low light intensities the increase in the rate of photosynthesis is linear
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At very high light intensities, photosynthesis is slowed and then inhibited, but these light intensities do not occur in nature.
Carbon dioxide concentration
Carbon dioxide - with water - is one of the reactants in photosynthesis.
If the concentration of carbon dioxide is increased, the rate of photosynthesis will therefore increase.
Again, at some point, a factor may become limiting.
Temperature
The chemical reactions that combine carbon dioxide and water to produce glucose are controlled by A protein which catalyses or speeds up a chemical reaction.. As with any other enzyme-controlled reaction, the rate of photosynthesis is affected by temperature.
At low temperatures, the rate of photosynthesis is limited by the number of molecular collisions between enzymes and substrates. At high temperatures, enzymes are If a substance is denatured, its structure and function is altered. This can be caused by heat, altered pH or by chemical agents..
Chlorophyll
Chlorophyll absorbs the light energy required to convert carbon dioxide and water into glucose.
Chlorophyll is green - so absorbs the red and blue parts of the The different types of electromagnetic radiation, arranged in order of frequency or wavelength. and reflects the green part of the spectrum.
Leaves with more chlorophyll are better able to absorb the light required for photosynthesis.
The compensation point
These graphs have been plotted with rate of photosynthesis against the factor under investigation.
If oxygen production or carbon dioxide uptake is used as a measure of photosynthetic rate, the graphs are slightly different. The line does not go through the The point where the x-axis and the y-axis meet is the point O, which is called the origin.. This is because oxygen production and carbon dioxide uptake are affected by The chemical change that takes place inside living cells, which uses glucose and oxygen to release the energy that organisms need to live. Carbon dioxide is a by-product of respiration. as well as photosynthesis. Respiration uses oxygen and produces carbon dioxide. Below is a graph measuring the rate of photosynthesis plotted on carbon dioxide against light intensity:
This graph shows that with no light, the carbon dioxide uptake is below zero (point A on the graph). With no light, there is no photosynthesis. So the rate of respiration is greater than the rate of photosynthesis. So this means there is an overall excess of carbon dioxide produced during respiration. An excess is the same as a negative uptake.
As the light intensity increases the rate of photosynthesis follows (point B on the graph). This means more carbon dioxide is being absorbed by the plant for photosynthesis. So this means more carbon dioxide uptake occurs. The graph shows this increase in carbon dioxide uptake.
There will come a point when the rate of photosynthesis has increased to match the rate of respiration. This is called the compensation point (point C on the graph). This is shown on the graph as the point the line crosses zero on the x-axis.
Beyond the compensation point, further increases in light intensity cause an increase in photosynthesis. This means the rate of photosynthesis increases beyond the rate of respiration (point D). Now more carbon dioxide is being taken in by the plant for photosynthesis than is produced by the plant during respiration. So the carbon dioxide uptake increases.
After a specific point, further increases in light intensity give ever smaller increases in photosynthesis. This results in the rate of carbon dioxide uptake slowing (point E on the graph).
A similar graph will be obtained if oxygen production is plotted against light intensity.