Factors Affecting Photosynthesis

Variables that affect the rate of photosynthesis: There are several factors that affect the rate of photosynthesis (or how fast sugars are produced in plants). These include: (a) light intensity (brightness), (b) carbon dioxide concentration, (c) temperature, and (d) light wavelength. The relationship between light intensity and photosynthesis is shown in the graph below.

Light intensity (amount/brightness of light). When there is no light, no photosynthesis takes place. As the light intensity increases from zero, the rate of photosynthesis increases. After a certain point, the rate of photosynthesis will not increase any more with increasing light intensity.

This happens because there is just not enough chlorophyll (and other molecules needed for photosynthesis) in plant cells to increase the rate of photosynthesis any more.

Q3.5: For low levels of light intensity, what is the relationship between light intensity and photosynthesis rate?

a) There is no relationship.

b) As light intensity increases, photosynthesis rate decreases.

c) As light intensity increases, photosynthesis rate increases.

d) I have no clue.

You are right. With low levels of light intensity (close to the origin of the graph), as light intensity increases, the rate of photosynthesis also increases. So, (c) is the correct response.

Not quite. With low levels of light intensity (close to the origin of the graph), as light intensity increases, the rate of photosynthesis also increases. So, (c) is the correct response.

Carbon Dioxide concentration. When there is no CO₂, photosynthesis does not occur. Increasing the concentration of Carbon Dioxide will increase the rate of photosynthesis. But after a certain point, the rate of photosynthesis will not increase any more as CO₂ concentration increases.

So, after a certain point, more carbon dioxide in the air will no longer help plants grow.

Q4: According to the graph, as the CO₂ levels increase, what happens to the rate of photosynthesis?

a) It keeps increasing.

b) It increases up to a certain point then remains the same.

c) It remains the same for all CO₂ levels.

d) This information cannot be determined from this graph.

Right! As the CO₂ level increases, the photosynthesis rate also increases. However, once it reaches a certain CO₂ level, the rate of photosynthesis does not increase (or decrease) any more.

Actually, as the CO₂ level increases, the photosynthesis rate also increases. However, once it reaches a certain CO₂ level, the rate of photosynthesis does not increase (or decrease) any more. So, (b) is the correct answer.

Temperature. The relationship between temperature and rate of photosynthesis is different from the others. Again, when the temperature is very low, no photosynthesis occurs. The increase in photosynthesis rate is slow at first, but starts to increase more quickly at a certain point. The rate of photosynthesis is highest between 10 and 20°C (or 50-68°F). After this maximum, the photosynthesis rate drops off quickly as the temperature increases. At around 40°C (or 104°F), photosynthesis stops. As a result, plants cannot grow anymore.

Why does photosynthesis stop at high temperatures? There are certain molecules called "enzymes" made by living organisms. Enzymes speed up chemical reactions. Chlorophyll is one enzyme that helps cause photosynthesis. Another enzyme, called RuBisCo, has the job of capturing CO₂. When the temperature is too high, RuBisCo gets very bad at capturing CO₂, which is needed for photosynthesis. As a result, photosynthesis stops.

Q4.5: What is the relationship between temperature and rate of photosynthesis between 21°C (70°F) and 38°C (100°F)? Base your answer on the graph and other information in this unit.

a) The rate of photosynthesis increases from 21-38°C.

b) The rate of photosynthesis decreases from 21-38°C.

c) The rate of photosynthesis does not change from 21-38°C.

d) How should I know?

That's right! The rate of photosynthesis is maximum a bit below room temperature (between 10 and 20°C) but decreases at temperatures higher than about 20 degrees. So, (b) is the correct answer.

Well, actually, the rate of photosynthesis is maximum a bit below room temperature (between 10 and 20°C) but decreases at temperatures higher than about 20 degrees. So, (b) is the correct response.

Light wavelength.

frequencies of visible light

*This is why plants' leaves look green to us: green light is not absorbed; instead, it is reflected by the molecules that make up the plants to our eyes.

Table of Contents: Factors that Affect Photosynthesis

Relationship between light intensity and photosynthesis
Relationship between CO2 and photosynthesis
Relationship between temperature and photosynthesis
Question
Relationship between light frequency and photosynthesis
Understanding Check
Light intensity & algae growth