5A - Limiting factors in photosynthesis

Question 1

What are the factors that affect photosynthesis?

Answer 1

Light intensity, temperature and carbon dioxide.

Question 2

What are the optimum conditions for photosynthesis?

Answer 2

High light intensity of a certain wavelength, temperature around 25oC and carbon dioxide at 0.4%.

Question 3

Why is high light intensity of a certain wavelength needed for photosynthesis?

Answer 3

Light is needed to provide the energy for the light-dependent reaction - the higher the light intensity, the more energy it provides.

Only certain wavelength of light are used for photosynthesis. The photosynthetic pigments chlorophyll a, chlorophyll b and carotene only absorb the red and blue light in sunlight. (Green light is reflected, which is why plants look green.)

Question 4

What are the optimum conditions regarding light intensity for photosynthesis?

Answer 4

High light intensity of a certain wavelength.

Question 5

Why is a temperature around 25oC needed for photosynthesis?

Answer 5

Photosynthesis involves enzymes (e.g. ATP synthase, rubisco). If the temperature falls below 10oC the enzymes become inactive, but if the temperature is more than 45oC they may start to denature.

Also, at high temperatures stomata close to avoid losing too much water. This causes photosynthesis to slow down because less CO2 enters the leaf when the stomata are closed.

Question 6

What are the optimum conditions regarding temperature for photosynthesis?

Answer 6

Temperature around 25oC.

Question 7

Why is a carbon dioxide concentration of 0.4% needed for photosynthesis?

Answer 7

CO2 makes up 0.04% of the gases in the atmosphere.

Increasing this to 0.4% gives a higher rate of photosynthesis, but any higher and the stomata start to close.

Question 8

What are the optimum conditions regarding CO2 concentration for photosynthesis?

Answer 8

CO2 at 0.4%.

Question 9

Apart from optimum levels of CO2 concentration, light intensity and temperature, what is also needed for photosynthesis?

Answer 9

A constant supply of water.

Question 10

Why is a constant supply of water needed for photosynthesis?

Answer 10

Because too little water means that photosynthesis has to stop but too much and the soil becomes waterlogged (reducing the uptake of minerals such as magnesium, which is needed to make chlorophyll a).

Question 11

What is usually the limiting factor of photosynthesis on a warm, windless day?

Answer 11

CO2

Question 12

What is usually the limiting factor of photosynthesis at night?

Answer 12

Light intensity

Question 13

What is the saturation point?

Answer 13

The point where a factor is no longer limiting the reaction - something else has become the limiting factor.

Question 14

Why do growers need to create an environment where plants get the right amount of everything that they need for photosynthesis?

Answer 14

To increase growth and therefore increase yield.

Question 15

How can growers create optimum conditions for photosynthesis?

Answer 15

In glasshouses and polytunnels (tunnels made of polyethene, under which plants can be grown).

Question 16

How can carbon dioxide concentration be managed in a glasshouse?

Answer 16

CO2 is added to the air, e.g. by burning a small amount of propane in a CO2 generator.

Question 17

How can light intensity be managed in a glasshouse?

Answer 17

Light can get in through the glass.

Lamps provide light at night-time.

Question 18

How can temperature be managed in a glasshouse?

Answer 18

Glasshouses trap heat energy from sunlight, which warms the air.

Heaters and cooling systems can also be used to keep a constant optimum temperature, and air circulation systems make sure the temperature is even throughout the glasshouse.