EN: Limiting Factors in Photosynthesis

Question 1

For plants in the UK/climates like it, what conditions would be ideal?

Answer 1

• High intensity of a certain wavelength.
• Temperature around 25^oC.
• Carbon dioxide at 0.4%.

Question 2

What wavelengths of light can photosynthetic pigments used in photosynthesis absorb and not absorb?

Answer 2

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 3

In photosynthesis, why is it optimum for temperature to be around 25^oC?

Answer 3

• Photosynthesis involves enzymes.
• If temp falls below 10^oC the enzymes become inactive, but if temperature is more than 45^oC they may start to denature.
• Also, at high temps the stomata close to avoid losing water - causes photosynthesis to slow down as less CO₂ enters the leaf when stomata are closed.

Question 4

For photosynthesis, why is it optimum for carbon dioxide levels to be at 0.4%?

Answer 4

• Carbon dioxide 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 5

Apart from light, heat and carbon dioxide, what else do plants need?

Answer 5

A constant supply of water - too little and photosynthesis stops, but too much and the soil becomes waterlogged (reduces the uptake of minerals like magnesium, which is needed to make chlorophyll a).

Question 6

What are the limiting factors in photosynthesis?

Answer 6

• Light
• Temperature
• Carbon dioxide

Question 7

Between points A and B what is the limiting factor?

Answer 7

Light intensity.

• As light intensity increases, so can the rate of photosynthesis.*
• B is the saturation point - increasing light intensity after this point makes no difference, as something else has become the limiting factor - the graph levels off.*

Question 8

What is the limiting factor in this graph?

Answer 8

Graphs level off when light intensity is no longer limiting.

Graph at 25 degrees levels off at a higher point than 15 degrees, showing that temperature must have been a limiting factor at 15 degrees.

Question 9

How is carbon dioxide concentration managed in a glasshouse?

Answer 9

CO₂ added to the air, eg. burning a small amount of propane in a CO₂ generator.

Question 10

How is light managed in a glasshouse?

Answer 10

Light can get in through glass

Lamps provide light at night-time

Question 11

How is temperature managed in a glasshouse?

Answer 11

Glasshouse traps heat energy from sunlight, which warms the air.

Heaters and cooling systems keep constant optimum temperature.

Air circulation systems make sure temperature is even throughout the glasshoue.