2. Organisation (limiting factors for photosynthesis and factors affecting transpiration)

Question 1

What are the limiting factors for photosynthesis?

Answer 1

• Temperature
• Light intensity
• Carbon dioxide

Question 2

How is temperature a limiting factor for photosynthesis?

Answer 2

• As temperature increases, the enzymes gain more kinetic energy so there is more frequent and successful collisions
• So more enzyme-substrate complexes are able to form
• So the rate of photosynthesis increases up to the optimum temperature (37-40 degrees)
• After exceeding the optimum temperature, the enzyme denatures
• This means that the enzymes active site changes shape
• So this means the active site is no longer specific and complementary to the substrate
• So fewer enzyme-substrate complexes can form
• So the rate of photosynthesis decreases

Question 3

How is carbon dioxide a limiting factor?

Answer 3

• As the carbon dioxide concentration increases, the rate of photosynthesis also increases
• This means it is a limiting reactant, as more carbon dioxide means more substrates so more enzyme-substrate complexes form
• As the carbon dioxide concentration continues to increases, the rate of photosynthesis stays the same/plateus
• This is because the enzymes active sites are occupied
• So the maximum rate of reaction has been reached
• So carbon dioxide is no longer the limiting reactant, another factor is limiting the rate of photosynthesis

Question 4

How is light intensity a limiting factor?

Answer 4

• As the light intensity increases, the rate of photosynthesis also increases
• This means it is a limiting factor, as more light energy means that more carbon dioxide and water can be converted into glucose and oxygen
• As the light intensity continues to increase, the rate of photosynthesis remains the same/plateaus
• This could be because there is not enough chlorophyll to absorb the sunlight
• So the maximum rate of reaction has been reached
• So light intensity is no longer the limiting reactant, another factor is limiting the rate of photosynthesis.

Question 5

What is the function of the xylem cell?

Answer 5

To allow water, minerals and ions to be taken up from the roots through the stem into the leaves

Question 6

How does water move up the leaf through the xylem?

Answer 6

• Transpiration happens against gravity
• So there is a pulling force of water upwards, created by the concentration gradient formed between the xylem and the inside of the leaf
• This is because some of the water is used for photosynthesis while some is lost via transpiration through the stomata
• So this makes water potential more negative inside the leaf
  So this maintains the concentration gradient he water potential inside the leaf is more negative than the water potential in the xylem
• So water is pulled into the leaf from the xylem

Question 7

What is transpiration?

Answer 7

The loss of water from the leaves via diffusion/evaporation

Question 8

What are the 4 factors that affect the rate of transpiration?

Answer 8

• Light intensity
• Temperature
• Wind
• Humidity

Question 9

What happens to the rate of transpiration as light intensity increases?

Answer 9

It increases

Question 10

What happens to the rate of transpiration as temperature increases?

Answer 10

It increases

Question 11

What happens to the rate of transpiration in windier conditions?

Answer 11

It increases

Question 12

What happens to the rate of transpiration in humid conditions?

Answer 12

It decreases/is slower

Question 13

How does the rate of transpiration increase as light intensity increases?

Answer 13

Light intensity increases
- This means the rate of photosynthesis will increases
- So the stomata will open wider to allow for my carbon dioxide to diffuse into the leaf via gas exchange
- So more water can leave the leaf via diffusion
So transpiration increases

Question 14

How does the rate of transpiration increase as temperature increases?

Answer 14

As temperature increases
- The water particles will gain more kinetic energy
- So this means there is a faster rate of diffusion/evaporation of water particles leaving the leaf
So transpiration increases

Question 15

How does the rate of transpiration increase in windier conditions?

Answer 15

As the level of wind increases
- The water particles next to the leaf outside of the stomata, get blown away
- So this decreases the concentration of water outside or next to the leaf
- So this means there is a greater difference in water concentration inside the leaf compared to outside
- So this means there is a larger concentration gradient
- So this increases the rate of diffusion of the water particles
So increases transpiration

Question 16

How does the rate of transpiration decrease in more humid conditions?

Answer 16

As humidity increases
- The concentration of water particles outside, next to the leaf increases
- So there is a smaller difference in the concentration of water particles inside the leaf compared to the outside
- So there is a smaller concentration gradient
- So the rate of diffusion of the water particles decrease
So transpiration decreases