4.1 Photosynthesis

Question 1

What is photosynthesis?

Answer 1

The process by which plants synthesise glucose using light energy, which is converted into chemical energy, from the Sun.

Question 2

Where does photosynthesis take place?

Answer 2

Within chloroplasts, which contains chlorophyll (a pigment which absorbs light energy), in leaf palisade cells.

Question 3

State the equations for photosynthesis (word and symbol)

Answer 3

Word Equation
Carbon dioxide + Water → Glucose + Oxygen

Symbol Equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Question 4

Is photosynthesis an endothermic or exothermic reaction, and why?

Answer 4

Endothermic - energy is transferred from the environment to chloroplasts by light.

Question 5

How can you show that a plant gives off oxygen during photosynthesis?

Answer 5

Using a water plant (eg. Elodea), collect gas bubbles produced during photosynthesis. The gas will relight a glowing splint as it contains oxygen.

Question 6

What are examples of leaf adaptations that maximise the rate of photosynthesis? (6)

Answer 6

• Broad leaves maximise surface area.
• Thin leaves create a short diffusion distance.
• Chlorophyll trap light energy.
• Veins transport water to leaves by way of the xylem, remove photosynthesis products by way of the phloem.
• Air spaces allow CO₂ to enter and O₂ to leave.
• Guard cells control opening of stomata for gaseous exchange and prevent water loss.

Question 7

What are the four main factors that affect the rate of photosynthesis?

Answer 7

• Temperature - The higher the temperature, the faster the rate (until a certain temperature is reached)
• Light intensity
• Carbon dioxide concentration
• Amount of chlorophyll

Question 8

How does temperature affect the rate of photosynthesis? (5)

Answer 8

As the temperature increases, the rate of photosynthesis increases, as the kinetic energy of particles is increased.
However, the rate decreases past a certain temperature as enzymes become denatured.

Question 9

How does carbon dioxide concentration affect the rate of photosynthesis? (4)

Answer 9

As the carbon dioxide concentration increases, the rate of photosynthesis increases (until another factor becomes limiting) as CO₂ is required to make glucose.

Question 10

How does the amount of chlorophyll affect the rate of photosynthesis? (3)

Answer 10

As the amount of chlorophyll decreases (eg. due to a lack of magnesium), the rate of photosynthesis decreases as chlorophyll is required to absorb light energy.

Question 11

What is a limiting factor?

Answer 11

An environmental factor which can restrict the rate of photosynthesis eg. light intensity.

Question 12

Explain how you can calculate the rate of photosynthesis by measuring oxygen production. (3)

Answer 12

1. Set up bubble potometer apparatus (pondweed in a sealed tube of water, attached to a capillary tube and a gas syringe).
2. Oxygen gas produced causes the bubble in the capillary tube to move.
3. Calculate the volume of oxygen produced using the distance the bubble has moved by.

Question 13

How can farmers use their knowledge of limiting factors to increase their profits? (3)

Higher only

Answer 13

They can control temperature, light intensity and CO₂ concentration to achieve the fastest possible rate of photosynthesis, leading to a greater yield.

Question 14

State the law which describes the relationship between the distance of a light source from a plant and light intensity?

Higher only

Answer 14

Inverse square law:
Light intensity ∝ 1 ÷ distance²

Question 15

What are some of the uses of the glucose produced during photosynthesis? (5)

Answer 15

• Respiration.
• Starch for storage.
• Cellulose for strength.
• Amino acid and protein synthesis (combined with nitrates).
• Lipids for energy storage in seeds.