Biology B8 - Photosynthesis

Write the balanced symbol equation for photosynthesis

[ 1 MARK ]

Where does photosynthesis occur in plant cells and why?

[ 2 MARKS ]

Photosynthesis occurs in the chloroplasts.

They contain chlorophyll which absorbs light energy for photosynthesis

Explain why light intensity affects the rate of photosynthesis

[ 3 MARKS ]

Light produces energy for photosynthesis

As light intensity increases rate increase - up to a point

Then another factor ( CO2 or TEMPERATURE ) becomes limiting

Explain the lock and key theory of enzyme action

[ 3 MARKS ]

Enzyme binds to the substrate because they are complementary shapes

The substrate binds to the enzyme and is broken down into products.

The products are released and the enzyme is unchanged.

Describe how the students would find out if the liquid from the leaf contained glucose

[ 3 MARKS ]

The students would use Benedicts solution and heat the sample.

To test for glucose, a few drops of Benedict's solution are added to liquid from leaf then mixture is heated in water bath. If glucose is present the solution will change colour from blue to brick red

Describe how the students would find out if the liquid from the leaf contain starch

[ 3 MARKS ]

The students would use iodine solution.

To test for starch, a few drops of iodine solution are added to the liquid from the leaf if starch is present iodine solution will change from brown / orange to blue - black

Describe an experiment to investigate how light intensity affects rate of photosynthesis

[ 6 MARKS ]

1. Place pondweed in sodium hydrocarbonate

2. Place a lamp at measured distances

3. Count bubbles per minute or collect oxygen gas

4. Repeat at several distances

5. Keep temperature and CO2 the same

6. Plot rate against distance or light intensity

Explain the shape of a photosynthesis rate - temperature graph

[ 3 MARKS ]

Rate increases as temperature rises ( Enzymes work faster )

Peaks at optimum temperature

Then falls sharply because enzymes denature

Describe how farmers use greenhouses to maximise photosynthesis

[ 4 MARKS ]

1. Use heaters to maintain optimum temperature

2. Add CO2 to the air

3. Use lamps to extend light hours

4. Keep enclosed to reduce pests and maintain humidity

How is glucose from photosynthesis used by plants?

[ 4 MARKS ]

1. Respiration ( Energy )

2. Converted to starch for storage

3. Made into cellulose for cell walls

4. Used to produce fats and proteins

What is meant by a “limiting factor”

[ 2 MARKS ]

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

Is photosynthesis an endothermic or exothermic reaction and why

Endothermic - energy is transferred from the environment to chloroplast by light

Give examples of leaf adaptions which maximise the rate of photosynthesis

• Broad leaves - maximise surface area.

• Thin leaves - short diffusion distance.

• Chlorophyll present - trap light energy.

• Veins - transport water to leaves via xylem, remove photosynthesis products via phloem.

• Air spaces - allow CO₂ to enter and O₂ to leave.

• Guard cells - control opening of stomata for gaseous exchange and prevent water loss.

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

• Temperature

• Light intensity

• Carbon dioxide concentration

• Amount of chlorophyll

How does temperature affect the rate of photosynthesis?

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

How does light intensity affect the rate of photosynthesis?

Increasing the light intensity increases the rate of photosynthesis until another factor becomes limiting

How does carbon dioxide concentration affect the rate of photosynthesis?

Increasing the rate of carbon dioxide concentration increases the rate of photosynthesis ( until another fact becomes limiting ) as CO2 is required to make glucose

How does the amount of chlorophyll affect the rate of photosynthesis?

Decreasing the amount of chlorophyll ( e.g. due to lack of magnesium ) decreases the rate of photosynthesis as chlorophyll is required to absorb light energy

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

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

State 5 uses of the glucose produced during photosynthesis

• Respiration

• Starch for storage

• Cellulose for strength

• Amino acid and protein synthesis (combined with nitrates)

• Lipids for energy storage in seeds