Photosynthesis

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The key idea

Photosynthesis transfers light energy into chemical energy stored in glucose.Its rate can be limited by light intensity, carbon dioxide concentration and temperature.

Photosynthesis in a leafCarbon dioxide enters stomata; water arrives through xylem.

Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

The photosynthesis equation

Photosynthesis is an endothermic reaction that transfers energy from the environment to the chloroplasts by light.Carbon dioxide and water react to produce glucose and oxygen.The word equation is carbon dioxide + water, in the presence of light energy, produces glucose + oxygen.The light is usually absorbed by the green $pig$ ment chlorophyll.

Limiting factors

A limiting factor is the factor in shortest supply that holds back the rate of photosynthesis.The three main limiting factors are light intensity, carbon dioxide concentration and temperature.As you increase a limiting factor the rate rises until another factor becomes limiting and the graph levels off.Temperature acts differently because at high temperatures enzymes controlling photosynthesis denature, so the rate falls.

Uses of glucose

Glucose made in photosynthesis is used for respiration to release energy.It is also converted into insoluble starch for storage, into cellulose to strengthen cell walls, and into lipids or oils for storage in seeds.Glucose is combined with nitrate ions absorbed from the soil to make amino acids for protein synthesis.

Investigating photosynthesis

The rate of photosynthesis can be measured by counting bubbles of oxygen released by pondweed such as Cabomba, or by collecting the gas in a measuring cylinder.Light intensity is changed by moving a lamp different distances from the plant.The inverse square law means light intensity is proportional to one divided by the distance squared, so doubling the distance gives one quarter of the intensity.

Key terms

Definitions to learn

The endothermic process where plants make glucose from carbon dioxide and water using light energy.

Chlorophyll

The green $pig$ ment in chloroplasts that absorbs light energy for photosynthesis.

Limiting factor

The factor in shortest supply that restricts the rate of a process such as photosynthesis.

Endothermic reaction

A reaction that takes in energy from the surroundings.

Inverse square law

Light intensity is proportional to one over the distance squared from the source.

Worked example

A graph of photosynthesis rate levels off as light intensity rises. Suggest one reason.

At low intensity, light limits the rate.

At high intensity, light is no longer limiting.

Another factor such as carbon dioxide or temperature limits the rate.

Final answer

The graph levels off because another factor becomes limiting.

Exam habit

Include both reactants (CO₂ and water) and both products (glucose and oxygen) in every photosynthesis answer.State that light energy is needed. Missing any one component loses a mark.

Watch out

Plants do not get food from soil. They make glucose by photosynthesis.

Examiner tips

How to score full marks

1Always write both reactants and both products in the equation and state that light energy is needed.
2When describing a rate graph, name which factor is limiting in each part of the curve, do not just say the rate increases.
3If asked why a plant is given extra carbon dioxide in a greenhouse, link it to it being a limiting factor that increases yield.

Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Write the word equation for photosynthesis.

Mark scheme

1.Place reactants on the left and products on the right.

carbon dioxide + water → glucose + oxygen

2Give two uses of glucose in plants.

Mark scheme

1.Think storage, respiration and building materials.

Any two from: respiration, starch storage, cellulose production, amino acid production, fat or oil production.

3Why is chlorophyll important?

Mark scheme

1.Link chlorophyll to light energy.

It absorbs light energy needed for photosynthesis.

4Write the word equation for photosynthesis.[1 mark]

Mark scheme

1.Put reactants on the left and products on the right.
2.Remember light energy is required.

carbon dioxide + water, in the presence of light energy, produces glucose + oxygen (1)

5Name the green

pig

ment that absorbs light for photosynthesis.[1 mark]

Mark scheme

1.Recall the $pig$ ment found in chloroplasts.

Chlorophyll (1)

6State two ways a plant uses the glucose it makes.[2 marks]

Mark scheme

1.Recall storage and structural uses.

Any two from: used in respiration (1); converted to starch for storage (1); used to make cellulose for cell walls (1); used to make lipids or oils (1); combined with nitrate ions to make amino acids and proteins (1)

7A student measures the rate of photosynthesis of pondweed at different light intensities. Explain the shape of the graph as light intensity increases.[3 marks]

Mark scheme

1.Describe the early steep part.
2.Describe the plateau and say why it happens.

As light intensity increases the rate of photosynthesis increases because light is the limiting factor (1); the graph then levels off (1) because another factor such as carbon dioxide concentration or temperature becomes the limiting factor (1)

8A greenhouse grower wants to maximise crop yield. Explain how controlling temperature, carbon dioxide and light could increase the rate of photosynthesis, and why simply raising the temperature is not always beneficial.[4 marks]

Mark scheme

1.Discuss each limiting factor.
2.Explain the enzyme limit on temperature.

Increasing light intensity increases the rate while light is limiting (1); increasing carbon dioxide concentration increases the rate while carbon dioxide is limiting (1); increasing temperature increases enzyme activity and rate up to an optimum (1); but above the optimum the enzymes controlling photosynthesis denature so the rate falls (1)

9State the symbol equation for photosynthesis.[1 mark]

Mark scheme

1.Recall the chemical formulae for each reactant and product.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (1) (all four correct formulae required)

10A lamp is moved from 10 cm to 20 cm from a pondweed. Use the inverse square law to calculate the new light intensity as a fraction of the original.[2 marks]

Mark scheme

1.Apply intensity is proportional to 1 / distance squared.
2.Calculate the ratio.

At 10 cm, intensity ∝

\frac{1}{1}00

; at 20 cm, intensity ∝

\frac{1}{4}00

(1); the new intensity is

\frac{1}{4}

of the original (1).

11Explain why a plant kept in a closed glass box in the dark will eventually die, even if it was photosynthesising normally before.[4 marks]

Mark scheme

1.Link the absence of light to glucose production.
2.Consider respiration still occurring.

In the dark the plant cannot photosynthesise (1); no light energy means no glucose is made (1); the plant continues to respire, using up stored glucose (1); without glucose for energy and building materials the plant cannot grow or maintain its cells and eventually dies (1).

12A student investigates the effect of carbon dioxide concentration on the rate of photosynthesis. Describe a method they could use and explain how they would know the rate has increased.[4 marks]

Mark scheme

1.Name the pondweed method or another valid method.
2.State the measurable variable.

Place pondweed in sodium hydrogencarbonate solution of different concentrations (1); count the number of oxygen bubbles produced per minute (1) or collect and measure the volume of gas; increasing bubble rate indicates a higher rate of photosynthesis (1); keep temperature and light intensity constant as control variables (1).

13Explain why a plant grown in conditions of low nitrate ions will have stunted growth even if light intensity and carbon dioxide are not limiting.[4 marks]

Mark scheme

1.Identify what nitrate ions are needed for.
2.Link to protein synthesis and growth.

Nitrate ions are needed to make amino acids (1); amino acids are the building blocks of proteins (1); without enough protein the plant cannot build new cells or tissues (1); so growth is stunted even if photosynthesis is producing glucose at a normal rate (1).

14Evaluate the economic and environmental considerations of a grower who wants to increase the carbon dioxide concentration inside a large commercial greenhouse to boost crop yield.[5 marks]

Mark scheme

1.Give the benefit linked to limiting factors.
2.Discuss financial cost and any environmental concerns.

Increasing carbon dioxide removes it as a limiting factor (1), increasing the rate of photosynthesis and therefore crop yield (1); the benefit is greater food production and profit (1); however, the cost of providing the extra carbon dioxide adds to running costs (1); if fossil fuels are burned to generate heat and carbon dioxide, this contributes to atmospheric carbon dioxide and climate change (1); the grower must weigh the economic gain against these costs (1).

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