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GCSE Biology · GCSEChapter 9 · Photosynthesis
GCSE · Chapter 9

Photosynthesis

By the end of this chapter you will be able to

  • Write the word equation and the balanced symbol equation for photosynthesis
  • Name the products of photosynthesis and say where each is used
  • Identify the limiting factor on a graph, and explain the effect of each factor
  • Carry out and interpret the leaf-disc practical, and calculate a rate
  • [H] Use the inverse square law to compare light intensities at different distances

You should already know

  • Chloroplasts and the structure of leaves (Chapter 8)
  • Enzymes and temperature (Chapter 5)
  • Aerobic respiration (Chapter 10 links back)

Where this leads

  • Aerobic and anaerobic respiration (Chapter 10)
  • Carbon cycle (Chapter 17)
  • Nitrate and proteins (Chapter 8)

GCSE Biology. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Write the word and symbol equations for photosynthesis, Describe chloroplasts and chlorophyll, and the products of photosynthesis, Explain limiting factors (light, carbon dioxide, temperature) and interpret graphs of them, Investigate the effect of light intensity on the rate of photosynthesis, using leaf discs, Describe the uses of glucose made by photosynthesis, and how greenhouses control the factors, [H] Apply the inverse square law to light intensity.

Exercise P  Do you remember?

1
FWhat is 1000 divided by 20? Write the number.
2
FWhich organelle contains chlorophyll? Write the word.
3
FWhich gas do plants take in for photosynthesis? Write the words.
4
FName the mineral ion needed for proteins that comes from the soil. Write the word.
5
FWhat is 2 x 2 x 2 x 2? Write the number.
9.1

The photosynthesis equation and where it happens

GCSE
Write the word and symbol equations for photosynthesis, Describe chloroplasts and chlorophyll
Video lesson: The photosynthesis equation and where it happensComing soon
Context. Ali grows a tomato plant. It gets bigger, and its leaves stay green. “Where does the plant get the material to build all that new stuff?” he asks. Most of it comes from the air and the water, using energy from the light.

Photosynthesis is the process by which plants make glucose from carbon dioxide and water, using light energy. It releases oxygen as a by-product. It happens in chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs the light energy needed for the reaction.

Word equation: carbon dioxide + water -> glucose + oxygen (light energy and chlorophyll are needed).

Symbol equation: 6CO2 + 6H2O -> C6H12O6 + 6O2.

leafchlorophyll in chloroplastscarbon dioxidewater (from roots)light energyglucoseoxygencarbon dioxide + water -> glucose + oxygen (in the light, using chlorophyll)
Photosynthesis: carbon dioxide and water go in, and glucose and oxygen come out, using light
chloroplast: chlorophyll traps light
A chloroplast with stacks of chlorophyll-containing grana

Key ideas

  • Photosynthesis makes glucose and oxygen from carbon dioxide and water, using light.
  • It takes place in chloroplasts, which contain chlorophyll.
  • Chlorophyll absorbs light energy.
  • The symbol equation has 6 carbon dioxide and 6 water molecules on the left.
Worked example 9.1.1
Write the word equation for photosynthesis, using the word glucose for the sugar. Write the words.

Method. Name the reactants and the products.

  1. Reactants. The reactants are carbon dioxide and water.
    carbon dioxide and water
  2. Products. The products are glucose and oxygen.
    glucose and oxygen
  3. Write it. Carbon dioxide + water gives glucose + oxygen, in light and with chlorophyll.
    in light
  4. Write the answer.
    carbon dioxide + water -> glucose + oxygen

Check. The equation balances in atoms with six of each. ✓

Worked example 9.1.2
Name the gas released as a by-product of photosynthesis. Write the word.

Method. Recall the by-product.

  1. Recall. Photosynthesis releases oxygen into the air.
    releases oxygen
  2. Name it. So the by-product is oxygen.
    oxygen
  3. Write the answer.
    oxygen

Check. Carbon dioxide is taken in, not released. ✓

Worked example 9.1.3
Name the pigment that absorbs light for photosynthesis. Write the word.

Method. Recall the green pigment.

  1. Recall. The green pigment in chloroplasts absorbs light.
    absorbs light
  2. Name it. This pigment is chlorophyll.
    chlorophyll
  3. Write the answer.
    chlorophyll

Check. Chlorophyll is green because it reflects green light. ✓

Watch out

Watch out: photosynthesis needs light, but it is not the same as respiration. Photosynthesis makes glucose; respiration releases energy from glucose.

Exam tip

Exam tip. In a symbol equation, count the atoms on each side. A balanced equation has the same number of each atom on each side.

Try it (with your teacher). With your teacher, look at a leaf under a lamp for a few minutes, then test it with iodine solution after a day, or use a model of a chloroplast. Describe what chlorophyll does, in one sentence.

Exercise 9A  The photosynthesis equation and where it happens

1
FWhat are the two reactants of photosynthesis? Write the words.
2
FWhat is the sugar made by photosynthesis? Write the word.
3
FWhich gas is released by photosynthesis? Write the word.
4
FName the part of the cell where photosynthesis happens. Write the word.
5
FWrite the word equation for photosynthesis, with the products glucose and oxygen. Write the words.
6
FHow many molecules of carbon dioxide are needed to make one molecule of glucose? Write the number.
7
FHow many oxygen molecules are made for each glucose molecule? Write the number.
8
FWhich pigment in the chloroplast absorbs light energy? Write the word.
9
FIs light a reactant of photosynthesis? Write yes or no.
10
PSIn which part of the leaf is most photosynthesis done: the palisade or the lower epidermis? Write the word.
11
PSExplain why a plant needs light for photosynthesis, in one sentence. Accept any sentence that says the light energy is absorbed by chlorophyll and used to make glucose.
12
RChallenge: explain why the chloroplasts in a leaf are placed near the upper surface, in two sentences. Accept any answer that says the top gets the most light, so the chloroplasts there make the most glucose.
9.2

Limiting factors

GCSE
Explain limiting factors: light intensity, carbon dioxide and temperature, Interpret graphs showing limiting factors
Video lesson: Limiting factorsComing soon
Context. Ruth grows a plant in a sealed box with a lamp. She increases the light, and the plant grows faster. Then she increases the light even more, and nothing changes. “Something else must be stopping it now,” she says.

A limiting factor is the factor in shortest supply, which stops the rate of photosynthesis from increasing further. Three main factors can be limiting: the light intensity, the carbon dioxide concentration and the temperature.

When light is the limiting factor, the rate rises as the light increases, then levels off once something else limits it. When carbon dioxide is limiting, adding more carbon dioxide raises the rate. Temperature affects the enzymes that control photosynthesis; the rate rises with temperature up to an optimum, then falls sharply when the enzymes denature.

Rate of photosynthesis at different light intensities024681012012345light intensity (arbitrary units)rate (arbitrary units)
Rate of photosynthesis against light intensity: the rate levels off at high light
Rate of photosynthesis at different carbon dioxide levels02468101201234carbon dioxide (arbitrary units)rate (arbitrary units)
Rate of photosynthesis against carbon dioxide: the rate levels off at high carbon dioxide
Rate of photosynthesis at different temperatures0246810121401020304050temperature (degrees C)rate (arbitrary units)
Rate of photosynthesis against temperature: the rate rises to an optimum and then falls

Key ideas

  • A limiting factor is the factor in shortest supply.
  • Light, carbon dioxide and temperature can each limit the rate.
  • A graph levels off when a different factor becomes limiting.
  • Temperature above the optimum denatures the enzymes.
Worked example 9.2.1
Read the light graph. At what light intensity does the rate first level off? Write the number, in arbitrary units.

Method. Find the first point where the line becomes flat.

  1. Look. The rate is 10 at light 3, and it stays at 10 at 4 and 5.
    stays at 10
  2. Read. So the rate first levels off at light intensity 3.
    3
  3. Write the answer.
    3

Check. The rate does not change after 3. ✓

Worked example 9.2.2
On the CO2 graph, the rate is 9 at a carbon dioxide level of 2. Write the rate at a level of 4. Write the number.

Method. Read the point at 4.

  1. Read. At a carbon dioxide level of 4 the rate is 10.
    10
  2. Compare. The rate has levelled off, so another factor now limits it.
    levelled off
  3. Write the answer.
    10

Check. It is the same as at level 3. ✓

Worked example 9.2.3
Name the factor that limits the rate at a temperature of 50 C in the temperature graph. Write the words.

Method. Think about what happens above the optimum.

  1. Look. At 50 C the rate has fallen, because the enzymes have denatured.
    denatured
  2. Name it. The limiting factor is the temperature, which has denatured the enzymes.
    temperature
  3. Write the answer.
    temperature

Check. Temperature can be limiting at both low and high values. ✓

Watch out

Watch out: a graph levels off because a different factor is limiting, not because the factor you changed has stopped working. Name the factor that is limiting.

Exam tip

Exam tip. When you name a limiting factor from a graph, quote the point where the line levels off, and say which factor is now limiting.

Try it (with your teacher). With your teacher, sketch the light graph on squared paper. Mark the point where the line levels off, and write the limiting factor at that point.

Exercise 9B  Limiting factors

1
FName the three factors that can limit the rate of photosynthesis. Write the words.
2
FWhat is a limiting factor? Write the words.
3
FOn the light graph, what is the rate at light intensity 2? Write the number.
4
FOn the light graph, what is the rate at light intensity 4? Write the number.
5
FOn the temperature graph, at which temperature is the rate highest? Write the number, in degrees C.
6
PSOn the CO2 graph, the rate has levelled off at 10 when the carbon dioxide level is 4. Name a factor that could now be limiting. Accept light or temperature.
7
PSWhy does the rate of photosynthesis increase when light intensity rises from 0 to 3? Write one sentence. Accept any sentence that says more light energy is absorbed by chlorophyll, so more glucose is made.
8
PSWhy does the temperature graph fall sharply above 40 degrees C? Write one sentence. Accept any sentence that says the enzymes denature, so the rate of photosynthesis falls.
9
PSA pupil says that increasing the light always increases the rate. Write yes or no. Accept no.
10
RChallenge: explain why a greenhouse with a lamp still needs carbon dioxide and a suitable temperature to keep the rate of photosynthesis high. Write two sentences. Accept any answer that says each factor can be limiting, so all of them must be kept at a high level.
9.3

Required practical: light intensity and the rate of photosynthesis

GCSE
Investigate the effect of light intensity on the rate of photosynthesis, Calculate a rate from a time
Video lesson: Required practical: light intensity and the rate of photosynthesisComing soon
Context. Ali places leaf discs in a solution of sodium hydrogen carbonate. The discs sink at first. As photosynthesis makes oxygen in the leaf, the discs float. “The faster they float, the faster the photosynthesis,” his teacher says. “Let us test that with a lamp at different distances.”

In this practical the independent variable is the distance of the lamp from the leaf discs, which changes the light intensity. The dependent variable is the time for half the discs to float. The rate is calculated from the time: rate = 1000 / time (a convenient scale for this data). The control variables include the number of discs, the volume of solution, the temperature and the carbon dioxide level (the bicarbonate solution).

Light intensity falls as the lamp moves away, so a shorter time to float means a higher rate.

Distance from lamp (cm)Time to float (s)Rate = 1000 / time
105020
2010010
302005
405002
Time for half the leaf discs to float, and the rate, at four distances (invented data)

Key ideas

  • Independent variable: distance of the lamp (light intensity). Dependent variable: time to float.
  • Rate = 1000 / time, so a shorter time gives a higher rate.
  • Keep the number of discs, the volume and the temperature the same.
  • Light intensity falls as the distance from the lamp increases.
Worked example 9.3.1
At 20 cm, the discs float in 100 seconds. Find the rate, using rate = 1000 / time. Write the number.

Method. Divide 1000 by the time.

  1. Read. The time to float at 20 cm is 100 seconds.
    100 s
  2. Divide. Rate = 1000 / 100.
    1000 / 100 = 10
  3. Write the answer.
    10

Check. 10 x 100 = 1000. ✓

Worked example 9.3.2
At 10 cm, the rate is 20. At 40 cm, the rate is 2. How many times faster is the rate at 10 cm than at 40 cm? Write the number.

Method. Divide the larger rate by the smaller.

  1. Read. The rate at 10 cm is 20, and at 40 cm it is 2.
    20 and 2
  2. Divide. Divide the larger rate by the smaller: 20 / 2 = 10.
    20 / 2 = 10
  3. Write the answer.
    10

Check. 2 x 10 = 20. ✓

Worked example 9.3.3
State the independent variable of the leaf-disc practical. Write the words.

Method. Find the variable changed on purpose.

  1. Find it. The distance of the lamp is changed on purpose.
    changed on purpose
  2. Name it. So the independent variable is the distance from the lamp, which sets the light intensity.
    distance from the lamp
  3. Write the answer.
    distance from the lamp

Check. The time to float is the dependent variable. ✓

Watch out

Watch out: a shorter time gives a higher rate. Do not multiply the time by 1000, because the rate is 1000 divided by the time.

Exam tip

Safety. The lamp is set and checked by the teacher. Keep electrical equipment away from water, and handle the leaf discs and solutions with care.

Try it (with your teacher). With your teacher, set up leaf discs in sodium hydrogen carbonate solution at two lamp distances. Record the time for half the discs to float, and calculate the rate for each.

Exercise 9C  Required practical: light intensity and the rate of photosynthesis

1
FIn the leaf-disc practical, what is the dependent variable? Write the words.
2
FCalculate the rate when the discs float in 500 seconds. Use rate = 1000 / time. Write the number.
3
FCalculate the rate when the discs float in 50 seconds. Use rate = 1000 / time. Write the number.
4
FAt which distance from the lamp is the rate lowest? Write the number, in cm.
5
FWhich gas is produced by the leaf discs, causing them to float? Write the word.
6
PSWhy are the leaf discs cut from the same leaf and the same size? Write one sentence. Accept any sentence that says they are control variables, so only the light changes between tests.
7
PSThe discs float in 200 seconds at 30 cm. Find the rate, and say whether it is higher or lower than at 20 cm. Accept rate 5, lower.
8
PSDescribe the pattern in the leaf-disc results in one sentence. Accept any sentence that says the rate falls as the distance from the lamp increases.
9
PSSuggest one way to improve the leaf-disc practical. Accept any sensible improvement, such as testing more distances or repeating each distance.
10
RChallenge: explain why the leaf discs float faster when the lamp is closer, in two sentences. Accept any answer that says more light is absorbed, so more oxygen is made and the discs float sooner.
9.4

Uses of glucose and greenhouses

GCSE
Describe the uses of glucose made in photosynthesis, Explain how greenhouses control the factors that limit photosynthesis
Video lesson: Uses of glucose and greenhousesComing soon
Context. A tomato grower wants fast growth and a good crop. “We will add carbon dioxide and heat, and we will keep the lamps on,” she says. Each of these is a way of removing a limiting factor from photosynthesis.

Glucose made by photosynthesis is used in several ways. It is used in respiration to release energy. It is converted to starch for storage. It is made into cellulose for cell walls. It is made into fats and oils for storage in seeds. With nitrate ions from the soil, it is used to make proteins.

A greenhouse can raise the rate of photosynthesis by adding carbon dioxide, providing extra light, and keeping the temperature at an optimum for the enzymes.

Use of glucoseWhat happens to it
respirationreleased as energy in the mitochondria
starchstored in the leaves and roots
cellulosemade into cell walls for growth
fats and oilsmade for storage in seeds
proteinsmade using glucose plus nitrate ions
The uses of glucose made by photosynthesis

Key ideas

  • Glucose is used in respiration, stored as starch, made into cellulose, fats and oils, and proteins.
  • Greenhouses can add carbon dioxide, extra light and heat.
  • Raising the limiting factor raises the rate of photosynthesis.
  • Glucose is also converted to starch for storage in leaves and roots.
Worked example 9.4.1
Name the storage carbohydrate that glucose is converted into in plants. Write the word.

Method. Recall the storage carbohydrate.

  1. Recall. Glucose is stored in plants as starch.
    stored as starch
  2. Name it. So the storage carbohydrate is starch.
    starch
  3. Write the answer.
    starch

Check. Cellulose is for cell walls, not for storage. ✓

Worked example 9.4.2
Name the substance that glucose and nitrate ions are used together to make. Write the word.

Method. Recall the product.

  1. Recall. Glucose and nitrate ions are used together to make proteins.
    make proteins
  2. Name it. So the product is protein.
    protein
  3. Write the answer.
    protein

Check. Nitrate is the mineral from the soil. ✓

Worked example 9.4.3
Explain why a greenhouse adds carbon dioxide. Write one sentence.

Method. Think about the limiting factor.

  1. Think. Carbon dioxide can be the limiting factor in a sealed greenhouse.
    limiting factor
  2. Conclude. Adding more lets the rate of photosynthesis increase.
    rate increase
  3. Write the answer.
    Adding carbon dioxide removes it as the limiting factor, so the rate of photosynthesis can increase.

Check. Greenhouses control several factors at once. ✓

Watch out

Watch out: adding more of one factor only helps if that factor is the limiting one. Above that point, the rate levels off.

Exam tip

Exam tip. When you explain a greenhouse measure, name the limiting factor it removes, and say how that raises the rate.

Try it (with your teacher). Write a list of the things a greenhouse controls, and the limiting factor each one controls. Check your list with your teacher.

Exercise 9D  Uses of glucose and greenhouses

1
FName the process in which glucose is broken down to release energy. Write the word.
2
FWhat is the storage carbohydrate made from glucose in plants? Write the word.
3
FGlucose is made into which substance for cell walls? Write the word.
4
FGlucose is made into fats and oils for storage in which part of a plant? Write the word.
5
FWhich mineral from the soil is needed with glucose to make proteins? Write the word.
6
FName one factor a greenhouse can control to increase the rate of photosynthesis. Accept any one: carbon dioxide, light, or temperature.
7
FIn a greenhouse, what is the purpose of keeping the temperature at an optimum? Write the words.
8
PSWhich product of photosynthesis is used to release energy in respiration? Write the word.
9
PSA plant makes 5 units of glucose each hour. How many units does it make in 6 hours? Write the number.
10
PSExplain why a greenhouse grower might not add extra light if carbon dioxide is still the limiting factor. Write one sentence. Accept any sentence that says extra light does not raise the rate when another factor is the limit.
11
RChallenge: a grower adds carbon dioxide and finds that the rate of photosynthesis does not rise. Explain why, and say what to check. Write two sentences. Accept any answer that says light or temperature is now limiting, so the grower should check those factors.
9.5

The inverse square law for light intensity [H]

GCSE
[H] Apply the inverse square law to light intensity, [H] Compare light intensities at different distances
Video lesson: The inverse square law for light intensity [H]Coming soon
Context. Hana moves the lamp twice as far from the pondweed. “Does that halve the light, or does it do something more?” she asks. The answer is given by the inverse square law.

[H] Higher tier only. This section is higher-tier content. It applies the inverse square law to the light from a point source, such as a lamp.

[H] For a point source of light, the intensity falls with the square of the distance. Doubling the distance cuts the intensity to a quarter, because the light spreads over four times the area. Tripling the distance cuts it to a ninth. This is the inverse square law: intensity is proportional to 1 / distance squared.

Distance (cm)Relative light intensity
10100
2025
406.25
Relative light intensity at three distances (relative to 10 cm = 100)

Key ideas

  • [H] Intensity is proportional to 1 / distance squared.
  • [H] Doubling the distance gives a quarter of the intensity.
  • [H] Tripling the distance gives a ninth of the intensity.
  • [H] Light intensity is the limiting factor for the rate at low light.
Worked example 9.5.1
[H] The intensity at 10 cm is 100 units. What is the intensity at 20 cm? Write the number.

Method. Doubling the distance gives a quarter of the intensity.

  1. Double. The distance doubles from 10 cm to 20 cm.
    10 to 20
  2. Divide. The intensity becomes 100 / 4 = 25 units.
    100 / 4 = 25
  3. Write the answer.
    25

Check. 25 x 4 = 100. ✓

Worked example 9.5.2
[H] The intensity at 10 cm is 100 units. What is the intensity at 30 cm? Write the number, to the nearest whole number.

Method. Tripling the distance gives a ninth of the intensity.

  1. Triple. The distance triples from 10 cm to 30 cm.
    10 to 30
  2. Divide. The intensity becomes 100 / 9, which is about 11.
    100 / 9 = 11.1
  3. Write the answer.
    11

Check. 11 x 9 = 99, which is close to 100. ✓

Worked example 9.5.3
[H] Explain why moving a lamp further away reduces the light more than in proportion. Write one sentence.

Method. Think about how the light spreads.

  1. Think. The light spreads over a larger area as the distance increases.
    larger area
  2. Conclude. So the intensity falls with the square of the distance, not in direct proportion.
    square of the distance
  3. Write the answer.
    The light spreads over a larger area, so intensity falls with the square of the distance, not in direct proportion.

Check. Doubling distance does not halve the intensity. ✓

Watch out

Watch out: doubling the distance does not halve the intensity. It cuts it to a quarter, because the distance is squared.

Exam tip

Exam tip. [H] Use the square of the distance in the calculation: 2 squared is 4, and 3 squared is 9.

Try it (with your teacher). With your teacher, use a torch or a lamp and a ruler. Measure the spot of light on a card at 10 cm and 20 cm. Compare the brightness of the spots, and say which result matches the inverse square law.

Exercise 9E  [H] The inverse square law

1
F[H] Intensity is proportional to what, for a point source of light? Write the words.
2
F[H] If the distance doubles, the intensity becomes what fraction of the original? Write the fraction.
3
F[H] If the distance triples, the intensity becomes what fraction of the original? Write the fraction.
4
PS[H] The intensity at 10 cm is 100 units. What is the intensity at 40 cm? Write the number.
5
PS[H] A lamp at 20 cm gives an intensity of 25 units. What intensity would it give at 40 cm? Write the number.
6
PS[H] Why does a lamp that is twice as far away not halve the light? Write one sentence. Accept any sentence that says the light spreads over four times the area, so the intensity falls to a quarter.
7
PS[H] A pupil says that doubling the distance halves the light, so the intensity at 20 cm is 50 units. Write yes or no, and give the correct number. Accept no, 25 units.
8
R[H] Challenge: explain why the leaf-disc rate at 40 cm is much lower than at 10 cm, using the inverse square law. Write two sentences. Accept any answer that says the light at 40 cm is 1/16 of the light at 10 cm, so less photosynthesis happens and the discs float more slowly.

Chapter summary

SkillMethod
EquationCarbon dioxide + water -> glucose + oxygen (light and chlorophyll); 6CO2 + 6H2O -> C6H12O6 + 6O2.
ChloroplastContains chlorophyll, which absorbs light energy for photosynthesis.
Limiting factorsLight, carbon dioxide and temperature; the rate levels off when another factor limits it.
Leaf-disc practicalRate = 1000 / time to float; a closer lamp gives more light and a faster rate.
Glucose usesRespiration, starch storage, cellulose for cell walls, fats and oils, and proteins with nitrate.
GreenhousesControl carbon dioxide, light and temperature to remove the limiting factor.
[H] Inverse squareIntensity is proportional to 1 / distance squared; doubling the distance gives a quarter.

Can you…?

  • I can write the word and symbol equations for photosynthesis
  • I can name the products of photosynthesis and say where they are used
  • I can identify the limiting factor on a graph and explain the effect of each factor
  • I can carry out the leaf-disc practical and calculate a rate
  • I can explain how greenhouses control the factors that limit photosynthesis
  • [H] I can apply the inverse square law to light intensity

Key terms

photosynthesis
the process by which plants make glucose from carbon dioxide and water, using light
chloroplast
the organelle where photosynthesis happens, containing chlorophyll
chlorophyll
the green pigment that absorbs light energy
limiting factor
the factor in shortest supply, which stops the rate increasing further
optimum
the value at which an enzyme works best
starch
the storage carbohydrate that plants make from glucose
cellulose
a strong carbohydrate that forms plant cell walls
[H] inverse square law
intensity is proportional to 1 divided by the distance squared

Exercise 9Z  Check-up

1
Write the word equation for photosynthesis. Write the words.
2
Name the organelle where photosynthesis happens. Write the word.
3
Name the green pigment that absorbs light for photosynthesis. Write the word.
4
Name the gas released by photosynthesis. Write the word.
5
Name the limiting factor on a graph that has levelled off after light intensity 3, if carbon dioxide is high. Write the word. Accept light.
6
On the light graph, what is the rate at light intensity 1? Write the number.
7
Calculate the rate of photosynthesis in the leaf-disc practical when the discs float in 100 seconds. Use rate = 1000 / time. Write the number.
8
Name the storage carbohydrate that glucose is converted into. Write the word.
9
Name the mineral needed with glucose to make proteins. Write the word.
10
Explain in one sentence why a rate can stop rising even when light increases. Accept any sentence that says another factor, such as carbon dioxide or temperature, is now the limiting factor.

Total 22 marks. Allow about 22 minutes.

Exercise 9X  Exam-style questions

1
FName the process by which plants make glucose from carbon dioxide and water. Write the word.[1 mark]
2
FName the two products of photosynthesis. Write the words.[1 mark]
3
PSThe rate of photosynthesis has levelled off at 10 when light is high and carbon dioxide is at level 3 or more. Name one other factor that could now be limiting the rate. Write the word. Accept temperature.[1 mark]
4
FWrite the balanced symbol equation for photosynthesis. Write the equation using 6CO2, 6H2O, C6H12O6 and 6O2.[2 marks]
5
FExplain how a greenhouse uses carbon dioxide to increase the rate of photosynthesis. Write two sentences. Accept any answer that says carbon dioxide may be limiting, and adding it raises the rate until another factor limits.[2 marks]
6
FDescribe how the glucose made by photosynthesis is used in a plant. Write three points. Accept any three of: respiration to release energy; stored as starch; made into cellulose for cell walls; made into fats and oils; made into proteins with nitrate.[3 marks]
7
PSIn a leaf-disc experiment, the discs float in 50 seconds at 10 cm and in 500 seconds at 40 cm. Calculate both rates, using rate = 1000 / time, and say how many times faster the rate is at 10 cm. Accept 20, 2, and 10 times faster.[3 marks]
8
RExplain why the rate of photosynthesis falls when the temperature rises above the optimum. Write two sentences. Accept any answer that says the enzymes denature at high temperature, so the active site changes shape and the rate falls.[3 marks]
9
PSThe rate of photosynthesis is 0, 4, 8 and 10 at light intensities 0, 1, 2 and 3, and stays at 10 at higher intensities. Describe how these results show that light is the limiting factor at low intensities. Write two sentences. Accept any answer that says the rate rises as light rises from 0 to 3, showing light is limiting.[2 marks]
10
R[H] A lamp gives an intensity of 100 units at 10 cm. Find the intensity at 30 cm, to the nearest whole number, and explain why the value is so much lower. Accept 11 and an answer that says the intensity falls with the square of the distance.[4 marks]
← Chapter 8: Plant tissues and transportChapter 10: Aerobic and anaerobic respiration →

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