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KS3 Science · Year 8Chapter 3 · Respiration and photosynthesis
Year 8 · Chapter 3

Respiration and photosynthesis

By the end of this chapter you will be able to

  • Write the word and symbol equations for aerobic respiration and photosynthesis
  • Explain anaerobic respiration in muscles and in yeast
  • Describe the structure of a leaf and how it is adapted for photosynthesis
  • Explain what a limiting factor is, and read a graph that shows one
  • Compare respiration and photosynthesis

You should already know

  • Mitochondria and chloroplasts (Year 7 Chapter 2)
  • Word equations and energy changes (Year 7 Chapter 9)

Where this leads

  • Energy transfers and stores (Year 8 Chapter 11)

National Curriculum for England, Key Stage 3 science, Year 8. Content: 8B4 aerobic respiration in cells, with word and symbol equations (releases energy, exothermic); anaerobic respiration in muscles and yeast (brief), 8B5 photosynthesis, with word and symbol equations and leaf structure; limiting factors of light, carbon dioxide and temperature; the two processes compared.

Exercise P  Do you remember?

1
FWhat is the name of the green pigment in chloroplasts that absorbs light? Write the word.
2
FWhich gas do plants take in from the air for photosynthesis? Write the words.
3
FWhich part of a cell is the site of aerobic respiration? Write the word.
4
FWhat is 12 x 4? Write the number.
5
FWhich gas is released by plants during photosynthesis? Write the word.
3.1

Aerobic respiration

Year 8
8B4: aerobic respiration in cells releases energy from glucose; word and symbol equations; exothermic
Video lesson: Aerobic respirationComing soon
Story. Sam cycles up a long hill. His breathing speeds up, and his muscles start to feel hot. “Where does the energy come from?” he asks. His teacher says, “From glucose, which reacts with oxygen inside your cells. That reaction is respiration.”

Respiration is the chemical reaction in cells that releases energy from glucose. In aerobic respiration the reaction needs oxygen. It happens in the mitochondria of almost every cell. It releases a large amount of energy, so it is exothermic.

The word equation is: glucose + oxygen → carbon dioxide + water.

glucose + oxygen &rarr; carbon dioxide + water (energy released)C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + 6O<sub>2</sub> &rarr; 6CO<sub>2</sub> + 6H<sub>2</sub>O
Word and symbol equations for aerobic respiration.

Key ideas

  • Aerobic respiration releases energy from glucose, using oxygen.
  • Its products are carbon dioxide and water.
  • It is exothermic: energy is released to the surroundings and to the body.
  • Respiration happens in the mitochondria of living cells.
Worked example 3.1.1
What are the products of aerobic respiration? Write the words.

Method. Read the right side of the word equation.

  1. Read. The products are written after the arrow.
    after the arrow
  2. Name them. The products are carbon dioxide and water.
    carbon dioxide and water
  3. Write the answer.
    carbon dioxide and water

Check. Glucose and oxygen are the reactants. ✓

Worked example 3.1.2
In the symbol equation, how many oxygen molecules are needed for one glucose molecule? Write the number.

Method. Read the coefficient in front of O2.

  1. Read. The symbol equation has 6O2 on the left.
    6O2
  2. Name the number. So 6 oxygen molecules are needed.
    6
  3. Write the answer.
    6

Check. Six molecules of O2 are needed for one glucose. ✓

Worked example 3.1.3
Is aerobic respiration exothermic or endothermic? Write the word.

Method. Decide from the energy change.

  1. Think. Respiration releases energy, so the surroundings gain energy.
    releases energy
  2. Decide. A reaction that gives energy out is exothermic.
    exothermic
  3. Write the answer.
    exothermic

Check. Energy is released when glucose is broken down. ✓

Watch out

Watch out: respiration is not breathing. Breathing brings oxygen in and takes carbon dioxide out; respiration is the reaction inside the cells.

Exam tip

Exam tip. In a symbol equation, the number in front of a formula multiplies every atom in it: 6O2 has 12 oxygen atoms.

Try it (with your teacher, or a grown-up at home). Look at a labelled diagram of a mitochondrion. Say where the reactants and products of aerobic respiration go. Do not try to measure respiration yourself; your teacher will explain the class data.

Exercise 3A  Aerobic respiration

1
FWhere in a cell does aerobic respiration happen? Write the word.
2
FWhat does aerobic respiration release from glucose? Write one word.
3
FWhich gas does aerobic respiration need? Write the word.
4
FName the gas that is one of the products of aerobic respiration. Write the words.
5
FName the other product of aerobic respiration. Write the word.
6
FIs aerobic respiration exothermic or endothermic? Write the word.
7
FIn the symbol equation for aerobic respiration, how many carbon dioxide molecules are made from one glucose? Write the number.
8
FIn the symbol equation for aerobic respiration, how many water molecules are made from one glucose? Write the number.
9
FWrite the reactants of aerobic respiration. Write the words.
10
PSAli says aerobic respiration takes in carbon dioxide from the air. Is Ali right? Write yes or no.
11
PSWorking scientifically: in the symbol equation, 6O2 is written on the left. How many oxygen atoms are there in 6O2? Write the number.
12
RExplain why breathing rate rises during hard exercise. Write one sentence.
3.2

Anaerobic respiration

Year 8
8B4: anaerobic respiration in muscles and in yeast (brief); energy released with no oxygen
Video lesson: Anaerobic respirationComing soon
Story. Ruby runs the last part of a race very hard. Her legs start to ache and feel heavy. “Why can I not keep this up?” she asks. Her teacher says, “Your muscles are respiring without enough oxygen. The product builds up and makes them tired.”

Anaerobic respiration releases energy from glucose without oxygen. It releases much less energy per glucose molecule than aerobic respiration.

In muscles, anaerobic respiration makes lactic acid: glucose → lactic acid. The lactic acid builds up during fast exercise. Afterwards, the body breathes hard to supply oxygen that helps to break it down.

In yeast, anaerobic respiration is called fermentation. It makes ethanol and carbon dioxide: glucose → ethanol + carbon dioxide. Bakers use the carbon dioxide to make bread rise.

Key ideas

  • Anaerobic respiration does not use oxygen.
  • In muscles it makes lactic acid, which builds up during fast exercise.
  • In yeast it makes ethanol and carbon dioxide; this is fermentation.
  • It releases less energy per glucose than aerobic respiration.
Worked example 3.2.1
What product builds up in muscles during fast exercise? Write the words.

Method. Recall the muscle product.

  1. Recall. Anaerobic respiration in muscles makes lactic acid.
    lactic acid
  2. Name it. Lactic acid is the product that builds up.
    lactic acid
  3. Write the answer.
    lactic acid

Check. Lactic acid makes the muscles tired. ✓

Worked example 3.2.2
Name the two products of anaerobic respiration in yeast. Write the words.

Method. Recall the two products.

  1. Recall. Yeast makes ethanol and carbon dioxide.
    ethanol and carbon dioxide
  2. Check. Both are written after the arrow in the word equation.
    after the arrow
  3. Write the answer.
    ethanol and carbon dioxide

Check. Yeast fermentation is anaerobic. ✓

Worked example 3.2.3
Does anaerobic respiration release more or less energy per glucose than aerobic respiration? Write the word.

Method. Compare the two processes.

  1. Compare. Anaerobic respiration releases less energy per glucose.
    less energy
  2. Decide. So the answer is less.
    less
  3. Write the answer.
    less

Check. Aerobic respiration releases more energy. ✓

Watch out

Watch out: lactic acid does not come from aerobic respiration. Muscles make it only when oxygen supply cannot keep up with the demand.

Exam tip

Exam tip. Write “lactic acid” in full, and name the process as anaerobic respiration.

Try it (with your teacher, or a grown-up at home). Bake or look at a labelled picture of bread dough rising. Explain the gas that makes it rise, and name the microorganism. Do not taste or handle raw dough with yeast without your teacher or a grown-up.

Exercise 3B  Anaerobic respiration

1
FDoes anaerobic respiration use oxygen? Write yes or no.
2
FName the product that builds up in muscles during fast exercise. Write the words.
3
FWhich gas makes bread dough rise? Write the word.
4
FName the other product of yeast fermentation, besides carbon dioxide. Write the word.
5
FWhat is the name of anaerobic respiration in yeast? Write the word.
6
FDoes anaerobic respiration release more or less energy per glucose than aerobic respiration? Write the word.
7
FWhich is an example of an organism that respires anaerobically: yeast or a leaf? Write the word.
8
FIn the word equation for lactic acid in a muscle, how many products are formed? Write the number.
9
FIn the word equation for fermentation in yeast, how many products are formed? Write the number.
10
PSWorking scientifically: a runner breathes hard after a sprint. Name one reason why. Accept any reason: to supply oxygen to break down the lactic acid built up.
11
PSAli says anaerobic respiration is the same as aerobic respiration without the carbon dioxide. Is Ali right? Write yes or no.
12
RExplain why a muscle gets tired during fast exercise. Write one sentence.
3.3

Photosynthesis

Year 8
8B5: photosynthesis with word and symbol equations; leaf structure and its adaptations
Video lesson: PhotosynthesisComing soon
Story. Ruby puts a pondweed in a jar of water on a sunny windowsill. Tiny bubbles rise from the cut stem. “Where is the gas coming from?” she asks. Her teacher says, “The plant is making oxygen by photosynthesis, using light energy.”

Photosynthesis is the reaction in green plants and algae that uses light energy to make glucose from carbon dioxide and water. Oxygen is released as a by-product. The reaction takes place in the chloroplasts, which contain the green pigment chlorophyll to absorb the light.

The word equation is: carbon dioxide + water → glucose + oxygen, with light and chlorophyll as conditions.

carbon dioxide + water &rarr; glucose + oxygen (light, chlorophyll)6CO<sub>2</sub> + 6H<sub>2</sub>O &rarr; C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + 6O<sub>2</sub>
Word and symbol equations for photosynthesis.

A leaf is adapted for photosynthesis. Its upper layer is transparent, letting light through to the palisade layer, which has many chloroplasts. The stomata on the lower surface let carbon dioxide in and oxygen out. Xylem vessels bring water to the leaf.

upper epidermispalisade mesophyllspongy mesophylllower epidermisstoma (gas exchange)light enters from abovelight
Cross-section of a leaf showing the layers and a stoma.

Key ideas

  • Photosynthesis uses light energy to make glucose from carbon dioxide and water.
  • It releases oxygen as a by-product.
  • Chlorophyll in chloroplasts absorbs the light.
  • Stomata let carbon dioxide in and oxygen out; xylem brings water.
Worked example 3.3.1
Which gas does a leaf take in for photosynthesis? Write the words.

Method. Recall the reactant.

  1. Recall. Carbon dioxide is taken in through the stomata.
    carbon dioxide
  2. Name it. So the gas is carbon dioxide.
    carbon dioxide
  3. Write the answer.
    carbon dioxide

Check. The gas is written CO2. ✓

Worked example 3.3.2
Name the gas released by photosynthesis. Write the word.

Method. Recall the product.

  1. Recall. Photosynthesis releases oxygen as a by-product.
    oxygen
  2. Name it. The gas released is oxygen.
    oxygen
  3. Write the answer.
    oxygen

Check. Oxygen leaves the leaf through the stomata. ✓

Worked example 3.3.3
Which layer of the leaf has the most chloroplasts? Write the words.

Method. Find the layer with the most chloroplasts.

  1. Look. The palisade layer is packed with chloroplasts, close to the light.
    packed with chloroplasts
  2. Name it. So the palisade mesophyll has the most.
    palisade mesophyll
  3. Write the answer.
    palisade mesophyll

Check. The spongy layer has fewer. ✓

Watch out

Watch out: plants respire as well as photosynthesise. Respiration happens in every living cell, day and night.

Exam tip

Exam tip. Name the conditions, light and chlorophyll, above the arrow, not as reactants or products.

Try it (with your teacher, or a grown-up at home). Look at a labelled picture of a leaf and find the palisade layer, the stomata and the xylem. Say why each is placed where it is. Do not put any plant in a sealed container with a lamp; observe only the picture or a leaf from outdoors.

Exercise 3C  Photosynthesis

1
FWhat is the green pigment in chloroplasts called? Write the word.
2
FWhat is the energy source for photosynthesis? Write the word.
3
FName the sugar made by photosynthesis. Write the word.
4
FWhich gas enters through the stomata for photosynthesis? Write the words.
5
FWhat is the name of the tiny openings on the underside of a leaf? Write the word.
6
FIn the symbol equation, how many oxygen molecules are made from one glucose molecule? Write the number.
7
FWhich layer of the leaf has the most chloroplasts? Write the words.
8
FName the tubes that carry water from the roots to the leaf. Write the word.
9
FIs photosynthesis an endothermic or exothermic reaction? Write the word.
10
FIn the word equation for photosynthesis, how many reactants are there? Write the number.
11
PSRuby says plants do not respire because they only photosynthesise. Is Ruby right? Write yes or no.
12
PSWorking scientifically: how many carbon atoms are there in one glucose molecule, C6H12O6? Write the number.
13
RExplain why a leaf needs light to make glucose. Write one sentence.
3.4

Limiting factors

Year 8
8B5: limiting factors of photosynthesis: light, carbon dioxide and temperature
Video lesson: Limiting factorsComing soon
Story. Omar grows tomato plants in a greenhouse. He adds more carbon dioxide, but the plants stop growing faster after a point. “Why not?” he asks. His teacher says, “The factor in shortest supply limits the rate. Once carbon dioxide is plentiful, something else is limiting.”

A limiting factor is the factor in shortest supply, which limits the rate of a reaction. For photosynthesis, the three main limiting factors are light intensity, carbon dioxide concentration and temperature.

When one factor is increased and the rate levels off, the graph shows a plateau. Another factor is then limiting the rate. Very high temperatures stop the enzymes in photosynthesis working.

Rate of photosynthesis at different carbon dioxide levels0.040.10.20.30.405101520253035Carbon dioxide (%)Bubbles per minute
Rate of photosynthesis levels off at high carbon dioxide levels.

The table shows class data for the rate of photosynthesis at five carbon dioxide levels, with light and temperature kept the same.

Carbon dioxide (%)Bubbles per minute
0.0410
0.120
0.230
0.330
0.430

Key ideas

  • A limiting factor is the one in shortest supply.
  • The main limiting factors are light, carbon dioxide and temperature.
  • A plateau on a graph shows that another factor is now limiting.
  • Very high temperatures stop photosynthesis by denaturing its enzymes.
Worked example 3.4.1
Using the table, at which carbon dioxide level does the rate first reach 30? Write the number.

Method. Look for the first time the rate is 30.

  1. Read. The rate is 10, then 20, then 30 at 0.2%.
    30 at 0.2
  2. Name it. The rate first reaches 30 at 0.2%.
    0.2
  3. Write the answer.
    0.2

Check. The rate stays at 30 after this. ✓

Worked example 3.4.2
Using the table, what is the rate at 0.1% carbon dioxide? Write the number.

Method. Read the row for 0.1%.

  1. Read. The row for 0.1% shows 20 bubbles per minute.
    20
  2. Name it. The rate is 20 bubbles per minute.
    20
  3. Write the answer.
    20

Check. The rate is 20 bubbles per minute. ✓

Worked example 3.4.3
At 0.3% carbon dioxide the rate stays at 30. What is the most likely limiting factor now? Accept light, or temperature.

Method. Think about what is still in short supply.

  1. Think. Carbon dioxide is plentiful, so it is no longer the limit.
    no longer the limit
  2. Name it. Light, or temperature, is now limiting the rate.
    light, or temperature
  3. Write the answer.
    Accept light, or temperature.

Check. Test one factor at a time. ✓

Watch out

Watch out: the factor that limits the rate changes. A plateau shows that the limit has moved to a different factor.

Exam tip

Exam tip. Say which factor is limiting: “at high carbon dioxide, light is the limiting factor”.

Try it (with your teacher, or a grown-up at home). Ask your teacher to show a demonstration of pondweed with a lamp, in a safe place away from any water and sockets. Count the bubbles per minute at two distances. Do not set up the lamp or water yourself.

Exercise 3D  Limiting factors

1
FWhat is the name for a factor in short supply that limits the rate? Write the words.
2
FName three limiting factors of photosynthesis. Accept any three: light, carbon dioxide, temperature.
3
FUsing the table, what is the rate at 0.04% carbon dioxide? Write the number.
4
FUsing the table, at which carbon dioxide level does the plateau start? Write the number, as a percentage.
5
FUsing the table, what is the difference in rate between 0.1% and 0.04%? Write the number.
6
FUsing the table, what is the rate at 0.4% carbon dioxide? Write the number.
7
FDoes the rate keep rising above 0.2% carbon dioxide? Write yes or no.
8
FWhat is the name for a graph section that stays flat, showing the rate has stopped rising? Write the word.
9
FWhich gas is the reactant that changes in this table? Write the words.
10
PSA greenhouse adds more carbon dioxide, but the rate does not rise. Name a limiting factor to check. Accept light, or temperature.
11
PSRuby says doubling the carbon dioxide from 0.2% to 0.4% doubles the rate. Is Ruby right? Write yes or no.
12
RExplain why a plant at a very high temperature may stop photosynthesising, even when light and carbon dioxide are plentiful. Write one sentence.
3.5

Comparing respiration and photosynthesis

Year 8
8B4 and 8B5: compare the reactants, products, energy changes and where the two processes happen
Video lesson: Comparing respiration and photosynthesisComing soon
Story. Sam says, “Plants do the opposite of animals, so they must breathe in the other gas.” His teacher laughs and says, “Almost. Both processes are linked, but they are not simply opposites. One stores energy and the other releases it.”

The two processes are linked. The products of one are the reactants of the other. Photosynthesis stores energy in glucose, from light. Respiration releases energy from glucose, for the cell to use.

The table compares the two processes.

FeatureAerobic respirationPhotosynthesis
reactantsglucose and oxygencarbon dioxide and water
productscarbon dioxide and waterglucose and oxygen
energyenergy released (exothermic)energy stored in glucose
where it happensin every living cell, in mitochondriain cells with chloroplasts, mainly leaves
light needed?noyes

Key ideas

  • Respiration releases energy; photosynthesis stores energy in glucose.
  • Respiration happens in all living cells; photosynthesis happens in cells with chloroplasts.
  • Photosynthesis needs light; respiration does not.
  • The equations are linked: the products of one are the reactants of the other.
Worked example 3.5.1
Which process releases energy from glucose? Write the word.

Method. Recall the energy change.

  1. Recall. Respiration releases energy from glucose.
    releases energy
  2. Name it. So the process is respiration.
    respiration
  3. Write the answer.
    respiration

Check. Photosynthesis stores energy. ✓

Worked example 3.5.2
Which gas is a product of respiration and a reactant of photosynthesis? Write the words.

Method. Compare the two equations.

  1. Look. Carbon dioxide is written after the arrow in respiration, and before the arrow in photosynthesis.
    after the arrow
  2. Name it. So the gas is carbon dioxide.
    carbon dioxide
  3. Write the answer.
    carbon dioxide

Check. Oxygen is the other linked gas. ✓

Worked example 3.5.3
Which process needs light? Write the word.

Method. Recall the condition.

  1. Recall. Photosynthesis uses light energy.
    light energy
  2. Name it. So the process is photosynthesis.
    photosynthesis
  3. Write the answer.
    photosynthesis

Check. Respiration does not need light. ✓

Watch out

Watch out: photosynthesis is not the reverse of respiration in every way. Photosynthesis needs light and chlorophyll; respiration does not.

Exam tip

Exam tip. Compare the processes using “both”, “but” and a clear reason: for example, “both use the same gases, but photosynthesis needs light”.

Try it (with your teacher, or a grown-up at home). Draw a two-column table with the headings “respiration” and “photosynthesis”. Fill it with reactants, products, energy change and light. Check it with a partner.

Exercise 3E  Comparing respiration and photosynthesis

1
FWhich process uses light energy? Write the word.
2
FWhich process happens in all living cells? Write the word.
3
FWhich process releases oxygen? Write the word.
4
FWhich process stores energy in glucose? Write the word.
5
FWhich process uses oxygen? Write the word.
6
FWhat are the reactants of photosynthesis? Write the words.
7
FWhat are the products of aerobic respiration? Write the words.
8
FIs the overall equation of respiration the reverse of the equation for photosynthesis? Write yes or no.
9
FIn the symbol equation for photosynthesis, how many water molecules are needed to make one glucose molecule? Write the number.
10
FIn the symbol equation for photosynthesis, how many oxygen molecules are released for one glucose molecule? Write the number.
11
PSAli says a plant does not respire at night because it has no light. Is Ali right? Write yes or no.
12
PSWorking scientifically: name the process that is still happening in a plant at night, when there is no light. Write the word.
13
RExplain why photosynthesis and respiration are described as linked processes. Write one sentence.

Chapter summary

SkillMethod
Aerobic respirationGlucose + oxygen -> carbon dioxide + water, with energy released (exothermic).
Anaerobic respirationMuscles: glucose -> lactic acid. Yeast (fermentation): glucose -> ethanol + carbon dioxide.
PhotosynthesisCarbon dioxide + water -> glucose + oxygen, using light and chlorophyll.
LeafPalisade layer has most chloroplasts; stomata let gases in and out; xylem carries water.
Limiting factorsLight, carbon dioxide or temperature; a plateau shows another factor is limiting.
Linked processesProducts of one are reactants of the other; photosynthesis stores energy, respiration releases it.

Can you…?

  • I can write the word and symbol equations for aerobic respiration and photosynthesis
  • I can explain anaerobic respiration in muscles and in yeast
  • I can describe the structure of a leaf and say how it is adapted
  • I can explain a limiting factor and read a graph that shows a plateau
  • I can compare respiration and photosynthesis clearly

Key terms

respiration
the chemical reaction in cells that releases energy from glucose
aerobic respiration
respiration using oxygen, giving carbon dioxide and water
anaerobic respiration
respiration without oxygen
fermentation
anaerobic respiration in yeast, giving ethanol and carbon dioxide
photosynthesis
the reaction in green plants that uses light to make glucose and oxygen
chlorophyll
the green pigment that absorbs light in chloroplasts
stomata
tiny openings on a leaf that let gases in and out
limiting factor
the factor in short supply that limits the rate of a reaction
plateau
a flat section of a graph where the rate has stopped rising

Exercise 3Z  Check-up

1
Which gas does aerobic respiration need? Write the word.
2
Name the other product of aerobic respiration, besides carbon dioxide. Write the word.
3
What product builds up in muscles during fast exercise? Write the words.
4
What is the name of anaerobic respiration in yeast? Write the word.
5
Name the green pigment that absorbs light in a leaf. Write the word.
6
Which layer of the leaf has the most chloroplasts? Write the words.
7
Using the table, what is the rate at 0.2% carbon dioxide? Write the number.
8
What is the name for a flat section of a graph that shows the rate has stopped rising? Write the word.
9
Which process stores energy in glucose: photosynthesis or respiration? Write the word.
10
Explain why anaerobic respiration releases less energy per glucose than aerobic respiration. Write one sentence.

Total 14 marks. Allow about 14 minutes.

Exercise 3X  Practice test questions

1
FWrite the word equation for aerobic respiration. Write the reactants and the products.[2 marks]
2
FName the product of anaerobic respiration in muscles. Write the words.[1 mark]
3
RExplain why a plant needs light to make glucose. Write one sentence.[2 marks]
4
FWhich gas is released by photosynthesis? Write the word.[1 mark]
5
FName the openings in a leaf that let gases in and out. Write the word.[1 mark]
6
PSUsing the table of rates, at what carbon dioxide level does the rate stop rising? Write the number, as a percentage.[2 marks]
7
FName one limiting factor of photosynthesis, other than carbon dioxide. Accept light, or temperature.[1 mark]
8
RExplain why the symbol equation for photosynthesis has 6O2 on the right. Write one sentence.[2 marks]
9
FWhich process happens in every living cell: photosynthesis or respiration? Write the word.[1 mark]
10
FName the layer of the leaf that has the most chloroplasts. Write the words.[1 mark]
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