AQA GCSE Combined Science (8464), Higher tier · Chemistry › Chemistry of the atmosphere › The composition and evolution of the Earth's atmosphere
Practise How oxygen increased. 12 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How algae and then plants produced the oxygen in today's atmosphere by photosynthesis, and how this allowed animals to evolve. Expect the word equation for photosynthesis, the key dates, and 'describe' or 'explain' questions, sometimes as part of a 6-mark answer on how the atmosphere has changed.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name the process that produced the oxygenPhotosynthesis by algae and plants produced the oxygen that is now in the atmosphere.
4
Write the word equation for photosynthesiscarbon dioxide + water → glucose + oxygen, with light as the energy source.
5
State when algae first produced oxygenAlgae first produced oxygen about 2.7 billion years ago, and oxygen appeared in the atmosphere soon after.
6
Describe how the percentage of oxygen increasedOver the next billion years plants evolved and oxygen gradually increased to a level that let animals evolve.
6
Explain two effects of photosynthesis on airPhotosynthesis adds oxygen to the atmosphere and removes carbon dioxide from it at the same time.
7
Balance the symbol equation for photosynthesisThe balanced equation is 6CO2 + 6H2O → C6H12O6 + 6O2.
Notes
Photosynthesis made the oxygen
The oxygen in today's atmosphere was produced by algae and plants through photosynthesis.
Word equation: carbon dioxide + water → glucose + oxygen. Light is needed, so write 'light' above the arrow.
Balanced symbol equation: 6CO2 + 6H2O → C6H12O6 + 6O2. Check: 6 C, 12 H and 18 O atoms on each side. grade 7+
Photosynthesis adds oxygen to the atmosphere and removes carbon dioxide from it at the same time.
The timeline
The early atmosphere had little or no oxygen.
Algae first produced oxygen about 2.7 billion years ago. Soon after this, oxygen appeared in the atmosphere.
Over the next billion years, plants evolved and the percentage of oxygen gradually increased.
Eventually the percentage of oxygen was high enough for animals to evolve. Animals need oxygen for respiration.
Today oxygen is about 20% of the atmosphere, and the proportions of the gases have been much the same for about 200 million years.
Putting it in order
Volcanoes release mainly carbon dioxide → algae evolve and photosynthesise → oxygen appears in the atmosphere → plants evolve → oxygen gradually increases → animals evolve.
The increase was very slow: it took hundreds of millions of years.
The dates are estimates, worked out from evidence in very old rocks and fossils.
Algae first produced oxygen about 2.7 billion years ago
Over the next billion years: plants evolved and oxygen gradually increased
Enough oxygen → animals could evolve
Photosynthesis adds O2 AND removes CO2
Oxygen today: about 20% of the atmosphere
How to answer each type of question
Complete the word equation for photosynthesis
1 to 2 marks4
Reactants: carbon dioxide and water. Products: glucose and oxygen.
Use names, not formulae, when the question asks for a word equation.
Example. Complete the word equation for photosynthesis. carbon dioxide + __________ → glucose + __________
Show the model answer
water (1) oxygen (1)
Describe how the percentage of oxygen increased
3 to 4 marks6
Start with who: algae, by photosynthesis.
Add when: about 2.7 billion years ago.
Then plants evolved over the next billion years and oxygen gradually increased.
Finish with the result: enough oxygen for animals to evolve.
Example. The Earth's early atmosphere had little or no oxygen. Describe how the percentage of oxygen in the atmosphere increased.
Show the model answer
Algae evolved and produced oxygen by photosynthesis (1), starting about 2.7 billion years ago (1). Over the next billion years plants evolved and the percentage of oxygen gradually increased (1), until there was enough oxygen for animals to evolve (1).
Explain how living things changed the atmosphere
3 to 5 marks7
Name the process: photosynthesis.
Give BOTH effects: carbon dioxide used up, oxygen released.
For animals, link to their need for oxygen for respiration.
Example. (a) Explain how the evolution of algae changed the composition of the atmosphere. (b) Suggest why animals evolved only after plants had existed for a long time.
Show the model answer
(a) Algae photosynthesised (1). This used carbon dioxide, so the percentage of carbon dioxide in the atmosphere decreased (1), and released oxygen, so the percentage of oxygen increased (1). (b) Animals need oxygen for respiration (1), and it took a long time for photosynthesis to raise the percentage of oxygen to a high enough level (1).
Balance the symbol equation for photosynthesis
1 mark7
Glucose, C6H12O6, has 6 C, so put 6 in front of CO2.
It has 12 H, so put 6 in front of H2O.
Count O on the left (12 + 6 = 18) and balance the O2 on the right: 18 − 6 = 12 atoms, so 6O2.
Example. Balance the equation for photosynthesis. __CO2 + __H2O → C6H12O6 + __O2
Show the model answer
6CO2 + 6H2O → C6H12O6 + 6O2 (1)
Shortcuts and memory tricks
Order of life: A-P-A, Algae, then Plants, then Animals.
Balance photosynthesis with 'six, six, one, six'.
Photosynthesis is the reverse of respiration: swap the two sides of the word equation.
One process, two effects: photosynthesis puts O2 in and takes CO2 out.
Where marks are lost
Saying plants produced the first oxygen. Algae came first, about 2.7 billion years ago; plants evolved later.
Saying the oxygen came from volcanoes. The volcanic early atmosphere had little or no oxygen.
Writing the photosynthesis equation the wrong way round (that is respiration).
Adding 'energy' as a product of photosynthesis. Light energy is taken in, so write 'light' above the arrow.
Mixing up billion and million: algae about 2.7 billion years ago, today's proportions for about 200 million years.
Exam technique
In 'describe how oxygen increased' answers, give the organisms, the process and the time scale.
If asked for a word equation, don't write formulae; if asked for a balanced symbol equation, don't write words.
In 6-mark questions on the history of the atmosphere, link each change to its cause, e.g. 'oxygen increased because algae and plants photosynthesised'.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Complete the word equation for this process. carbon dioxide + .................... → glucose + oxygen
water
Balance the symbol equation for photosynthesis. ....CO2 + ....H2O → C6H12O6 + ....O2
6CO2 + 6H2O → C6H12O6 + 6O2
Name the gas that plants and algae take in during photosynthesis.
carbon dioxide
About how many billion years ago did algae first produce oxygen?
2.7 billion years ago
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy3 marks
(a) Which process produced the oxygen that is now in the atmosphere? Tick (✓) one box.[1]
Combustion
Photosynthesis
Respiration
Volcanic activity
(b) Complete the word equation for this process. carbon dioxide + .................... → glucose + oxygen[1]
(c) Which organisms first produced oxygen, about 2.7 billion years ago?[1]
Show the answer and mark scheme
(a)Answer: Photosynthesis
(b)Answer: water
water
(c)Answer: algae
algae
Question 2Medium5 marks
Algae and plants produced the oxygen that is now in the atmosphere.
(a) Balance the symbol equation for photosynthesis. ....CO2 + ....H2O → C6H12O6 + ....O2[1]
(b) Where does the energy needed for photosynthesis come from?[1]
(c) Explain how the evolution of plants led to an increase in the percentage of oxygen in the atmosphere.[2]
(d) Suggest why animals could only evolve after the percentage of oxygen in the atmosphere had increased.[1]
Show the answer and mark scheme
(a)Answer: 6CO2 + 6H2O → C6H12O6 + 6O2
6CO2 + 6H2O → C6H12O6 + 6O2
(b)
light / sunlight
(c)
plants photosynthesise, taking in carbon dioxide and giving out oxygen
as plants evolved and spread over the next billion years, oxygen gradually built up
(d)
animals need oxygen for (aerobic) respiration
Question 3Hard7 marks
An area of forest uses 5.5 tonnes of carbon dioxide in photosynthesis each year. 6CO2 + 6H2O → C6H12O6 + 6O2 Relative formula masses (Mr): CO2 = 44 O2 = 32 C6H12O6 = 180
(a) Calculate the mass of oxygen released each year.[3]
(b) Calculate the mass of glucose produced each year.[2]
(c) Equal numbers of molecules of oxygen and carbon dioxide are involved in photosynthesis, but the mass of oxygen released is less than the mass of carbon dioxide used. Explain why.[2]
Show the answer and mark scheme
(a)Answer: 4.0 tonnes
amount of O2 = amount of CO2 / 6 : 6 ratio
5.5 × 32 ÷ 44
4.0 (tonnes)
(b)Answer: 3.75 tonnes
5.5 × 180 ÷ (6 × 44) / 5.5 × 180 ÷ 264
3.75 (tonnes)
(c)
6 molecules / moles of O2 are produced for every 6 molecules / moles of CO2 used
an oxygen molecule has a smaller mass than a carbon dioxide molecule (Mr 32 compared with 44)