AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemistry of the atmosphere › Composition and evolution of the atmosphere
Practise How oxygen increased. 7 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.
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
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.
Photosynthesis adds oxygen to the atmosphere and removes carbon dioxide from it at the same time.
The timeline
diagram
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.
Sketch, not to scale: photosynthesis slowly raised oxygen to about 20%.
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.
How to answer each type of question
Complete the word equation for photosynthesis
1 to 2 marksGrade 4
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 answerHide the model answer
water (1) oxygen (1)
Don’t lose marks
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.
More tips
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.
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'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the process that produced the oxygenPhotosynthesis by algae and plants produced the oxygen that is now in the atmosphere.
Grade 4
Write the word equation for photosynthesiscarbon dioxide + water → glucose + oxygen, with light as the energy source.
Grade 5
State when algae first produced oxygenAlgae first produced oxygen about 2.7 billion years ago, and oxygen appeared in the atmosphere soon after.
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