Carbon cycle

Learn it by playing

Answer these questions to earn energy, then fish and explore. No account needed.

For teachers: ready-to-use lesson slides, revision notes, diagrams for Carbon cycle (KS3 Science, Chemistry) — use them in your lesson, or run the topic as an interactive class activity your students play as a live game.

Lesson notes

What is the Carbon Cycle?

The **carbon cycle** describes the continuous movement of carbon into and out of the atmosphere, animals, plants, and the ground.
Carbon in the atmosphere is mainly in the form of **carbon dioxide (CO₂)**.

Processes that Remove CO₂ from the Atmosphere

**Photosynthesis** removes CO₂ from the atmosphere.
Photosynthesis is carried out by **plants**.
In photosynthesis, plants use **carbon dioxide and water** (in the presence of light) to produce **glucose and oxygen**.
Overall, plants remove more CO₂ than they release.

Processes that Release CO₂ into the Atmosphere

**Respiration** in plants and animals releases CO₂.
**Decomposition** of dead organisms by decomposers releases CO₂.
**Burning fossil fuels** (combustion) releases CO₂.
Decomposers include **bacteria** and **fungi**.

Decomposition and Fossil Fuels

Decomposers break down dead organisms into simpler substances, releasing CO₂.
When conditions do not allow decomposition, remains can form **fossil fuels** over millions of years.
Fossil fuels include **coal, oil, and natural gas**.
Fossil fuels are burned in **power plants** and **vehicles**.

Impact of Burning Fossil Fuels

Burning fossil fuels releases CO₂, a **greenhouse gas**.
Increased greenhouse gases contribute to **global warming**.
Reducing fossil fuel use can help lower CO₂ levels and slow global warming.

States of matter: solid, liquid, gas. (Not directly about carbon cycle, but useful for understanding decomposition and combustion.)

Slides

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

Practice questions

Free preview — 8 of 40 questions. Sign up to see them all.

1.What is the carbon cycle?
Easy
- AThe continuous movement of carbon into and out of the atmosphere, animals, plants and the ground
- BThe process of carbon dioxide being released only by animals
- CThe movement of carbon only between plants and the atmosphere
- DThe formation of carbon compounds in the Earth's crust
2.Which process removes carbon dioxide from the atmosphere?
Easy
- APhotosynthesis
- BRespiration
- CDecomposition
- DBurning fossil fuels
3.Which of the following processes releases carbon dioxide into the atmosphere?
Medium
- ARespiration, decomposition, and burning fossil fuels
- BPhotosynthesis, respiration, and decomposition
- CPhotosynthesis, burning fossil fuels, and decomposition
- DRespiration, photosynthesis, and burning fossil fuels
4.Name two decomposers.
Easy
- ABacteria and fungi
- BPlants and animals
- CViruses and protists
- DInsects and worms
5.What do decomposers do?
Easy
- ABreak down dead organisms into simpler substances
- BProduce oxygen through photosynthesis
- CConsume carbon dioxide from the atmosphere
- DCreate fossil fuels from living matter
6.Fossil fuels are formed from:
Medium
- AThe remains of plants and animals over millions of years
- BThe decomposition of rocks
- CVolcanic eruptions
- DThe burning of trees
7.Which of the following is a fossil fuel?
Easy
- ACoal
- BWood
- CSunlight
- DWater
8.What effect does burning fossil fuels have on atmospheric carbon dioxide levels?
Easy
- AIncreases them
- BDecreases them
- CNo effect
- DConverts carbon dioxide to oxygen

More questions

9.Which of the following is a use of burning fossil fuels?
Medium
- AGenerating electricity in power plants
- BPhotosynthesis
- CDecomposition
- DPlant growth
10.Large amounts of greenhouse gases in the atmosphere lead to:
Medium
- AGlobal warming
- BIncreased photosynthesis
- CMore decomposition
- DCooling of the Earth
11.Planting more trees would most likely:
Medium
- ADecrease atmospheric carbon dioxide levels
- BIncrease atmospheric carbon dioxide levels
- CHave no effect on carbon dioxide levels
- DIncrease the rate of decomposition
12.If humans reduce the amount of fossil fuels burned, what would be the expected effect on carbon dioxide levels and global warming?
Hard
- ACarbon dioxide levels would decrease, reducing global warming
- BCarbon dioxide levels would increase, worsening global warming
- CCarbon dioxide levels would stay the same, no effect on global warming
- DCarbon dioxide levels would decrease, but global warming would increase
13.A student measured the carbon dioxide concentration in a sealed greenhouse over 24 hours. The graph showed a decrease during the day and an increase at night. Which process explains the decrease during the day?
Medium
- ARespiration
- BPhotosynthesis
- CDecomposition
- DCombustion
14.In an experiment, 10 g of wood was burned completely. The mass of carbon dioxide released was measured as 14.7 g. Assuming all carbon in the wood was converted to CO₂, what is the approximate percentage by mass of carbon in the wood? (Ar: C=12, O=16)
Hard
- A40%
- B50%
- C60%
- D70%
15.The table shows the relative amounts of carbon stored in different reservoirs. Which reservoir contains the most carbon?
Medium
- AAtmosphere
- BFossil fuels
- COceans
- DTerrestrial plants
16.Which of the following is a process that returns carbon to the atmosphere?
Easy
- APhotosynthesis
- BRespiration
- CFormation of fossil fuels
- DCarbonate precipitation
17.A forest fire burns 1000 tonnes of biomass containing 40% carbon by mass. How many tonnes of carbon dioxide are released into the atmosphere? (Ar: C=12, O=16)
Hard
- A400 tonnes
- B733 tonnes
- C1467 tonnes
- D2200 tonnes
18.Scientists measured the rate of photosynthesis in a plant at different carbon dioxide concentrations. Which graph shape best represents the expected relationship?
Medium
- AA straight line increasing
- BA curve that levels off at high CO₂
- CA curve that decreases after a peak
- DA straight horizontal line
19.Which of the following is NOT a fossil fuel?
Easy
- ACoal
- BNatural gas
- CWood
- DOil
20.A student set up two sealed jars with a plant and a sensor. Jar A was kept in light, Jar B in darkness. After 24 hours, which jar had a lower carbon dioxide concentration?
Medium
- AJar A
- BJar B
- CBoth the same
- DCannot determine
21.The graph shows atmospheric CO₂ levels measured at Mauna Loa since 1958. The upward trend is mainly due to:
Hard
- AIncreased volcanic activity
- BBurning of fossil fuels
- CIncreased plant growth
- DDecreased ocean uptake
22.In the carbon cycle, which process converts carbon in fossil fuels into atmospheric carbon dioxide?
Medium
- ARespiration
- BPhotosynthesis
- CCombustion
- DDecomposition
23.What is the main form of carbon in the atmosphere?
Easy
- ACarbon monoxide
- BCarbon dioxide
- CMethane
- DCarbon soot
24.A sample of limestone (calcium carbonate) was heated strongly, producing 11.2 g of calcium oxide and 8.8 g of carbon dioxide. What mass of limestone was used?
Hard
- A2.4 g
- B11.2 g
- C20.0 g
- D8.8 g
25.A student set up an experiment to measure the rate of photosynthesis in pondweed. They placed a lamp at different distances from the pondweed and counted the number of bubbles produced per minute. Which variable should be kept constant in this experiment?
Medium
- ADistance of the lamp from the pondweed
- BTemperature of the water
- CNumber of bubbles per minute
- DLight intensity
26.In an experiment, a student placed a live snail and a plant in a sealed container in the light. After 24 hours, the concentration of carbon dioxide in the container was measured. Which of the following is the most likely outcome?
Medium
- ACarbon dioxide concentration increased because the snail respired more than the plant photosynthesised.
- BCarbon dioxide concentration decreased because the plant photosynthesised more than the snail respired.
- CCarbon dioxide concentration remained constant because respiration and photosynthesis balanced exactly.
- DCarbon dioxide concentration increased because the plant respired more than it photosynthesised.
27.A student investigated the effect of temperature on the rate of decomposition of dead leaves by microorganisms. They measured the mass loss over 10 days. Which of the following graphs best represents the expected results?
Hard
- AMass loss increases steadily with temperature up to about 40°C, then decreases sharply.
- BMass loss decreases steadily with increasing temperature.
- CMass loss is highest at 0°C and lowest at 60°C.
- DMass loss is constant at all temperatures.
28.A student wants to investigate the effect of light intensity on the rate of photosynthesis in aquatic plants. Which piece of equipment is most suitable for measuring the rate of photosynthesis?
Medium
- AA pH meter
- BA gas syringe to collect oxygen bubbles
- CA thermometer
- DA balance to measure mass change
29.In an experiment to measure the carbon dioxide concentration in a sealed greenhouse over 24 hours, a student obtained a graph that decreased during the day and increased at night. Which of the following conclusions is correct?
Hard
- APhotosynthesis occurs only at night, causing CO₂ to decrease.
- BRespiration occurs only during the day, causing CO₂ to increase.
- CDuring the day, photosynthesis removes CO₂ faster than respiration adds it; at night, only respiration occurs, adding CO₂.
- DThe greenhouse was not sealed properly, causing CO₂ to leak out during the day.
30.Which of the following is a process that releases carbon dioxide into the atmosphere?
Easy
- APhotosynthesis
- BRespiration
- CFormation of fossil fuels
- DPlant growth
31.A student measured the mass of carbon dioxide released when burning 10 g of wood. The wood contained 40% carbon by mass. Assuming all carbon is converted to CO₂, what mass of CO₂ is released? (Ar: C=12, O=16)
Medium
- A14.7 g
- B4.0 g
- C36.7 g
- D10.0 g
32.In the carbon cycle, which of the following processes returns carbon to the atmosphere over a very short timescale (days to years)?
Hard
- AFormation of fossil fuels
- BCombustion of fossil fuels
- CRespiration and decomposition
- DWeathering of rocks
33.A student set up two sealed jars: Jar A contained a plant and a CO₂ sensor, Jar B contained a plant, a snail, and a CO₂ sensor. Both jars were placed in light for 6 hours. Which jar is likely to have a lower CO₂ concentration after 6 hours?
Medium
- AJar A, because the plant photosynthesises without any respiration from animals.
- BJar B, because the snail adds CO₂, increasing the rate of photosynthesis.
- CBoth jars will have the same CO₂ concentration because the plant balances the CO₂.
- DJar B, because the snail respires, adding CO₂, so CO₂ is higher.
34.Carbon dioxide is a greenhouse gas. Which of the following is a consequence of increased greenhouse gases in the atmosphere?
Easy
- AGlobal cooling
- BReduced plant growth
- CGlobal warming
- DIncreased oxygen levels
35.A student wants to compare the amount of carbon stored in a tree versus in the atmosphere directly above it. Which method would best estimate the carbon stored in the tree?
Hard
- AMeasure the height and diameter of the tree and use a formula to estimate biomass.
- BWeigh a leaf and multiply by the number of leaves.
- CMeasure the volume of the tree and multiply by the density of wood.
- DCount the number of branches and estimate their mass.
36.Decomposers play a key role in the carbon cycle. Which of the following best describes what decomposers do?
Medium
- AThey convert carbon dioxide into glucose through photosynthesis.
- BThey break down dead organisms, releasing carbon dioxide into the atmosphere.
- CThey convert fossil fuels into carbon dioxide through combustion.
- DThey store carbon in the ground for millions of years.
37.In an experiment, a sealed container with a plant and a CO₂ sensor is placed in bright light for 6 hours, then in darkness for 6 hours. Which graph best shows the expected change in CO₂ concentration over the 12-hour period?
Medium
- AA steady decrease during the light period, then a steady increase during the dark period
- BA steady increase during the light period, then a steady decrease during the dark period
- CNo change during the light period, then a sharp decrease during the dark period
- DA sharp increase during the light period, then a slow increase during the dark period
38.A student measured the carbon content of three soil samples: Sample A from a forest, Sample B from a grassland, and Sample C from a desert. Which sample is most likely to have the highest carbon content?
Medium
- ASample A (forest)
- BSample B (grassland)
- CSample C (desert)
- DAll samples have the same carbon content
39.A power station burns 1000 tonnes of coal per day. The coal contains 80% carbon by mass. Assuming all carbon is converted to carbon dioxide, what mass of carbon dioxide is released per day? (Ar: C=12, O=16)
Hard
- A2933 tonnes
- B800 tonnes
- C1000 tonnes
- D1467 tonnes
40.The table shows the mean residence time of carbon in different reservoirs. Which reservoir has the longest residence time?
Medium
- AFossil fuels (millions of years)
- BAtmosphere (5 years)
- CTerrestrial vegetation (10 years)
- DOcean surface (20 years)

Revision notes

## Carbon Cycle Revision Notes The carbon cycle describes the continuous movement of carbon into and out of the atmosphere, animals, plants, and the ground. ### Key Processes - **Photosynthesis**: Plants remove CO₂ from the atmosphere. They make glucose and oxygen from carbon dioxide and water (in the presence of light). Overall, plants remove more CO₂ than they release. - **Respiration**: Plants and animals release CO₂ into the atmosphere. - **Decomposition**: Decomposers (bacteria and fungi) break down dead organisms, releasing CO₂. - **Formation of fossil fuels**: When conditions do not allow decomposition, remains of plants and animals slowly (over millions of years) form fossil fuels (coal, oil, gas). - **Burning fossil fuels**: Humans burn fossil fuels in power plants and vehicles, releasing CO₂ (a greenhouse gas). ### Effects - Greenhouse gases (like CO₂) contribute to **global warming**. - Planting more trees **reduces** CO₂ levels (more photosynthesis). - Reducing fossil fuel burning **decreases** CO₂ emissions, slowing global warming. ### Key Definitions - **Photosynthesis**: Chemical process where plants make glucose and oxygen from carbon dioxide and water (under light). - **Decomposers**: Bacteria and fungi that break down dead organisms into simpler substances. - **Fossil fuels**: Coal, oil, and gas formed from remains of plants and animals over millions of years. - **Greenhouse gas**: A gas (e.g., CO₂) that traps heat and contributes to global warming.

Carbon cycle: The continuous movement of carbon into and out of the atmosphere, animals, plants and the ground.
Photosynthesis: A chemical process carried out by plants where glucose and oxygen are made from carbon dioxide and water (under the presence of light).
Respiration: A process that releases carbon dioxide into the atmosphere, carried out by plants and animals.
Decomposition: The breakdown of dead organisms into simpler substances by decomposers, releasing carbon dioxide.
Decomposers: Bacteria and fungi that break down dead organisms.
Fossil fuels: Fuels formed from the remains of plants and animals over millions of years when conditions do not allow decomposition; examples are coal, oil, and gas.
Greenhouse gas: A gas, such as carbon dioxide, that contributes to global warming when present in large amounts in the atmosphere.
Global warming: The increase in Earth's temperature caused by large amounts of greenhouse gases in the atmosphere.

Flashcards

What is the carbon cycle?

The carbon cycle describes the continuous movement of carbon into and out of the atmosphere, animals, plants and the ground.

In what form is carbon found in the atmosphere?

Carbon in the atmosphere is in the form of carbon dioxide.

Which process removes carbon dioxide from the atmosphere?

Photosynthesis removes carbon dioxide from the atmosphere.

What is photosynthesis?

Photosynthesis is a chemical process carried out by plants where glucose and oxygen are made from carbon dioxide and water (under the presence of light).

Do plants remove more or less carbon dioxide than they release?

Plants remove more carbon dioxide from the atmosphere than they put back into it.

How do plants release carbon dioxide back into the atmosphere?

Plants release carbon dioxide through respiration.

How do animals release carbon dioxide into the atmosphere?

Animals release carbon dioxide through respiration.

What is decomposition?

Decomposition is the breakdown of dead organisms into simpler substances by decomposers.

Name two types of decomposers.

Bacteria and fungi are decomposers.

What do decomposers do?

Decomposers break down dead organisms into simpler substances, releasing carbon dioxide.

How are fossil fuels formed?

Fossil fuels are formed from the remains of plants and animals when conditions do not allow for decomposition, a slow process taking millions of years.

Name three fossil fuels.

Coal, oil, and gas are fossil fuels.

What happens when fossil fuels are burned?

Burning fossil fuels releases carbon dioxide into the atmosphere.

What is a greenhouse gas?

Carbon dioxide is a greenhouse gas that contributes to global warming.

What does a large amount of greenhouse gases in the atmosphere lead to?

A large amount of greenhouse gases leads to global warming.

What effect would planting more trees have on carbon dioxide levels?

Planting more trees would remove more carbon dioxide from the atmosphere through photosynthesis.

What effect would reducing fossil fuel burning have on carbon dioxide levels and global warming?

Reducing fossil fuel burning would decrease carbon dioxide levels in the atmosphere and slow down global warming.

What are two uses of burning fossil fuels?

Burning fossil fuels is used in power plants and vehicles.

What process releases carbon dioxide from dead organisms?

Decomposition of the remains of plants and animals releases carbon dioxide.

Unlock all 40 questions, flashcards & more

Create a free account to see every question, the slides, flashcards and revision notes for this topic.

Past papers

Past-paper practice for this topic is coming soon.

Coming soon