IGCSE Biology 0610: Photosynthesis, Respiration, Decomposition and Combustion in the Carbon Cycle

A small, undisturbed woodland contains oak trees, deer that browse on low leaves, and fallen leaves and twigs that build up on the woodland floor. Fungi and bacteria in the leaf litter break down this dead plant material over time. Occasionally, a fallen branch is collected by a park ranger and burned on a campfire.

(a) Name the process by which the oak trees remove carbon dioxide from the atmosphere and use the carbon to build organic compounds in their tissues. [1]

(b) State two different living processes, occurring in this woodland, that return carbon dioxide to the atmosphere. [2]

(c) Explain what happens to the carbon stored in the dead leaves and twigs as fungi and bacteria break them down in the leaf litter. [3]

(d) Name the process occurring when the fallen branch is burned on the campfire, and state what happens to the carbon it contained. [2]

Part (a): Removing carbon dioxide from the air

Green plants, including the oak trees, remove carbon dioxide from the atmosphere and convert it into organic compounds such as glucose during photosynthesis. This is the main process by which carbon enters the living parts of the woodland.

Part (b): Returning carbon dioxide to the air

Every living organism in the woodland respires continuously, releasing carbon dioxide as a waste product. In this woodland, this includes:

Respiration by the oak trees themselves,
Respiration by the deer that browse on the leaves,
Respiration by the fungi and bacteria decomposing the leaf litter.

Any two of these count as correct answers.

Part (c): Decomposition of the leaf litter

Fungi and bacteria (decomposers) feed on the dead leaves and twigs, using the organic compounds in them as a source of energy. As they respire, they release carbon dioxide into the atmosphere, just as the living trees and deer do. Decomposition also breaks the complex organic molecules of the dead plant material down into simpler substances, releasing nutrients such as nitrogen compounds back into the soil, but the carbon that was locked up in the dead tissue is returned mainly as carbon dioxide gas.

Part (d): Burning the fallen branch

Burning the branch is an example of combustion. During combustion, the carbon that was stored in the organic compounds of the wood combines rapidly with oxygen from the air, releasing it into the atmosphere as carbon dioxide gas. Combustion returns carbon to the atmosphere much more quickly than the slower process of decomposition.

Final answers

(a) Photosynthesis
(b) Any two of: respiration by the oak trees; respiration by the deer; respiration by the fungi and bacteria in the leaf litter
(c) Decomposers respire the organic compounds in the dead leaves and twigs, releasing carbon dioxide into the atmosphere, while also releasing other nutrients into the soil
(d) Combustion: the carbon in the branch combines with oxygen and is released as carbon dioxide