Lesson 4 of 5 / Organisms and their environment
Following carbon through an ecosystem
How can the same carbon atom enter a plant, an animal and the air?
I can trace carbon transfers and explain the roles of forests and oceans as carbon sinks.
I can describe carbon cycling and forest/ocean carbon sinks.Syllabus
Syllabus K327 / K328, Topic 9(e). Describe carbon cycling and forest/ocean carbon sinks.
About 5 min
Make a guess. You are not marked.
A forest absorbs carbon dioxide by photosynthesis. Does it also give carbon dioxide out?
Show the answer
It does, through respiration and decay.
Trees, animals and decomposers all respire. A forest is a carbon sink only when uptake is greater than release.
The key idea
Carbon cycles between organisms and their surroundings. A carbon sink takes in more carbon than it releases over the period considered.
A carbon store holds carbon. A transfer moves it between stores. A sink describes a net uptake, so the balance of incoming and outgoing transfers matters.
Explore the idea
Name the process moving the carbon
Photosynthesis incorporates carbon from carbon dioxide into organic molecules. The transfer requires an energy input from light.
These original transfer models connect into a cycle of matter. Forest and ocean sinks can still release carbon; sink status depends on net uptake over a period. Energy does not cycle with the carbon.
Explanation
Plants take in carbon dioxide during photosynthesis and use its carbon to build organic molecules. Feeding transfers this carbon into consumers. Carbon is also transferred to decomposers in dead material and waste.
Plants, animals and decomposers respire, releasing carbon dioxide. As decomposers break down organic material, their respiration returns carbon to the atmosphere or water. Some organic carbon can remain stored in soils or sediments rather than returning immediately.
Combustion releases carbon dioxide from fuels and biomass. Fossil fuels contain carbon stored over geological timescales; burning them transfers this stored carbon into the atmosphere much faster than those fuels form.
Forests can be carbon sinks when uptake through photosynthesis exceeds releases through respiration, decomposition and disturbance. Carbon is held in trees and other biomass and in soils. A forest still respires; being a sink means net uptake, not the complete absence of release.
Oceans absorb carbon dioxide through exchange with the atmosphere. Marine photosynthesis incorporates some carbon into organisms, and some reaches deeper water and sediments. Oceans can act as a net carbon sink, but carbon exchange occurs both ways and uptake is not unlimited. Carbon cycles, whereas energy flowing through these processes is eventually dispersed as heat.
Step by step
- 1
Name the store
Locate carbon in atmospheric carbon dioxide, organisms, soil, fuels or the ocean.
- 2
Name the transfer process
Use photosynthesis, feeding, respiration, decomposition, combustion or air-sea exchange.
- 3
Compare inputs and outputs
A store acts as a sink when it gains more carbon than it releases over a defined period.
Worked example
A forest carbon balance
In an original annual balance, a forest takes up 100 carbon units and releases 75. Is it a sink during that year, and does it release no carbon?
One way to explain it
It is a net sink of 25 carbon units because uptake exceeds release by 100 - 75. It still releases 75 units; a sink is defined by the net balance, not by zero release.
Why this answer works
- Subtract total release from uptake.
- Specify the time period.
- Keep net uptake separate from gross release.
Watch out for this
A student says: "A carbon sink never releases carbon dioxide." What is wrong with this?
Show the answer
A sink may both absorb and release carbon. It is a sink when total uptake is greater than total release over the period considered.
Check your understanding
Which process transfers carbon from atmospheric carbon dioxide into a plant's organic molecules?
Show the answer
Photosynthesis.
The carbon in carbon dioxide becomes part of organic molecules formed by the plant.
Quick recall
How can the same carbon atom enter a plant, an animal and the air?