1.8 Chemical merry-go-round - The Carbon Cycle
Plants are producers they make their own food by the process of photosynthesis.
Photosynthesis converts light energy from the sun to glucose a chemical store of
energy from the raw materials of carbon dioxide and water. Oxygen is produced as a
waste product and released into the atmosphere. Animals are consumers they obtain
their food by eating plants or other animals.
Plants and animals store energy in carbohydrates and fats. When energy is needed it is
released from carbohydrates by the process of respiration. To do this aerobically
plants and animals need to take in oxygen and combine it with glucose. This reaction
produces carbon dioxide and water and releases energy. The amount of carbon
dioxide produced by the millions of animals and plants respiring aerobically on the
planet has been estimated at 5 trillion tonnes every year. When animals and plants die
they stop respiring.
Organic materials (which consist of the waste products from living organisms and the
remains of dead organisms) are broken down by decomposers- bacteria and fungi in
the soil. Bacteria and fungi release enzymes, these digest and breakdown the organic
compounds, into soluble compounds. They then take up the substances they require
for respiration and growth but also release some substances back into the soil. The
process of breaking down organic matter into smaller soluble compounds is called
decomposition. Decomposition involves respiration it therefore requires oxygen and
produces carbon dioxide and water; in addition it releases soluble compounds
containing essential plant nutrients into the soil. Decomposers need suitable
conditions to be able to rot dead material and to reproduce. They need a suitable pH,
together with sufficient oxygen and water. Gardeners make compost heaps to rot
down the dead plant material from the garden. The rotted material is rich in nutrients
that plants require and so is returned to the soil.
Sometimes the conditions are not right for decomposition. Fossil fuels like coal, oil,
and gas are the remains of plants and animals which lived millions of years ago.
Instead of being decomposed they have been compressed under layers of mud and
rock. The carbon in the dead organisms remains locked up in the fuel. This process is
called fossilisation. On burning, carbon in the fuel reacts with oxygen in the air to
produce carbon dioxide which is released into the atmosphere. This process is called
combustion.
Carbon circulates in the environment. Plants photosynthesize; removing carbon
dioxide from the atmosphere to make glucose. Animals eat plants to obtain
carbohydrates. Animals and plants use carbohydrates in respiration which produces
carbon dioxide again. Dead organisms are decomposed which also releases carbon
dioxide into the atmosphere. There is a balance between respiration, photosynthesis
and decomposition that keeps the amount of carbon dioxide in the air at a constant
level. There is concern that the increased combustion of fossil fuels is increasing the
amount of carbon dioxide in the atmosphere. A gradual increase in atmospheric
carbon dioxide has been measured in detection stations on remote islands. Carbon is
constantly moving from place to place in the environment and we call this the carbon
cycle.
The Carbon Cycle
Read the text 1.8 Chemical merry-go-round through once and then do the following tasks:
1. Highlight or underline the gases in one colour.
2. Highlight or underline the processes in another colour, check you agree with your neighbour.
3. Sort the highlighted processes into the table below
Processes that remove carbon Processes that add carbon dioxide Processes that neither add nor
dioxide from the atmosphere. to the atmosphere. remove carbon dioxide from the
atmosphere.
4. Complete the table below to describe the four processes that either add or remove carbon
dioxide from the atmosphere.
Process Description of process
5. Write word or symbol equations for the processes of photosynthesis, respiration and
combustion of carbon.
6. While you are reading this you are producing about 250 cm of carbon dioxide every minute.
How much will a class of 30 produce in the next hour?