Energy flow, pyramids and cyclesIB MYP Sciences: Revision notes

Almost all the energy in food chains comes from the Sun. Producers capture a small part of the light energy by photosynthesis and store it in glucose and other compounds. When animals eat the producers, this energy passes along the chain.

At each step, energy is lost. Only about 10 % is passed on to the next trophic level. The rest is lost as:

• Heat from respiration
• Movement (energy used by muscles)
• Waste such as faeces
• Uneaten parts such as bones, roots and fur

Worked example: Grass in a field captures 8000 kJ per square metre. A primary consumer receives 10 % of 8000 = 800 kJ. A secondary consumer receives 10 % of 800 = 80 kJ.

Because only about 10 % of energy passes on at each step, the energy left falls rapidly. After four or five trophic levels there is not enough energy to support another consumer. This is why most food chains have no more than five links, and why there are fewer large predators than herbivores.

It also explains why eating plants directly feeds more people than eating animals fed on those plants: one fewer step means less energy lost.

A pyramid of numbers shows how many organisms are at each trophic level. A pyramid of biomass shows the dry mass of the organisms at each level.

Biomass nearly always decreases at each level, giving a pyramid shape, because energy is lost. Numbers can give a strange shape: one large oak tree can feed thousands of insects, so the producer level has fewer organisms than the next. A pyramid of biomass does not have this problem, so it is usually more reliable.

Carbon moves between the air, living things and the ground in the carbon cycle.

• Photosynthesis removes carbon dioxide from the air and builds carbon into glucose in plants.
• Feeding passes carbon compounds along food chains.
• Respiration by plants, animals and decomposers releases carbon dioxide.
• Decomposition of dead organisms and waste releases carbon dioxide as decomposers respire.
• Combustion of wood and fossil fuels releases carbon dioxide.

Fossil fuels formed from remains of ancient organisms. Burning them returns carbon that was locked away for millions of years, which is why atmospheric carbon dioxide is rising.

Water moves between the sea, land, air and living things.

1. Evaporation from the sea and lakes, and transpiration from plants, put water vapour into the air.
2. The vapour cools and condenses into clouds.
3. Precipitation (rain, snow) returns water to the land and sea.
4. Water flows back to the sea as run-off in rivers or soaks into the ground.

The Sun's energy drives the cycle.

Decomposers, mainly bacteria and fungi, break down dead plants and animals and waste. This releases nutrients, such as nitrates and phosphates, into the soil. Plants take them up through their roots and use them to grow, so the nutrients are recycled.

Without decomposers, dead material would pile up and the nutrients would stay locked inside it, so plants would run out of minerals. Decomposers also release carbon dioxide as they respire.