A food chain shows what eats what in a particular habitat. For example, grass seed is eaten by a vole, which is eaten by a barn owl. The arrows between each item in the chain always point in the direction of energy transfer.

The amount of available energy decreases from one stage to the next. Energy is lost as you go up the food chain. Energy released by respiration is used for movement and other life processes, and is eventually lost as heat to the surroundings. Energy is also lost in waste materials such as faeces. 

Biomass means the mass of living material at a stage in a food chain. The biomass goes down as you go from one stage to the next, just like the amount of energy.

The efficiency of food production can be improved by reducing the amount of energy lost to the surroundings. This can be done by Preventing animals moving round alot and Keeping their surrondings warm. Mammals and birds maintain a constant body temperature using energy released by respiration. As a result, their energy losses are high. Keeping pigs and chickens in warm sheds with little space to move around allows more efficient food production. This raises moral concerns about the lives of such animals. A balance must be reached between the needs of farmers / consumers and the welfare animals.

This Cow has eaten 100 kJ of energy in the form of grass,excreted 63 kJ in the form of faeces, urine and gas.Only 4 kJ of the original energy is available to the next stage.The energy stored in its body tissues is 4 kJ.  Energy released by respiration = 100 - 63 - 4 = 33 kJ. The Energy Transfer is 4/100 x 100 = 4%.

Food production is more efficient if the food chain is short, because a higher percentage of energy is available to us.

Removing Carbon dioxide from the atmosphere - Green plants remove carbon dioxide from the atmosphere by photosynthesis. The carbon becomes part of complex molecules such as proteins, fats and carbohydrates in the plants.

Returning carbon dioxide to the atmosphere - Organisms return carbon dioxide to the atmosphere by respiration. Animals, Plants and Microorganisms all respire.