(a)State the second law of thermodynamics. (ii) Use the second law of thermodynamics to explain the flow of energy across different trophic levels in a food...

(a)(i) The second law of thermodynamics

The second law of thermodynamics states that whenever energy is converted from one form to another, some of the energy is always lost as heat, so that no energy transfer or transformation is 100% efficient; the entropy (disorder) of the universe always tends to increase.

(a)(ii) Applying it to energy flow in a food chain

Energy enters a food chain when green plants (producers) trap sunlight in photosynthesis. As energy passes from one trophic level to the next (producer to primary consumer to secondary consumer, and so on), only about 10% is passed on and stored in the next level. The other roughly 90% is lost as heat through respiration and in movement, and some is lost in undigested waste and death. Because of this loss at each transfer, less and less energy is available at higher trophic levels. This is why energy flow is one-directional (it does not cycle) and why food chains usually have only four or five links, and why there are fewer organisms at the top than at the bottom (the pyramid of energy).

(b)(i) Three factors that limit the size of human populations

1. Food supply (availability of food).
2. Disease and epidemics.
3. Space / shelter and availability of water (war and natural disasters also accepted).

(b)(ii) Two ways each of overcoming the factors

• Food supply: increase food production through mechanised and improved farming; improve storage, preservation and distribution of food.
• Disease: provide immunisation/vaccination and good medical services; improve sanitation, clean water and public health education.
• Space / water: build more houses and develop new settlements/urban planning; provide dams, boreholes and pipe-borne water and manage resources wisely.