Energy Transfer

GPP and NPP

Photosynthesis is very inefficient. Green plants only capture a small amount of the solar energy that actually reaches them, and turn it into gross primary production (GPP). GPP is the total energy fixed by photoautotrophs during photosynthesis.

Some sunlight passes straight through a leaf, and some is reflected from the surface of the leaves. Certain parts of the visible spectrum cannot be utilised by the plant for photosynthesis.

Net Primary Production (NPP) is the energy stored as biomass.

NPP = GPP – Energy lost in respiration

Energy is transferred between trophic levels when one organism eats another. However, these transfers are inefficient, and this inefficiency limits the length of food chains. Only a small proportion of the available energy is transferred from one trophic level to the next. Much energy is lost during the respiratory processes of each organism in the chain. It is this loss of energy, at each stage, which limits the length of food chains (number of trophic levels).

Explain why not all of the energy in producer biomass can be converted into energy in primary consumer biomass.

Lost as heat; in respiration; movement; excreted material: egested/not all digested: not all eaten.

Give two reasons why the percentage of energy transferred between consumers tends to be low.

  • (Energy lost via):
    • Heat/respiration;
    • movement/muscle contraction/named activity;
    • faeces/indigestible material/not all eaten;
    • excretion/examples.

Explain, in terms of energy, why food chains with the fewest steps are most efficient.

  • Energy lost at each trophic level
  • due to respiration/heat loss/other valid reason
  • fewest steps means least energy loss.