Energy is lost at every step in the trophic structure where energy is transferred—for example, ...

A) Gross primary productivity (GPP) is a measure of the total amount of light energy used in photosynthesis per unit time. It is usually expressed in joules per square meter per year (J/m2/yr). Net primary productivity (NPP) equals GPP minus the energy the photosynthetic organisms use for cellular respiration. NPP can similarly be expressed in J/m2/yr, or it can be expressed as the total biomass of primary producers added per year to an area. Biomass is usually expressed in g/m2/yr. (The assumption is that each gram of biomass contains a given amount of energy, which can be released by oxidation.)

The information given in the question states that, of the sunlight that reaches plants, only 1% is converted to chemical energy. This is a measure of GPP. Only 50 to 90% of this 1%—that is, 0.5 to 0.9%—of the energy that reaches plants becomes actual biomass, or NPP. If all the NPP were consumed by primary consumers, you would expect to see 10% of it end up in biomass of secondary consumers. That is, 0.05 to 0.09% of the sunlight that reaches plants is actually converted into biomass of primary consumers.

B) The energy that is “lost” in these transfers is energy used by the organisms in their own metabolism. For example, a primary consumer needs to eat 10 g of plant food to gain 1 g of biomass. The other 9 g of plant food are oxidized (via cellular respiration) to produce the ATP needed to build more biomass and to maintain the life functions of the organism.