Organisms and their Environment - Energy flow and food webs

The Sun is the principal source of energy input to biological systems, where light energy is captured by producers during photosynthesis to create chemical energy in the form of $C_6H_{12}O_6$.

Energy flow is linear and non-cyclical; it enters ecosystems as light and is eventually lost to the environment as heat energy.

A food chain shows the transfer of energy from one organism to the next, beginning with a producer (Trophic Level 1).

Food webs are networks of interconnected food chains, representing the complex feeding relationships within an ecosystem.

Energy is lost at each trophic level through metabolic processes such as aerobic respiration ($C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$), excretion, and as heat.

Only approximately $10\%$ of the energy from one trophic level is passed to the next, which limits the number of trophic levels in a food chain to usually no more than five.

Pyramids of numbers represent the count of individuals at each level, while Pyramids of Biomass represent the dry mass of living material ($g/m^2$). Pyramids of Energy represent the total energy available ($kJ/m^2/year$) and are always upright.

Decomposers (fungi and bacteria) play a vital role by breaking down dead organic matter and returning nutrients to the soil, though the energy they extract is also eventually lost as heat.