Transfers of energy and matter

Biology · C Interaction and interdependence. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology topics

The idea

An ecosystem is an open system: both energy and matter enter and leave it. For most ecosystems the energy comes from sunlight. Producers fix it into carbon compounds, and it then passes along food chains when one organism eats another.

Energy flows one way and is not recycled. At each step most of it is lost, mainly as heat from respiration, so only about 10% reaches the next trophic level. This is why food chains are short.

Matter is different: it is recycled. In the carbon cycle, photosynthesis takes carbon dioxide out of the air, and respiration and combustion put it back. Decomposers release the carbon held in dead matter.

Rules to remember

Arrows in a food chain point in the direction of energy flow, from the organism eaten to the eater.
Trophic levels: producer (1), primary consumer (2), secondary consumer (3), tertiary consumer (4).
Photoautotrophs use light as their energy source; chemoautotrophs use the oxidation of inorganic chemicals; heterotrophs take carbon compounds from other organisms.
efficiency of transfer (%) = energy in the higher level ÷ energy in the level below × 100.
Energy is lost through respiration (heat), parts not eaten and faeces (egestion).
Primary production: biomass made by autotrophs. Secondary production: biomass made by heterotrophs; it is always lower. Units such as g m⁻² yr⁻¹.
Carbon sink: takes in more carbon than it releases. Carbon source: releases more than it takes in. The Keeling Curve shows CO₂ rising year by year, with a small fall each northern summer when photosynthesis is high.

Common mistake

Students say energy is recycled by decomposers. Decomposers recycle matter (nutrients). Energy is lost as heat and cannot be reused.

Worked example

Primary consumers in a lake store 3000 kJ m⁻² yr⁻¹ and secondary consumers store 360 kJ m⁻² yr⁻¹. Calculate the efficiency of the energy transfer.

Efficiency = energy in secondary consumers ÷ energy in primary consumers × 100.
Efficiency = 360 ÷ 3000 × 100 = 0.12 × 100 = 12.

Answer: 12%.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

C4.2 Transfers of Energy and Matter

Keep going

← Populations and communities DNA replication →Take a Biology test