C4.2 Transfers of energy and matterIB Biology HL: Revision notes

An ecosystem is an open system: both energy and matter can enter and leave (e.g. leaves blow in, animals migrate out). In a closed system, only energy passes in and out.

Sunlight is the principal energy source for most ecosystems. Exceptions include caves and the deep ocean below the reach of light, where energy comes from chemoautotrophs or from organic matter carried in.

NOS: a scientific law is a generalisation that describes a pattern ("most ecosystems rely on sunlight"). Unlike a theory it does not explain, but like a theory it can be used to predict. Useful generalisations apply widely and have few exceptions.

Autotrophs use an external energy source to make carbon compounds from simple inorganic substances such as CO₂. Energy is needed for carbon fixation and for anabolic reactions building macromolecules.

• Photoautotrophs use light.
• Chemoautotrophs use energy from oxidation reactions — e.g. iron-oxidizing bacteria oxidise Fe²⁺ to Fe³⁺.

Heterotrophs obtain carbon compounds from other organisms. Complex molecules (proteins, nucleic acids) are digested externally or internally, then assimilated into the carbon compounds the organism needs.

Both release energy by oxidising carbon compounds in cell respiration.

Chemical energy passes to a consumer when it feeds on the previous stage in a food chain. In food chains and food webs, arrows show the direction of transfer of energy and biomass (pointing to the feeder).

Trophic levels: producer, primary consumer, secondary consumer, tertiary consumer. Many organisms have varied diets and occupy different trophic levels in different chains.

Decomposers and detritus feeders obtain energy from carbon compounds in dead organic matter: faeces, dead parts (e.g. shed leaves) and whole dead organisms. They are not usually drawn in food chains, but they are central to energy transformation and recycling.

Energy pyramids show the energy flowing through each trophic level per unit area per unit time (e.g. kJ m⁻² yr⁻¹). Each bar is smaller than the one below.

Energy is lost between levels because:

• Heat is lost in cell respiration in both autotrophs and heterotrophs — energy transfers are not 100 % efficient, so heat is produced both when ATP is made and when it is used.
• Not all of an organism is eaten; not all that is eaten is digested (faeces). This energy passes to decomposers.

Typically only about 10 % passes to the next level, which restricts the number of trophic levels. At higher levels there are fewer or smaller organisms, so less biomass — but the energy content per unit mass is not reduced.

Primary production is the accumulation of carbon compounds in biomass by autotrophs; units are mass of carbon per unit area per unit time, usually g m⁻² yr⁻¹. Biomes differ in their capacity to accumulate biomass (tropical rainforest high, desert and tundra low).

Secondary production is accumulation of carbon compounds in biomass by heterotrophs. It is lower than primary production because carbon compounds are lost as CO₂ and water in cell respiration.

Biomass accumulates when organisms grow or reproduce.

In a carbon cycle diagram, carbon moves from atmospheric CO₂ into producers by photosynthesis, to consumers by feeding, and back to CO₂ by respiration (including decomposers) and combustion.

An ecosystem is a carbon sink if photosynthesis exceeds respiration (net CO₂ uptake) and a carbon source if respiration exceeds photosynthesis.

Combustion of biomass, peat, coal, oil and natural gas releases CO₂. These sinks formed at very different times (coal and oil millions of years ago, peat thousands). Lightning causes some natural fires, but humans have greatly increased combustion.

Keeling Curve (Mauna Loa): annual fluctuation — CO₂ falls in the northern summer as photosynthesis exceeds respiration and rises in winter as respiration exceeds photosynthesis; long-term rise — mainly fossil fuel combustion.

Aerobic respiration depends on atmospheric oxygen produced by photosynthesis, and photosynthesis depends on atmospheric carbon dioxide released by respiration. These yearly fluxes are huge, making this a major interaction between autotrophs and heterotrophs.

All chemical elements used by living organisms — not just carbon but nitrogen, phosphorus and others — are recycled in ecosystems. Decomposers play a key role by breaking down dead matter and releasing inorganic nutrients for producers to reabsorb. Energy flows through ecosystems and is lost as heat; matter is recycled.