Big picture: energy flow vs matter cycling

• Ecosystems are open systems: both energy and matter can enter and leave.

• In a closed system, energy can pass in and out, but matter does not.

• Energy flows one way through ecosystems and is ultimately lost as heat.

• Matter is recycled because atoms in carbon compounds and other nutrients can be reused.

• Sunlight is the main energy source for most ecosystems.

• Important exceptions: ecosystems in caves and deep ocean communities below light penetration can depend on chemoautotrophs instead.

This diagram shows how chemoautotrophic microbes can make organic molecules without sunlight by using energy from chemical reactions. It is useful for remembering the syllabus exception to sunlight as the principal energy source. Source

Autotrophs, heterotrophs and decomposers

• Autotrophs make carbon compounds from simple inorganic substances.

• They need an external energy source for carbon fixation and other anabolic reactions.

• Photoautotrophs use light energy.

• Chemoautotrophs use energy from oxidation reactions; example: iron-oxidizing bacteria.

• Heterotrophs obtain carbon compounds from other organisms, then digest and assimilate them to build their own biomass.

• Both autotrophs and heterotrophs release energy by cell respiration.

• Decomposers obtain energy from organic matter from dead organisms, dead body parts and faeces.

• Decomposers are essential for recycling matter back into ecosystems.

Food chains, food webs and trophic levels

• A food chain shows a linear feeding sequence.

• A food web shows the full network of feeding relationships in a community.

• In both diagrams, arrows show the direction of energy transfer and biomass transfer.

• Standard trophic levels:

  • Producer

  • Primary consumer

  • Secondary consumer

  • Tertiary consumer

• Many organisms feed at more than one trophic level, so they can occupy different trophic levels in different food chains.

• Decomposers and detritus feeders are often not drawn as part of a simple food chain, but they still play a major role in energy transformations and matter recycling.

This image shows multiple organisms linked in a food web, helping you see why real ecosystems are more complex than a single food chain. Use it to practise reading arrow direction and identifying trophic relationships. Source

Energy transfer and energy pyramids

• Chemical energy passes from one organism to the next when a consumer feeds on the previous organism in a food chain.

• At each transfer, there is a large loss of energy, so less energy is available at the next trophic level.

• Causes of energy loss include:

  • Heat loss during cell respiration

  • Energy remaining in uneaten parts

  • Energy lost in faeces and other waste

  • Energy used for movement, maintenance and other life processes

• Energy transfer is never 100% efficient.

• This explains why energy pyramids always get narrower at higher trophic levels.

• Because less energy is available higher up, ecosystems can support only a limited number of trophic levels.

• Higher trophic levels therefore usually have fewer organisms or smaller total biomass.

• Important distinction: biomass decreases up the food chain, but the energy content per unit mass is not lower.

This diagram is useful for visualizing how energy enters at the producer level and decreases between trophic levels. It also helps explain why long food chains are rare. Source

Primary and secondary production

• Primary production = accumulation of carbon compounds in biomass by autotrophs.

• Units are usually g m⁻² yr⁻¹ (mass of carbon per unit area per unit time).

• Biomes differ in how much biomass they can accumulate, so primary production varies between ecosystems.

• Secondary production = accumulation of carbon compounds in biomass by heterotrophs.

• Secondary production is lower than primary production because some biomass is lost when carbon compounds are broken down in cell respiration to carbon dioxide and water.

• Biomass increases when organisms grow or reproduce.

The carbon cycle

• Carbon is recycled through ecosystems by photosynthesis, feeding and respiration.

• Photosynthesis removes carbon dioxide from the atmosphere and converts it into organic carbon compounds.

• Feeding transfers carbon compounds between trophic levels.

• Respiration returns carbon dioxide to the atmosphere.

• Ecosystems are carbon sinks when photosynthesis exceeds respiration.

• Ecosystems are carbon sources when respiration exceeds photosynthesis.

• Combustion of biomass, peat, coal, oil and natural gas releases carbon dioxide to the atmosphere.

• Some combustion occurs naturally, such as after lightning strikes, but human activity has greatly increased combustion rates.

This diagram summarizes the carbon cycle reservoirs and transfers. It is especially useful for linking photosynthesis, respiration, combustion and carbon sinks/sources in one view. Source

The Keeling Curve and atmosphere interactions

• The Keeling Curve shows the long-term rise in atmospheric carbon dioxide concentration.

• The long-term upward trend is explained mainly by increasing combustion of fossil fuels.

• The annual zigzag fluctuation is caused by seasonal changes in photosynthesis and respiration, especially in the Northern Hemisphere.

• Aerobic respiration depends on atmospheric oxygen produced by photosynthesis.

• Photosynthesis depends on atmospheric carbon dioxide produced by respiration.

• This shows a major interdependence between autotrophs and heterotrophs.

This graph shows the seasonal fluctuations and the overall long-term increase in atmospheric carbon dioxide. It is the key image for explaining how photosynthesis, respiration and combustion affect atmospheric CO₂. Source

Recycling of matter beyond carbon

• Not only carbon, but all chemical elements needed by living organisms are recycled in ecosystems.

• This includes elements used to build biomolecules and cell structures.

• Decomposers are central because they break down organic matter and return chemical elements to the environment.

• You do not need detailed knowledge of the nitrogen cycle or other nutrient cycles for this subtopic.

Exam traps and high-yield distinctions

• Energy is not recycled; it is transferred and eventually lost as heat.

• Matter is recycled because atoms are reused.

• Do not confuse food chains with food webs: chains are linear, webs are networked.

• Do not forget that arrows in food chains/webs show the direction of energy transfer, not “who is bigger” or “who eats backwards”.

• Decomposers may be missing from a simple food chain diagram, but they are still essential in ecosystems.

• Secondary production is always lower than primary production in an ecosystem.

• A decrease in biomass up a pyramid does not mean lower energy per gram of tissue.

One-minute synthesis

• Autotrophs capture energy and build biomass.

• Heterotrophs and decomposers obtain carbon compounds from other organisms.

• Energy flows through trophic levels and is lost as heat, so it cannot be recycled.

• Matter cycles because carbon and other elements move between organisms and the environment.

• The core exam idea: ecosystems depend on continuous energy input but internal recycling of matter.