Biology IGCSE - Organisms and their environment

Sun’s energy

The Sun is the principal source of energy input to biological systems

Describe the flow of energy through living organisms

Light energy from the Sun → becomes photosynthesis and stored in plants → animals eat plants→ using energy stored in plants to convert to chemical energy → used for life processes (e.g. respiration, reproduction, excretion)

Food chain

A diagram showing the flow of energy from one organism to the next, beginning with a producer

Simple food chain

Food web

A network of interconnected food chains

Producer

An organism that makes its own organic nutrients → using energy from sunlight by photosynthesis

Consumer

An organism that gets its energy by feeding on other organisms

How consumers are categorised

Consumers may be primary, secondary and tertiary according to their position in a food chain

Herbivore

An animal that gets its energy by eating plants

Carnivore

An animal that gets its energy by eating other animals

Decomposer

An organism that gets its energy from dead or waste organic material

Use food chains and food webs to describe the impact humans have through overharvesting of food species and through introducing foreign species to a habitat

Over-harvesting → damages food chains as other organisms which consume these organisms will not have enough food to survive → many of them will die

New species → it disrupts food chains → leading to loss of native species and ecosystem imbalance

Trophic level

The position of an organism in a food chain, food web or pyramid of biomass or numbers

Identify the trophic levels in food webs and food chains

Explain why the transfer of energy from one trophic level to another is often not efficient

Only 10% of energy taken in is used for growth (building proteins and carbohydrates that will be eaten) – this is all that is available for the next trophic level

Lots of energy is wasted because it remains stored in inedible parts, e.g. bones, fur.

Explain, in terms of energy loss, why food chains usually have fewer than five trophic levels 

There is only 10% flow of energy from one trophic level to the nex → loss of energy at each step is so great that very little usable remains after four or five trophic levels → so only 4 to 5 trophic levels are present in each food chain

Explain why it is more energy efficient for humans to eat crop plants than to eat livestock that have been fed on crop plants

Plant-based diets are more energy efficient than livestock-based diets → livestock animals must be fed lots of plants. When a person eats a plant → 20% of energy passed on to the person to use as fuel or rebuild body tissue

Carbon cycle

The exchange of carbon between living organisms and their atmosphere. In the carbon cycle, carbon is constantly removed from, and returned to, the environment.

Describe the carbon cycle

Photosynthesis → Plants absorb carbon dioxide from atmosphere and use it make their own energy → storing carbon in their tissues

Respiration → Organisms release CO2 back into the atmosphere by breaking down glucose for energy during respiration

Feeding → Carbon moves through the food chain as animals eat → incorporating carbon into their own bodies

Decomposition → When organisms die → decomposers break down bodies → releasing carbon back into the atmosphere as CO2 or into soil as carbon compounds

Formation of Fossil Fuels → Over millions of years, the carbon in dead organic matter can be converted into fossil fuels like coal and oil under the Earth’s surface.

Combustion → Burning fossil fuels releases stored carbon back into the atmosphere as CO2 → completing the cycle.