Ecosystems and Energy Flow Revision Notes | GCSE AQA Higher Biology

1. The Living World - From Individuals to Ecosystems

Picture a single oak tree in your local park. Now zoom out - that tree is part of a woodland community with squirrels, birds, fungi and countless other species. Zoom out further and you see the entire ecosystem where all these organisms interact with soil, climate and water. This is the hierarchy of life that ecologists study.

Here's the key insight: organisms don't live in isolation. A rabbit population depends on grass populations for food. Fox populations depend on rabbit populations. Even the soil bacteria that decompose dead leaves are part of this web of interactions.

2. Interdependence and Adaptation

Living organisms are interdependent - they rely on each other for survival. This creates a delicate balance. Remove one species and the effects ripple through the entire community.

Think about bees and flowering plants. Bees need nectar for energy and pollen for protein. Plants need bees to transfer pollen between flowers for reproduction. Neither can survive without the other - this is called mutualism.

Adaptations arise because organisms face environmental pressures. Arctic foxes have thick white fur (structural adaptation) and hunt at night (behavioural adaptation). Cacti store water in thick stems and have spines instead of leaves to reduce water loss.

3. Photosynthesis - Capturing the Sun's Energy

Here's the remarkable truth: almost all life on Earth depends on photosynthesis. This process captures light energy from the Sun and converts it into chemical energy stored in glucose. Without photosynthesis, there would be no food, no oxygen, no life as we know it.

The process happens in two stages. First, chlorophyll absorbs light energy and splits water molecules, releasing oxygen. Second, this energy is used to combine carbon dioxide with hydrogen to make glucose. The glucose stores the captured energy in its chemical bonds.

Trap: Many students think plants only photosynthesise during the day. Actually, plants respire 24/7 but only photosynthesise when light is available. During the day, photosynthesis rate usually exceeds respiration rate.

4. Cellular Respiration - Releasing Stored Energy

Glucose is like a rechargeable battery - it stores energy until organisms need it. Cellular respiration is the process that 'discharges' this battery, releasing the stored energy for life processes like movement, growth and maintaining body temperature.

Notice something important: respiration is essentially photosynthesis in reverse. The glucose and oxygen that photosynthesis produces become the reactants for respiration. The carbon dioxide and water that respiration produces become the reactants for photosynthesis. It's a perfect cycle.

This happens in every living cell - plant, animal, fungus and bacteria. Even plants respire, using some of the glucose they made during photosynthesis to power their own life processes.

5. Energy Flow Through Ecosystems

Energy flows through ecosystems in one direction - from the Sun, through producers, to consumers, and finally lost as heat. This creates the feeding relationships we call food chains and food webs.

Worked example: Calculating Energy Transfer Efficiency

A food chain shows: Grass (10,000 kJ) → Rabbit (1,000 kJ) → Fox (100 kJ). Calculate the energy transfer efficiency from grass to rabbit.

1. Energy in grass = 10,000 kJ Energy in rabbit = 1,000 kJ — Always identify what energy goes in and what comes out
2. Efficiency = $\frac{\text{Energy transferred}}{\text{Energy available}} \times 100\%$ — This gives the percentage of energy that moves to the next level
3. Efficiency = $\frac{1,000}{10,000} \times 100\%$ — Energy transferred = 1,000 kJ, Energy available = 10,000 kJ
4. Efficiency = $0.1 \times 100\% = 10\%$ — Only 10% of the grass's energy reaches the rabbit - typical for ecosystems

Answer: 10% energy transfer efficiency

Energy transfer between trophic levels is typically only 10% efficient. The other 90% is lost as heat during respiration, movement, and in waste products. This explains why food chains rarely have more than 4-5 levels - there isn't enough energy left to support higher levels.

6. Exam Technique - Ecosystems and Energy Flow

Common Exam Mistakes: Students often confuse respiration with breathing (respiration is a chemical reaction in cells). They also forget that plants do both photosynthesis AND respiration. Finally, many think energy is recycled in ecosystems - it's not, it flows through and is lost as heat.

For calculation questions, examiners award method marks even if your final answer is wrong. Always show your working clearly: formula first, then substitution, then calculation with units.