Energy Flow

The chain of energy:

1. SUN — ultimate source. Constantly delivered.
2. PRODUCERS (green plants, algae, some bacteria) — capture ~1% of incoming sunlight via PHOTOSYNTHESIS, store as glucose.
3. PRIMARY CONSUMERS (herbivores) eat producers — get some of the captured energy.
4. SECONDARY CONSUMERS (carnivores) eat primary consumers.
5. TERTIARY / TOP CONSUMERS eat secondary consumers.
6. DECOMPOSERS (bacteria + fungi) break down dead remains at every level.

Why FLOW not CYCLE.

• Energy enters as light → ultimately leaves as HEAT (radiated to space).
• One-way: cannot be reused.
• Sun must keep providing → if sun stopped, ecosystems would die in days/weeks.

Compare with NUTRIENT CYCLES (carbon, nitrogen, water): these atoms ARE recycled — the same atoms in your body have been in countless other organisms.

Worked qualitative. Why is the SUN's energy so important to LIFE on Earth?

• Photosynthesis powers nearly all food chains.
• Without sun → no producers → no consumers → ecosystem collapses.
• Some exceptions: deep sea hydrothermal vents use CHEMICAL energy (chemosynthesis) — but these are tiny in comparison.

Cambridge tip. Always say energy 'FLOWS' (not 'cycles'). Cambridge marks the term.

Only ~10% of the energy at one trophic level passes to the next. The other ~90% is LOST in several ways:

1. Respiration → HEAT.

• Most energy used for movement, growth, body temperature.
• Most ends up as HEAT released to surroundings.
• Largest single loss.

2. Excretion.

• Urea, faeces contain energy that doesn't continue up the chain.
• Decomposers eventually break these down.

3. Not all parts eaten.

• Bones, fur, feathers, roots, woody bits often left behind.
• Energy in these doesn't pass to the predator.

4. Not all eaten parts digested.

• Some nutrients pass through gut undigested (cellulose for non-herbivores).
• Energy lost in faeces.

Why food chains are short.

• 100% energy at producer.
• 10% at primary consumer.
• 1% at secondary consumer.
• 0.1% at tertiary consumer.
• After 4-5 levels, energy is too low to support another → typical food chains are 3-5 organisms long.

Worked qualitative. Why is it 'more efficient' to eat plants directly than to eat meat?

• One step in food chain → less energy lost.
• 10 kg of grain → eaten by humans gives ~10 kg human energy.
• 10 kg of grain → eaten by cattle → only 1 kg of meat → eaten by humans gives ~0.1 kg human energy.
• More people can be fed from same land if they eat plants.
• Why vegetarian diets are 'land-efficient'.

Cambridge tip. Cambridge often gives a calculation: '5000 kJ at producers, what's at primary consumers if 10% efficient?' Answer: 500 kJ. Memorise the 10% rule.

Pyramid of NUMBERS.

• Number of organisms at each trophic level.
• Often pyramid-shaped (lots of grass, fewer rabbits, fewer foxes).
• CAN BE INVERTED if e.g. one tree supports many caterpillars supporting fewer birds.

Pyramid of BIOMASS.

• Total mass of organisms at each level.
• Almost always upright (mass usually decreases).
• Sometimes inverted in oceans (many small phytoplankton fast-replicating).

Pyramid of ENERGY.

• Total energy stored at each level.
• ALWAYS UPRIGHT — energy must decrease.
• The most reliable pyramid.

Worked qualitative. A single oak tree (1 producer) supports thousands of caterpillars (many primary consumers). Draw the pyramid of numbers — what shape is it?

• INVERTED — narrow at producer (1 tree), wide at consumers (many caterpillars).
• The pyramid of energy would still be UPRIGHT — the tree contains far more energy than all caterpillars combined.

Cambridge tip. Memorise: NUMBER pyramid CAN invert; BIOMASS sometimes; ENERGY NEVER inverts.