Sec 4 Combined Science (Biology) Organisms & their Environment notes

Matter recycles; energy does not. Sunlight enters the ecosystem and is captured by producers, flows through the animals that eat them, and at every step some of it leaves the living system as heat released by respiration. That heat warms the surroundings and radiates away — it can never be gathered back and re-used by plants. This one-way street is what the syllabus means by the non-cyclical nature of energy flow: unlike carbon or water, energy passes through the ecosystem once and is gone.

The consequence is a hierarchy: every ecosystem needs a huge input of sunlight to support even a modest mass of top predators, because only a fraction of each level's energy survives to the next.

Producers — green plants and algae — make their own food by photosynthesis and are the only doorway for energy into the ecosystem. Consumers cannot make food; primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary ones. Decomposers — bacteria and fungi — break down dead organisms and waste, releasing substances back to the environment and themselves becoming food for nothing less than the soil's recycling economy.

A food chain (grass → grasshopper → frog → snake) traces one line of who-eats-whom. Real ecosystems are tangled: most organisms eat and are eaten by several others, so chains interconnect into a food web. Reading a web means tracing every arrow into and out of a species — remove one strand and the tension redistributes everywhere.

Only about 10% of the energy at one trophic level is built into the bodies of the next. The rest is lost because organisms are not efficient: they respire, releasing most of their food energy as heat; they move, spending energy on muscle work; whole parts are never eaten (roots, bones, bark); and material is egested or excreted before its energy can be captured. Decomposers also claim a share of every death and dropping.

A pyramid of numbers simply counts the organisms at each level — bars proportional to how many. A pyramid of biomass weighs the total living material at each level, which tracks energy better: biomass is broadest at the base and shrinks upward, because mass embodies energy. The two can disagree. One tree feeds hundreds of aphids, which feed a few ladybirds: the numbers pyramid is upside-down at its base (1 → 800 → 5), yet the biomass pyramid stays honest — the tree's mass dwarfs its aphids combined.

Interpretation rules: a wide base means many producers; an inverted numbers pyramid signals one large producer supporting many small consumers; a biomass pyramid that narrows upward is the normal energy-shrink made visible.

Carbon travels in a true cycle. Photosynthesis pulls carbon dioxide from the air into carbohydrates. Respiration — by plants, animals and decomposers — returns carbon dioxide. Feeding passes carbon along food chains; decomposition releases it from dead matter; combustion of wood or fuel sends it straight back to the atmosphere. And two giant carbon sinks park carbon for long stays: forests, locking carbon into wood and soil, and oceans, dissolving carbon dioxide and storing carbon in the bodies and shells of sea life. A sink absorbs more carbon than it releases — which is why keeping them intact matters for the atmosphere.

Two human habits push atmospheric carbon dioxide up. Deforestation removes trees that were photosynthesising — so less carbon dioxide is removed — and clearing often burns the wood, releasing its stored carbon at once. Fossil-fuel burning (coal, oil, natural gas) returns to the air carbon that had been locked underground for hundreds of millions of years. The rising carbon dioxide thickens the atmosphere)s heat-trapping blanket: the planet retains more of the Sun)s energy, and global warming follows — rising temperatures, shifting weather patterns, melting ice.

The levers for pulling back: burn fewer fossil fuels by switching to renewable energy and using energy efficiently; protect existing forests and plant new ones, restoring the sink; and reduce, reuse, recycle to cut the demand that drives extraction and burning in the first place. Each action works on one side of the equation — less carbon in, or more carbon absorbed.