IGCSE Biology Organisms & environment notes

The Sun is the principal source of energy input to biological systems. Producers (green plants and algae) capture light energy in photosynthesis and build organic nutrients; that chemical energy flows along food chains — each arrow meaning “is eaten by” — and is eventually transferred to the environment, mostly as heat released by respiration. Energy flows in one direction: it is not recycled.

A labelled example to picture: grass → grasshopper → frog → snake → eagle. The grass is the producer; the grasshopper a primary consumer (and a herbivore — an animal eating plants); the frog a secondary consumer; the snake a tertiary consumer; the eagle a quaternary consumer (a carnivore — an animal eating other animals). A decomposer — a fungus or bacterium — gets its energy from dead or waste organic material, breaking down organisms at every level.

A food web is a network of interconnected food chains — most real ecosystems, because most organisms eat more than one thing. Reading webs answers “what happens if…” questions: overharvesting a food species removes energy and food from every consumer above it; introducing a foreign species can give it a competitor-free feast, crashing native populations — humans alter habitats through both routes.

A trophic level is the position of an organism in a food chain, food web or ecological pyramid — producer is level 1, primary consumer level 2, and so on. Pyramids of numbers simply count organisms at each level; pyramids of biomass measure the dry mass of organisms at each level. Supplement adds pyramids of energy — the energy available per level per unit time.

Why biomass beats numbers (Core): a pyramid of numbers can invert — one tree (level 1) feeds thousands of aphids (level 2), so the “pyramid” points the wrong way. A pyramid of biomass shows the tree’s great mass against the aphids’ small total, giving the expected shape and a truer picture of the energy available.

Supplement — why energy pyramids are best — and why chains stay short: energy transfer between trophic levels is inefficient: organisms egest (never eat everything), excrete wastes, and respire — releasing energy as heat to the environment — while much energy simply stays locked in uneaten parts. Typically only a small fraction of the energy passes to the next level, so by the fourth or fifth transfer almost nothing remains — food chains rarely exceed five trophic levels. The same logic answers the diet question: eating crop plants directly uses one transfer; feeding crops to livestock and eating the meat uses two — wasting far more energy per unit of food.

The carbon cycle (Core) has six named processes: photosynthesis removes carbon dioxide from the air into organic molecules; feeding passes carbon along food chains; respiration returns carbon dioxide to the air; decomposition returns carbon from dead material to the soil and air; some dead organisms became fossil fuels over millions of years; and combustion of those fuels (and of wood) releases carbon dioxide again.

Supplement — the nitrogen cycle, told as a journey: proteins in dead plants and animals are decomposed to ammonium ions; nitrifying bacteria perform nitrification, converting ammonium ions to nitrites then nitrates; plant roots absorb nitrate ions and use them to build amino acids and proteins; animals obtain nitrogen by feeding and digesting proteins; excess amino acids are deaminated in the liver (Topic 11), and the nitrogen leaves as urea; finally, denitrifying bacteria convert nitrates back to nitrogen gas. Nitrogen gas also re-enters the cycle by nitrogen fixation — by lightning and by nitrogen-fixing bacteria.

Supplement — the microbes’ four roles: decomposition (breaking proteins down to ammonium ions), nitrification (ammonium → nitrites → nitrates), nitrogen fixation (N₂ into usable compounds) and denitrification (nitrates back to nitrogen gas). Generic names only are required — no individual bacterium names.

The level words: a population is a group of organisms of one species living in the same area at the same time; a community is all of the populations of different species in an ecosystem; an ecosystem is a unit containing the community of organisms and their environment, interacting together. The rate of population growth depends on food supply, competition, predation and disease.

A labelled graph to picture the sigmoid (S-shaped) growth curve of a population in an environment with limited resources: a flat lag phase, a steep exponential (log) phase, a stationary phase where the line levels off, and finally a death phase if conditions worsen.

Supplement — explaining each phase with limiting factors: in the lag phase, organisms are adjusting to the new habitat and few are reproducing at full rate; in the exponential phase, resources (food, space) are abundant and there is little competition, so the population grows at its maximum rate; in the stationary phase, limiting factors — dwindling food, rising competition, predation and disease — pull the birth rate down to match the death rate; in the death phase, resources are exhausted or waste/pollution accumulates, so deaths exceed births and the population falls.

Humans have increased food production through: agricultural machinery (working larger areas more efficiently), chemical fertilisers (replacing soil nutrients, improving yields), insecticides (protecting quality and yield from pests), herbicides (killing weeds that would compete with crops) and selective breeding (improving crop plants and livestock — Topic 15).

Large-scale monocultures — huge areas of a single crop — have advantages: efficient machinery use, uniform planting, spraying and harvesting, and high total yields. The disadvantages: low biodiversity and lost habitats, easy spread of pests and diseases through the identical crop, and soil nutrient depletion (the same crop drains the same minerals year after year).

Intensive livestock production keeps animals at high density for maximum output. Advantages: high production per animal and per hectare, low cost per unit of food, efficient feeding and disease control. Disadvantages: disease can spread rapidly in crowded conditions, animals may show poor welfare (restricted movement, unnatural behaviour), antibiotic overuse fuels resistance, and large amounts of feed and waste are concentrated in one place.

Biodiversity is the number of different species that live in an area. Habitats are destroyed for housing, crop plant production and livestock production, for extraction of natural resources, and damaged by freshwater and marine pollution. Because every species is a node in a food web, altering food webs and food chains lets humans harm whole habitats, not just single species.

Deforestation — clearing forest, often by burning — has five undesirable effects the syllabus names: it reduces biodiversity, drives extinctions of species with nowhere else to live, causes loss of soil (roots no longer hold it), flooding (rain runs off instead of being absorbed), and an increase of carbon dioxide in the atmosphere (burning releases CO₂ and fewer trees remain to photosynthesise it away).

Supplement — eutrophication, the chain that kills lakes: fertiliser (or sewage) increases nitrate and other ions in the water; producers (algae) grow more — an algal bloom; the algae die and decomposition increases; decomposers respire aerobically, using up dissolved oxygen; the reduced oxygen kills organisms that need dissolved oxygen — fish and many invertebrates suffocate. At Core you need the effects of untreated sewage and excess fertiliser on water; at Supplement you reproduce this six-step chain in order.

Non-biodegradable plastics persist for decades: in the ocean they entangle and are swallowed by marine animals (blocking guts, leaching chemicals), and they break into microplastics that enter food chains; on land they accumulate in soil and dumps, harming organisms and taking up space without decomposing. Air pollution by methane and carbon dioxide — from livestock digestion, rotting waste and fossil-fuel combustion — strengthens the enhanced greenhouse effect: these gases trap more heat in the atmosphere, driving climate change (rising temperatures, changing rainfall, melting ice, rising seas).

A sustainable resource is one which is produced as rapidly as it is removed from the environment, so that it does not run out. Forests and fish stocks are the two syllabus examples that can be managed this way. Species become endangered or extinct through climate change, habitat destruction, hunting, overharvesting, pollution and introduced species.

Endangered species can be conserved by monitoring and protecting the species and their habitats, education (changing human behaviour), captive breeding programmes and seed banks (storing the genetic diversity of plants against the worst). Supplement: captive breeding can use artificial insemination (AI) and in vitro fertilisation (IVF) to breed animals that will not mate naturally in captivity, and to move genes between populations without moving animals.

Supplement — forests can be conserved with education, protected areas (national parks where logging is banned), quotas (legal limits on timber removed) and replanting (replacing every tree felled). Fish stocks can be conserved with education, closed seasons (no fishing while species breed), protected areas, controlled net types and mesh size (letting young fish escape), quotas on catches and monitoring of stocks.

Supplement — why conserve at all: to maintain or increase biodiversity, reduce extinction, protect vulnerable ecosystems, and maintain ecosystem functions — nutrient cycling and resource provision: food, drugs (many medicines come from species), fuel and genes (wild relatives of crops carry useful traits). Supplement: a shrinking population loses genetic variation — fewer alleles in the gene pool — leaving the species less able to adapt to change (and more prone to harmful recessive conditions from interbreeding).