Topic C: Organisms and Environment

This wide-ranging part connects the workings of individual organisms to the ecosystems they live in. You study human nutrition, gas exchange and transport, the parallel transport and support systems of plants, and then scale up to energy flow, nutrient cycles and the impact of humans on the environment. Many questions ask you to interpret diagrams of systems and data on feeding relationships, so diagram literacy and clear cause-and-effect explanation are essential.

Human nutrition and digestion

A balanced diet supplies carbohydrates, proteins, fats, vitamins, minerals, water and dietary fibre in the correct proportions. Large insoluble food molecules must be digested into small soluble ones that can be absorbed. Digestion is both mechanical, by teeth and churning, and chemical, by enzymes: amylase digests starch to maltose, proteases digest protein to amino acids, and lipase digests fats to fatty acids and glycerol. Digestion occurs along the alimentary canal, with absorption mainly in the small intestine, whose lining is folded into villi to give a large surface area, a thin wall, and a good blood supply for efficient absorption. You should be able to relate each adaptation of the villus to its function and trace a named nutrient through the gut.

Gas exchange in humans

Cells need oxygen for respiration and must remove carbon dioxide. In humans, air passes through the trachea and bronchi to the alveoli in the lungs, where gas exchange occurs. Alveoli are adapted for efficient diffusion: they provide a very large surface area, have thin walls one cell thick, are moist, and are surrounded by many capillaries that maintain a steep concentration gradient. Oxygen diffuses from the alveolar air into the blood and carbon dioxide diffuses out. Ventilation refreshes the air: during inhalation the diaphragm and intercostal muscles contract, increasing the volume of the thorax and lowering its pressure so air flows in, and the reverse occurs during exhalation. Smoking damages this system and is linked to bronchitis, emphysema and lung cancer.

Transport in humans: blood and circulation

Blood transports materials around the body and consists of plasma, red blood cells, white blood cells and platelets. Red blood cells contain haemoglobin, which combines with oxygen to form oxyhaemoglobin in the lungs and releases it in the tissues; their biconcave shape and lack of a nucleus suit oxygen carriage. White blood cells defend against disease and platelets help clotting. The heart pumps blood through a double circulation: the right side sends deoxygenated blood to the lungs and the left side sends oxygenated blood to the body, the left ventricle having a thicker muscular wall to generate higher pressure. Arteries carry blood away from the heart at high pressure, veins return it with the help of valves, and capillaries allow exchange with tissues.

Transport in flowering plants

Plants transport water, minerals and food through two tissues. Xylem carries water and dissolved minerals upward from the roots to the leaves and also helps support the plant; it consists of dead, hollow, lignified vessels. Phloem transports sugars made in the leaves to other parts of the plant in a process called translocation, using living sieve tube cells. Water is absorbed by root hair cells, which have a large surface area, moves across the root and up the xylem, and is lost as vapour from the leaves in transpiration. Transpiration creates a pull that draws the column of water up, and its rate increases with higher temperature, light intensity, air movement and lower humidity, as you can show with a potometer.

Support in plants

Plants are supported by several means without a skeleton. In herbaceous plants, turgor pressure provides much of the support: when cells are full of water they become turgid and press against their cell walls, keeping stems and leaves firm; loss of water makes cells flaccid and the plant wilts. Lignified xylem and other strengthening tissues give mechanical support, especially in older stems and in trees, where wood provides rigidity. The arrangement of vascular bundles and the cellulose cell wall of every cell also contribute to support. You should be able to explain wilting in terms of water loss exceeding water uptake and the resulting loss of turgor, and link plant support to the distribution of strengthening tissue.

Ecosystems, food chains and energy flow

An ecosystem is a community of organisms together with their physical environment. Energy enters as sunlight and is captured by producers, mainly green plants, in photosynthesis. It then passes along a food chain to primary consumers, secondary consumers and so on, and food webs show the many interlinked chains in a community. At each transfer much energy is lost as heat from respiration and in undigested material, so only about ten per cent passes to the next level; this is why food chains are short and why pyramids of energy always narrow upwards. Decomposers break down dead matter and release energy. You should be able to construct food webs, draw ecological pyramids, and explain the consequences of removing one organism.

Nutrient cycles

Unlike energy, nutrients are recycled. In the carbon cycle, carbon dioxide is removed from the air by photosynthesis and returned by respiration, decomposition and combustion of fuels, so the level stays roughly in balance unless humans disturb it. In the nitrogen cycle, nitrogen-fixing bacteria convert nitrogen gas into nitrogen compounds, plants absorb nitrate to make protein, animals obtain protein by feeding, decomposers release ammonium from dead matter and wastes, nitrifying bacteria convert this to nitrate, and denitrifying bacteria return nitrogen gas to the air. Understanding these cycles explains why decomposers are vital and why human activities such as burning fuels and using fertilisers can shift the natural balance and cause environmental problems.

Human impact on the environment

Human activities disturb ecosystems in many ways. Burning fossil fuels releases extra carbon dioxide, an important greenhouse gas linked to global warming and climate change, and produces sulfur dioxide and nitrogen oxides that cause acid rain. Deforestation reduces photosynthesis, destroys habitats and lowers biodiversity. Releasing sewage and fertiliser into water causes eutrophication: nutrients trigger algal blooms, the algae die and bacteria decomposing them use up oxygen, killing fish and other organisms. Overfishing and pollution further reduce biodiversity. You should be able to describe a named example of human impact, explain the mechanism step by step, and suggest realistic conservation or control measures, supporting your points with data when a question provides it.