SPM Biology key terms

A thin, flexible layer of phospholipids and proteins surrounding every cell. It controls which substances enter and leave, allowing selective movement of ions, nutrients, water and waste.

The net movement of particles from a region of higher concentration to a region of lower concentration, driven by random molecular motion. It needs no energy input and stops when concentrations equalise.

The diffusion of water molecules across a partially permeable membrane from a dilute solution to a more concentrated solution. It is passive and central to water uptake by roots and cell turgidity.

Movement of ions or molecules across a membrane against their concentration gradient, using carrier proteins and energy from ATP. It lets cells accumulate substances they need, such as mineral ions in root hair cells.

The difference in concentration of a substance between two regions. A steeper gradient increases the rate of diffusion, while active transport can work against a gradient.

A protein that speeds up a specific biochemical reaction by lowering its activation energy. Each enzyme has an active site whose shape is complementary to its substrate, and is affected by temperature and pH.

The specific region of an enzyme where the substrate binds and the reaction occurs. Its shape is complementary to the substrate, explaining enzyme specificity and loss of function when denatured.

Loss of an enzyme's three-dimensional shape, especially at its active site, caused by high temperature or extreme pH. The enzyme can no longer bind its substrate, so reaction rate falls sharply.

A membrane-bound organelle where aerobic respiration occurs. Its folded inner membrane increases surface area for the reactions that release energy as ATP, so active cells contain many mitochondria.

A green organelle containing chlorophyll, found in plant cells. It is the site of photosynthesis, absorbing light energy to convert carbon dioxide and water into glucose and oxygen.

A rigid layer of cellulose outside the cell membrane of plant cells. It gives support and shape and withstands internal pressure, preventing plant cells from bursting when they take in water.

The outward pressure of cell contents against the cell wall when a plant cell is full of water. It keeps soft plant tissues firm and supports stems and leaves.

Shrinking of the cytoplasm and cell membrane away from the cell wall when a plant cell loses water by osmosis in a concentrated solution. Severe plasmolysis causes wilting.

The process by which green plants use light energy to convert carbon dioxide and water into glucose and oxygen, catalysed in chloroplasts. It stores energy in organic molecules and releases oxygen.

The green pigment in chloroplasts that absorbs mainly red and blue light for photosynthesis. It reflects green light, which is why most leaves appear green.

The environmental condition in shortest supply that restricts the rate of a process such as photosynthesis. Light intensity, carbon dioxide concentration and temperature can each act as the limiting factor.

Loss of water vapour from plant leaves through stomata, mainly by evaporation from internal leaf surfaces. It pulls water up the xylem and cools the plant.

Small pores in the leaf epidermis, each bordered by two guard cells, through which gases and water vapour pass. They open for carbon dioxide uptake and close to reduce water loss.

One of a pair of specialised epidermal cells surrounding a stoma. Changes in their water content alter their shape, opening or closing the pore and regulating gas exchange and water loss.

A tissue of dead, hollow, lignified vessels and tracheids that transports water and dissolved minerals from roots to leaves. Its walls give mechanical support to the plant.

A living transport tissue of sieve tubes and companion cells that carries dissolved sugars and amino acids from sources such as leaves to sinks such as roots and growing tissues.

The movement of dissolved organic substances, especially sucrose and amino acids, through phloem tissue from where they are made or stored to where they are used or stored.

The controlled release of energy from organic molecules such as glucose, producing ATP for cell activities. Aerobic respiration uses oxygen; anaerobic respiration occurs without it and releases less energy.

Adenosine triphosphate, the short-term energy currency of cells. Energy released from respiration is stored in its phosphate bonds and released where needed for processes such as active transport.

Breakdown of glucose without oxygen, releasing much less energy than aerobic respiration. In human muscle it forms lactic acid; in yeast it produces ethanol and carbon dioxide during fermentation.

Maintenance of a stable internal environment, such as body temperature, blood glucose and water content, despite external changes. It relies on negative feedback control systems.

A control mechanism in which a change from the normal level triggers a response that reverses that change. It keeps variables such as temperature and blood glucose within narrow limits.

A hormone secreted by the pancreas when blood glucose rises. It increases glucose uptake by cells and storage as glycogen in liver and muscle, lowering blood glucose concentration.

A hormone secreted by the pancreas when blood glucose falls. It stimulates the breakdown of liver glycogen into glucose, raising blood glucose and opposing the action of insulin.

Control of the water and solute content of body fluids so that cells neither swell nor shrink excessively. The kidneys adjust water reabsorption, guided by hormones such as ADH.

A nerve cell that conducts electrical impulses. It has dendrites receiving signals, a cell body, and a long axon carrying impulses to other neurons, muscles or glands.

The junction between two neurons, or between a neuron and a muscle, where a gap separates them. Impulses cross as chemical neurotransmitters diffuse across the gap, ensuring one-way transmission.

A chemical released from the end of a neuron into a synapse when an impulse arrives. It diffuses across the gap and triggers an impulse in the next cell.

The nerve pathway that produces a rapid, automatic response to a stimulus without conscious control. It usually involves a receptor, sensory neuron, relay neuron, motor neuron and effector.

One of two or more alternative versions of a gene occupying the same position on homologous chromosomes. Different alleles may be dominant or recessive and produce different phenotypes.

The combination of alleles an organism carries for a particular gene, such as homozygous dominant, heterozygous or homozygous recessive. It determines the potential phenotype.

The observable characteristics of an organism, resulting from its genotype interacting with the environment. Examples include eye colour, blood group and plant height.

An allele that produces its effect in the phenotype even when only one copy is present. It masks the expression of a recessive allele in a heterozygous individual.

A change in the base sequence of DNA, which may alter the protein produced. Mutations can be spontaneous or caused by mutagens and are a source of genetic variation.

Deliberate transfer of a gene from one organism to another, often using plasmids or viruses as vectors. It produces genetically modified organisms with desired traits such as insulin-producing bacteria.

A small circular piece of DNA found in bacteria, separate from the main chromosome. Plasmids are widely used as vectors to carry foreign genes into host cells.

A community of interacting organisms together with the non-living factors of their environment, such as soil, water and light. Energy and matter flow through it between organisms.

A sequence showing the feeding relationships in a community, from producer through primary and secondary consumers. Arrows indicate the direction of energy transfer between trophic levels.

A feeding position in a food chain, such as producer, primary consumer or secondary consumer. Energy decreases at each higher level because much is lost as heat and in waste.

The variety of living organisms in an area, including variation within species, between species and of ecosystems. It supports ecosystem stability and is threatened by habitat loss and pollution.

Protecting and managing natural resources and species so they persist for future generations. It includes habitat protection, breeding programmes and sustainable use of resources.

Enrichment of water with nutrients, often from fertiliser runoff, causing rapid algal growth. Decay of the algae depletes oxygen, killing fish and other aquatic organisms.