MATSEC IM Biology key terms

A thin, flexible layer of phospholipids and proteins surrounding every cell. It controls which substances enter and leave, and it separates the cell's contents from the outside environment.

The net movement of particles from a region where they are more concentrated to one where they are less concentrated, caused by random motion. It needs no energy from the cell and stops when concentrations equalise.

The movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated one. It is a special case of diffusion and explains why plant cells become turgid in pure water.

A protein that speeds up a specific chemical reaction by lowering its activation energy. Each enzyme has an active site whose shape fits particular substrate molecules, and it is denatured by high temperature or extreme pH.

The small region of an enzyme molecule where the substrate binds and the reaction occurs. Its precise three-dimensional shape is complementary to the substrate, which explains why enzymes are highly specific.

The loss of a protein's normal three-dimensional shape, usually caused by high temperature or extremes of pH. It destroys the active site of enzymes so they can no longer catalyse reactions, and it is usually irreversible.

The molecule or molecules that an enzyme acts on. The substrate fits into the enzyme's active site and is converted into products during the reaction.

The release of energy from glucose using oxygen, producing carbon dioxide and water. It yields far more ATP per glucose molecule than anaerobic respiration and takes place mainly in the mitochondria.

The release of energy from glucose without oxygen. In human muscle it produces lactic acid, and in yeast it produces ethanol and carbon dioxide; both yield much less energy than aerobic respiration.

Adenosine triphosphate, the short-term energy-carrier molecule of cells. Energy released from glucose is used to build ATP, and breaking it down releases energy for processes such as muscle contraction and active transport.

The process by which green plants use light energy to convert carbon dioxide and water into glucose and oxygen. Chlorophyll in chloroplasts absorbs the light, and the glucose formed supplies energy and building material.

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

The factor in shortest supply that holds back the rate of a process such as photosynthesis. Common examples are light intensity, carbon dioxide concentration and temperature; increasing the others has little effect until this one is raised.

The movement of substances across a cell membrane against their concentration gradient, using energy from ATP and carrier proteins. It allows cells to take up ions and nutrients even when they are already more concentrated inside.

A section of DNA that carries the instructions for making a particular protein or characteristic. Genes are found at fixed positions on chromosomes and are passed from parents to offspring.

One of the different versions of the same gene, such as the alleles for brown or blue eye colour. An organism inherits two alleles for each gene, one from each parent.

The combination of alleles an organism carries for a particular gene, for example homozygous dominant, heterozygous or homozygous recessive. It is written with letters and is not always visible in the organism's appearance.

The observable characteristics of an organism, such as flower colour or blood group. It results from the organism's genotype interacting with environmental factors.

Describes an allele whose effect is shown even when only one copy is present. A dominant allele masks the effect of a recessive allele in a heterozygous organism.

Describes an allele whose effect appears only when two copies are present and no dominant allele is there to mask it. Recessive characteristics can therefore skip generations in a family.

Having two identical alleles for a particular gene, for example two dominant alleles or two recessive alleles. A homozygous organism always passes the same allele to its offspring for that gene.

Having two different alleles for the same gene, one dominant and one recessive. A heterozygous organism shows the dominant characteristic but can pass either allele to its offspring.

A long thread-like structure of DNA and protein found in the nucleus. Human body cells contain 23 pairs, and chromosomes carry genes in a fixed order.

Cell division that produces two genetically identical daughter cells with the same chromosome number as the parent. It is used for growth, repair and asexual reproduction.

A type of cell division that produces four gametes, each with half the normal chromosome number. It creates genetic variation through independent assortment and crossing over, and it occurs in the testes and ovaries.

A sex cell such as a sperm, egg or pollen grain. Gametes carry half the normal number of chromosomes and fuse at fertilisation to form a new individual.

The process by which individuals better adapted to their environment are more likely to survive and reproduce, so their alleles become more common in later generations. It is the main mechanism of evolution.

A random change in the base sequence of DNA. Mutations may have no effect, be harmful or occasionally be beneficial, and they are the original source of new alleles in a population.

A feature of an organism that improves its chance of surviving and reproducing in its habitat, such as thick fur in a cold climate or a streamlined body in water. Adaptations arise over many generations by natural selection.

The maintenance of a stable internal environment within narrow limits, despite changes outside the body. Examples include controlling blood glucose, body temperature and water balance, mostly by negative feedback.

A control mechanism in which a change from the normal level triggers a response that reverses that change. It keeps conditions such as body temperature and blood glucose close to a set point.

A hormone secreted by the pancreas when blood glucose rises. It causes cells, especially in the liver and muscles, to take up glucose and store it as glycogen, lowering blood glucose.

The control of water and dissolved salt concentrations in body fluids. The kidneys adjust how much water is reabsorbed and excreted in urine, helping to keep cells at the correct concentration.

The firmness of a plant cell caused by water pushing the cell membrane against the rigid cell wall. Turgid cells support soft plant parts, and loss of turgor causes wilting.

The loss of water vapour from the leaves of plants, mainly through stomata. It pulls water up the xylem from the roots and is faster in warm, dry, windy or bright conditions.

A sequence showing the feeding relationships in a community, beginning with a producer and followed by consumers. The arrows show the direction in which energy passes from one organism to the next.

An organism that makes its own food by photosynthesis, usually a green plant or alga. Producers form the first level of every food chain and supply energy to all other organisms in the community.

A feeding position in a food chain, such as producer, primary consumer or secondary consumer. Energy is lost at each transfer, so higher trophic levels support far fewer organisms.

A diagram of many interlinked food chains in a habitat, showing the range of organisms each animal eats. It gives a more realistic picture of feeding than a single chain.

The particular place where an organism lives, such as a rock pool, hedgerow or woodland. It provides the food, water, shelter and conditions the organism needs.

All the individuals of one species living in the same area at the same time. Population size is affected by birth rate, death rate, immigration and emigration.

The warming of the Earth caused by gases such as carbon dioxide and methane trapping heat that would otherwise escape into space. Human activities have increased these gases and strengthened the effect.

The enrichment of water with nutrients such as nitrates from fertiliser runoff, causing rapid algal growth. When the algae die and decompose, oxygen is used up and fish and other animals may suffocate.

A fuel such as coal, oil or natural gas formed from the remains of organisms over millions of years. Burning fossil fuels releases carbon dioxide, a major greenhouse gas.