HKDSE Biology key terms

A thin, selectively permeable layer of phospholipids and proteins surrounding the cytoplasm, controlling which substances enter or leave the cell.

The net movement of particles from a region of higher concentration to a region of lower concentration, driven by random molecular motion and requiring no energy input.

The net movement of water molecules across a selectively permeable membrane from a region of higher water potential to one of lower water potential.

The movement of substances across a membrane against their concentration gradient using carrier proteins and energy released from ATP.

A protein that acts as a biological catalyst, lowering the activation energy of a reaction by binding its specific substrate at the active site.

The small region of an enzyme molecule, shaped to fit a particular substrate, where the catalytic reaction takes place.

Loss of an enzyme's three-dimensional shape and hence its catalytic function, usually caused by high temperature or extremes of pH.

Adenosine triphosphate, the universal energy currency of cells; hydrolysis of its terminal phosphate bond releases energy for cellular work.

The process in chloroplasts by which light energy is used to convert carbon dioxide and water into glucose, releasing oxygen as a by-product.

The green pigment in chloroplasts that absorbs mainly red and blue light and converts light energy into chemical energy during photosynthesis.

The factor in shortest supply, such as light intensity or carbon dioxide concentration, that restricts the rate of a process such as photosynthesis.

The controlled release of energy from organic molecules such as glucose in cells, producing ATP; aerobic respiration uses oxygen while anaerobic respiration does not.

Release of a small amount of energy from glucose without oxygen, producing lactic acid in muscles or ethanol and carbon dioxide in yeast.

Nuclear division producing two genetically identical daughter cells with the same chromosome number as the parent, used in growth and repair.

Reduction division producing four haploid cells with half the chromosome number, involving crossing over and independent assortment to generate genetic variation.

Exchange of corresponding segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis, increasing genetic variation.

One of two or more alternative forms of the same gene, occupying the same locus on homologous chromosomes and giving rise to different traits.

The genetic makeup of an organism with respect to a particular gene or set of genes, described by the alleles it carries.

The observable characteristics of an organism resulting from the interaction between its genotype and the environment.

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

An allele whose effect on the phenotype appears only when two copies are present, i.e. in a homozygous individual.

Deoxyribonucleic acid, a double-stranded helix of nucleotides that stores genetic information in the sequence of its bases.

A length of DNA that codes for a particular polypeptide or functional RNA, and hence controls a characteristic.

A permanent change in the base sequence of DNA, which may alter the protein produced and provide a source of new alleles.

A haploid sex cell, such as a sperm or egg cell in humans, formed by meiosis and able to fuse with another gamete at fertilisation.

The process by which individuals with inherited features better suited to their environment survive and reproduce more, so those alleles become more common.

An inherited feature of an organism that increases its chance of survival and reproduction in its particular habitat.

Maintenance of a relatively stable internal environment, such as constant body temperature or blood glucose concentration, by negative feedback mechanisms.

A control mechanism in which a change from the normal level triggers a response that counteracts the change and restores the set point.

A chemical messenger secreted by an endocrine gland into the blood, which travels to target cells bearing specific receptors and alters their activity.

A hormone secreted by the pancreas that lowers blood glucose concentration by promoting uptake of glucose into cells and its conversion to glycogen.

A nerve cell specialised for transmitting electrical impulses, consisting of a cell body, dendrites and an axon.

The junction between two neurons or between a neuron and an effector, across which impulses are transmitted by chemical neurotransmitters.

A rapid, automatic and involuntary response to a stimulus, mediated by a reflex arc without involving conscious thought.

A protein produced by lymphocytes in response to a specific antigen, binding to it and helping to destroy the pathogen.

A molecule, often on the surface of a pathogen or foreign cell, that is recognised as non-self and triggers a specific immune response.

The body's ability to resist infection by a particular pathogen, either through antibodies produced after exposure or through vaccination.

Introduction of weakened or harmless antigens into the body to stimulate antibody production and memory cells, giving protection against later infection.

A community of interacting organisms together with the non-living components of their environment, through which energy and nutrients flow.

A feeding position in a food chain, such as producer or primary consumer, defined by the number of energy-transfer steps from the start.

A sequence of organisms in which each is eaten by the next, showing the direction in which energy passes through a community.

An organism such as a bacterium or fungus that breaks down dead organic matter, releasing nutrients back into the environment.

The variety of living organisms, encompassing variation within species, between species and of ecosystems within a given area.

The protection and sustainable management of natural resources and habitats so that species and ecosystems are maintained for the future.

Loss of water vapour from the aerial parts of a plant, mainly through stomata, which draws water up the xylem.

A small pore in the leaf epidermis, bounded by two guard cells, through which gases and water vapour are exchanged.

Plant tissue of dead, hollow vessels and tracheids that transports water and dissolved minerals upward from the roots to the leaves.

Plant tissue of living sieve tube elements that transports sugars and other organic solutes from sources to sinks.

Shrinking of the cytoplasm and cell membrane away from the cell wall when a plant cell loses water in a hypertonic solution.

The tendency of water to move from one place to another; pure water at atmospheric pressure has the highest value, zero.

A difference in the concentration of a substance between two regions, which drives diffusion and active transport.

The specific molecule on which an enzyme acts, binding to the active site to form an enzyme-substrate complex.

The ordered sequence of growth, DNA replication and division through which a cell passes between one mitosis and the next.

The fusion of a haploid male gamete with a haploid female gamete to form a diploid zygote, restoring the full chromosome number.