Cambridge IAL Biology key terms

How many times larger an image appears compared with the real object, calculated as image size divided by actual size. It is used to interpret measurements taken from drawings and micrographs.

The smallest distance between two points at which they can still be seen as separate. Electron microscopes have far higher resolution than light microscopes because they use shorter-wavelength electrons.

The detailed internal structure of a cell and its organelles as revealed by electron microscopy, including membranes, ribosomes and the cytoskeleton. It explains how cell structure relates to specialised function.

The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of random motion. It requires no energy input from the cell.

The net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential. It is a passive process.

A measure of the tendency of water to move out of a solution, expressed in kilopascals. Pure water at atmospheric pressure has a water potential of zero; adding solute makes it negative.

Passive movement of substances across a membrane through channel or carrier proteins, down a concentration gradient. It is used for polar or charged molecules that cannot cross the phospholipid bilayer unaided.

Movement of ions or molecules across a membrane against a concentration gradient using carrier proteins and energy from ATP hydrolysis. It allows cells to accumulate substances in higher concentration than their surroundings.

Adenosine triphosphate, the universal energy currency of cells. Hydrolysis of its terminal phosphate bond releases energy that drives processes such as active transport, synthesis and muscle contraction.

A globular protein that acts as a biological catalyst by lowering the activation energy of a reaction. Its specificity arises from the shape of its active site.

The region of an enzyme where substrate molecules bind and the reaction is catalysed. Its shape is complementary to the substrate, forming a temporary enzyme-substrate complex.

Loss of the three-dimensional shape of a protein, and therefore of its function, caused by breaking hydrogen and ionic bonds. Heat, extremes of pH and heavy metals can all denature enzymes.

A small molecule that can be joined to many identical or similar molecules to form a polymer. Examples include glucose, amino acids and nucleotides.

A reaction that joins two molecules together with the formation of a chemical bond and the release of one molecule of water. It builds polymers such as polysaccharides, polypeptides and nucleic acids.

A reaction in which a chemical bond is broken by the addition of a water molecule. It is the reverse of condensation and is used to digest polymers such as starch and proteins.

A large molecule built from many repeating subunits called monomers joined by covalent bonds. Starch, proteins and DNA are all biological polymers.

The covalent bond formed between two monosaccharides by a condensation reaction, as in maltose or the branches of amylopectin. It is broken by hydrolysis during digestion.

The covalent bond between the carboxyl group of one amino acid and the amino group of another, formed by condensation. A chain of such bonds forms the primary structure of a polypeptide.

The sequence of amino acids in a polypeptide chain, held together by peptide bonds. It determines all higher levels of protein folding and therefore the protein's function.

The overall three-dimensional shape of a single polypeptide chain, stabilised by hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions. It creates the specific shape needed for function.

The double layer of phospholipid molecules that forms the basis of cell membranes, with hydrophilic heads facing outwards and hydrophobic tails inwards. It acts as a barrier to most water-soluble substances.

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

Two successive nuclear divisions that produce four haploid cells from one diploid cell, with genetic variation arising from crossing over and independent assortment. It forms gametes in sexually reproducing organisms.

Exchange of corresponding sections of chromatids between homologous chromosomes during prophase I of meiosis. It produces recombinant chromatids and so increases genetic variation in the gametes formed.

The random arrangement and separation of homologous chromosome pairs during meiosis I, so that maternal and paternal chromosomes are distributed randomly into gametes. It generates many possible allele combinations.

The sequence of stages a cell passes through between one division and the next: interphase, nuclear division and cytokinesis. Interphase includes DNA replication in the S phase.

A specialised surface where oxygen and carbon dioxide are exchanged between an organism and its environment. Effective surfaces are thin, moist, permeable and have a large area and steep diffusion gradient.

The movement of air or water over a gas exchange surface to maintain a steep concentration gradient. In mammals it is achieved by pressure changes in the thorax produced by the diaphragm and intercostal muscles.

The movement of dissolved assimilates, especially sucrose and amino acids, through the phloem from sources to sinks. It is driven by pressure differences between loading and unloading regions.

Vascular tissue that transports water and dissolved mineral ions upwards from the roots to the shoots. Its dead, lignified vessel elements and tracheids also provide structural support.

The loss of water vapour from the aerial parts of a plant, mainly through stomata. It pulls water up the xylem and is affected by light, temperature, humidity and wind.

A pore in the leaf epidermis bounded by two guard cells, through which gases and water vapour pass. Guard cells change shape with water uptake to open or close the pore.

A source is a region that produces or releases assimilates, such as a photosynthesising leaf; a sink is a region that uses or stores them, such as a root or developing fruit. Direction of phloem transport depends on their relative positions.

The first stage of photosynthesis, occurring in the thylakoid membranes, in which light energy drives the photolysis of water, produces ATP and reduced NADP, and releases oxygen.

The Calvin cycle, occurring in the stroma, in which carbon dioxide is fixed by rubisco and reduced using ATP and reduced NADP to form triose phosphate and then carbohydrate.

The synthesis of ATP driven by the flow of protons down an electrochemical gradient through ATP synthase in a membrane. It occurs in chloroplast thylakoids and in the inner mitochondrial membrane.

The controlled release of energy from organic molecules, such as glucose, to synthesise ATP. Aerobic respiration uses oxygen and yields far more ATP than anaerobic respiration.

The maintenance of a stable internal environment around a set point despite changes in external conditions. It relies on negative feedback control systems involving receptors, coordination centres and effectors.

A control mechanism in which a deviation from the set point triggers a response that reduces that deviation and restores the original level. It stabilises conditions such as blood glucose and body temperature.