Biology Definitions & Key Facts

Characteristics of living things

Movement — an action by an organism or part of an organism causing a change of position or place.

Respiration — the chemical reactions in cells that break down nutrient molecules and release energy.

Sensitivity — the ability to detect and respond to changes in the environment.

Excretion — removal from organisms of toxic materials, the waste products of metabolism and substances in excess of requirements.

Movement in and out of cells

Diffusion — 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 their random movement.

Osmosis — the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

Active transport — the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration.

• Diffusion and osmosis are passive (no energy from the cell). Active transport needs energy and works against the gradient.
• A high surface area, a thin barrier and a steep gradient all make diffusion faster.

Enzymes and nutrition

Enzyme — a protein that functions as a biological catalyst.

Catalyst — a substance that speeds up a chemical reaction and is not changed by the reaction.

Photosynthesis — the process by which plants manufacture carbohydrates from raw materials using energy from light.

Digestion — the breakdown of large insoluble molecules into small soluble molecules.

Absorption — movement of digested food molecules through the wall of the intestine into the blood.

Assimilation — movement of digested food molecules into the cells of the body where they are used, becoming part of the cells.

Egestion — passing out of food that has not been digested or absorbed, as faeces, through the anus.

• Denaturing: high temperature or the wrong pH changes the shape of the active site, so the substrate no longer fits. This is permanent.
• Optimum: each enzyme works fastest at one temperature and one pH. Body enzymes work best at about 37 °C.

Transport in plants and humans

Transpiration — the loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells, followed by diffusion of water vapour through the stomata.

Translocation — the movement of sucrose and amino acids in the phloem, from sources to sinks.

Homeostasis — the maintenance of a constant internal environment.

Hormone — a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Reflex action — a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors.

• Humans have 46 chromosomes (23 pairs) in body cells and 23 in gametes. XX is female and XY is male, so each child has a 50% chance of being a boy.
• Mutation is a random change in a gene or chromosome. It can be caused by ionising radiation and some chemicals.
• Variation: continuous (a range, such as height) or discontinuous (distinct groups, such as blood group).
• Natural selection steps: variation exists → more offspring are born than survive → individuals with suited features survive and reproduce → they pass on the alleles.

Ecology and the environment

• Energy is lost between trophic levels as heat, movement and waste, so food chains are short and there is less biomass at each higher level.
• Eutrophication: fertiliser washes into water → algae grow quickly → they die and bacteria decompose them → oxygen in the water is used up → fish die.
• Deforestation reduces biodiversity, increases carbon dioxide in the air and can cause soil erosion and flooding.
• The greenhouse effect: carbon dioxide and methane trap heat, which causes global warming.