MSCE Biology key terms

A thin, flexible boundary around the cytoplasm made of a phospholipid bilayer with embedded proteins. It controls which substances enter and leave the cell and carries receptor molecules for signalling.

A specialised structure inside a cell, such as a mitochondrion, chloroplast or nucleus, that performs a particular job. Each organelle is often enclosed by its own membrane and works with others to keep the cell alive.

A rod-shaped organelle with a folded inner membrane where aerobic respiration takes place. It releases energy from glucose as ATP, so cells with high energy demands contain many of them.

A green organelle found in plant cells containing chlorophyll, stacked membranes and starch grains. It absorbs light energy and carries out photosynthesis, converting carbon dioxide and water into glucose and oxygen.

A protein that speeds up a specific chemical reaction by lowering its activation energy. Its active site binds particular substrate molecules, and the enzyme is unchanged after the reaction.

Loss of an enzyme's normal three-dimensional shape, usually caused by high temperature or extreme pH. The active site can no longer fit its substrate, so the reaction rate falls and the change is usually irreversible.

The net movement of particles from a region of higher concentration to a region of lower concentration, driven by their random motion. It needs no energy input and is faster over short distances and at higher temperatures.

The movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated one. It explains water uptake by root hairs and the effect of salt solutions on cells.

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

The process in which green plants use light energy to combine carbon dioxide and water into glucose, releasing oxygen. It occurs in chloroplasts and stores light energy as chemical energy in organic molecules.

The environmental condition in shortest supply that holds back a process such as photosynthesis or growth. Raising light intensity, carbon dioxide or temperature increases the rate only until another factor becomes limiting.

The breakdown of glucose in the presence of oxygen to release energy, producing carbon dioxide and water. It yields far more ATP per glucose molecule than anaerobic respiration and occurs mainly in mitochondria.

Release of energy from glucose without oxygen. In muscles it forms lactic acid, and in yeast it produces ethanol and carbon dioxide during fermentation, yielding much less energy per glucose molecule.

The extra oxygen needed after vigorous exercise to break down the lactic acid that built up in muscles. It explains why breathing and heart rate stay raised for a while after exercise stops.

A tiny air sac in the lungs where gaseous exchange occurs. Its large total surface area, thin walls, moist lining and dense blood supply allow rapid diffusion of oxygen in and carbon dioxide out.

The mechanical movement of air into and out of the lungs through changes in thorax volume and pressure. It maintains steep concentration gradients of oxygen and carbon dioxide across the alveolar surface.

Loss of water vapour from the leaves of a plant, mainly through stomata. It pulls water up the xylem and is affected by humidity, wind, temperature and light intensity.

The movement of dissolved sugars and other organic solutes through the phloem from sources such as leaves to sinks such as roots, fruits and growing tips, where they are used or stored.

A small pore in the leaf epidermis, bounded by two guard cells, that allows carbon dioxide to enter and water vapour and oxygen to leave. Guard cells open and close it in response to water loss and light.

The maintenance of a stable internal environment within narrow limits despite changes outside. Examples include control of body temperature, blood glucose concentration and water balance 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 temperature and blood glucose close to a set point.

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

Control of the water and dissolved salt content of body fluids. The kidneys adjust how much water is reabsorbed from filtrate, guided by hormones, so that cells neither swell nor shrink excessively.

The microscopic filtering unit of the kidney, consisting of a glomerulus, Bowman's capsule and a coiled tubule. It filters blood and reabsorbs useful substances to form urine.

The uptake of useful substances such as glucose, amino acids and some water from kidney tubule fluid back into the blood. It uses active transport and diffusion so that only waste leaves as urine.

Shrinking of the cytoplasm and vacuole away from the cell wall when a plant cell loses water in a concentrated solution. It causes wilting and shows that water has left by osmosis.

Cell division that halves the chromosome number and produces four genetically different gametes. Independent assortment and crossing over during meiosis create variation in the offspring.

Cell division producing two daughter cells genetically identical to the parent, used for growth, repair and asexual reproduction. Chromosomes replicate, line up and are pulled to opposite poles before the cell divides.

A sex cell such as a sperm, egg or pollen nucleus that carries half the normal chromosome number. Two gametes fuse at fertilisation to restore the full chromosome number in the zygote.

One of two or more alternative forms of the same gene, occupying the same position on a chromosome. Alleles may be dominant or recessive and determine which characteristic is expressed.

The combination of alleles an organism carries for a particular gene, written as letters such as AA, Aa or aa. It determines the potential characteristics, which may be modified by the environment.

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

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

A random change in the base sequence of DNA or in chromosome structure. Mutations create new alleles and are the ultimate source of genetic variation on which natural selection acts.

An inherited feature that improves an organism's chance of survival and reproduction in its habitat. Examples include streamlined bodies in fish and water-storing stems in desert plants.

A sequence showing the direction of energy transfer between organisms, starting with a producer and ending with a top consumer. Arrows point from the organism eaten to the one that eats it.

An organism, usually a green plant or alga, that makes its own organic food by photosynthesis. Producers form the base of every food chain and supply energy to all other trophic levels.

An organism such as a bacterium or fungus that breaks down dead organisms and waste, releasing nutrients back into the soil. Decomposers recycle minerals so producers can use them again.

Nutrient enrichment of water, often by fertiliser or sewage runoff, that causes algal blooms. Decay of the algae removes oxygen from the water, killing fish and other aerobic organisms.

A microorganism or virus that causes disease, such as a bacterium, fungus or protoctist. Pathogens may spread by air, water, food, contact or vectors, and are controlled by hygiene and immunity.

A protein produced by white blood cells that binds specifically to an antigen on a pathogen, helping to destroy it. Antibodies provide immunity and give protection against future infection by the same organism.

Introducing weakened, killed or harmless parts of a pathogen to stimulate an immune response and produce antibodies and memory cells. It gives protection against later infection without causing the disease.