Biology key terms

Topic 1 · The nature and variety of living organisms

What Makes Something Alive

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Respiration

The chemical process, occurring in all living cells, that releases energy from glucose. It is not breathing.

Excretion

The removal of the waste products of metabolism (for example carbon dioxide and urea) from the body.

Egestion

The removal of undigested food from the body as faeces. It is not excretion, because the material was never absorbed into the body's cells.

Exam-exact meanings.

Nutrition

taking in materials for energy and growth

Respiration

releasing energy from glucose in every cell

Excretion

removing the waste products of metabolism

Growth

a permanent increase in size

Movement

some part of the organism moves

Sensitivity

detecting and responding to the surroundings

Reproduction

making more organisms of the same kind

Control of internal conditions

keeping the inside steady as the outside changes

The Four Eukaryote Groups

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Saprotrophic nutrition

Nutrition in which digestive enzymes are secreted onto dead organic material outside the organism, and the soluble products of digestion are then absorbed.

Exam-exact meanings.

What they are

microscopic single-celled eukaryotes; the group for those that fit none of the other three

Amoeba

features like an animal cell; lives in pond water

Chlorella

has chloroplasts, so it is plant-LIKE — but single-celled, so not a plant

Plasmodium

the pathogen that causes malaria — the named pathogenic example

Bacteria, Viruses and Pathogens

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Pathogen

An organism that causes disease. Pathogens may be fungi, bacteria, protoctists or viruses.

Prokaryote

An organism whose cell has no nucleus. Its DNA is a single circular chromosome lying in the cytoplasm, with plasmids alongside, and it has no membrane-bound organelles.

Exam-exact meanings.

What an antibiotic attacks

Structures and processes a bacterium has and a human cell does not — chiefly the bacterial cell wall, and the bacterium's own protein-making machinery.

Why a virus is unaffected

A virus has no cell wall, no cytoplasm and no metabolism of its own, so it possesses none of those targets.

Why no antibiotic could be designed for it

A virus reproduces inside a host cell using the HOST'S machinery, so a drug stopping that process would have to damage the human cell too.

Who becomes resistant

Bacteria, not viruses. A virus was never affected in the first place, so there is nothing for it to become resistant to.

Pneumococcus

bacterium — causes pneumonia

Plasmodium

protoctist — causes malaria

Influenza virus

virus — causes flu

HIV

virus — causes AIDS

Topic 2 · Structures and functions in living organisms

Levels of Organisation

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Organ

A group of different tissues working together to perform a particular function. The number of DIFFERENT tissues is what makes it an organ — never its size or its importance.

Exam-exact meanings.

Organelle

a specialised structure inside a cell

Cell

the basic unit of a living organism

Tissue

a group of cells with similar structure and function working together

Organ

a group of different tissues working together to perform a function

System

a group of organs with related functions working together

Nucleus

contains the chromosomes and controls the cell's activities

Cytoplasm

site of most chemical reactions

Cell membrane

partially permeable; controls what enters and leaves

Mitochondria

site of aerobic respiration, releasing energy

Ribosomes

site of protein synthesis

Cell wall

cellulose; supports the cell and stops it bursting

Chloroplasts

contain chlorophyll; absorb light for photosynthesis

Permanent vacuole

contains cell sap; keeps the cell turgid

Many mitochondria

a high rate of aerobic respiration, so a high energy demand — muscle, sperm, kidney tubule, small-intestine lining

Many ribosomes

a high rate of protein synthesis, so the cell builds or secretes a lot of protein — glandular and enzyme-secreting cells

Many chloroplasts

a high rate of photosynthesis — palisade mesophyll near the upper leaf surface

A large permanent vacuole

the cell is kept turgid by cell sap, which is how non-woody plants stand up

THE LIMIT ON ALL OF THESE

an abundance gives you the SCALE of an activity, never its purpose. Three unrelated cell types are all packed with mitochondria.

Comparing Cells, and Stem Cells

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Differentiation

The process by which a cell becomes specialised for a particular function, developing a structure suited to that function.

Stem cell

An undifferentiated cell that can divide to produce more cells of the same type, and which can differentiate into other types of cell.

Carbohydrates, Proteins and Lipids

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Protein

A large molecule built from a chain of amino acids. Proteins contain carbon, hydrogen, oxygen and nitrogen, and it is the number and order of the amino acids that decides which protein it is.

Exam-exact meanings.

Starch and glycogen

built from simple sugars (glucose)

Protein

built from amino acids — about twenty different ones

Lipid

built from three fatty acids and one glycerol

Amino acid

the basic unit a protein is built from; about twenty different kinds exist

Simple sugar

the basic unit a carbohydrate is built from — glucose is the one to name

Fatty acid and glycerol

the units a lipid is built from: THREE fatty acids to ONE glycerol

Reducing sugar

a sugar that gives a positive Benedict's test — glucose is one; starch is not

Emulsion

the cloudy white suspension formed in the fat test when ethanol solution is poured into water

Enzymes: Catalysts With a Shape

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Enzyme

A biological catalyst: a protein that speeds up the rate of a reaction without being changed or used up itself.

Denaturation

A change in the shape of an enzyme's active site, so the substrate is no longer complementary and can no longer bind. Denaturation by heat is not reversed by cooling.

Exam-exact meanings.

Metabolic reactions

the chemical reactions taking place inside a living organism; 'metabolism' is all of them together

Active site

the region of the enzyme the substrate binds to

Substrate

the molecule the enzyme acts on

Complementary

the substrate shape fits the active site exactly

Enzyme–substrate complex

what exists while the substrate is bound

Specific

one enzyme acts on one substrate, because of the shape fit

Optimum

the temperature or pH at which the rate is maximum

Catalyst

speeds up a reaction and is unchanged and not used up by it, so one molecule works over and over

An enzyme is a protein

a chain of amino acids, folded into one particular three-dimensional shape

The sequence decides the fold

the order of the amino acids determines how the chain folds, and therefore the shape of the active site

So shape decides function

change the shape and the substrate no longer fits — which is exactly what heat and pH do

Amylase

starch → maltose (mouth, pancreas)

Maltase

maltose → glucose (small intestine)

Protease

protein → amino acids (stomach, pancreas)

Lipase

lipid → fatty acids + glycerol (pancreas)

Catalase

hydrogen peroxide → water + oxygen

Enzymes and pH

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Optimum pH

The pH at which an enzyme's rate of reaction is at its maximum. It is a property of that particular enzyme, and differs from one enzyme to another.

Exam-exact meanings.

Optimum pH

the pH at which that enzyme's rate is highest; different for each enzyme

Active site

the region of the enzyme the substrate binds to — a shape, made by the folding

Complementary

the substrate's shape fits the active site; this fit is reduced as the pH moves away from the optimum

Denatured

the active site has changed shape so much — at extreme pH or high temperature — that the substrate can no longer bind; this is not reversed by restoring the conditions

Buffer

a solution that holds the pH steady, so the recorded pH is the pH the reaction ran at

Diffusion, Osmosis and Active Transport

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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 concentration (a dilute solution) to a region of lower water concentration (a concentrated solution), through a partially permeable membrane.

Active transport

The movement of particles against a concentration gradient, from a lower to a higher concentration, using carrier proteins in the cell membrane and energy released by respiration.

Exam-exact meanings.

…net movement…

particles cross both ways; 'net' names the difference between the two flows, which is why movement does not stop at equilibrium

…higher to lower water concentration…

a dilute solution holds more water per unit volume than a concentrated one, so 'dilute → concentrated' is the same statement in other words

…partially permeable…

the membrane lets water through and holds the dissolved solute back; this is the specification's word, so use it rather than 'semi-permeable'

Concentration gradient

A steeper gradient means more particles on the high-concentration side, so more cross per second than cross back; the net rate is the difference between the two flows.

Surface area

More membrane area means more particles can cross at the same time, so roughly doubling the area doubles the net rate.

Diffusion distance

A thinner barrier means each particle completes the crossing sooner, so more cross per second. Exchange surfaces are one cell thick for exactly this reason.

Temperature

Higher temperature gives particles more kinetic energy, so their mean speed rises and they reach and cross the surface more often; the net rate increases.

Surface area to volume ratio

Volume rises faster than surface area, so a larger organism has less surface per unit of volume to supply it — and diffusion alone becomes too slow.

Combining them — NOT an Edexcel equation

For DIFFUSION only: rate ∝ (surface area × concentration difference) ÷ distance. A way to think, never a formula to quote, and it says nothing about active transport.

Alveoli (lungs)

millions of them → very large surface area; wall one cell thick → short diffusion distance; dense capillaries and ventilation → steep oxygen and carbon dioxide gradients maintained

Villi and microvilli (small intestine)

folds and projections → large surface area; epithelium one cell thick → short distance; dense blood supply carries absorbed food away → gradient stays steep

Root hair cell

long narrow projection → large surface area for absorbing water by osmosis and mineral ions by active transport; many mitochondria supply the energy

Photosynthesis

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Photosynthesis

The process by which plants use light energy, absorbed by chlorophyll, to convert carbon dioxide and water into glucose. Light energy is transferred to chemical energy stored in the glucose, and oxygen is released as a by-product.

Exam-exact meanings.

Carbon dioxide

diffuses into the leaf from the air through the stomata, then through the air spaces to the mesophyll cells

Water

absorbed from the soil by root hair cells and carried up to the leaf in the xylem

Light

absorbed by chlorophyll in the chloroplasts; an energy source, not a raw material, so none of it appears in the glucose

Glucose

the product; the light energy is now chemical energy stored in its bonds

Oxygen

the by-product; most diffuses out through the stomata

Magnesium ions

needed to make chlorophyll. Without them the plant cannot make chlorophyll, so the leaves turn yellow and the rate of photosynthesis falls.

Nitrate ions

combined with glucose to make amino acids, which are built into proteins. Without them growth is stunted and the older leaves may yellow.

What Limits Photosynthesis

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Limiting factor

The factor that is in shortest supply at a given moment, and which therefore determines the rate of photosynthesis. Increasing any other factor has no effect until the limiting factor itself is increased.

Exam-exact meanings.

Light intensity

raises the rate until light is no longer the factor in shortest supply; the graph then levels off at a plateau

Carbon dioxide concentration

raises the rate until carbon dioxide is no longer in shortest supply; the graph then levels off at a plateau

Temperature

raises the rate to an optimum, after which the rate FALLS as the enzymes controlling photosynthesis denature

Distance and intensity — a way to think, NOT an Edexcel equation

intensity is proportional to 1 ÷ distance², so halving the lamp distance gives four times the intensity rather than twice

The Leaf: Structure and Gas Exchange

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Stoma (plural: stomata)

A pore in the epidermis of a leaf, usually on the lower surface, surrounded by two guard cells, through which gases diffuse into and out of the leaf.

Exam-exact meanings.

Waxy cuticle

a waterproof layer that reduces evaporation of water from the upper surface

Upper epidermis

thin and transparent, so light passes through it to the cells below

Palisade mesophyll

tall, closely packed cells holding most of the chloroplasts, placed at the top to absorb the most light

Spongy mesophyll

loosely packed cells with large air spaces, giving a large moist surface area and letting gases diffuse throughout the leaf

Lower epidermis

contains the stomata and their guard cells, where gases enter and leave

Vascular bundle (vein)

xylem brings water to the leaf; phloem carries the sugars made there away to the rest of the plant

A Balanced Diet

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Balanced diet

A diet containing all the required nutrients — carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre — in the correct proportions, and supplying an amount of energy that matches the energy the body uses.

Exam-exact meanings.

Carbohydrate

bread, rice, pasta, potatoes. Respired to release energy; the body's main energy source.

Protein

meat, fish, eggs, beans, dairy. Supplies amino acids for growth and repair of tissues; deficiency stunts growth and slows healing.

Lipid (fat and oil)

butter, oils, nuts, oily fish. An energy store, insulation, and part of every cell membrane.

Water

drinks and most foods. The solvent for all reactions in cells, and the transport medium of the blood.

Dietary fibre

wholegrains, vegetables, fruit skins. Not digested or absorbed; provides the bulk the gut muscles grip during peristalsis.

Minerals

needed in small amounts and built into structures — see the mineral and vitamin lists below.

Vitamin A

liver, fish oils, carrots. Needed to make the pigment that allows vision in dim light; deficiency causes night blindness.

Vitamin C

citrus fruit, blackcurrants, peppers. Needed to make collagen, which holds tissues together; deficiency causes scurvy.

Vitamin D

oily fish, eggs, and made in the skin in sunlight. Needed to absorb calcium; deficiency causes rickets.

Calcium

milk, cheese, green vegetables. Builds bones and teeth and is needed for blood clotting; deficiency weakens bone.

Iron

red meat, liver, dark green vegetables. Built into haemoglobin to carry oxygen; deficiency causes anaemia.

The Alimentary Canal

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Peristalsis

Waves of muscular contraction that move food along the alimentary canal, produced by circular and longitudinal muscles in the gut wall contracting and relaxing alternately as an antagonistic pair.

Exam-exact meanings.

Ingestion

taking food into the body through the mouth

Digestion

breaking large insoluble molecules into small soluble ones, physically and then chemically by enzymes

Absorption

the movement of small soluble molecules through the gut wall into the blood

Assimilation

the use of absorbed molecules by the body's cells to build tissue or release energy

Egestion

removal of undigested food as faeces; it was never absorbed, so it is NOT excretion

Mouth

teeth break food up physically, increasing surface area; saliva adds amylase and lubricates the bolus

Oesophagus

carries the bolus from mouth to stomach by peristalsis; no digestion occurs here

Stomach

churns food and secretes hydrochloric acid and protease; holds the meal for several hours

Small intestine

receives bile and pancreatic juice, completes digestion, and absorbs the soluble products into the blood

Large intestine

absorbs water from the remaining material back into the blood

Rectum and anus

store the faeces and egest them

Digestive Enzymes and Bile

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Emulsification

The breaking up of large lipid droplets into many smaller droplets by bile. It is a physical change that increases the surface area available to lipase; no chemical bonds are broken.

Exam-exact meanings.

Amylase

starch → maltose. Made in the salivary glands and the pancreas; acts in the mouth and small intestine.

Maltase

maltose → glucose. Made in the small intestine, where it completes the digestion of starch.

Protease

protein → amino acids. Made in the stomach, the pancreas and the small intestine.

Lipase

lipid → fatty acids and glycerol. Made in the pancreas and the small intestine.

Absorption and the Energy in Food

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Villus (plural: villi)

A finger-like projection of the lining of the small intestine, one cell thick, containing a blood capillary and a lacteal, which greatly increases the surface area for absorption.

Exam-exact meanings.

Villi and microvilli

increase the surface area enormously, so more molecules can be absorbed at the same time

Wall one cell thick

gives a very short diffusion distance between the gut contents and the blood

Dense capillary network

carries absorbed glucose and amino acids away, keeping the concentration gradient steep

Lacteal

absorbs the fatty acids and glycerol, which are not water-soluble and so cannot enter the blood directly

Many mitochondria

release the energy needed for active transport of the last glucose against its gradient

Respiration and ATP

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Respiration

The chemical process, taking place in all living cells, in which glucose is broken down and the energy released is used to produce ATP. It is not the same as breathing.

ATP

The molecule that acts as the immediate energy supply in cells. Energy released by respiration is used to make it, and it is broken down to release that energy wherever a cell needs it.

Where aerobic respiration happens

Aerobic respiration takes place in the MITOCHONDRIA of the cell. A cell with an unusually large ATP bill — one doing a great deal of active transport, or dividing rapidly — contains unusually large numbers of them, so the count of mitochondria is evidence of how much ATP that cell has to produce.

Anaerobic Respiration

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Anaerobic respiration

The release of energy from the incomplete breakdown of glucose in the absence of oxygen. It releases much less energy per glucose molecule than aerobic respiration.

Oxygen debt

The extra oxygen taken in after exercise, used to break down the lactic acid that built up in the muscles while anaerobic respiration was taking place.

The Thorax and Ventilation

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Ventilation

The movement of air into and out of the lungs, produced by movements of the ribs, intercostal muscles and diaphragm that change the volume, and therefore the pressure, inside the thorax.

Exam-exact meanings.

Trachea

carries air from the throat to the bronchi; held open by rings of cartilage so it cannot collapse

Bronchi

two tubes, one to each lung, dividing from the trachea

Bronchioles

the fine branching tubes that deliver air to the alveoli

Alveoli

the tiny air sacs where gas exchange actually happens; nowhere else in the lung exchanges gases

Ribs

form a protective cage and are moved by the intercostal muscles to change the volume of the thorax

Intercostal muscles

between the ribs; the external ones contract to raise the ribcage for breathing in

Diaphragm

a sheet of muscle forming the floor of the thorax; contracts and flattens to increase the volume

Pleural membranes

two sheets with fluid between them, reducing friction and holding the lung against the chest wall

Alveoli, Exercise and Smoking

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Gas exchange

The diffusion of oxygen from the alveolar air into the blood, and of carbon dioxide from the blood into the alveolar air, each moving down its own concentration gradient.

Exam-exact meanings.

Hundreds of millions of alveoli

an enormous total surface area, so many molecules can diffuse at the same time

Wall one cell thick

a very short diffusion distance between air and blood

Dense capillary network

constantly removes oxygen and delivers carbon dioxide, keeping the concentration gradients steep

Ventilation

constantly replaces the alveolar air, maintaining the gradients from the air side

Moist lining

gases must dissolve before they can diffuse across the cell membrane

Tar

paralyses the cilia so mucus collects, causing smoker's cough and bronchitis; also contains carcinogens that cause lung cancer

Carbon monoxide

binds strongly to haemoglobin, so the blood carries less oxygen; the heart works harder and a fetus receives less oxygen

Nicotine

the addictive component; raises heart rate and blood pressure and narrows blood vessels

Smoke particles generally

damage and break down alveolar walls, causing emphysema and a permanent loss of surface area

Why Transport Systems Exist, and the Two Plant Vessels

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Xylem

Dead, hollow, lignified tubes that transport water and dissolved mineral ions in one direction only, from the roots up to the leaves.

Phloem

Living cells joined end to end by perforated sieve plates, which transport dissolved sucrose and amino acids in both directions, from sources to sinks.

Water Uptake and Transpiration

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Transpiration

The evaporation of water from the surfaces of the mesophyll cells inside a leaf, followed by its diffusion out through the stomata as water vapour.

Exam-exact meanings.

Temperature ↑ → rate ↑

molecules gain kinetic energy, so water evaporates more readily and the vapour diffuses away faster

Wind speed ↑ → rate ↑

moving air removes the humid layer outside the stomata, keeping the water-vapour gradient steep

Humidity ↑ → rate ↓

the air outside is already near saturation, so the gradient out of the leaf is shallow

Light intensity ↑ → rate ↑

light causes the stomata to open, so there is a larger route for water vapour to escape; the effect is indirect

Blood

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Plasma

The pale yellow liquid part of blood, mostly water, in which the blood cells are suspended and in which dissolved substances and heat are transported.

Haemoglobin

The red, iron-containing protein inside red blood cells that binds oxygen where its concentration is high and releases it where its concentration is low.

Exam-exact meanings.

Contains haemoglobin

binds oxygen in the lungs and releases it in respiring tissues, allowing far more oxygen to be carried than could dissolve in plasma

No nucleus

leaves more room for haemoglobin, so each cell carries more oxygen; the cost is a lifespan of about four months and no ability to divide

Biconcave disc shape

a larger surface area to volume ratio than a sphere, so oxygen diffuses in and out faster; also flexible enough to pass through narrow capillaries

Immunity and Clotting

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Antigen

A molecule, usually on the surface of a pathogen, that the body recognises as foreign and that triggers an immune response.

Antibody

A protein produced by a lymphocyte with a shape complementary to one specific antigen, which binds to that antigen and marks the pathogen for destruction.

Vaccination

Introducing dead or weakened pathogens, or their antigens, so that lymphocytes produce antibodies and memory cells without the person suffering the disease.

The Heart, and What Goes Wrong With It

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Double circulation

A circulatory system in which blood passes through the heart twice on each complete circuit of the body — once to be pumped to the lungs, and once to be pumped to the rest of the body.

Exam-exact meanings.

Thick left ventricle wall

generates the high pressure needed to pump blood all round the body

Thinner right ventricle wall

only has to pump blood as far as the lungs, so less pressure is needed

Thin atrium walls

only push blood down into the ventricles below, a very short distance

Atrioventricular valves

prevent backflow from the ventricles into the atria when the ventricles contract

Semilunar valves

prevent backflow from the arteries into the ventricles when the ventricles relax

Coronary arteries

supply the heart muscle itself with oxygen and glucose; the heart cannot use the blood inside its chambers

Blood Vessels and the Circulation

Exam-exact meanings.

Aorta

carries oxygenated blood from the left ventricle to the whole body

Vena cava

returns deoxygenated blood from the body to the right atrium

Pulmonary artery

carries DEOXYGENATED blood from the right ventricle to the lungs — an exception

Pulmonary vein

returns OXYGENATED blood from the lungs to the left atrium — the other exception

Renal artery and renal vein

carry blood to and from the kidneys

Hepatic artery and hepatic vein

carry blood to and from the liver

Hepatic portal vein

carries blood from the small intestine directly to the liver, so absorbed food is processed first

Excretion: Getting Rid of What You Made

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Excretion

The removal from the body of the waste products of metabolism — substances made by the body's own chemical reactions — together with substances in excess of requirements.

Exam-exact meanings.

Lungs

carbon dioxide from respiration, and water vapour

Kidneys

urea, excess water and excess mineral ions, removed together as urine

Skin

water, mineral ions and a little urea, in sweat — though sweating exists to cool the body, not to excrete

Plants (through the stomata)

carbon dioxide from respiration, and oxygen produced in excess by photosynthesis

The Kidney and the Nephron

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Osmoregulation

The control of the water and dissolved ion content of the body fluids, keeping their concentration constant regardless of how much is taken in or lost.

Exam-exact meanings.

Renal artery

brings blood to the kidney at high pressure, direct from the aorta

Kidney

filters the blood and reclaims what is needed, producing urine

Renal vein

carries the filtered blood away, now containing far less urea

Ureters

two tubes, one from each kidney, carrying urine down to the bladder

Bladder

stores urine until it is released

Urethra

the single tube carrying urine out of the body

Glomerulus

a knot of capillaries where the blood is filtered, driven by high blood pressure

Bowman's capsule

the cup surrounding the glomerulus that collects the filtrate

Proximal convoluted tubule

the first coiled section, where most reabsorption takes place

Loop of Henlé

a hairpin dipping into the kidney's centre, central to concentrating the urine

Distal convoluted tubule

where fine adjustments to the ion content are made

Collecting duct

where the amount of water reabsorbed is controlled, settling the concentration of the urine

Ultrafiltration and Selective Reabsorption

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Ultrafiltration

Filtration at a molecular scale, in which high blood pressure in the glomerulus forces small molecules out through the basement membrane into Bowman's capsule, while blood cells and plasma proteins are too large to pass.

Selective reabsorption

The return to the blood of useful substances from the filtrate, notably all of the glucose, which is reabsorbed at the proximal convoluted tubule by active transport using carrier proteins and energy from respiration.

ADH and the Water Balance of the Blood

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ADH (antidiuretic hormone)

A hormone released by the pituitary gland which increases the permeability of the collecting duct walls to water, so that more water is reabsorbed into the blood.

Negative feedback

A control mechanism in which a change from the normal level triggers a response that reverses that change, returning the system towards its normal level.

Homeostasis and the Control of Body Temperature

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Homeostasis

The maintenance of a constant internal environment, within narrow limits, despite changes in the external environment and in the organism's own activity.

Negative feedback

A control mechanism in which a change away from the normal level triggers a response that reverses that change and returns the condition towards normal.

Exam-exact meanings.

Stimulus

A detectable change in the internal or external environment — a rise in temperature, a fall in light intensity, a touch.

Receptor

A cell or organ that detects a stimulus. Temperature receptors in the skin and hypothalamus; light receptors in the retina.

Co-ordinator

The part that receives the information and determines the response — the brain or spinal cord. For temperature, the hypothalamus.

Effector

The part that carries out the response. ONLY a muscle (which contracts) or a gland (which secretes). Naming anything else loses the mark.

Temperature (about 37 °C)

Enzymes work fastest near 37 °C; much colder and reactions are too slow, much hotter and enzymes are denatured and cannot be recovered.

Water content of the blood

Cells lose or gain water by osmosis if the blood's concentration changes, and either shrivels or bursts them.

Blood glucose concentration

Glucose is the respiratory substrate; too little starves the cells of energy, and too much draws water out of them by osmosis.

pH of the blood

Enzymes have an optimum pH and are denatured outside a narrow range around it.

Carbon dioxide concentration

Dissolved carbon dioxide makes the blood more acidic, so removing it is part of holding the pH steady.

Plant Responses, Tropisms and Auxin

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Tropism

A growth response of a plant in which the direction of growth is determined by the direction of the stimulus. Positive means growth towards the stimulus, negative means growth away from it.

Auxin

A plant hormone produced in the tip of a shoot which stimulates the elongation of cells in the region behind the tip.

Exam-exact meanings.

Phototropism

A growth response in which the direction of growth is determined by the direction of light.

Geotropism

A growth response in which the direction of growth is determined by gravity.

Positive tropism

Growth TOWARDS the stimulus. It does not mean 'a good response'.

Negative tropism

Growth AWAY FROM the stimulus.

Coleoptile

The pale protective sheath around the first shoot of a grass seedling — the standard subject of tropism experiments because it is straight, fast-growing and easy to cut.

Elongation

An existing cell becoming longer by taking in water, not the production of new cells. This is what auxin causes in a shoot.

The Nervous System, and the Two Ways a Body Sends a Message

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Central nervous system (CNS)

The brain and the spinal cord. It receives information from receptors, processes it, and sends impulses out to effectors — it does not include the nerves themselves.

Receptor

A cell or group of cells that detects a stimulus and converts it into an electrical impulse. Receptors are grouped together in sense organs.

Exam-exact meanings.

Sensory neurone

Carries impulses FROM a receptor TO the central nervous system.

Relay neurone

Lies entirely within the central nervous system and connects a sensory neurone to a motor neurone.

Motor neurone

Carries impulses FROM the central nervous system TO an effector — a muscle or a gland.

Eye

Light — its receptors are in the retina.

Ear

Sound, and the position and movement of the head.

Nose

Chemicals in the air.

Tongue

Chemicals in food and drink.

Skin

Touch, pressure, temperature and pain — four kinds of receptor in one organ.

Synapses and the Reflex Arc

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Synapse

The junction between two neurones, where a small gap separates the ending of one from the membrane of the next.

Neurotransmitter

A chemical released from vesicles at the ending of a neurone, which diffuses across the synapse and binds to receptor molecules on the next neurone, generating a new impulse in it.

Reflex action

A rapid, automatic response to a stimulus that does not involve conscious control by the brain, usually protective in function.

Exam-exact meanings.

Rapid

The impulse takes the shortest available route, through the spinal cord rather than via the brain.

Automatic / involuntary

It happens without any decision, and cannot be prevented by deciding not to.

Protective

Most reflexes prevent damage — withdrawal from heat, blinking, the pupil reflex in bright light.

The same every time

The pathway is fixed, so the same stimulus produces the same response in everyone.

The brain is informed, not consulted

An impulse does travel to the brain, which is why you become aware of it — but the response has already happened.

The Eye — Focusing, and the Pupil Reflex

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Accommodation

The change in the shape of the lens that allows the eye to focus light from objects at different distances onto the retina.

Refraction

The change in direction of light as it passes from one transparent medium into another of different density — in the eye, at the cornea and at the lens.

Exam-exact meanings.

Cornea

Transparent front layer; refracts light, and does most of the refraction. Its shape cannot change.

Iris

Ring of muscle containing circular and radial muscles; alters the size of the pupil and so controls how much light enters.

Pupil

The hole in the centre of the iris through which light passes. It is a gap, not a structure.

Lens

Transparent and elastic; changes shape to make the fine adjustment to focusing.

Ciliary muscles

A ring of muscle around the lens; contracting makes the ring smaller and slackens the suspensory ligaments.

Suspensory ligaments

Join the ciliary muscles to the edge of the lens; when tight they pull the lens thin.

Retina

Contains the light receptors, which convert light into electrical impulses.

Optic nerve

Carries the impulses from the retina to the brain.

The Hormones — Sources, Roles and Effects

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Hormone

A chemical messenger produced by a gland and released into the blood, which is carried to its target organs and alters their activity.

Target organ

An organ whose cells carry receptors for a particular hormone, and which therefore responds to it while other organs do not.

Exam-exact meanings.

Adrenaline · adrenal glands

Prepares the body for action: raises heart rate and breathing rate, causes the liver to release glucose from glycogen, and diverts blood to the muscles.

Insulin · pancreas

Lowers blood glucose concentration: causes cells, especially in the liver, to take up glucose from the blood and store it as glycogen.

Testosterone · testes

Causes the male secondary sexual characteristics to develop at puberty, and is required for sperm production.

Oestrogen · ovaries

Causes the female secondary sexual characteristics to develop at puberty, and causes the lining of the uterus to thicken and repair.

Progesterone · ovaries

Maintains the thickened lining of the uterus after ovulation; when its level falls the lining breaks down.

ADH · pituitary gland

Increases the permeability of the kidney's collecting ducts to water, so more water is reabsorbed into the blood and less urine is produced.

FSH · pituitary gland

Causes an egg to mature in the ovary, and stimulates the ovary to produce oestrogen.

LH · pituitary gland

Causes ovulation — the release of the mature egg from the ovary.

Gas Exchange in Plants, Day and Night

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Net gas exchange

The overall movement of a gas between a leaf and the air — the difference between the amount used by one process and the amount produced by the other, not the rate of either process on its own.

Exam-exact meanings.

Respiration

In every living cell, day and night, in light and darkness. Uses OXYGEN, produces CARBON DIOXIDE.

Photosynthesis

Only in cells with chloroplasts, and only in light. Uses CARBON DIOXIDE, produces OXYGEN.

The point where they balance

The light intensity at which the rate of photosynthesis exactly equals the rate of respiration, so there is no net exchange of either gas with the air.

Topic 3 · Reproduction and inheritance

Sexual and Asexual Reproduction

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Sexual reproduction

Reproduction involving the fusion of the nuclei of two gametes, one from each of two parents, producing offspring that are genetically different from the parents and from each other.

Asexual reproduction

Reproduction involving only one parent and no fusion of gametes, producing offspring that are genetically identical to the parent.

Fertilisation

The fusion of the nucleus of a male gamete with the nucleus of a female gamete to produce a zygote.

Exam-exact meanings.

Gamete

A sex cell, containing half the normal number of chromosomes. In animals, the sperm and the egg cell; in flowering plants, the pollen nucleus and the ovule.

Fertilisation

The fusion of a male gamete nucleus with a female gamete nucleus.

Zygote

The single cell produced by fertilisation, with the full number of chromosomes — half from each parent.

Embryo

The ball of cells formed when the zygote divides repeatedly, which develops into the new organism.

Clone

A group of genetically identical organisms produced from one parent by asexual reproduction.

Resistance

The ability of some individuals to survive a disease or condition that kills others. It is inherited, and must already be present in the population — it is not produced in response to the threat.

Flowers, Pollination and Fertilisation

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Pollination

The transfer of pollen from an anther to a stigma. It is the delivery of the male gamete to the female part of a flower — not the fusion of the gametes.

Exam-exact meanings.

Anther

Produces pollen grains, which contain the male gametes.

Filament

The stalk that holds the anther in position. Anther + filament = the stamen.

Stigma

Receives pollen grains. Sticky in insect-pollinated flowers, feathery in wind-pollinated ones.

Style

Connects the stigma to the ovary; the pollen tube grows down through it.

Ovary

Contains the ovules. After fertilisation it becomes the fruit.

Ovule

Contains the female gamete. After fertilisation it becomes the seed.

Petals

Large and brightly coloured to attract insects; small, green or absent where the wind does the work.

Sepals

Protected the flower while it was in bud.

Seeds and Germination

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Germination

The beginning of growth of a seed, in which the embryo uses the seed's food store to develop into a seedling, until photosynthesis can begin.

The Human Reproductive Systems

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Secondary sexual characteristic

A physical feature that develops at puberty under the influence of testosterone or oestrogen, and which is not directly involved in producing or delivering gametes.

Exam-exact meanings.

1 · Released into the blood

A gland releases the hormone into the bloodstream, which is its only means of transport. The blood carries it to every organ in the body; it is not aimed at anything.

2 · Only target organs respond

Only cells able to respond to that particular hormone are affected. Every other cell ignores it, however much of it arrives.

3 · The effect lasts while the concentration does

The response continues for as long as the hormone's concentration in the blood is maintained, which is why hormonal changes are slow and long-lasting.

Negative feedback

Once the effect has been achieved, the response reduces further release of the hormone.

Testes

Produce sperm and the hormone testosterone. Held outside the body in the scrotum, because sperm production needs a temperature slightly below body temperature.

Scrotum

The sac that holds the testes outside the body cavity, keeping them cooler than 37 °C.

Sperm duct

Carries sperm from the testis to the urethra.

Prostate and other glands

Add fluid containing nutrients to the sperm, forming semen — the nutrients supply energy for respiration in the sperm.

Urethra

Carries semen out of the body through the penis. It also carries urine, but not at the same time.

Penis

Delivers semen into the vagina, so sperm are released close to the female gametes.

Ovaries

Contain the ova and release one about every month; also produce oestrogen and progesterone.

Oviduct (fallopian tube)

Lined with cilia that sweep the egg towards the uterus. Fertilisation normally occurs here.

Uterus

Muscular organ with a thick lining in which the embryo implants and develops; its wall contracts during birth.

Cervix

The ring of muscle at the base of the uterus; holds the fetus in place and dilates during birth.

Vagina

Receives semen, and is the passage through which the baby is born.

The Menstrual Cycle

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Menstrual cycle

The approximately monthly cycle in which an egg matures and is released from an ovary, and the lining of the uterus is prepared to receive an embryo and then either maintained or shed.

Ovulation

The release of a mature egg from the ovary into the oviduct, caused by a surge of LH at about day 14 of the cycle.

The Placenta and the Amniotic Fluid

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Placenta

The organ that develops in the uterus during pregnancy, in which the mother's blood and the fetus's blood come close enough for substances to be exchanged by diffusion without the two bloods mixing.

Amniotic fluid

The liquid contained within the amnion, which surrounds and protects the developing fetus.

Exam-exact meanings.

Large surface area

Finger-like villi push into the uterus wall, so many substances can cross at the same time.

Short diffusion distance

The barrier between the two circulations is very thin, so each substance completes the crossing quickly.

Steep concentration gradients

Both circulations keep flowing, so substances are continually delivered and removed and the gradients are never used up.

The extra constraint

The two bloodstreams must NOT mix — the mother's blood pressure would damage the fetus's vessels, and their blood groups may be incompatible.

DNA, Genes and Chromosomes

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Gene

A section of DNA that codes for a particular protein.

Genome

The entire DNA of an organism — all of its chromosomes and all of the genes they carry.

Chromosome

A structure found in the nucleus, made of a long coiled molecule of DNA, along which genes are located at fixed positions.

Exam-exact meanings.

Genome

The entire DNA of an organism.

Chromosome

One long coiled DNA molecule in the nucleus. Humans have 46 in a body cell, in 23 pairs.

Gene

A section of a chromosome's DNA coding for one protein, at a fixed position.

DNA

The molecule itself: a double helix of two strands held together by paired bases.

Base

One of the four chemical letters A, T, C and G. Their ORDER carries the information.

Protein Synthesis

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Transcription

The process, occurring in the nucleus, in which one strand of the DNA of a gene is used as a template to make a complementary strand of messenger RNA.

Translation

The process, occurring at a ribosome, in which the sequence of codons on the mRNA is used to assemble amino acids in the correct order to make a protein.

Exam-exact meanings.

mRNA (messenger RNA)

A single-stranded copy of one gene, made in the nucleus, which carries the code out to a ribosome.

Ribosome

The structure in the cytoplasm that reads the mRNA three bases at a time and joins amino acids together.

tRNA (transfer RNA)

Carries one specific amino acid, and has an anticodon that pairs with the matching codon on the mRNA.

Codon

A triplet of three bases ON THE mRNA, coding for one amino acid.

Anticodon

A triplet of three bases ON THE tRNA, complementary to the codon it binds to.

Alleles, Genotype and Phenotype

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Allele

One of the alternative forms of a gene, occupying the same position on a chromosome and giving rise to inherited differences.

Codominance

Where neither allele is dominant over the other, so that a heterozygous individual shows both characteristics in its phenotype.

Exam-exact meanings.

Genotype

The alleles an organism has for a particular gene, such as RR, Rr or rr.

Phenotype

The observable characteristics of an organism, resulting from its genotype and its environment.

Dominant

An allele that is expressed in the phenotype whenever it is present, even in a heterozygote. Written as a capital letter.

Recessive

An allele that is only expressed in the phenotype when both alleles present are recessive. Written as a lower-case letter.

Homozygous

Having two identical alleles for a gene — RR or rr.

Heterozygous

Having two different alleles for a gene — Rr.

Monohybrid Inheritance and Probability

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Monohybrid inheritance

The inheritance of a single characteristic controlled by one gene with two alleles.

Polygenic inheritance

Inheritance in which a characteristic is controlled by several genes acting together, usually producing a continuous range of phenotypes rather than distinct categories.

Pedigrees and Sex Determination

Exam-exact meanings.

Circle

A female.

Square

A male.

Shaded symbol

An individual affected by the condition — showing the phenotype in question.

Unshaded symbol

An unaffected individual. This does NOT mean their genotype is known.

Horizontal line between two symbols

A couple who have had children together.

Vertical line leading down

Leads to their offspring, who are shown side by side on the generation below.

Mitosis and Meiosis

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Mitosis

A type of cell division in which one cell divides once to produce two cells, each diploid and genetically identical to the parent cell and to each other.

Meiosis

A type of cell division in which one cell divides twice to produce four haploid gametes, each genetically different from the others and from the parent cell.

Exam-exact meanings.

Diploid

Having the full number of chromosomes, arranged in pairs. In humans this is 46 — 23 pairs. All body cells are diploid.

Haploid

Having a single set of chromosomes, one of each pair. In humans this is 23. Gametes are haploid.

Human diploid number

46.

Human haploid number

23.

Why gametes must be haploid

So that fertilisation restores the diploid number rather than doubling it every generation.

Variation and Mutation

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Mutation

A rare, random change in the genetic material — in the sequence of bases in DNA, or in the number or structure of chromosomes — which can be inherited if it occurs in a cell that gives rise to gametes.

Exam-exact meanings.

Meiosis

The pairs of chromosomes separate independently, so each gamete gets a different combination. Rearranges existing alleles; creates nothing new.

Random fertilisation

Any sperm may fertilise any egg, so the zygote's combination is a matter of chance. Rearranges existing alleles; creates nothing new.

Mutation

A random change in the genetic material itself. The ONLY source of new alleles, and therefore the ultimate origin of all genetic variation.

Rare

Mutations at any particular gene occur infrequently, which is why populations are stable between generations.

Random

They are not directed towards a need. An organism does not mutate because a characteristic would be useful.

Heritable

Passed on only if the mutation occurs in a cell that produces gametes. A mutation in a body cell is not inherited.

How Mutations Affect Proteins

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Mutagen

An agent that increases the rate at which mutations occur — such as ionising radiation or a chemical mutagen. It raises the probability of a mutation but does not determine which mutation occurs.

Carcinogen

A mutagen that causes mutations leading to uncontrolled cell division, and so increases the risk of cancer.

Exam-exact meanings.

Most DNA is not a gene

A change in a non-coding stretch usually alters no protein at all.

The code has spares

There are 64 codons for 20 amino acids, so several codons specify the same amino acid — a change between them alters nothing.

Position matters

An amino acid changed far from an enzyme's active site may leave the working shape unchanged, or affect it only slightly.

The other allele

In a diploid organism, a recessive mutated allele is masked by a working dominant allele on the other chromosome.

The rare case

A mutation in a critical position of a gene for an important protein can change the phenotype significantly, which is why the effects range from none to severe.

Natural Selection and Antibiotic Resistance

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Natural selection

The process by which individuals with characteristics best suited to their environment are more likely to survive and reproduce, so that the alleles for those characteristics become more common in the population over generations.

Evolution

The change in the inherited characteristics of a population over successive generations, as a result of natural selection acting on genetic variation.

Exam-exact meanings.

Variation

Name the characteristic that varies, and say the variation is genetic and arose by mutation.

Competition

Name what the individuals are competing FOR — food, light, territory, mates — or what is killing them.

Survival

Say which individuals are MORE LIKELY to survive and reproduce, and why that characteristic helps in these conditions.

Inheritance

State that the survivors pass the alleles to their offspring.

Over generations

State that the proportion of the population with the characteristic increases. Populations change; individuals do not.

Topic 4 · Ecology and the environment

Populations, Ecosystems and Sampling with Quadrats

Exam-exact meanings.

Population

All the organisms of ONE species living in a habitat at a given time.

Community

All the populations of ALL the species living in a habitat.

Habitat

The place where an organism lives.

Ecosystem

A community of organisms together with the non-living components of their environment, and the interactions between them.

Interdependence

Species in a community depend on each other for food, shelter, pollination and seed dispersal, so a change in one population can affect the others.

The distinction that is tested

A community is living organisms only; an ecosystem also includes the non-living conditions — soil, water, light, temperature.

Biodiversity, Distribution and Limiting Factors

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Biodiversity

The variety of different species present in an area.

Exam-exact meanings.

What is counted

The number of different SPECIES found across all the quadrats — not the number of individuals.

How the quadrats are placed

At random, using coordinates from random numbers, and the same number and size in each area compared.

Why high biodiversity matters

A disease, drought or new predator is unlikely to affect every species alike, so the community is more likely to survive a change.

The limitation of the measure

It treats a species represented by one straggling plant as equal to one covering half the area.

Food Chains, Webs and Pyramids

Exam-exact meanings.

Producer

A green plant or alga that makes its own food by photosynthesis. Every food chain starts with one.

Primary consumer

An organism that eats producers — a herbivore.

Secondary consumer

An organism that eats primary consumers — a carnivore.

Tertiary consumer

An organism that eats secondary consumers.

Decomposer

A bacterium or fungus that feeds on dead organisms and waste, releasing mineral ions back into the soil.

Trophic level

The position an organism occupies in a food chain. The same species may occupy different levels in different chains.

Its predators

Lose one of their food sources. Their population may fall — but say whether they have an alternative food in the web before predicting they die out.

Its food

Is eaten less, so that population is likely to increase, at least at first.

Its competitors

Face less competition for shared food, so their population may increase.

Species with no direct link

Can still be affected, through competition with a predator that has switched food, or through a change further down the web. This is what a web shows and a single chain cannot.

How to phrase it

Say 'is likely to' rather than 'will'. Two routes through a web often push a population in opposite directions.

Energy Transfer Along a Food Chain

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Trophic efficiency

The percentage of the energy in one trophic level that is transferred to the next. It is about 10 % as a rule of thumb, calculated as energy in the higher level ÷ energy in the lower level × 100.

Exam-exact meanings.

Food chains are short

After four or five transfers, too little energy remains to support a further population — usually only four or five trophic levels are possible.

Fewer organisms at each level

Less energy available means less biomass can be supported, which is why pyramids of biomass and energy are never inverted.

Eating plants feeds more people

Each transfer loses about 90 %, so eating crops directly involves one transfer rather than two, and the same land supports far more people.

The honest limits of that argument

Much grazing land cannot grow crops, so livestock there convert grass people cannot eat into food they can; animal products also supply protein, iron and vitamin B12.

The Carbon and Nitrogen Cycles

Exam-exact meanings.

Photosynthesis

Removes carbon dioxide FROM the atmosphere, fixing the carbon into glucose and then into other plant compounds. The only process that takes carbon out.

Respiration

Returns carbon dioxide TO the atmosphere. Carried out by every living organism, continuously.

Decomposition

Returns carbon dioxide TO the atmosphere. Decomposers feed on dead organisms and waste and respire, releasing the carbon they contained.

Combustion

Returns carbon dioxide TO the atmosphere. Burning wood or fossil fuels oxidises the carbon they contain.

Fossil fuels

Formed from organisms that died where decomposers could not act, so their carbon was locked away for hundreds of millions of years. Burning them returns it rapidly.

Nitrogen-fixing bacteria

Convert nitrogen GAS from the air into nitrogen compounds. Found free in the soil and in root nodules of legumes such as peas, beans and clover.

Decomposers

Break down proteins in dead organisms, urea and faeces, releasing AMMONIUM COMPOUNDS into the soil.

Nitrifying bacteria

Convert ammonium compounds into NITRATES — the form plants can absorb.

Denitrifying bacteria

Convert nitrates back into nitrogen GAS, returning it to the atmosphere. They thrive in waterlogged, poorly aerated soil.

How plants obtain nitrogen

They absorb NITRATES through their roots by active transport, and use them to make amino acids and then proteins.

Air Pollution and the Greenhouse Gases

Exam-exact meanings.

Carbon dioxide

Burning fossil fuels in power stations, vehicles and industry; deforestation, which releases carbon and removes the photosynthesis that absorbed it. Dominant because of the quantity released and because it persists for centuries.

Methane

Cattle and other livestock during digestion; rice paddy fields; decaying waste in landfill. Traps far more heat per molecule than carbon dioxide but is released in smaller amounts and breaks down within decades.

Nitrous oxide

Nitrogen fertilisers used on farmland, and vehicle exhausts.

CFCs

Aerosols, refrigerants and older manufacturing processes, now largely banned by international agreement.

Water vapour

The most abundant greenhouse gas, largely outside direct human control; its concentration rises as the atmosphere warms, amplifying the warming caused by the others.

Global Warming and Deforestation

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Enhanced greenhouse effect

The increase in the natural greenhouse effect caused by rising concentrations of greenhouse gases, so that more of the radiation emitted by the Earth is absorbed and re-radiated back to the surface.

Global warming

The increase in the mean surface temperature of the Earth caused by the enhanced greenhouse effect.

Exam-exact meanings.

Rising sea levels

Ice sheets and glaciers melt and seawater expands as it warms, flooding low-lying land and destroying habitats.

Changed species distribution

Species move towards the poles or up mountains as their range shifts; those that cannot move, or have nowhere further to go, decline or become extinct.

Changed seasonal timing

Flowering, migration and breeding shift — but not all by the same amount, so relationships such as a plant and its pollinator can fall out of step.

More extreme weather

A warmer atmosphere holds more water vapour and more energy, giving more intense storms, floods and droughts that destroy habitats and crops.

Reduced biodiversity

The population of any species unable to move or adapt quickly enough falls until it becomes extinct, so the variety of species in the community is reduced.

Less carbon dioxide absorbed

The trees no longer photosynthesise, so that absorption stops and the carbon dioxide stays in the atmosphere.

Why mature trees matter most

A mature tree holds far more biomass, and therefore far more stored carbon, than a young one.

More carbon dioxide released

Burning the felled trees releases their stored carbon at once; decomposers respiring on the remains release more.

Soil erosion

Roots no longer bind the soil and the canopy no longer shelters it, so topsoil — which holds the nutrients — is washed and blown away.

Leaching

Rain passes through the soil instead of being taken up by trees, carrying dissolved nitrates and other mineral ions below the reach of roots.

Reduced evapotranspiration

Trees drew up water and released it as vapour from their leaves; without them less water enters the atmosphere locally, so cloud formation and rainfall fall.

Loss of habitat and biodiversity

Species lose their habitat and food supply, and many become extinct — tropical forest holds a large share of the world's species.

Water Pollution and Eutrophication

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Eutrophication

The process in which mineral ions, especially nitrates leached from fertiliser, enrich a body of water, causing algae to grow rapidly and ultimately leading to a fall in dissolved oxygen and the death of aerobic organisms.

Topic 5 · Use of biological resources

Increasing Crop Yield

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Pest

Any organism that reduces the yield of a crop — including insects that eat it, fungi that infect it, and weeds that compete with it for light, water and mineral ions.

Biological control

Controlling a pest by deliberately introducing a natural predator, parasite or pathogen of that pest, rather than by using a chemical.

Yeast, Bread and Fermentation

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Fermentation

Anaerobic respiration in yeast, in which glucose is broken down into ethanol and carbon dioxide, releasing a small amount of energy.

Exam-exact meanings.

Yeast

A single-celled fungus, used in baking and brewing because it respires anaerobically.

Anaerobic respiration

The release of energy from glucose without oxygen. In yeast the products are ethanol and carbon dioxide.

Gluten

The network of stretchy proteins developed by kneading dough, which traps the carbon dioxide as bubbles.

Proving

Leaving dough in a warm place so that the yeast respires and the trapped gas makes it rise.

Optimum temperature

The temperature at which an enzyme works fastest — about 40 °C for the enzymes of respiration in yeast.

Yoghurt and the Industrial Fermenter

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Industrial fermenter

A large sterile vessel in which microorganisms are grown under carefully controlled conditions to produce a useful product.

Exam-exact meanings.

Sterilisation with steam

Kills unwanted microorganisms before use, so they cannot compete with the culture or contaminate the product.

Water jacket

Removes the heat released by the culture's respiration, keeping the temperature at the optimum so the enzymes are not denatured.

Paddles or stirrer

Keep the microorganisms suspended and distribute nutrients, oxygen and heat evenly through the vessel.

Air inlet with filtered air

Supplies oxygen for aerobic respiration, and the filter keeps contaminating organisms out.

Nutrient supply

Provides the carbohydrate, nitrogen source and mineral ions the microorganisms need to grow and make the product.

Temperature and pH probes

Monitor conditions continuously so they can be corrected automatically rather than being allowed to drift.

Fish Farming and Selective Breeding

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Selective breeding

Choosing which individuals reproduce, so that alleles for a desired characteristic become more common in the population over successive generations. Also called artificial selection.

Exam-exact meanings.

Plants — yield

Wheat and other cereals bred for larger grains and higher yield per plant.

Plants — structure

Cereals bred for shorter, stronger stems so the crop does not fall over in wind and rain.

Plants — disease

Crops bred for resistance to fungal and viral diseases, reducing the need for chemical control.

Animals — yield

Cattle bred for higher milk yield or greater meat production; chickens for faster growth.

Animals — product quality

Sheep bred for finer or more plentiful wool.

Genetic Engineering

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Genetic engineering

Cutting a gene out of the DNA of one organism and inserting it into the DNA of another, so that the second organism makes the protein the gene codes for.

Transgenic

Describes an organism containing genetic material transferred to it from a different species.

Exam-exact meanings.

Restriction enzyme

An enzyme that cuts DNA at a specific sequence of bases, usually leaving short single-stranded sticky ends.

Sticky ends

The short run of unpaired bases left at a cut end, which pairs with a complementary run on another piece.

DNA ligase

An enzyme that seals the sugar-phosphate backbone, permanently joining two pieces of DNA.

Vector

Something that carries DNA into a host cell and gets it replicated there — a plasmid or a virus.

Plasmid

A small circle of DNA in a bacterium, separate from its main chromosome and copied independently.

Recombinant DNA

A DNA molecule made from pieces that came from more than one organism.

GM Insulin and GM Crops

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Genetically modified organism

An organism whose DNA has been deliberately altered by genetic engineering, usually by inserting a gene that gives it a characteristic it did not have.

Micropropagation

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Micropropagation

Producing very large numbers of genetically identical plants from a small piece of tissue, grown on a sterile nutrient medium containing plant hormones.

Exam-exact meanings.

Explant

The small piece of tissue cut from the parent plant, usually from a shoot tip, that the culture is started from.

In vitro

Grown in glass — that is, in a container outside the living organism, rather than in the plant itself.

Callus

The shapeless mass of unspecialised cells the explant first divides into, before roots and shoots form.

Growth medium

The sterile jelly or liquid supplying nutrients, sugar and plant hormones for the culture.

Aseptic technique

Working so that no microorganism enters the culture — sterile instruments, sterile medium, sterilised explant.

Clone

An organism genetically identical to the one it came from, because it was produced by mitosis alone.

Cloning Mammals

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Nuclear transfer

Placing the nucleus of a body cell into an egg cell whose own nucleus has been removed, so that the egg develops into an animal genetically identical to the nucleus donor.

Exam-exact meanings.

Clone

An organism genetically identical to another because it developed from that organism's cells without fertilisation.

Body cell

Any cell of the organism other than a gamete — the source of the nucleus that is transferred.

Enucleated egg

An unfertilised egg cell whose own nucleus has been removed, leaving the cytoplasm to direct development.

Electric pulse

The brief current used to fuse the nucleus into the egg and to stimulate the egg to start dividing.

Surrogate mother

The female into whose uterus the embryo is implanted, which carries the pregnancy but contributes no nuclear DNA.

Transgenic animal

An animal carrying a gene transferred to it from a different species, such as a human gene for a protein.