1
Q
Abiotic
A
- An ecological factor that is part of the
non-living or physical environment of an
organism. - Abiotic factors include climatic
features (such as rainfall and temperature), solar energy input and edaphic factors (soil).
2
Q
Acetyl coenzyme A, Acetyl CoA
A
- An intermediate product formed during respiration.
- Acetyl coenzyme A is produced by the link reaction when coenzyme A combines with a two-carbon fragment formed from pyruvate.
- Acetyl coenzyme A carries this fragment into
the Krebs cycle.
3
Q
Acetylcholine
A
- Acetylcholine is a neurotransmitter. It is involved in the transmission of impulses across synapses.
- When an impulse arrives at the presynaptic membrane, acetylcholine is released into the synaptic cleft.
- It diffuses across the cleft and attaches to receptors on the postsynaptic membrane.
- This triggers an impulse in the postsynaptic neurone.
4
Q
Acetylcholinesterase
A
- An enzyme found in synapses that have acetylcholine as a neurotransmitter.
- Acetylcholinesterase breaks down acetylcholine once it has triggered an impulse in the postsynaptic neurone.
- This is important as unless acetylcholine is removed rapidly from a synapse there would be continuous transmission along the postsynaptic neurone.
5
Q
Achondroplasia
A
- A genetic condition caused by a dominant allele.
- Someone who is heterozygous for this condition has very restricted growth.
6
Q
Acinar cell
A
- A cell from the pancreas.
- It produces enzymes that are used in digestion.
7
Q
Acrosome, Acrosome reaction
A
- An organelle in the head of a sperm that contains digestive enzymes.
- When fertilisation takes place, the membrane surrounding these enzymes bursts.
- The enzymes digest the follicle cells; they also digest the jelly-like layer surrounding the egg cell allowing the sperm to fuse with the egg membrane.
- This process is known as the acrosome reaction.
8
Q
Actin
A
- A protein found in many cells.
- Actin plays an important part in muscle contraction.
- In a muscle fibre, actin forms the thin filaments.
- These filaments slide between the thicker filaments of another protein, myosin, bringing about muscle contraction.
- It is also thought to be involved in cell cleavage.
9
Q
Action potential
A
- The changes which take place across the cell surface membrane of a nerve cell during the passage of an impulse.
- These changes are brought about by the opening and closing of voltage-dependent Na+ and K+ gated channels in the membrane.
10
Q
Activation energy
A
- Before a chemical reaction can take place, bonds must be broken.
- This requires energy.
- This activation energy is normally provided by heating the substances involved in the reaction.
- Enzymes reduce the amount of activation energy necessary, so reactions in living organisms can take place at relatively low temperatures.
11
Q
Active artificial immunity
A
- Immunity that develops following immunisation.
- Antigens in the vaccine trigger a specific immune response by the body’s immune system.
12
Q
Active natural immunity
A
- Immunity that develops following an infection.
- The specific immune response to the foreign antigens helps to destroy the pathogens and produces a supply of antibodies and B memory and T memory cells that will respond quickly if re-infected with the same pathogen.
13
Q
Active site
A
- The part of an enzyme molecule into which a substrate molecule fits during a chemical reaction.
- It is like a pocket on the surface of the enzyme and it has a specific shape.
- Only a substrate molecule with the complementary shape will be able to fit into this active site.
14
Q
Active transport
A
- A process that involves the movement of substances from where they are in a low concentration to where they are in a higher concentration; in other words, it involves the movement of substances against a concentration gradient.
- Active transport involves the use of specific carrier proteins in cell membranes.
- It also requires energy in the form of ATP.
15
Q
Adaptation
A
- Any features of an organism that make it well suited to survive in its environment.
- Adaptations can be anatomical, physiological or behavioural.
16
Q
Adaptive radiation
A
- The way in which a common ancestor may give rise to different species, each occupying a different ecological niche.
- For example, it is thought that all molluscs evolved from a worm-like ancestor.
- There are now many different species which are adapted to a wide range of niches.
- Present-day molluscs include slow-moving, land-dwelling herbivores such as slugs and snails, as well as rapid swimming carnivores like squid.
17
Q
Adenine
A
- One of the nucleotide bases found in nucleic acids.
- When the two strands of nucleotides which make up a molecule of DNA come together, adenine always pairs with thymine.
- The atoms of the two bases are arranged in such a way that two hydrogen bonds form between them.
- Adenine is also a constituent of ATP and ADP.
18
Q
Adhesion
A
- A force resulting from attraction between molecules of different substances.
- In xylem, water molecules adhere to the molecules of lignin and to other substances which make up the walls of the vessel.
19
Q
ADP, Adenosine diphosphate
A
- A substance found in all living cells where it is involved in the transfer of energy.
- Adenosine diphosphate (ADP) is produced when a phosphate group is removed from ATP; energy is released.
- In this way energy is transferred and made available for movement, active transport and the synthesis of large molecules.
20
Q
Adrenaline
A
- A hormone produced by the adrenal glands at times of stress such as when we are angry or frightened.
- It has a range of effects on the body.
- These include increasing the stroke volume and rate of beating of the heart, dilating blood vessels supplying muscles, and stimulating the conversion of glycogen to glucose.
- Scientists have shown that there are links between stress, adrenaline and an increased risk of heart disease.
21
Q
Aerobic
A
- Aerobic means requiring the presence of oxygen.
- Aerobic bacteria are bacteria that can only live and multiply in the presence of oxygen.
- Aerobic respiration is respiration that requires the presence of oxygen.
22
Q
Aerobes
A
- Aerobes are organisms that respire using oxygen.
23
Q
Aerobic capacity
A
- The ability to take in, transport and use oxygen.
- Our aerobic capacity determines our ability to undertake long periods of vigorous exercise such as running steadily.
24
Q
Agonist
A
- A substance which mimics the action of a neurotransmitter at a synapse.
- The molecules of an agonist have a similar shape to the molecules of a particular neurotransmitter.
- As a result they also fit into the receptor molecules in the postsynaptic membrane of the synapse.
- Neurotransmitters only exert their effects for a short time because they are rapidly broken down.
- Agonists on the other hand are not broken down so readily and can continue to stimulate the postsynaptic nerve cell.
- Agonists that mimic the neurotransmitter dopamine are useful in the treatment of Parkinson’s disease.
25
AIDS, Acquired Immune Deficiency Syndrome
- A collection of disease symptoms relating to the same cause, damage to the immune system caused by HIV.
- AIDS involves a number of opportunistic infections.
26
Albinism
- A genetic condition caused by a recessive allele that results in the non-production of pigment.
27
Alga, Algal, Algae
- A group of plant-like organisms that include the one-celled organisms that form phytoplankton as well as the seaweeds.
- Although algae photosynthesise and their cells are surrounded by a cell wall, they are not plants.
- They belong to a group of organisms called the Protoctista.
28
Algal bloom
- The rapid growth of a population of the microscopic freshwater or marine algae that make up the phytoplankton.
- Algal blooms often result from an increase in the concentration of nitrates or phosphates associated with pollution.
29
Allele
- One of the different forms of a particular gene.
- In humans, a gene codes for the protein that transports chloride ions across the cell surface membrane.
- We all have this gene, the CF gene.
- The CF gene has two alternative forms or alleles.
- The normal allele, F, produces a functioning protein.
- The other allele, f, produces a protein which is very slightly different and does not function.
- If a person has two copies of the f allele, he or she will have cystic fibrosis.
30
Allele frequency
- The relative frequency of alleles in a population of organisms changes over time
with evolution.
- Alleles that are for advantageous characteristics will become more frequent, with those that do not provide an advantage becoming less frequent.
31
Allergen
- A substance that produces an allergic reaction in a sensitive person.
- Many different substances can act as allergens.
- One concern over the widespread use of genetically modified foods is that they may make allergens more widespread.
32
Allergic reaction
- In a person suffering from an allergy, a reaction takes place involving a particular substance, called an allergen, to which the person is sensitive.
- As a result, large amounts of histamine are produced.
- Some allergens produce localised effects such as those that occur with hay fever and asthma.
- Others are more widespread and can be very serious.
- Anaphylactic shock, for example, may occur in response to nuts.
- It affects the lungs and the circulatory system, and may result in death unless medical help can be given.
33
Allopatric speciation
- The formation of a new species due to reduced gene flow between two populations as a result of geographical isolation.
- The two groups are unable to mate so are reproductively isolated.
- Random mutations in each population and different selection pressures lead to changes in allele frequencies.
- If members of the two groups change in ways that prevent individuals from the two populations from interbreeding and producing fertile offspring then they are considered to belong to different species.
34
All-or-nothing
- An expression used to describe an action potential.
- When an action potential is produced in a nerve cell, it is always the same size.
- It does not matter how big the initial stimulus is, the action potential will always involve the same change in potential difference across the cell surface membrane.
- Because of this, the only way that information about the strength of a stimulus can be carried is by varying the number of nerve impulses in a given time.
35
Alveolus, Alveoli
- One of the tiny air sacs in the lungs where gas exchange takes place.
- The alveoli are lined by a layer of cells known as squamous epithelium.
- These cells are very thin and this helps to ensure efficient diffusion of oxygen from the alveoli into the blood, and of carbon dioxide from the blood into the alveoli.
- The total surface area of all the alveoli in the lungs is very large.
- This large surface area also helps to ensure efficient diffusion.
36
Amino acid
- The basic unit or monomer from which proteins are formed.
- There are twenty different amino acids which may be linked by condensation to form proteins.
- All of these amino acids have the same basic chemical structure.
37
Amniocentesis
- A way of obtaining cells from a developing fetus.
- A developing fetus is wrapped in a membrane called the amnion.
- The space between the amnion and the fetus is filled with a fluid called amniotic fluid.
- Amniotic fluid contains some fetal cells.
- These cells can be examined and their chromosomes observed or the DNA that they contain may be investigated.
38
Amniotic fluid
- The space between the amnion and the fetus is filled with a fluid called amniotic fluid.
- This cushions and supports the delicate tissues of the developing fetus.
- Amniotic fluid contains some fetal cells.
- These cells can be examined and their chromosomes observed or the DNA that they contain may be investigated.
39
Amphibian
- A member of the class of animals that includes frogs and toads.
- Amphibians have moist skin that acts as an additional gas exchange surface.
- They breed in water and produce aquatic larvae called tadpoles, which change into terrestrial adults.
40
Amylase
- An enzyme that digests starch into soluble sugars.
- This reaction involves the addition of water molecules so it is an example of hydrolysis.
- Amylases are important digestive enzymes in animals but they are also found in many microorganisms and plant tissues.
- Amylose is a polysaccharide.
-Amylase is the enzyme that breaks down amylose to maltose.
41
Amylopectin
- Amylopectin is a polysaccharide formed of branched chains of glucose molecules.
- Starch consists of a mixture of amylopectin and amylose.
42
Amyloplast
- An organelle found in plant cells which stores starch.
- Each amyloplast consists of a tiny grain of starch surrounded by a doublelayered cell membrane.
43
Amylose
- A polysaccharide formed from a long straight chain of glucose molecules.
- This chain winds into a spiral shape.
- Starch consists of a mixture of amylose and amylopectin.
44
Anabolic
- An anabolic reaction is a chemical reaction in which smaller molecules combine together to produce larger ones.
- Examples of anabolic reactions include photosynthesis and protein synthesis.
45
Anabolic steroid
- Testosterone is described as an anabolic steroid because it is a steroid hormone that builds up substances such as muscle proteins, leading to an increase in the size and strength of muscle.
46
Anaemia
- A condition in which there is a reduced amount of haemoglobin, the oxygen-carrying pigment found in red blood cells.
- People suffering from anaemia tire easily and soon get out of breath if they exert themselves.
- There are several causes of anaemia.
- One of them involves a shortage of iron in the diet.
47
Anaerobic, anaerobically
- Anaerobic means the absence of oxygen.
- Anaerobic bacteria are bacteria which only live and multiply in the absence of oxygen.
- Anaerobic respiration is respiration which takes place in the absence of oxygen.
- In most animals, it results in the formation of lactate.
- In plants and microorganisms such as yeast, ethanol and carbon dioxide are produced.
48
Anaerobes
- Anaerobes are organisms that respire without using oxygen.
A-level Biology - Edexcel SNAB
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