AQA Biology Revision

What has the specification got to say about these lovely questions?

 The structured question in Unit 5 tests data handling skills and requires candidates to apply knowledge, understanding and skills from the AS and A2 specification. In the essay in Unit 5, candidates are required to use knowledge and understanding from across all units. The requirement that Stretch and Challenge is included at A2 will be met in the externally assessed units by the following

•using a variety of stems in questions to avoida formulaic approach through the use of such words as: evaluate.

• avoiding assessments being too atomistic, connections between areas of content being used where possible and appropriate

• having some requirement for extended writing

• using a range of question types to address different skills, i.e. not just short answer/structured questions

• asking candidates to bring to bear knowledge and the other prescribed skills in answering questions rather than simply demonstrating a range of content coverage.

The requirement that Stretch and Challenge isincluded at A2 is met in the long, structured questions
in Units 4 and 5 which are designed with an incline of difficulty such that the later sub-questions will offer a genuine challenge to the most able candidates. Differentiation by outcome will be used. The mark descriptors for the essay in Unit 5 include marks that will be gained only by those candidates who have included material beyond that expected of a good A Level candidate. This technique has been successful in rewarding the most able candidates.



 The stuff you need to know for the final question needs to be relevant i.e in the book/spec. This is just something I did whilst bored so thought it can't hurt to stick it on here.

3.5 Unit 5 BIOL5 Control in cells and in organisms

Blah blah blah  summary of the units, well done. 

Multicellular organisms are able to control the activities of different tissues and organs within their bodies. They do this by detecting stimuli and stimulating appropriate effectors: plants use specificgrowth factors; animals use hormones, nerve impulses or a combination of both. By responding to internal and external stimuli, animals increase theirchances of survival by avoiding harmful environments and by maintaining optimal conditions for theirmetabolism. Cells are also able to control their metabolic activities by regulating the transcription and translation oftheir genome. Although the cells within an organism carry the same genetic code, they translate only part of it. In multi-cellular organisms, this control of translation enables cells to have specialised functions, forming tissues and organs. The sequencing and manipulation of DNA has many medical and technological applications. Consideration of control mechanisms underpins the content of this unit. Students who have studied it should develop an understanding of the ways in which organisms and cells control their activities. This should lead to an appreciation of common ailments resulting from a breakdown of these control mechanisms and the use of DNA technology in the diagnosis and treatment of human diseases.


3.5.1 Stimuli, both internal and external, are detected and lead to a response.

Survival and response

Organisms increase their chance of survival by responding to changes in their environment.MOVE AWAY FROM STUFF THAT HURTS...genius

Tropisms as responses to directional stimuli that can maintain the roots and shoot of flowering plants in a favourable environment. ( Growth movement of part of a plant in response to a direcional stimulus...Stem grows towards light positive phototropism, roots grow away from light negative phototropism. WOW DO I SEE A PATTERN.)

Taxes and kineses as simple responses that can maintain a mobile organism in a favourable environment.(Roots grow down into the soil with gravity,stems grow up towards light. Need to be able to name these like phototropic and geotropic and hydrotropic WOW DO  SEE A PATTERN. Kinesis is for movement towards or away from stimulus)

A simple reflex arc involving three neurones.(Sensory from sensor to intermediate, intermediate to motor, motor to effector i.e muscle)

The importance of simple reflexes in avoiding damage to the body.(avoid danger and harm fast, involuntary )



Control of heart rate

The role of chemoreceptors and pressure receptors, the autonomic nervous system and effectors in controlling heart rate. okay so thats a mouthfull of a sentence.

Chemoreceptors : in wall of carotid arteries and sensitive to  pH in blood which changes as a result of CO2

Pressure receptors : carotid arteries and aorta. Does what it says on the tin. blood pressure changes it sends impulse to brain (medulla oblongata to be exact) 

Autonomic system :involuntary control of muscles and glands... 2 types:

Sympathetic nervous system : STIMULATES AND SPEEDS STUFF UP, allows us to cope with stress

Parasympathetic nervoues system : INHIBITS AND SLOWS STUFF DOWN, conserves energy and replenish reserves.



Receptors The basic structure of a Pacinian corpuscle as an example of a receptor. The creation of a generator potential on stimulation. page 150, looks abit like a finger with blood round the outside and neurone down the middle.

The Pacinian corpuscle should be used as an example to illustrate the following.

• Receptors only respond to specific stimuli :in this case mechanical pressure ONLY

• Stimulation of receptor membranes produces deformation of stretch-mediated sodium channels, leading to the establishment of a generator potential.With pressure channels open allowing sodium in creating a generator potantial.

Differences in sensitivity and visual acuity as explained by differences in the distribution of rods and cones and the connections they make in the optic nerve.Named cause of how they look, how superficial

Rod cells : many rod cells share a single sensory neurone so can respond to low light by mulitple cells generating a small potential which combines to create an action potential. The stuff in them ( rhodopsin) breaks down easily in low light and rod cells are round the edge of the eye.

Cone cells: each has it's own wavelenght of light it responds to. The pigmentin cone cells requires high light intensity to break down. As each has its own sensory neurone the brain can distinguish between  2 separate light source giving high visual acuit, found in the fovea, focus point.


3.5.2 Coordination may be chemical or electrical in nature.

Principles Nerve cells pass electrical impulses along their length. They stimulate their target cells by secreting chemical neurotransmitters directly on to them. This results in rapid, short-lived and localised responses.Unlike hormones


Mammalian hormones are substances that stimulate their target cells via the blood system. This results in slow, long-lasting and widespread responses.

Histamine and prostaglandins are local chemical mediators released by some mammalian cells and affect only cells in their immediate vicinity.Histamine : released from white blood cells following allergen causing dilation of small arteries and arterioles causing swelling and redness/itching. Prostaglandins are found in cell membranes also cause dilation and affect blood pressure/neruotransmitters and affect pain. THESE ARE LOCALISED

In flowering plants, specific growth factors diffuse from growing regions to other tissues. They regulate growth in response to directional stimuli. The role of indoleacetic acid (IAA) in controlling tropisms in flowering plants. Stem tips produce IAA : it moves away from light and induces growing, rest is self explanatory.

Nerve impulses The structure of a myelinated motor 162 , nodes of Ranvier in between Schwann cells which are made of myelin sheath (lipid)

The establishment of a resting potential in terms of differential membrane permeability, electrochemical gradients and the movement of sodium and potassium ions. Remember that the pump is 3 Na out and 2 K in with sodium gates closed, so Na can't re-enter axon.Suppose you could remeber this by the word "kin" meaing "k-in" or some mixture of pot and in like point, usually the ruder the more likely you are to remeber but i didn't tell you that and im not suggesting any risque ideas ;)

Changes in membrane permeability lead to depolarisation and the generation of an action potential. The all-or-nothing principle. Stimulus causes sodium gates to open which diffuse into axon and have POSITIVE FEEDBACK leading to greater influx. Sodium gates close at  +40mV and potassium open and move along electrical gradient out of axon.Oh and the all or nothing principle is basically means there either is an action potential or there isn't, no in the middle or sitting on the fence. BUT HOW DO WE PERCEIVE size of impulse i hear you shout. Different neurones have different thresholds stimulus levels, so will only create an impulse if a large stimulus occurs.

The passage of an action potential along non-myelinated and myelinated axons, resulting in nerve impulses.

The nature and importance of the refractory period in producing discrete impulses. Sodium gates staying closed for a small period of time prevents continuous action potentials.

Meaning: The impulses are discrete (separate)

 Impulses are in one direction as it can only move from an active region to a resting region preventing action potential in 2 directions.

Limits the number of action potentials possible in a given time.


 Factors affecting the speed of conductance: myelination and saltatory conduction;axon diameter; temperature.

Temperature: GCSE stuff probably enzymes are involved and ions are diffusing so temperature affects it.

Diameter of axon: ions "leak" out and the larger the axon the less leakage supposedly as it can't maintain the potential difference aas ions slip to the wrong side.

Myelin sheath: Prevents action potential forming so can only form in the nodes of Ranvierso the impulse jumps from one node to the next tripling the speed of the impulse.


 Will try to continue, but wanted to get site up and going as quick as possible


3.5.6 The sequence of bases in DNA determines the structure of proteins, including enzymes.


The genetic code

 The genetic code as base triplets in mRNA which code for specific amino acids. A codon is 3 nucleotide bases which code for an amino acid.


The genetic code is universal, non-overlapping and degenerate.

Universal: Same codon codes for the same amino acid in all organisms (mostly)

Non-overlapping: each base is read only once

Degenerative: Most amino acids have more than one codon.


The structure of molecules of messenger RNA (mRNA) and transfer RNA (tRNA).

mRNA : long strand of single helix. Just like a single strand of DNA it has phosphate/ribose sugar backbone with bases attached. Small enough to fit through nuclear pores out of nucleus and is easily broken down.


tRNA: Several types of this, each one able to carry a single amino acid. One side of the molecule has an anti-codon loop with the other havin the place for an amino acid to attach. Suited for the job by having the two specific ends so good at aligning amino acids to make a protein


Candidates should be able to compare the structure and composition of DNA, mRNA and tRNA. Instead of AT,GC its AU and GC.


Polypeptide synthesis

Transcription as the production of mRNA from DNA. DNA helicase breaks open a chunk of the DNA chain . between the bases (hydrogen bonds)  RNA polymerase moves alon one of the strands causing the nucleotides on the strand to join with free nucleotides (there’s a quite a few spare hanging around like a bad smell) DNA strand closes up behind the RNA polymerase. THIS MAKES PRE-mRNA


The role of RNA polymerase. See above


The splicing of pre-mRNA to form mRNA in eukaryotic cells. DNA is made of useful bits (exons) which code for proteins and useless bits (introns) These bits alternate, and so would interfere with protein synthesis so splicing removes the useless bits.


Translation as the production of polypeptides from the sequence of codons carried by mRNA. 14.3 in a sentence Figure 4 on page 227.

The two amino acids are joined by an enzyme and ATP forming a peptide bond.

Ribosome is made of 2 units that sit either side of the mRNA and all the stuff happens inside.

Secondary structure : polypeptide is folded

Tertiary structure : secondary structure is folded

Quaternary structure: polypeptide chains and other non-protein groups added.



The role of ribosomes and tRNA.


Candidates should be able to

• show understanding of how the base sequences of nucleic acids relate to the amino acid sequence of polypeptides, when provided with suitable data.


• interpret data from experimental work investigating the role of nucleic acids. Sounds like a “how science works question”, Yippeee L


Recall of specific codons and the amino acids for which they code, and of specific experiments, will not be tested.(don’t learn the table)


Gene mutation

Gene mutations might arise during DNA replication. Mutations in body cells aren’t passed on, mutations in gametes could.


 The deletion and substitution of bases.

Substitution is changing one base for another and can have 3 effects

1)     Nonsense mutation : base change results in the formation of a stop codon, protein is short and doesn’t work, (slightly sizest but OK)

2)     Mis-sense mutation : base change results in  a different amino-acid so the polypeptide will be different by one amino acid, that amino acid might be vital in determining the tertiary structure and so will stop it working or it might not be.

3)     Silent mutation : substituted base codes for the same amino acid so no change.


Deletion is nasty: it means all the following triplet codes will be different so if it is at the start of a sequence then everything is screwed.

TGC       AGC         TAC          becomes

GCA      GCT          ACA


Gene mutations occur spontaneously.


The mutation rate is increased by mutagenic agents.i.e

High energy radiation

Chemicals that alter DNA structure.


 Some mutations result in a different amino acid sequence in the encoded polypeptide.


Due to the degenerate nature of the genetic code, not all mutations result in a change to the amino acid sequence of the encoded polypeptide. (Silent)


The rate of cell division is controlled by proto-oncogenes that stimulate cell division and tumour suppressor genes that slow cell division. If either mutates it means bad times


A mutated proto-oncogene, called an oncogene, stimulates cells to divide too quickly. A mutated tumour

suppressor gene is inactivated, allowing the rate of cell division to increase.