All the bullet points in this handout have been awarded a mark on A level mark schemes at some
stage. This handout shows the level of information you must give in your answers and what the
examiners are looking for. As the number of AS exams completed increases I will add to the document
making it a better resource to revise from.
General
You will in the exam get questions on unfamiliar topics! This is so you can apply principles you have
learnt into novel situations.
Components of a Triglyceride

Phosphoric acid/phosphate

Glycerol

Fatty acid
Protein Structure



(Specific) primary structure / order of amino acids;
(Specific) tertiary / 3D structure;
(So) Only binds to / fits / complementary to one (antigen);
Haemoglobin
The haemoglobins are a group of chemically similar molecules found in many different organisms.
Haemoglobin is a protein with a quaternary structure.



Protein has primary structure / amino acid sequence;
Therefore bonds always form in same position;
quaternary structure – haemoglobin incorporates ions for oxygen transport
Carbohydrates
The structure of b-glucose as
and the linking of b-glucose by glycosidic bonds formed by condensation to form cellulose.
The basic structure and functions of starch, glycogen and cellulose and the relationship of structure
to function of these substances in animals and plants.
Starch: helical shape provides compact store (in plants); insolubility linked to storage
(osmotically inactive), large size does not pass through membrane, provides large number of
glucose molecules for respiration.
Glycogen: similar to starch but more branches, insoluble storage compound in liver and muscles
(mammals). Conversion of glucose to glycogen for storage. Importance of control of blood
glucose, more branches so broken down quicker
Cellulose: long straight chains of glucose molecules, OH groups of chains linked by hydrogen bonds
forming microfibrils / macrofibrils. Layers of fibrils orientated in different directions are
interwoven and embedded in a matrix - providing rigid cell wall; gaps in layers provide
permeability.
Cells
There are fundamental differences between plant cells and animal cells.
The structure of a palisade cell from a leaf as seen with an optical microscope.
The appearance, ultrastructure and function of
• cell wall
Cellulose
Long chains of monomers/glucose/sugars/Straight chains;
Chains cross link by (hydrogen) bonds;
Chemically stable / inert / insoluble.
Function: support / strength / shape / prevents osmotic lysis;
Role of Hydrogen bonds in Cellulose
1. Holds chains/cellulose molecules together/forms cross links between chains/cellulose
molecules/forms microfibrils;
2. Providing strength/rigidity (to cellulose/cell wall);
3. Hydrogen bonds strong in large numbers;
Starch
 Helical /spiral/coiled - Compact / description e.g. ‘tightly packed’;
 Insoluble - Prevents osmosis/uptake of water / does not affect water potential / (starch)
does not leave cell;
 Large molecule / long chain - Does not leave cell;
Test for non-reducing sugars

test for reducing sugar first;

boil/heat with acid;

neutralise with alkali/sodium hydrogen carbonate

heat with Benedict’s (solution);

blue to orange colour change indicates non-reducing sugars
Digestion of starch overview
Amylase
Starch
Amylase;
(Starch) to maltose:
Maltase;
Maltose to glucose;
Hydrolysis;
(Of) glycosidic bond;
Maltase
Maltose
Glucose
Microscropy

Electron microscope has greater resolution (can distinguish between objects close together)

Because wavelength of electron beam smaller
TEM
Advantages:
1 Small objects can be seen;
2 TEM has high resolution;
3 Wavelength of electrons shorter;
Limitations:
4 Cannot look at living cells;
5 Must be in a vacuum;
6 Must cut section / thin specimen;
7 Preparation may create artefact
8 Does not produce colour image;
Differences between prokaryotic and eukaryotic ribosomes
 Eukaryotic ribosomes denser/heavier (not just ‘larger’)
Prokaryotes v Eukaryotes
 Any two from
 No membrane-bound organelles/no mitochondria / no golgi/ no endoplasmic reticulum/etc;
 Any two from
 Capsule/flagellum/plasmid / cell wall/etc;
Preparing a sample of organelles
 Homogenise;
 in ice cold;
 Reduce/prevent enzyme activity;
 isotonic solution;

Maintain concentrations/water potential same inside and outside

prevent osmosis;

Prevent bursting/shrinkage organelles (not cells)
 then centrifuge
Structure of the membrane
 Phospholipids and proteins
 Phospholipid bilayer
 Arrangement of phospholipid molecules ‘tails to tails’
 ”Floating protein molecules”/molecules can move in membrane
 Intrinsic proteins extend through bilayer
 Extrinsic proteins in outer layer only
 Detail of channel proteins/glycoproteins
Mitochondria

(Mitochondria) use aerobic respiration;


Mitochondria produce ATP/release energy;
Energy/ATP required for muscles (to contract);
Facilitated diffusion
 Movement from high to lower concentration
 Use of carrier/channel protein
 Proteins specific
 Changes shape of protein and passes through channel/membrane
 No energy/ATP needed
Active transport
 Movement against concentration gradient
 Use of carrier/intrinsic/pump proteins
 Protein specific (to ion)
 Energy/ATP required
 Energy used to change shape of proteins/attach ion to protein
 Ions moved through membrane as proteins change shape/position
Differences between active transport and diffusion
Active transport
Diffusion
May move substances against concentration
gradient;
Requires ATP/energy;
Requires membrane proteins/carriers
Substances moved down concentration gradient;
Does not require ATP/energy;
Does not (necessarily) require membrane
proteins/carriers
Active transport

Active transport requires energy/uses ATP;

Moves substances against concentration gradient
Mechanism of active transport

Carrier protein (in membrane); (reject: extrinsic protein, or just ‘protein’);

Molecule transported by change of shape/’flipping’ of carrier protein;

Energy used to attach molecule to carrier protein/change shape. (not just ‘ATP provides
energy’).
How the structure of DNA is related to its function

sugar – phosphate backbone gives strength;

coiling gives compact shape;

sequence of bases allows information to be stored;

long molecule stores large amount of information;

information can be replicated exactly because of complementary base pairing;

(double helix protects) weak hydrogen bonds/double helix makes molecule stable/prevents
code being corrupted;

Weak hydrogen bonds/chains can split for replication/transcription.
Mitosis

Chromosomes become shorter/condense/coiling

Nuclear membrane disappears

Spindle formation

The chromosomes arrange on equator of the spindle

Centromeres attach to spindle;

Centromeres divide;

Chromatids pulled apart;

Role of spindle fibres

Chromatids moved to opposite poles

Uncoiling/elongation;

(DNA) replication;

formation of another chromatid
DNA replication (semi conservative replication)


DNA strands separate/hydrogen bonds are broken (a labelled diagram could show this);
Each strand forms a template/is copied/one new strand and one old ( a labelled diagram
could show this);
Complementary base pairing;

Genes and polypeptides
A gene occupies a fixed position, called a locus, on a particular strand of DNA.
Genes are sections of DNA that contain coded information as a specific sequence of bases. Genes
code for polypeptides that determine the nature and development of organisms.
A sequence of three bases, called a triplet, codes for a specific amino acid. The base sequence of a
gene determines the amino acid sequence in a polypeptide.




Change in (sequence of) amino acids/primary structure;
Change in hydrogen/ionic/disulfide bonds;
Alters tertiary structure/active site (of enzyme);
Substrate cannot bind / no enzyme-substrate complexes form;
In eukaryotes, much of the nuclear DNA does not code for polypeptides. There are, for example,
introns within genes and multiple repeats between genes. Differences in base sequences of alleles of
a single gene may result in non-functional proteins, including non-functional enzymes.

Allele - (Different) form/type/version of a gene / different base sequence of a gene
 Intron - Region of non-coding DNA / degenerate DNA;
DNA and chromosomes
In eukaryotes, DNA is linear and associated with proteins. In prokaryotes, DNA molecules are
smaller, circular and are not associated with proteins.
Meiosis
The importance of meiosis in producing cells which are genetically different, caused by:
• the formation of haploid cells
• independent segregation of homologous chromosomes. Gametes are genetically different as a
result of different combinations of maternal and paternal chromosomes
• genetic recombination by crossing over.
Meiosis is important because:
 Meiosis halves the number of chromosomes;
 chromosome number is halved/haploid;
 allowing a constant number/diploid to be restored by fertilisation/over generations;
 Introduces variation;
 Correct reference to natural selection / survival;
Compare Mitosis and Meiosis
Mitosis
chromosome number remains same /
cells produced diploid
cells produced identical / no variation in
cells produced
only one division/2 cells produced
somatic/ body cell formation/ used in
AR/growth
Meiosis
chromosome number halved / cells produced
haploid
cells produced not identical / variation in cells
produced
two divisions / 4 cells produced
used in gamete formation / reproductive cell
formation / occurs in gonads/named gonad (reject
occurs in gametes)
Effect of cancer drugs
 Prevents/slows DNA replication/doubling;
 Prevents/slows mitosis;
 New strand not formed / nucleotides(of new strand) not joined together / sugar-phosphate
bonds not formed;
Reasons why eggs are larger than sperm

Larger amount of food can be stored;

For development of embryo
Structure of egg
 Cytoplasm of egg contains yolk/food stores
Needed for nourishment/development of (embryo/zygote)
Differences between trna and mrna
 tRNA short chain versus mRNA long chain
 tRNA clover leaf shape versus mRNA straight chain
 tRNA folded versus mRNA straight
 tRNA fixed length versus mRNA variable length
 tRNA held together with hydrogen bonds versus mRNA straight molecule
Polypeptide Synthesis













Sequence of bases is the code;
3 bases code for one amino acid;
DNA strands separate/Hydrogen bonds break;
Producing mRNA/transcription (linked to mRNA production);
Uracil replaces thymine;
Role of RNA polymerase;
Complementary base pairing;
mRNA small enough to leave nucleus and enter cytoplasm;
mRNA attaches to ribsome/rER;
tRNA bring specific amino acid;
anticodons of tRNA complementary to codons on mRNA/ translation;
amino acids join by peptide bonds/condensation reaction;
tRNA then free to pick up another amino acid
Mutations

change in code/base sequence;

detail eg substitution/addition/deletion;

of base(s);

different amino acid(s) inserted into protein/polypeptide.
Causes of mutation

High energy ionised particles/X-rays/ultraviolet light/high energy
radiation/uranium/plutonium/gamma rays/tobacco tar/ caffeine/pesticides/mustar
gas/base analogues/free radicals; (reject radiation)
Effect of mutation



Change in the sequence of nucleotides/bases/addition/deletion/substitution;
Changed order of amino acids/different protein/different tertiary; structure;
Inactive enzyme if shape of active site is changed/enzyme-substrate complex does not form;
Reliability



Repeat the results using same method/use someone else’s results
Anomalies to be identified / effect of anomalies to be reduced / effect of variation in data to
be minimised;
A mean to be calculated;
Importance of repeats


(Allows) anomalies to be identified / ignored / effect of anomalies to be reduced / effect of
variation in data to be minimised;
Makes the average / mean / line of best fit more reliable / allows concordant results;
Use of percentage change


Allows comparison / shows proportional change;
Idea that cylinders have different starting masses / weights;
Reliable data
 Large sample / wide range (of individuals tested);
 Random (sampling);
 Tested at different times/more than once;
 Mean/average value determined;
 Idea of establishing a range for the normal concentration / reference to use of standard
deviation;
Selecting volunteers
Consider:
1. Sex/gender;
2. Lifestyle;
3. Body mass;
4. Health;
5. Ethnicity;
6. Genetic factors / family history;