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questionbase.50megs.com AS-Level Revision Notes
AS Biology ­ Revision Notes
Unit 1 ­ Core Principles
Biochemistry
1. Carbohydrates have the general formula (CH2O)x.
2. The simplest carbohydrates are monosaccharides, e.g. glucose (C 6H12O6), and are monomers.
3. Monosaccharides can join together to form disaccharides in a condensation reaction, forming a
glycosidic bond. This can be broken by hydrolysis:
4. Polysaccharides are formed from multiple glycosidic bonds, and include starch, glycogen and
cellulose.
5. 1-4 glycosidic bonds create straight chains, whereas side chains are produced from 1-6 bonds.
6. Cellulose is fibrous, with 1-4 linkages, and is used in cell walls. Glycogen and starch are both
coiled, with 1-4 linkages, and 1-6 side chains, and are used for storage.
7. Amino acids are the monomers of proteins ­ they have an amino group at one end, and a
carboxylic acid group at the other.
8. The `R' group distinguishes one amino acid from another ­ there are about 20 in total; those that
are needed in the diet are called essential amino acids.
9. Some amino acids can be synthesised by transamination.
10. Amino acids bond like monosaccharides, creating peptide bonds.
11. Fibrous proteins, e.g. collagen and keratin, have long, straight, polypeptide chains.
12. Globular proteins, e.g. gels and enzymes, have a roughly rounded shape.
13. Proteins have four different structures:
a. The primary structure is the basic sequence of amino acids.
b. The secondary structure is how the chain becomes coiled and folded due to the bonding.
c. The tertiary structure is the 3D structure of the protein, due to disulphide bridges and
hydrogen bonding.
d. The quaternary structure is when more than one protein is joined together (e.g. in
haemoglobin).
14. The tertiary structure of an enzyme gives rise to the active site ­ when an enzyme is denatured the
hydrogen bonds are broken and so the active site changes shape.
15. Lipids are made of glycerol (propan-1,2,3-triol) and fatty acids. The fatty acids join via their
carboxylic acid group to the hydroxyl groups on glycerol, forming three ester bonds and a
triglyceride.
16. Saturated fatty acids are all single bonds, and are mainly fats and waxes, where as unsaturated fatty
acids contain double bonds and form oils. Polyunsaturated fatty acids contain many double bonds.
17. If a phosphate group replaces one fatty acid then a phospholipid is formed. The phosphate is
hydrophilic, but the fatty acids are hydrophobic, so they will form either a monolayered vesicle or
a phospholipid bilayer (as in plasma membranes).
18. Benedict's solution (containing copper sulphate) will test for a reducing sugar ­ it will turn
orange/red in its presence upon heating.
19. Non-reducing sugars (e.g. sucrose) are tested for by first hydrolysing with dilute HCl then using
Benedict's solution.
20. Iodine will produce a blue/black colour in the presence of starch.
21. Biuret solution (copper sulphate and sodium hydroxide) will test for proteins, by turning lilac in its
presence.
22. The emulsion test is for lipids ­ dissolve the lipid in ethanol, then pour into water. A fine emulsion
is formed if it is a lipid.
23. Chromatography is used to separate out soluble components of a mixture (e.g. amino acids). This
Distance moved by spot
is done by calculating the Rf value for a specific solvent ­ Rf = .
Distance moved by solvent
24. Two-way chromatography can be more accurate, as two different solvents are used, and so the
mixture can be more easily separated.
25. Water is essential to life due to the following properties:
a. It is polar, so will dissolve other polar substances.
b. Due to hydrogen bonding it has a much higher melting/boiling point than expected,
allowing it to be liquid within cells.
c. It can be a breathing medium ­ typically there is about 1% dissolved oxygen in water.
d. It has a high latent heat of vaporisation, and so evaporation removes a lot of energy and is
used as a cooling mechanism.

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AS-Level Revision Notes
e. It has a high specific heat capacity ­ it absorbs a lot of energy with little change in
temperature. So it is thermally very stable.
Cells And Transport
1. Prokaryotic cells have no nuclear membrane ­ the nucleic acids are free in the cytoplasm.
2. In a bacterium, there is a cell membrane, a cell wall (containing polysaccharide but not cellulose)
and a protective capsule. They may have one or more flagella that enable it to move.…read more

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AS-Level Revision Notes
c. The distance over which diffusion takes place.
d. The surface area.
surface area × concentration difference
25. Fick's Law says that: rate of diffusion =
distance between the two areas
26. Facilitated diffusion is still a passive process, but uses either:
a. Ion channels (i.e. pores with charges along them) that can open and close to allow or
prevent the movement of ions.
b.…read more

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AS-Level Revision Notes
14. Extracellular digestion takes place in saprophytic fungi. In the mycelium, enzymes are secreted
from the hyphae. Digestion takes place on the food surface, and the products of digestion are
absorbed. Note that fungi are multinuclear (i.e. there are no cell membranes or walls).
15. In the alimentary canal, digestion takes place by both mechanical and chemical breakdown. There
are five processes involved in feeding:
a. Ingestion ­ taking food in.
b. Digestion ­ breaking food down.
c.…read more

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c. Lungs ­ bronchial/alveolar network within the pleural cavity, e.g. most mammals.
4. All respiratory surfaces have the following properties:
a. Large surface area.
b. Short diffusion pathway (thin walls).
c. Well supplied with blood.
d. They are moist (as a consequence of thin walls).
5. In a dicotyledonous leaf, air enters through the stomata by simple diffusion into the air spaces
within the spongy mesophyll. Gaseous exchange takes place into the palisade and spongy
mesophyll cells.
6.…read more

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