Summary of AS Unit 2 Biology

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AS Biology Notes ­ AQA Unit 2: The Variety of Living Organisms
- Variation describes the differences in characteristics between organisms.
- Can be inter- or intraspecific:
o Inter = between members of different species.
o Intra = within the same species.
- Also continuous/discontinuous:
o Continuous = infinite number of possible values, e.g. height, weight.
o Discontinuous = finite number of possible values, e.g. eye colour, number of
- Continuous numerical data can be analysed using the mean or standard deviation:
o Mean gives the average value ( )
o Standard deviation gives a measure of how the data is spread around the
mean: low standard deviation means consistent data, high means
inconsistent ( ). N.B. calculation of standard deviation is not
required according to AQA specification.
- When data is sampled and analysed, there is always a possibility of trends being due
to chance; in biology a 95% confidence level is set: if the probability of something
not being due to chance is above 95% it is accepted.
- Phenotype describes an organisms physical characteristics; genotype describes its
genetic makeup.
- Variation can be caused by genetic or environmental factors. Often genetic factors
cause discontinuous variation whilst environmental factors cause continuous
variation, though this is not always true.
DNA structure:
- DNA is a long and complex molecule containing the genetic information of an
organism; it codes for protein synthesis. DNA is a polymer made up of many single
units called nucleotides.
- A nucleotide is made up of three parts:
o A pentose ­ i.e. 5-carbon ­ sugar (ribose in RNA, deoxyribose in DNA).
o A phosphate group.
o A nitrogen-containing base: either Adenine, Guanine, Cytosine or Thymine
(A,T,C or G)
- Nucleotides join together by phosphodiester bonds (condensation reactions) to form
polynucleotide chains.
- In DNA, two polynucleotide chains line up so that the sugar and phosphate groups
point outwards (the sugar-phosphate backbone). The bases point inwards and form
hydrogen bonds, joining the two strands. Base pairing is specific and
complementary: A will only bond with T; C will only bond with G. Complementary
base pairing is essential to the function of DNA.
- The ratio of pyrimidine (i.e. T and C) to purine (i.e. A and G) bases in DNA is thus 1:1.
One gene, one polypeptide:
- A gene or cistron is a sequence of bases in DNA which codes for the synthesis of a
specific polypeptide. Each triplet of bases codes for an amino acid; a sequence of
amino acids forms a polypeptide.

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In the 20th century Beadle and Tatum proved that each gene codes for a single
polypeptide. They cultured a fungus, and then caused a mutation in a single gene.
The mutated fungus only grew when a specific amino acid was present, showing that
the fungus lacked the enzyme (a polypeptide) for the synthesis of that amino acid.
- A gene for a particular characteristic or polypeptide occupies a specific position on a
chromosome, known as a gene locus.…read more

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A small group becomes separated from a larger population and is unable to
interact with the original population (so cannot breed with it).
o By chance, this small group contains an abnormally high frequency of certain
o Breeding within the group causes the frequency of these alleles to rise
further; the new population can have characteristics totally different from
the original one. This can result in the formation of a new species.…read more

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Individuals from low-oxygen habitats such as at high altitude usually have
oxygen dissociation curves to the left of others, i.e. oxygen will bind more
readily. This allows oxygen to bind even when there are low levels in the air.
However, it makes it harder for oxygen to be released to the cells.
o Individuals from high-oxygen habitats which require lots of movement (e.g.
ducks) have oxygen dissociation curves to the right ­ oxygen binds less
readily.…read more

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The matrix inside the chloroplast
is known as the stroma, similar to the cytoplasm in a cell.
- A palisade cell in a plant contains all the organelles commonly found in plants, so can
be used as a generic model of a cell. An intestinal epithelial cell in an animal can be
used similarly.
DNA replication:
- In order for cells to divide, the DNA in them must first replicate itself.…read more

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S (DNA synthesis) phase: growth continues and the DNA is replicated,
duplicating the chromosomes.
o G2 (Gap 2) phase: growth still continues. Organelles are replicated and the
apparatus for mitosis forms.
- Following interphase is mitosis ­ the division of the cell to from two diploid daughter
- This is the process by which the cell divides.…read more

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Mitosis and cancer:
- Cells in multicellular organisms only divide when required; otherwise they enter the
G0 phase of the cell cycle where no division occurs.
- If a cell is no longer required or a mutation is detected, the cell may enter apoptosis
­ a process of controlled cell death in which the organelles break down and the cell
disintegrates into membrane-bound packages.…read more

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Vascular tissues: the xylem, which transports water up the stem, and the
phloem, which transports food substances.
- All plant cells are totipotent ­ i.e. with the correct stimulus any plant cell or cutting
can be made into an entire plant. This is used to create genetically identical clones of
plants such as crops for orchards etc.
Effects of size on animals:
- In general, an increase in the size of an organism decreases the surface area to
volume ratio.…read more

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Efficient circulation and ventilation also help maintain the concentration
gradient ­ there is a constantly replenished blood supply, and the operculum,
a bony flap, pumps water over the gills.
o The gill plates of fish are very thin to decrease the diffusion distance.
- In plants:
o Gaseous exchange occurs through stomata: microscopic pores on the
surfaces of leaves. These lead into air spaces in the mesophyll layer of the
o The stomatal aperture (opening) can be controlled by the plant.…read more

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Upper limbs = subclavian vein/artery.
o Liver = hepatic vein/artery.
o Stomach and intestines = gastric and mesenteric arteries; deoxygenated
blood from the stomach and intestines passes through the hepatic portal vein
to the liver before returning to the heart.
o Kidneys = renal vein/artery.
o Genitals = genital vein/artery.
o Lower limbs = iliac vein/artery.
Arteries, capillaries and veins:
- Arteries carry blood under high pressure away from the heart.…read more


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