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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…

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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…

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o 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…

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o 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…

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of photosynthesis in starch or lipid droplets. 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.…

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o 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

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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…

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o 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…

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o 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…

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o 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…


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