AQA Biology Unit 4 notes

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  • Created on: 16-03-13 13:23
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Biology Unit 4
Variation terms
Genotype- Genetic constitution of the organism
Phenotype- The expression of a allele e.g. hair colour
Recessive- Only expressed in the phenotype when homozygous, rr
Dominant- Can be expressed either heterozygous or homozygous, RR or Rr
Co-dominant- Both alleles are expressed in the phenotype, RB
Hardy Weinberg
There are no mutations, so no new alleles are created.
There is no immigration, so no new alleles are introduced, and no emigration, so no alleles are lost.
There is no selection, so no alleles are favoured or eliminated.
Mating is random, so alleles are mixed randomly.
The population is large, so there are no genetic bottlenecks.
Gene pool (the sum of all the alleles of all the genes of all the individuals in a population) therefore remains
stable so the allele frequency does not change from generation to generation
Types of selection
Directional- When one extreme phenotype
(i.e. tall necks) is favoured over the other
extreme (short necks). Happens when an
environment changes, includes biotic and
abiotic factors so organisms adapt and
change. This happens in speciation.
Disruptive- When both extremes are selected
over the intermediate e.g. with finches, large
and small beaks selected but birds with
middle can't eat either size seed so aren't
Stabilising- When the environment does not
change too much, i.e. no pressure for a well-adapted species to change, the most common (intermediate)
phenotypes are selected for, and the extremes selected against. E.g. Intermediate weights for babies are
selected for, thin or fat babies against
Geographically and reproductively isolated
Separate gene pools- no interbreeding between populations-
no gene flow
Variation due to mutation
New area has different environmental/abiotic/biotic conditions
Selection for advantageous allele
Selected organisms more likely to survive and reproduce
This leads to a change in allele frequency
If physical barrier removed somehow and mate with original
species, will produce infertile offspring
Occurs over a long period of time
triphosphate) is
useful (E)

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Releases energy is small manageable amounts
It is broken down in one step
Provides an immediate supply of energy
It adds phosphate which makes these phosphorylated substances more reactive by lowering Ea
It can be reformed
Uses of ATP
Muscle contraction and other types of movement e.g. by cilia, flagella
Active transport- Nitrates into plant root
Biosynthesis- protein synthesis, DNA replication, starch synthesis.…read more

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ATP produced in Krebs cycle
Krebs produces NADH and FADH
Electrons are released from NADH and FADH
Electrons then pass through ECT through series of redox reactions
This releases energy and creates a proton gradient
Protons move into intermembrane space
Move back via ATP synthase creating ATP from ADP+Pi
Anaerobic respiration
Pyruvate(3C) Ethanal (2C) +CO2
Ethanal (2C) (reduced) + NADH(Oxidised) Ethanol (2C) +NAD
Because there is no O2, NADH cannot get rid of its H+ in ECT, so no NAD is produced so this…read more

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Occurs in light independent reaction, in the Stroma
Carbon cycle (from dead to leaf)
M.O that decompose are saprobionts
They decompose through extracellular digestion by secreting enzymes onto dead tissue
Absorb products of digestion (glucose)
The M.…read more

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Variation leads to a mutation
Allele for resistance is favourable
This allele is selected for
Pests with this allele more likely to survive and reproduce, passing on the allele to offspring so frequency of
the allele increases
Intensive rearing (How it increases net productivity (Net productivity = Gross productivity ­ respiratory loss) in
Fed on concentrate- high nutritive diet, low in plant fibres (cellulose) so easy to digest- higher proportion of
food is absorbed
Movement is restricted so less energy used
Kept inside in…read more

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Measuring tapes are placed at right angles after finding area on a grid/using coordinates and random
numbers (from tables or a computer) are used as coordinates to choose sampling points in the area.
Then either use a line transect, where organisms touching a piece of string stretched along the transect are
recorded or a belt transect, where quadrats are placed at regular intervals along the transect and organisms
in each quadrat are counted (percentage cover).…read more


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