A2 Revision Notes

My name is Sophie. I am currently in year 13 studying maths, chemistry and biology. i thought starting a blog to store my revision notes would be a cool idea. feel free to have a look, ask questions, correct me, whatever.

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Does anyone have the January 2012 question papers for C3 and/or C4?

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Would anybody like me to post notes on anything?

Anything from A2 Biology (AQA) Chemistry (AQA) or Maths (MEI)

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How did we feel about core 3 today?

For me, it was far from easy, but manageable. 

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C3, MEI? Possibly? Thanks in advance!

Any specifics? Differentiation? Integration? Logs?

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Is there any A Level maths anyone would like me too go over?

Anything from 

C1

C2

C3

S1

M1

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Nitration of Benzene

Conditions - Benzene is treated with a mixture of concentrated Nitric acid and concentrated Sulphuric acid (H2SO4 being the catalyst as it is regenerated at the end of the reaction)

Reaction to produce the reactive intermediate-

Mechanism:

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Eutrophication

Eutrophication can be caused by leeching or sewage pollution 

Leeching - Water soluble molecules are washed out of the soil by rain into nearby rivers and ponds. 

Excess nitrates=stimulated algae growth 

Lots of algae leads to sunlight becoming blocked from the plants growing below

Plants dies because the cannot photosynthesize enough

Bacteria feed on dead plant matter

More bacteria=more respiration=lower oxygen concentration in the water

Fish and other organisms die due to not enough oxygen being available for respiration

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The Nitrogen Cycle

Plants absorb nitrates

transported up the xylem from the roots (taken in by active transport)

they are used to synthesise proteins

-Animals get nitrogen from plants by consuming them

-The excrete excess amino acids as urea or ammonia into the soil.   

AMMONIFICATION

Nitrogen compound in the soil fro dead animals, plants and waste products

They are broken down by SAPROPHYTES into ammonia. They use saprophytic digestion by secreting enzymes 

NITRIFICATION

Ammonium converted into nitrates by nitrifying bacteria in a series of oxidative reactions

DENITRIFICATION

Done by denitrifying bacteria. 

This occurs on anaerobic conditions where there is very little to no oxygen available eg. bogs, waterlogged soils 

Nitrogen is released as a gas to the atmosphere 

NITROGEN FIXATION

This is done my nitrogen fixing bacteria eg. Rhizobium

Nitrogen gas is fixed into a form which plants can use such as ammonium (used to synthesise amino acids)

Bacteria exist in small nodules of LEGUMINOUS plants (clovers, peas) or can be free living in the soil

They use the enzyme nitrogenase to fix nitrogen gas at low temperatures. 

Bacteria in the nodules have a mutualistic relationship with the plants - the bacteria get sugars from the plant while the plant gets nitrates from nitrogen fixation

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RESPIRATION - The Kreb Cycle

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RESPIRATION - The Link Reaction

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RESPIRATION - Glycolysis

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Speciation

Two individuals can be bred to see if they're the same species. If they can produce fertile offspring then they are the same species.

Speciation is when two populations are isolated and are no longer able to interbreed fertile offspring. It happens in 3 steps:

I.N.S:

ISOLATION- geographical barrier separates a species 

NATURAL SELECTION - occurs differently in each population due to different mutations or adaptations.

SPECIATION - The two populations will have become so genetically different over time they can no longer breed. They are then considered different species.

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Natural Selection

Natural selection can summarised using V.I.B.E

Variation

Intraspecific competition 

Breeding

Evolution

VARIATION - all organisms in a population will vary. No two members of the population have the same genotype. (excluding asexual reproduction and twins of course)

INTRASPECIFIC COMPETITION - There is a 'struggle for survival'. Member of the population will compete for food and shelter to escape from predators and fid a mate.

BREEDING - The member of the population who possess the features that enable them to survive will breed and pass of these characteristics to their offspring. The ability to survive is called the 'fitness'

EVOLUTION - Over a number of generations, if there is still a selection pressure, the population will change. The population will on average have different features than previously due the frequency of alleles changing over time.  

EXAMPLE QUESTION

Explain how a population of polar bears dumped on a desert island might become brown in colour. 

Variation within a species will cause some polar bears to be darker than others

Darker bears will survive better because they are better camouflaged than their rivals.

The darker bears survive longer and will therefore have more opportunity to pass on the allele for darker fur to their offspring

The allele for darker fur becomes more frequent in the next generation. The bears will become darker on average over time. 

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Hardy-Weinberg

Hardy-Weinberg is based on frequencies. The frequency shows a proportion on a 0-1 scale.

They came up with two laws:

p+q=1            and           p2+2pq+q2=1 

p= frequency of the dominant allele

q= frequency of the recessive allele

p2= frequency of the homozygous dominant individuals 

2pq= frequency of the hetrozygous individuals 

q2= frequency of the homozygous recessive individuals 

I like to think of it this way-

Take two people, both hetrozygous pq. The probability of them actually having a child is 1.

Using a punnet square diagram, the second equation makes perfect sense. 

All these adding together is the total probability of having an offspring. This is the way i like to remember which part of the equation is for which genotype. 

For an example population:

p= 5/8 = 0.625                   p2= 2/4 = 0.5

q= 3/8 = 0.375                   q2= 1/4 = 0.25

                                       2pq= 1/4 = 0.25

EXAMPLE QUESTIONS (simple ones)

In a population, there are 100 people. The frequency of the dominant allele is 0.65.

1) Calculate the frequency of the recessive allele.

p=0.65

Using p+q=1:     0.65+q=1      1-0.65=q      q=0.35

2) Calculate the frequency of homozygous dominant alleles

p=0.65

p2=0.4225 = 0.42

3) Calculate the number of homozygous dominant individuals

By multiplying the frequency by the population number, you arrive with the number of individuals with that genotype. 

p2=0.42    0.42x100= 42

(harder one)

In a population of 2000 foxes, there are 322 homozygous recessive individuals. Calculate the number of hetrozygous dominants and hetrozygous individuals.

We are given the frequency of homozygous recessive (q2)

q2=322/2000= 0.161              q=0.401

p2=0.359                             p= 1-0.401 = 0.599

Homozygous dominant = 0.359x2000 = 718 individuals

Hetrozygous = (2x0.401x0.599) x 2000 = 960 individuals 

Principles of Hardy-Weinberg

Hardy-Weinberg predicts that a populations gene pool does not change from generation to generation. This means that the frequency of of alleles stays the same over time, provided that:

there is a large population

mating is done completely randomly

no introduction of new alleles by immigrants 

no genetic mutations 

Factors that disturb the principle

Certain characteristics will be susceptible to more predators eg. Slow runners who cannot out run the predator or bright colors which make them more visible

Emigration/immigration/mutations will always occur withing a population which will add or remove alleles

Founder effect can mean a new population has an imbalance of alleles