pearson1995
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Hello everyone,

I am fairly new to these forums and will be taking Biology Unit 2 in the summer. It is just under a month away and I am going to go over what I believe is the key parts to nailing this exam. As I am sure you already know, this exam is worth the most UMS Points out of all the AS Biology exams and for that reason as high score is very important. I personally am aiming for 124 UMS so basically 90% UMS. Make no mistake, this exam exam isn't easy, but it isn't impossible either

Variation

Difficulty 3/10

This topic isn't to hard. Basic ideas of a large sample size to find a representative mean etc. Repeat readings to find anomalous results and to calculate a mean. Interspecific and infraspecific variation. Know the difference between discontinuous data and continuous data. In short continuous data has the possibility of having any number e.g. height, where as discontinuous has set possibilities e.g. number of arms (you can't have 0.578 arms ). One of the most important things to know for this topic is how to calculate standard deviation using your calc. Every calc is different so make sure you know how to use yours!!!



DNA and Meiosis

Difficulty 8/10

A big topic this one. Make sure you know the basic information about DNA i.e. what each part of the nucleotide is called (phosphate group, deoxyribose etc etc.), what bases are complimentary to which (I remember it as AT Go Compare) DNA is a poly nucleotide because of its repeating units. H-Bonds between bases. All of the above is basic knowledge you must must know!

Introns are non coding DNA. You define a gene as a sequence of nucleotides forming part of a chromosome. Also Eukaryotic DNA is in a nucleus where as Prokaryotic DNA is free within the cytoplasm and is circular.

Meiosis is something which you just have to learn, make sure you know every single part of it. Try and do it back to front, if you can you really do know it. Homologous chromosomes determine the same genetic characteristic but aren't identical. This is because they have the same genes but different alleles (versions of a gene)

DNA is a very stable molecule and can pass on genetic info on without change. Its strands can separate during replication and act as templates. It is also a very large molecule and therefor can hold a lot of genetic information.


Genetic Diversity

Difficulty 6/10

3 parts to this topic:

Founder effect, where an individual(s) moves into an area with a small allele pool compared to the rest of the original population and therefore future generations have a small allele pool. the frequency of the alleles will increase in future generations

Bottleneck, where a large amount of organisms from a population die, and a few survivors remain, they have a small allele pool compared to the original population.

And the one everyone forgets..... Selective breading, as we select certain characterises, we reduce the variety of alleles in the population.



Variety of Life

Difficulty 6/10

2 main parts

1) Haemoglobin, 4 polypeptide chains, primary sequence is the order of bases. secondary sequence is the way it coils. Tertiatry sequence is the way it folds into a specific shape. (when an enzyme is denatured it is this tertiary shape which is altered and therefore ionic/hydrogen and disulphide bonds are broken). Quaternary structure is the way the polypeptide chains are linked.

Contains Fe2+ which is where oxygen can attach to (associate) and where oxygen leaves (dissociate). High affinity means it will associate easier and low affinity is the opposite.

When a muscle respire, it releases CO2, lowering pH, changing the Tertiary structure, so O2 dissociates, O2 can be used by muscle cells.

Make sure you know how to read Oxygen dissociation curves. Basically to the right means low affinity, therefore animals which have a crave to the right have a large SA:VOL so are small. High altitude means little oxygen, therefore curve shifts to left to have a higher affinity so Heam can associate with what little oxygen there is in the air.

2) Starch, Cellulose and glycogen, just read the text book, not too much tbh, remember that Startch is only found in plants and is insoluble so it doesn't effect water potential, also coiled so compact

Last thing in the topic is blood vessels, personally I find this kind of obvious so I won't go into detail, just read the text book if your not sure, it explains it fairly well.


The Cell Cycle

Difficulty 7/10

2 stages of cell division, Nuclear division and cell division.

Learn of by heart: DNA Helicase break down the H-Bonds between the bases so the two strand unwind. They then act as templates for free nucleotides to attach to. A to T and G to C. This process is helped by DNA Polymerase. This is semi conservative replication.

^Easy 6/7 marks there ^

Learn Mitosis in the same way as Meiosis. i.e. which stage does DNA get replicated?
Spoiler:
Show
Interphase


If you know Mitosis off by heart, you will find this whole topic kind of easy tbh.



Cellular Organisation

Difficulty 5/10


Easy stuff here. Tissue is a collection of similar cells. Organ is a group of different tissues and an organ system is a group of organs working together.

All cells mature by a process called cell specialisation. (different genes get switched on)


Exchange and Transport

Difficulty 10/10


This is the hardest topic in my opinion. So much knowledge required.I find you can break it down into 3 parts.

Diffusion - first of all learn the general principles. i.e. how SA:VOL, Difference in conc gradient, distance of pathway and time all effect diffusion. Then you have to apply your knowledge to different organisms.

First of all insects. Spiracles is where gas is exchanged, and they go into trachea and then tracheoles, this increases the SA. Insect can pump their abdomen causing air to be drawn in and this increases the conc difference. As insects are small they have a short diffusion pathway.

Ok, next are fish. Fish have girl which have gill filaments and gill lamella 90 degree to the filaments increasing the SA. Counter current blood flow which increases the conc gradient so much so that there is always a conc difference. Rich blood supply, so short diffusion difference.

Humans we know about from unit 1, I haven’t seen many questions on Unit 2 papers ask about human exchange system

Plants have stomata which is where gas is exchanged. They also have spongy mesophyll to increase SA.

For all these cases the site of gas exchange is a way for water to be lost, for that reason insect close their spiracles as can plants their stomata. Generally plants have stomata open in the day and closed at night.


Tissue Fluid, they always ask this usually a 3/6 marker and it really easy.

Tissue fluid leaves the capillary due to hydrostatic pressure which forces the water out as it overcomes the osmotic forces. Proteins remain in the capillary as they can’t pass through the membrane, this lowers the water potential of the capillary. O2 diffuse into cells and CO2 diffuses into tissue fluid. Water then re enters the capillary via osmosis as the capillary has a lower water potential. The remaining tissue fluid enters the lymphatic system.


For me this is the hardest part of the exam, how water moves through a plant. I find it helps to break it down into parts rather than just learning it all at once.

First of all water enters the plant via the root hair cells. This is because the root tip has mineral ions and amino acids, this lowers the water potential and water can enter the root hair cell via osmosis. Water then moves through the cortex via the apoplastic and symplastic pathway. The apoplastic pathway involves water moving through the cell walls, as it moves it pulls the water molecule behind it due to the cohesive properties of water. Water has cohesive properties as it has Hydrogen bonds between molecules. The symplastic pathway involves water moving from the cytoplasm of one cell to the next via osmosis, each neighbouring cell has a lower water potential, so the water molecule moves from cell to cell.

Once the water reaches the casperian strip, which is a waterproof barrier, water can only travel via the symplastic route. The water then has to enter the xylem, this is achieved by the xylem actively transporting mineral ions from the endodermis into the xylem lowering the water potential so water enters via osmosis. Due to water cohesive properties, the water molecules make one continuous water column from the bottom of the xylem to the leafs stomata. The xylem also contains nitrogen in its proteins so water can form hydrogen bonds with that, this is called capillary action.

As a water molecule leaves the leaf via stomata (transpiration) water molecules behind it gets pulled up due to waters cohesive properties. This phenomena is called cohesion tension theory. As water gets drawn up the root hair cell due to the root hair cell having a lower water potential and causing water to enter via osmosis, this phenomena is called root pressure.


Sorry if I haven’t explained this topic very well, I personally think this is the hardest as it requires so many facts. I have attached a youtube video below which explains the topic very well.





Classification

Difficulty 6/10


This isn't a too hard topic. We classify animals in 2 ways, artificial classification which classifies animals due to their phenotypes. Natural/Modern Classification classifies animals based on their phylogenetic relationships (their evolutionary relationships).

Taxonomy is identifying species and classifying them.
A hierarchy is groups within groups without any overlap.

Kingdom, Phylum, Class, Order, Family, Genus, Species.
(King Philip Came Over From German Soil)

We name animals with the binomial system. Two name, Generic name and specific name e.g. Homo sapien Remember italic and specific name has NO CAPITAL LETTER!. You will loose a mark.



Relationships Between Organisms

Difficulty 8/10


4 ways to find the relationships between organisms.

1) Compare Base Sequence
2) DNA Hybridisation
3) Compare Amino Acids
4) Immunological response.

(For 1 and 3)

-Proteins are made by polypeptide chain(s)
-A polypeptide is made by Amino Acids linking together via a condensation reaction forming a peptide bond.
-As the polypeptide has a tertiary structure it contains ionic,hydrogen and disulphide bonds.
-Amino Acids are coded by 3 bases (triplet code)
-More similar base sequence (primary sequence) = more closely related.

(For 2)

2 Different stand of DNA are heated to high temperature (one is radioactively labeled)
this break the H bonds between bases
As the strands cool they will combine with one another
Some of the new double helix formed will be made from the strands of different organisms
New H bonds between complimentary bases are formed
More H bonds means a higher temp is required to separate the 2 strands and therefore the organisms are more closely related.


(For 4)

-Serum from organism A is injected into organism B
-organism B produces antibodies
-Serum form organism B is extracted and mixed with blood from organism C
-Antigens respond and make a precipitate.
-The more ppt formed, the closer organism A and C are related.


Also you have to know about courtship behaviour, its kind of simple. Basically its to find opposite sex member of the same species that is sexually mature.



Bacteria

Difficulty 7/10

Antibiotics prevent the cell wall properly forming.
Water then can enter the bacteria as it has a lower water potential than the surrounding water.
This water causes the cell to burst. This is called osmotic lysis.

When antibiotics are given
-occasionally a bacteria will survive due to it having a mutated resistant allele.
-it survives and can then asexually reproduce passing on its DNA to the next generation. Vertical Gene Transmission
-It can also pass on its resistant allele to other species, by replicating the plasmid that the allele is on, via a conjugation tube. This is called Horizontal Gene Transmission.



Biodiversity

Difficulty 5/10

Make sure you know the equation d=N(N-1)/Σn(n-1) and how to use it.

If deforestation occurs, it decreases the variety of plant, decreasing the variety and number of habitats, and decreasing the variety and number of food sources. Easy 3 marks there.

Species Diversity is the number of different species and number of individuals of each species.











And thats it guys, I have written this myself and am only a student so there may be a couple of mistakes, please correct me!

The best advice I can give is do past papers


Good luck
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missuniverse23
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[QUOTE=pearson1995;47445920]
I LOVE YOU!!!! You are so helpful. Thanks. MAy god bless you
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pearson1995
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[QUOTE=missuniverse23;47446686]
(Original post by pearson1995)
I LOVE YOU!!!! You are so helpful. Thanks. MAy god bless you
No problem, any questions just ask
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jaszs
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do you have one for unit 1 or a2?
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pearson1995
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(Original post by jaszs)
do you have one for unit 1 or a2?
Sorry bud, I only wrote this because I'm sitting the exam. I did Unit 1 last year and am retaking unit 2 this year.

From what I can remember Unit 1 wasn't too hard. It required a lot of knowledge but nothing to hard to understand..... except for immunity, that was hard actually. I know I watched youtube videos by a channel called bozemanbiology. It may be called bozemanscience now. Just type in Bozemanscience and the topic your finding hard and he explains everything really well
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jaszs
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(Original post by pearson1995)
Sorry bud, I only wrote this because I'm sitting the exam. I did Unit 1 last year and am retaking unit 2 this year.

From what I can remember Unit 1 wasn't too hard. It required a lot of knowledge but nothing to hard to understand..... except for immunity, that was hard actually. I know I watched youtube videos by a channel called bozemanbiology. It may be called bozemanscience now. Just type in Bozemanscience and the topic your finding hard and he explains everything really well
thanks
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fudgebow
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Hi, so are the xylem walls (partially) made of protein then?
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pearson1995
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(Original post by fudgebow)
Hi, so are the xylem walls (partially) made of protein then?
Hi I didn't explain this part to well in the OP,


I can not find any mark scheme which requires you to say how adhesion occurs. You must know however that it does occur.

In the above video, the man explains how the Xylem is made out of protein, which contains nitrogen atoms for the water to form hydrogen bonds with, and therefore can bond with the xylem which results in adhesion.

I have only ever heard this mentioned in the above video, however you will know if you do Chemistry AS that what he says does make sense.

To be honest, I would write it in the exam, just say "Water also can stick to the sides of the xylem due to adhesion." (I believe that would get you the mark)
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chatty
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(Original post by pearson1995)
Hi I didn't explain this part to well in the OP,


I can not find any mark scheme which requires you to say how adhesion occurs. You must know however that it does occur.

In the above video, the man explains how the Xylem is made out of protein, which contains nitrogen atoms for the water to form hydrogen bonds with, and therefore can bond with the xylem which results in adhesion.

I have only ever heard this mentioned in the above video, however you will know if you do Chemistry AS that what he says does make sense.

To be honest, I would write it in the exam, just say "Water also can stick to the sides of the xylem due to adhesion." (I believe that would get you the mark)

Many thanks for the info.
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chatty
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Thank you so much. You are a star.
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hiphop97
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does anyone know if we need to know anything on the Meselson-Stahl experient. I cant find it in the spec but some people are saying you need to understand the results of it
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chocolattelee
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Wow thanks!~
Have you got any prediction what you think likely gon be on this paper?
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Afghan Warrior
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You are a total life saver, thank you!!! I was worrying so much because I have left revision so late. Very good summary notes! Thanks once again

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vicky.i
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I think you may have just saved my life. To put it simply
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daveperry
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(Original post by vicky.i)
I think you may have just saved my life. To put it simply
Same here, as a bit of last minute revision before the exam today this was SO helpful!
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lushmoo
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(Original post by pearson1995)
Hello everyone,

One of the most important things to know for this topic is how to calculate standard deviation using your calc. Every calc is different so make sure you know how to use yours!!!
um, we don't need to know how to calculate standard deviation - it specifically says so in the spec
just thought i better clear that up before anyone panics
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pearson1995
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(Original post by lushmoo)
um, we don't need to know how to calculate standard deviation - it specifically says so in the spec
just thought i better clear that up before anyone panics
Sorry, I didn't realise that. I think we need it in the ISA though right?
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lushmoo
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(Original post by pearson1995)
Sorry, I didn't realise that. I think we need it in the ISA though right?
Yeah maybe
but otherwise great guide!
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