Edexcel 6BIO2 ~ 3rd June 2013 ~ AS Biology

These are the points from the mark scheme:

1) Idea of selection pressure/change in environment.

2) Reference to competition/predation.

3) Mutation (in context).

4) idea of advantageous allele.

* What this means is that a change in the environment ( extreme temperature changes etc.)/ or competition (giraffes with longer necks have a better chance of reaching high leafs on a tree, than giraffes with shorter necks)/ or predation (a new predator is introduced to the environment or that there is an increase in a certain predator - more foxes than rabbits) will make that organism have to adapt to the new change. That is caused by a mutation, where an advantageous allele is produced, this allele contains the change(s) that the organism needs to adapt.

5) Idea that individuals with advantageous {alleles/characteristics} survive and breed.

6) idea of (advantageous) {allele/mutation} being passed on (to future generations);

7) idea of more individuals with this adaptation in the population/increased frequency of advantageous alleles in the population.

* This means that organisms with this advantageous allele will have an advantage over the organisms that don't have the allele. This is natural selection at its finest where the organisms at a disadvantage - without the special allele - will die out while the ones with the allele will survive to reproduce. For the last point, its hinting towards as the generations progress there will be more organisms with the advantageous allele - as more will reproduce - and less organisms with the original genetic makeup as they will have died out.

mutations are random, so can the environment infulence a mutation in a gene?

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Yup, mutations can be cause by either external (exogenous) or endogenous (native) factors. Those factors can induce mutations in DNA.

Exogenous factors include: Environmental factors such as sunlight, radiation, and smoking can cause mutations.
Endogenous factors include: Errors during DNA replication.

its not the microfibrils that are held by hydrogen bonds, its the cellulose chains that are held by hydrogen bonds.

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Yup, thats what i was thinking.

finally thats all i needed to get it thanks
so in the context of natural selection, if the environment changes it influences a mutation in the gene, and the gene maybe advantageous, therefore giving the individual with this gene a selective advantage of others towards this change in the environment?
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Then how would you define a microfibril?

A long chain of beta glucose monomers held together by glycosidic bonds??

bundles of cellulose molecules is more appropriate.

you could say, it consists roughly of 60~70 cellulose polymers, each roughly 1000~10000 b-glucose monomers long. Adjacent cellulose molecules are held together by hydrogen bonds, forming microfibrils..

Then talk about where you'd find them such as in the cell walls of plant cells, which are held together by pectin/polysaccharide glue

Wait woah isn't it parallel cellulose molecules by hydrogen bonds, adjacent via glycosidic? And I'm a bit confused too so cellulose molecules make up microfibrils that make up fibrils that make up fibres?

Thanks in advance.

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i dont understand how a predation/competition can lead to a mutation ?

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Predation - The story about the moths, where the white moths had a bad camouflage and died out due to birds finding them easily. While black moths had a better camouflage and could hide from predators.


Competition - The story about the giraffes, where the longer neck giraffes could eat from higher trees and therefore survived to reproduce.

Just think about it in terms of lots of random mutations can occur and one of these mutations can fit the need of the organism, which will make that specific mutation the one that is known as having the advantageous allele.

bundles of cellulose molecules is more appropriate.

you could say, it consists roughly of 60~70 cellulose polymers, each roughly 1000~10000 b-glucose monomers long. Adjacent cellulose molecules are held together by hydrogen bonds, forming microfibrils..

Then talk about where you'd find them such as in the cell walls of plant cells, which are held together by pectin/polysaccharide glue

In essence that's best way to understand it (not just environment, its also competition and predation). Just remember to mention all the following points when talking about adaptation.

1) Idea of selection pressure/change in environment.

2) Reference to competition/predation.

3) Mutation (in context).

4) idea of advantageous allele.

5) Idea that individuals with advantageous {alleles/characteristics} survive and breed.

6) Idea that advantageous {allele/mutation} being passed on (to future generations)

7) idea of more individuals with this adaptation in the population/increased frequency of advantageous alleles in the population.

can u give me an idea of how u revise these days? like do u only solve papers from edexcel board? which book u use now? Thanks

thanks hows revision going?

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adjacent as in molecules which are next to each other.. they may not be necsarily parallel.

Okay for that bit, don't remember it like that, just remember fibres consist of 2 cell walls, the primary cell wall and secondary cell wall. The secondary cell wall has more microfibrils (is thicker) making it stronger. Also, the microfibrils are arranged in nets which increases its strength and they are held together by pectin/hemicellulose