Edexcel 6BIO2 ~ 3rd June 2013 ~ AS Biology

A selective pressure is a 'force' that causes a particular organism to evolve in a certain direction; limits of resources, predators, disease, adverse weather etc etc
In the case where there is a selective pressure, those with an allele that proves to be advantageous will have the selective advantage- they will survive, reproduce, and the frequency of that allele will increase- it is a directional selection.

Does that help? :smile:

i dont get the term the change in the environment causes a change in the selection pressure? i thought the selection pressure was the change?
i dont make sense :/ lol

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Reply 1001

daniya12

Original post by Rubyturner94

it is :/ now im even confused as to where they are located.

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Im pretty sure it is definitely that primary= cellulose micro fibrils in the same direction and secondary= different direction.
Think about it: primary cell walls are thin and flexible, whereas a secondary wall is developed as the plant ages, and it is much more thick and rigid..... so the micro fibrils must be orientated at different angles.
As for their positions; yeah I'm kinda confused to but it seems to be
-Middle Lamell
-Primary cell wall
-Secondary cell
-Plasma membrane
...Essentially the middle lamella seems the top layer:

And then the primary cell wall, and eventually as the plant ages, a secondary cell wall wil develop under the primary cell wall, and before the plasma membrane

Does that help? :smile:

Reply 1002

Rubyturner94

Original post by daniya12

Im pretty sure it is definitely that primary= cellulose micro fibrils in the same direction and secondary= different direction.
Think about it: primary cell walls are thin and flexible, whereas a secondary wall is developed as the plant ages, and it is much more thick and rigid..... so the micro fibrils must be orientated at different angles.
As for their positions; yeah I'm kinda confused to but it seems to be
-Middle Lamell
-Primary cell wall
-Secondary cell
-Plasma membrane
...Essentially the middle lamella seems the top layer:

And then the primary cell wall, and eventually as the plant ages, a secondary cell wall wil develop under the primary cell wall, and before the plasma membrane

Does that help? :smile:

yh its make sense that way, thanks :smile:

but i dont understand why the text books say diffeeent things

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Reply 1003

maryam1996

Hi, Can someone please help me with question 8)b)v) from the January 2012 paper the question is to Describe and explain the effect on the mean error rate per rat for maze-bright rats when they grew up in normal cage conditions and enriched cage conditions. The answer in the mark scheme says -No difference -Having maximum potential/ genetic potential. What do they mean by (having maximum potential/ genetic potential). Thank you :smile:

Reply 1004

daniya12

Original post by maryam1996

Hi, Can someone please help me with question 8)b)v) from the January 2012 paper the question is to Describe and explain the effect on the mean error rate per rat for maze-bright rats when they grew up in normal cage conditions and enriched cage conditions. The answer in the mark scheme says -No difference -Having maximum potential/ genetic potential. What do they mean by (having maximum potential/ genetic potential). Thank you :smile:

As there is no difference between the error rate, you can assume that the environment did not have an effect on the rats ability, so it must be the genotype. But as they are genetically similar, both being maze bright, they will have the same ability and both have the same/ similar potential/

Reply 1005

Randy123

hey, does anyone know what we need to know about the taxonomic hierachical system?

Reply 1006

RoyalBlue7

19

Original post by Rubyturner94

in the.red snab book it doesnt say that. and also in the mark scheme.

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this is disturbing. Thats why I hate Biology. Cant there be more consistency?"? :angry:

Reply 1007

kimmykim1

hey can some one sum up the ethics of using stem cells? :biggrin:

Reply 1008

Branny101

Original post by StUdEnTIGCSE

this is disturbing. Thats why I hate Biology. Cant there be more consistency?"? :angry:

This.

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Reply 1009

DanielDaniels

14

Help....shall i write 3 or 4 or 5 or 6 points here?

Reply 1010

That_Clever_Guy

2

Original post by Daniel Atieh

Help....shall i write 3 or 4 or 5 or 6 points here?

i always try and write more just to be on the safe side but three valid points should be sufficient

Reply 1011

RoyalBlue7

19

Original post by Daniel Atieh

Help....shall i write 3 or 4 or 5 or 6 points here?

Unlike in Chemistry they don't penalise inaccurate information.
I guess :tongue:

So write as many as that come in your head

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Reply 1012

RoyalBlue7

19

Original post by mynameisntbobk

that diagram's wrong, where did it come from?

Original post by Branny101

Google and research, this is what is shown.
Maybe it is because plants grow from inside out ( hence the concentric rings in the bark)?

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The diagram not wrong.
If you see closely primary cell wall is actually the inner layer, secondary cell wall is the outer layer.

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Reply 1013

RoyalBlue7

19

Well between two daughter cells first is the middle lamella formed, then the primary wall then OVER it the secondary cell wall.
The diagram agrees.
The secondary wall is inner to the cell, the primary cell wall is outer to the cell.
The secondary cell wall is outer to the boundary of the cell and the primary cell wall is inner to the boundary.
We both mean the same thing... :tongue:

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Reply 1014

Ingamo

1

Has anyone got the jan 13 unit paper to hand? I would look through the thread but it may take a while.. :rolleyes:

Reply 1015

That_Clever_Guy

2

Original post by Ingamo

Has anyone got the jan 13 unit paper to hand? I would look through the thread but it may take a while.. :rolleyes:

Jan 2013 Unit 2 MS.pdf Jan 2013 Unit 2 QP.pdf

Reply 1016

Rubyturner94

so have we come to a conclusion as to where secondary and primary cell wall are? and the way the micrifbrils and arranged im each?

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Reply 1017

AbiHem

Does crossing over of chromatids happen and then independent assortment happen?
Or are they complete different things?! IM CONFUSED HELP ME PLEASE ANYONE

Reply 1018

Ellsxo

Anyone wanna give a guess at the grade boundaries for the Specimen paper?

Reply 1019

kevsamuel

Could someone list all the topics that we NEED to know, because I don't want to be studying topics like epistatis if it's not actually in the syllabus.

Thanks :smile:

Edexcel 6BIO2 ~ 3rd June 2013 ~ AS Biology

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