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# Edexcel A2 Biology Unit 4(6BIO4) 13/06/11 - OFFICIAL THREAD ! watch

1. (Original post by Maria1234)
guys how do they use 'molecular clocks' as evidence for evolution?
we don't need to know that for this unit, but anyways, Molecular clock is a technique used to deduce the time when two species diverged or when a species evolved.
Scientists mainly use fossil constraints to collect data - it is very similar to climate proxies, but rather than measuring temperature change, you measure changes in species.
2. does anyone know what the grade boundaries are? i.e. what do you have to get out of 90 to get a B?
3. (Original post by LibbyU)
does anyone know what the grade boundaries are? i.e. what do you have to get out of 90 to get a B?
im sure its out of 120 :s
4. (Original post by LibbyU)
does anyone know what the grade boundaries are? i.e. what do you have to get out of 90 to get a B?
UMS marks are out of 120 and you have to get 96 to get an A
5. (Original post by LibbyU)
does anyone know what the grade boundaries are? i.e. what do you have to get out of 90 to get a B?
it depends from year to year. But usually an A is about 58-60 Raw marks so a B would be around the beginning 50s!
6. Thanks guys Can someone explain what raw marks and UMS are? I get that the paper is marked out of 90 and that score is then converted to UMS but what is UMS? why do they convert it?

there's nooo way I'd ever get an A in bio, im predicted a C but really wanna get a B! does anyone know what a C is? (out of 90) and if B is the beginning 50's out of 90 then thats really good!
7. (Original post by LibbyU)
Thanks guys Can someone explain what raw marks and UMS are? I get that the paper is marked out of 90 and that score is then converted to UMS but what is UMS? why do they convert it?

there's nooo way I'd ever get an A in bio, im predicted a C but really wanna get a B! does anyone know what a C is? (out of 90) and if B is the beginning 50's out of 90 then thats really good!
they convert the raw mark (/90) to ums (/120) to take into account how hard the paper was. It would be unfair if they just gave your raw mark score as if it was a really hard paper no one would get an A. Previous boundaries:

jan10 jun10 jan11
A*63 63 58
A 59 58 54
B 55 53 50
C 51 49 46
D 47 45 42
E 44 41 38
8. Hi can someone please explain to me the difference between abundance and distribution.

Why do density independant factors affect distribution and density dependant factors affect abundance.
And why does intraspecific compettion affect abundance but interspecific competiton affects both abundance and distribution?
9. Anyone know where to get answers to these questions: http://vle.havant.ac.uk/Biology_web/..._questions.htm

Thanks
10. hey im stuck on plants atm, basically i know nitrate are used to make proteins in plants with glucose. Yet if there is a lack of nitrates that means there is a lack of protein being made, so what happens to the glucose that cant be converted to protein?
11. Can someone help me with this point on the specification.

13. Explain the roles of antigens and antibodies in the body's immune response including the involvement of plasma cells, macrophages and antigen-presenting cells.

Earlier on in the book it states that macrophages are part of the non-specific immune response, but then later on states that antibodies bind to antigens which label them for engulfment by macrophages, so basically what do we need to know in relation to antibodies and macrophages.
12. (Original post by lbavfc)
Can someone help me with this point on the specification.

13. Explain the roles of antigens and antibodies in the body's immune response including the involvement of plasma cells, macrophages and antigen-presenting cells.

Earlier on in the book it states that macrophages are part of the non-specific immune response, but then later on states that antibodies bind to antigens which label them for engulfment by macrophages, so basically what do we need to know in relation to antibodies and macrophages.
Macrophages engulf the pathogens and then display the foreign (or non-self) antigens on their surface. When any cell in the human body displays foreign antigens in this way it is known as an antigen-presenting cell (APC).

Macrophages are part of the non-specific and specific immune response. They are non-specific in that when they come across any foreign antigen pathogen they try and engulf and digest them off. They don't know what type of organism they are digesting (i.e. they can't say ooo this is Virus 'X', bad!! - kill it!). They say.. hey.. I don't know if you are Virus X or Y but I do know you're not meant to be part of the body let me kill you.

Then once they present the antigens, this activates the specific immune response (T-cells and B-cells etc. etc.), which have complementary receptors that are specific to that antigen and bind to the antigens via these receptors.

I've put my notes on the immune system response in the spoiler

Spoiler:
Show

• Viruses/bacteria have antigens which are recognised as ‘non-self’ and this recognition activates the non-specific and specific immune responses
• Phagocytes engulf virus/bacterium
- pathogen now contained in a vesicle in the phagocyte
- Vesicle containing pathogen fuses with a lysosome and enzymes in lysosome break down the pathogen
- Phagocyte then presents the pathogen’s antigens on it’s surface ? activates other immune system cells
• Phagocyte presents the antigen to a T-helper cell.
- T-helper cell binds onto the antigen via complementary surface receptors
- This activates the T-helper cell and it then divides to make, active T-helper cells and T-memory cells
• B-cells have antibodies on their surface and these binds to a pathogen with complimentary antigen. NOTE: Each B-cell will bind to a different antigen
- This antigen-antibody complex along with T helper cell activate the B-cell
- The B-cell then divides and differentiates into plasma cells and B-memory cells
- Plasma cells produce antibodies which are bind to the antigens of the pathogen that triggered the response in the first place which helps destroy the pathogen.

CELL-MEDIATED RESPONSE
Bacterium/virus infects a host cell ? Host cell displays antigens ? T-killer cells with complimentary receptor bind to the antigens ?If exposed to cytokines by T-helper cells the T-killer cells divide and differentiates into active T-killer cells and T-memory cells ? Activated T-killer cells bind to infected cells which are presenting the complementary antigen ? Release enzymes and chemicals that cause pores to form in the infected cell’s membrane which leads to cell lysis and cell death.

NOTE: Phagocyte ^ is a general term for any WBC which engulfs and digests stuff. A macrophage is a type of phagocyte.

Antibodies kill a pathogen in many ways:

1) reduce the pathogens' ability to infect cells (don't ask me how coz book doesn't say so and I doubt we need to know)

2) bind to the foreign antigens to make an antigen-antibody complex - which is readily digested by macrophages

3) antigen-antibody complex causes pathogens to clump together preventing them spreading to other parts of the body (don't ask me how again )
13. (Original post by InItToWinItGetIt?)
Macrophages engulf the pathogens and then display the foreign (or non-self) antigens on their surface. When any cell in the human body displays foreign antigens in this way it is known as an antigen-presenting cell (APC).

Macrophages are part of the non-specific and specific immune response. They are non-specific in that when they come across any foreign antigen pathogen they try and engulf and digest them off. They don't know what type of organism they are digesting (i.e. they can't say ooo this is Virus 'X', bad!! - kill it!). They say.. hey.. I don't know if you are Virus X or Y but I do know you're not meant to be part of the body let me kill you.

Then once they present the antigens, this activates the specific immune response (T-cells and B-cells etc. etc.), which have complementary receptors that are specific to that antigen and bind to the antigens via these receptors.

I've put my notes on the immune system response in the spoiler

Spoiler:
Show

• Viruses/bacteria have antigens which are recognised as ‘non-self’ and this recognition activates the non-specific and specific immune responses
• Phagocytes engulf virus/bacterium
- pathogen now contained in a vesicle in the phagocyte
- Vesicle containing pathogen fuses with a lysosome and enzymes in lysosome break down the pathogen
- Phagocyte then presents the pathogen’s antigens on it’s surface ? activates other immune system cells
• Phagocyte presents the antigen to a T-helper cell.
- T-helper cell binds onto the antigen via complementary surface receptors
- This activates the T-helper cell and it then divides to make, active T-helper cells and T-memory cells
• B-cells have antibodies on their surface and these binds to a pathogen with complimentary antigen. NOTE: Each B-cell will bind to a different antigen
- This antigen-antibody complex along with T helper cell activate the B-cell
- The B-cell then divides and differentiates into plasma cells and B-memory cells
- Plasma cells produce antibodies which are bind to the antigens of the pathogen that triggered the response in the first place which helps destroy the pathogen.

CELL-MEDIATED RESPONSE
Bacterium/virus infects a host cell ? Host cell displays antigens ? T-killer cells with complimentary receptor bind to the antigens ?If exposed to cytokines by T-helper cells the T-killer cells divide and differentiates into active T-killer cells and T-memory cells ? Activated T-killer cells bind to infected cells which are presenting the complementary antigen ? Release enzymes and chemicals that cause pores to form in the infected cell’s membrane which leads to cell lysis and cell death.

Antibodies kill a pathogen in many ways:

1) reduce the pathogens' ability to infect cells (don't ask me how coz book doesn't say so and I doubt we need to know)

2) bind to the foreign antigens to make an antigen-antibody complex - which is readily digested by macrophages

3) antigen-antibody complex causes pathogens to clump together preventing them spreading to other parts of the body (don't ask me how again )
Thank you, so in relation to macrophages and antibodies, is it true that the macrophage can engulf a bacterium more easily if an antibody is bound to one of its antigens?
14. (Original post by lbavfc)
Thank you, so in relation to macrophages and antibodies, is it true that the macrophage can engulf a bacterium more easily if an antibody is bound to one of its antigens?
I would think so, as when the antibody binds to a foreign antigen, it probably releases chemicals that somehow notify other immune cells/defence mechanisms to kill off the pathogen.
15. I need a clear conception abou "Binary Fission" in Bacterial reproduction.
I am also confused about "Founder effect"
16. got a point which i thought of sharing-
Scientific theory or knowledge has a disadvantage.It is always only tentative and can be disproved at any time.That means,a scientific theory only establishes temporarily on the basis of best explanation possible during the development of the theory.It is very much likely to be proved wrong in future.
17. (Original post by phetylheminthes)
I need a clear conception abou "Binary Fission" in Bacterial reproduction.
I am also confused about "Founder effect"
Binary is asexual reproduction, when one bacteria cell divides into two. And I've never heard of the ''founder effect''. Are you sure we need to know it coz I can't find it on the spec?
18. can anyone tell all a2 bio practicals we need to know about and maybe a site where I can go over the methods,results and conclusion. Because I havent done any practicals as Iam studying by myself.
19. can anyone give a decent explanation of the specific immune responses? every book i have explains it differently and often contradict each other :|
20. (Original post by InItToWinItGetIt?)
Binary is asexual reproduction, when one bacteria cell divides into two. And I've never heard of the ''founder effect''. Are you sure we need to know it coz I can't find it on the spec?
well....i just want to know the whole process of Binary fission.Can you tell me easily and elaborately?
talking about founder effect,we had in in u-2.just trying to know coz u-4 ques might include some topics from u-2(nothing from u-1 for sure).

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