A level Biology: Post your doubts here!

[My Name]

You thought well but you thought too late!

Sorry, I guess?
How am I late exactly?

 

[qwertypoiu]

People got any ideas on the topics and questions that may come up on this year's paper?
variant 22 please

Oh yeah there's definitely going to be something about plant transport. Also some stuff about diseases.
Also don't forget the DNA structure and cell division.
Gaseous exchange will be the main concentration.
Questions to do with enzymes are also very likely.
Yeah and remember heart structure too .
Just in case remember about Bohr effect as well.

 

[Mahnoorfatima]

Someone please explain the role of B and T lmphocytes properly as i am really confused and am mixing them up

also transcription and translation

please

Okay, so first thing u should know is that B and T lymphocytes work when an immune response is generated when a forgein pathogen enters the body. When a foreign pathogen enters the body, macrophages engulfs it and exposes the cell surface antigens on the pathogen's surface thus macrophages are called APC'S (antigen presenting cells) then Those B cells whose antigen binding site fits the cell interfere with the macrophages and are selected to respond i.e clonal selection takes place. Next these B cells divide by mitosis i.e clonal expansion to produce Plasma cells and memory cells. the plasma cells produce antibodies that kill the pathogen and the memory cells remain in th eblood for a long time so that a faster secondary response is produced if the same pathogen enters the body once again i.e the memory cells will divide to produce more plasma cels which secrete a large number of antibodies this is a reason why we don't catch measles twice.
T lymphocytes has 2 types
1.T helper cells which release cytokines that stimulate B cells to produce plasma cells
2.T killer cells search the body for any infected body cells and release toxins such as H2O2 to kill the cells.

 

[qwertypoiu]

Sorry, I guess?
How am I late exactly?

I'm sorry I didn't mean to be rude.
I just meant there were two days left but thanks for sharing we can still use it for paper 1 and 3.
Sorry again please forgive me.

 

[My Name]

Okay, so first thing u should know is that B and T lymphocytes work when an immune response is generated when a forgein pathogen enters the body. When a foreign pathogen enters the body, macrophages engulfs it and exposes the cell surface antigens on the pathogen's surface thus macrophages are called APC'S (antigen presenting cells) then Those B cells whose antigen binding site fits the cell interfere with the macrophages and are selected to respond i.e clonal selection takes place. Next these B cells divide by mitosis i.e clonal expansion to produce Plasma cells and memory cells. the plasma cells produce antibodies that kill the pathogen and the memory cells remain in th eblood for a long time so that a faster secondary response is produced if the same pathogen enters the body once again i.e the memory cells will divide to produce more plasma cels which secrete a large number of antibodies this is a reason why we don't catch measles twice.
T lymphocytes has 2 types
1.T helper cells which release cytokines that stimulate B cells to produce plasma cells
2.T killer cells search the body for any infected body cells and release toxins such as H2O2 to kill the cells.

There are total four lymphocytes right?
T helper cells
T killer cells
T suppressor
T memory

 

[Mahnoorfatima]

There are total four lymphocytes right?
T helper cells
T killer cells
T suppressor
T memory

Oh yes! thanks for reminding. What about T supresser? And, aren't T memory released as a reult of the action of T helper or no they're seperate?

 

[My Name]

I'm sorry I didn't mean to be rude.
I just meant there were two days left but thanks for sharing we can still use it for paper 1 and 3.
Sorry again please forgive me.

Nah its fine, you weren't rude. You were honest.
Yep. I got to know that yesterday and found this site today and posted here ASAP.
No need for sorry. But I think for last minute revision it'll help.

 

[Mahnoorfatima]

Nah its fine, you weren't rude. You were honest.
Yep. I got to know that yesterday and found this site today and posted here ASAP.
No need for sorry. But I think for last minute revision it'll help.

Better late than never! Thanks a BUNCH for sharing it. We'll have an idea about what to write!

 

[qwertypoiu]

Nah its fine, you weren't rude. You were honest.
Yep. I got to know that yesterday and found this site today and posted here ASAP.
No need for sorry. But I think for last minute revision it'll help.

Yeah alright thanks for sharing.

 

[My Name]

Oh yes! thanks for reminding. What about T supresser? And, aren't T memory released as a reult of the action of T helper or no they're seperate?

Welcome! Well,like when B divide Plasma and Memory are formed just like that the four types of T cells are formed.
T-suppressor suppress the activity of lymphocytes once an infection has been eliminated and the T-memory have the same function as the B-memory cells.

 

[My Name]

Better late than never! Thanks a BUNCH for sharing it. We'll have an idea about what to write!

Yep! hehe Welcome

Yeah alright thanks for sharing.

Welcome and thank you for your help here!

 

[Shazi Shazi]

how can i download papers on this site. its failing please help

 

[qwertypoiu]

how can i download papers on this site. its failing please help

Welcome to XPC. Paper section stopped working sometime ago. You may try alternatives like
http://www.onlineexamhelp.com

 

[fantastic girl]

http://www.studentbounty.com/pastpa...el/Biology (9700)/2012 Nov/9700_w12_qp_13.pdf
Can someone explain me Q21 - why replication is wrong? isnt a part of DNA being copied to form mRNA ? :/
nd Q40 was there in another pper but the asnwer was niche.....so wat is the correct answer ?

 

[My Name]

how can i download papers on this site. its failing please help

http://studentbounty.com/pastpapers/index.php?dir=Cambridge International Examinations (CIE)
http://onlineexamhelp.com/
http://www.papacambridge.com/
http://maxpapers.com
You can try these links for now.

 

[Mahnoorfatima]

When enzymes are inactive do their Hydrogen bonds break distorting the shape of it?Can someone explain the complete effect of temperature on enzyme activity i.e what bonds break if the enzyme's inactive(though I think bonds are not broken during inactivity) and when the temperature is increased above normal, the vibration of the enzyme 1st breaks H bonds and in the end the disulfide bonds right?

 

[princess Anu]

1.Can somebody explain me why alternate Beta glucose molecules need to flip during the formation of cellulose?
2.is it NADP or NADPH required by nitrogenase enzyme ?
3. why does nitrogenase require ATP, during nitrogen fixation? are enzyme catalysed reactions energy requiring?

 

[qwertypoiu]

http://www.studentbounty.com/pastpapers/Cambridge International Examinations (CIE)/International AS and A Level/Biology (9700)/2012 Nov/9700_w12_qp_13.pdf
Can someone explain me Q21 - why replication is wrong? isnt a part of DNA being copied to form mRNA ? :/
nd Q40 was there in another pper but the asnwer was niche.....so wat is the correct answer ?

Replication implies copying something. However the nucleotides that form mRNA are different to those that formed the DNA, so it wasn't exactly a replication.
What I mean is that the mRNA are not copies of the DNA stand, but rather have the complementary bases.
When we talk of DNA replication it's the final double helix molecule that is the exact copy of original - which is true.

As for ecological question perhaps it's because many organisms live in the tree.

 

[qwertypoiu]

When enzymes are inactive do their Hydrogen bonds break distorting the shape of it?Can someone explain the complete effect of temperature on enzyme activity i.e what bonds break if the enzyme's inactive(though I think bonds are not broken during inactivity) and when the temperature is increased above normal, the vibration of the enzyme 1st breaks H bonds and in the end the disulfide bonds right?

At low temperature, reaction rate is low because collision between enzymes and substrate is not effective. The collision rate is low as well. When temperature increases, the bonds that maintain the three dimensional shape of the enzyme start to break and get affected, so enzyme shape changes, too high a temperature and it will be denatured permanently. The bonds that break are probably hydrogen bonds.

 

[qwertypoiu]

1.Can somebody explain me why alternate Beta glucose molecules need to flip during the formation of cellulose?
2.is it NADP or NADPH required by nitrogenase enzyme ?
3. why does nitrogenase require ATP, during nitrogen fixation? are enzyme catalysed reactions energy requiring?

If beta glucose didn't flip the OH and H group would be too far away to form bonds.
Reduced NADP is required as far as I know.
Undoubtedly, even catalysed reactions require activation energy. In fact allreactions have activation energy, just that some reactions can take sufficient amounts from room temperature.
Remember that making the inert nitrogen into reactive NH4+ is obviously gonna require energy to be put into the reaction, like all other endothermic reactions where products have more energy in them than reactants (like photosynthesis)