A level Biology: Post your doubts here!

[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)

 

[Mahnoorfatima]

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.

So you mean that during cold denaturation bonds do not break?

 

[Mahnoorfatima]

Does someone have the Grade thresholds and ER's for Oct/NOv 14? Please I need it!

 

[qwertypoiu]

So you mean that during cold denaturation bonds do not break?

Yep

 

[Mahnoorfatima]

In a question asking about mutation, do we have to write about the change in tRNA due to a change in mRNA as well?
And, how there's an altered mRNA condon bring a wrong a.a attached to tRNA which has an altered anticodon complementary to the mRNA's codon? Also leading to a wrong sequence of bases on the polypeptide chain incorporating a wrong a.a on the growing polypeptide chain which alters the primary structure of the polypeptide. Is that all we write? Idk why but I find answering this question really hard. :/

 

[Abdul Hanan]

Please explain...M/J 2014 p22..^
Also please tell me one challenging P2 which i can do thanks...

 

[Abdul Hanan]

Does someone have the Grade thresholds and ER's for Oct/NOv 14? Please I need it!

http://onlineexamhelp.com/wp-content/uploads/2015/02/9700_w14_gt.pdf
gt only published yet i think...

 

[princess Anu]

View attachment 52999
Please explain...M/J 2014 p22..^
Also please tell me one challenging P2 which i can do thanks...

See if this makes sense to you..
also can u attach the marking scheme for this que

 

[princess Anu]

nicotine causes endothelial damage .. how? by increasing the B.P?
T.B exhibits antigenic variation & concealment or only concealment?
Can we see golgi apparatus on Light micrscope?
H+ are pumped by companion cells into their own cell walls or that of the source cell?

 

[qwertypoiu]

nicotine causes endothelial damage .. how? by increasing the B.P?
T.B exhibits antigenic variation & concealment or only concealment?
Can we see golgi apparatus on Light micrscope?
H+ are pumped by companion cells into their own cell walls or that of the source cell?

Both carbon monoxide and nicotine increases the chances of endothelial damage. It is not known how. High blood pressure also causes endothelial damage.
TB can conceal itself inside macrophages. It doesn't exhibit antigenic variation like Plasmodium does due to its life cycle, but it mutates and develops resistance to antibiotics. Isthis considered antigenic variation? Please tell me if you know.
Golgi apparatus may be seen under a light microscope when certain special stains are used.
I think H+ ions are pumped into the cell wall.

 

[Abdul Hanan]

See if this makes sense to you..
also can u attach the marking scheme for this que

Thanks helped alot... Here's the ms.

 

[princess Anu]

so what can never be seen on a light micro? and no antigenic variation is when it changes its antigen! during mutation to develop resistance it doesn't change its antigen! but it actually changes the target sites for antibiotics. for example the shape of active site of an enzyme the antibiotic used to inhibit etc.

 

[Laila39]

Why do people who have a deficiency of iron in their diet are often lacking in energy and feel tired?

 

[nehaoscar]

Any last minute key words and points that you must know for the structured exam?
And any tips on how to frame your answers and/or any other advice?

Good Luck to you all for all your exams BTW!

 

[Mahnoorfatima]

Why is there a greater resistance in the systemic circulation than pulmonary?

 

[hard_worker]

U mean pressure?

 

[princess Anu]

can somebody tell me how do the staggered ends of tropocollagen molecule contribute to tensile strength of collagen??

 

[Lola_sweet]

https://docs.google.com/viewerng/vi...wp-content/uploads/2012/11/9700_s14_qp_21.pdf

Can someone please explain Qs 4D)

 

[Mahnoorfatima]

U mean pressure?

No. there's this question where its says why there's difference in pressures in the ventricles but not the atria so in the MS it's written that there's greater sesistance in systemic circulation..

 

[nehaoscar]

Albumin is the most abundant plasma protein.
Suggest why it is important that capillary walls should not be permeable to albumin?