Biology a level help

I did AS biology and got a C, from an A at GCSE but I'm aware I should've worked harder. Stick at it if you really want, but do appreciate it is much much harder at AS/A2

What spec are you doing?

AQA - and the new spec starting from 2015.

Hi, I am a private biology tutor, and can hopefully help.

The principle of dialysis (of course, used effectively as a substitute for your kidneys, which have failed) is to filter out harmful substances (excess water, urea [end-product of protein breakdown (effectively of amino acid breakdown)], excess electrolytes, creattinine, uric acid [end product of DNA and RNA breakdown] out of the blood compartment into the "urine" (in this case, virtual) compartment. This requires the passage of these substances through a membrane that allows them to pass through.

(I) Suitable as it is semi-permeable = allows small molecules like urea and Na+, Cl- etc through.
Allows excess water to go through by osmosis
It has a good blood supply which means the process of "cleaning" is efficient.

(ii) The volume of liquid in haemodialysis is larger = allows quicker filtration

(iii) Dialysis fluid could be made to have lower water potential by dissolving extra solutes in it = more water filtered into it by osmosis

(iv) Greater distance for substances to travel by diffusion
peritoneal membrane may not be semi-permeable any longer = osmosis less efficient

A Level Biology tip: 1. keep it simple

2. check number of marks for each part of question: make sure you write as many (brief) points as marks no.

3. read Q carefully and only answer what is asked/needed nothing irrelevant

Hope this helps.
Mukesh