AQA AS Biology Unit 1

Step 1: PhagocytosisThe pathogen has a protein on its surface (antigen) which is recognised as foreign by the phagocyte, the phagocyte is then attracted to the pathogen by chemicals released from it called chemoattractants. When then phagocyte gets to the pathogen, it engulfs it in a phagosome (or phagocytic vacuole) and then the lysosome in the phagocyte fuses with the the vacuole and empties out its hydrolytic enzymes, which break down the pathogen. The phagocyte then places the antigen on its surface.Step 2: T and B cellsThe t cells have a receptor on their surface, when it binds to the antigen on the phagocytes membrane, they are activated and divide by mitosis to produce cloned T cells and these have 4 jobs to do. One, is to secrete a chemical which activates the B cell, Two, is to develop into memory cells, 3 is to stimulate the uptake of the pathogens by phagocytosis and four, destroy infected cells by producing a protein which creates a hole in the membrane of the infected cell making it permeable to substances.Now, in terms of the B cells, they have antibodies on their surface, when the b cells activate they divide by mitosis to produce plasma cells (important note: its mitosis so the plasma cells are genetically identical to the original b cell and so has the same antibodies). The antibody has a variable region which is composed of a specific sequence of amino acid (a specific primary structure) making it complementary to the shape of the antigen to form the antigen-antibody complex.Step 3: memory cellsAnyway, the B and T cells develop into memory cells and are present in the blood, so the person is now immune, on the second exposure the memory cells divide into the T cells and the correct B cells and can destroy the pathogen.Hopes this helps.

Thank you a great deal for this. Helped me so much! Just the understanding of the little bits was getting to me. However having said that there was just a tiny concept I think I am missing. I've tried to make it really simple to show you what I don't get in the word document below. Thanks once again honestly a life saver.

For your question on the B cell activation, The T cells activate them first, when their antibodies bind to the antigen they divide.

You can use the formula c1v1=c2v2 where c is conc. and v is volume.

C1 is 1.0
C2 is 2.0
and V2 is 20

All you do now is rearrange that formula to find V1 (0.2x20)/1 which gives you 4 cm3 of solution and the other 16 cm3 is water

Hope that helps!

this is a similar question i did in my ISA paper-we know from the question that it will add up to 20cm^3. 0.2moldm^3 of NaCl solution is 20% of 1moldm^3. so the volume will be 20% of 20cm^3 which is 4cm^3. To work out the remaining volume of distilled water is just 20 - 4 which is 16.

20cm^3 multiplied by 0.2mol dm^-3 would give you 4cm^3, and thats for the 'Volume of 1.0 mol dm-3 sodium chloride solution'
And because the student needed 20cm^3 in total, 20 - 4 = 16cm^3 for the volume of distilled water.

Hope it helped haha, bit of a pointless 1 marker though, bet they threw that in there just to reach the required marking total of 60

Thanks so much I finally get it. So essentially its two ways B cells work i.e. the membrane bound antibodies bind to antigen on the surface of the pathogen and via t cell activates the B cells to divide....

OR

B cells can take up the antigen from the surface of the pathogen and then T cells bind to the antigen on the surface of the B cell hence causing the B cells to divide in this way instead...

Right? Pleaseee

Posted from TSR Mobile

Thanks so much I finally get it. So essentially its two ways B cells work i.e. the membrane bound antibodies bind to antigen on the surface of the pathogen and via t cell activates the B cells to divide....

OR

B cells can take up the antigen from the surface of the pathogen and then T cells bind to the antigen on the surface of the B cell hence causing the B cells to divide in this way instead...

Right? Pleaseee

Posted from TSR Mobile

The antibody does three things1. coats the pathogen, making it easier for the phagocyte to engulf it2. coating the pathogen to prevent it from entering the cells3. binding to and neutralising (or inactivating) toxins produced by the pathogen

Righty thanks! But thats not what I meant lolLike the B-cells have two ways of working.One way they can take up the antigen from the pathogens surface and T-cells can actually bind to the antigen on the B-Cells surface and than ACTIVATE THE B CELL TO DIVIDE. OR the B-cells consisting of the antibody on it's surface binds to the antigen and then the T-helper cell produces a chemical activating the B-cell to divide rather than the need to bind to it to activate it.

The T cells bind to the antigen on the cell surface membrane of the phagocyte, the B cells are activated when their antibody binds with the antigen alongside the chemicals released by the T cells.

In addition to that, it states this: 'the B cells are activated when their membrane bound antibody binds with the antigen on the surface of a pathogen alongside the chemicals released by the T cells.

I feel very unprepared for this exam! I've neglected biology recently as I've had 7 exams thus far. I have a few days to revise- any recommendations on how to revise? e.g past papers, going through notes ect.

do all the past papers, and memorise the mark scheme for the 6 markers at the end. Look to see how the examiners want the questions answered etc

Also can someone make sense of 7(c) in the june 2013 paper for me? Just need some idea/understanding of what the hell they are talking about. SORRY FOR NO PARAGRAPHS BUT STUDENT ROOM IS NOT LETTING ME USE PARAGRAPHS.