B2 extended response

Question 1

Q1. Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells.

Answer 1

1. Cell homogenisation to break open cells;
2. Filter to remove large debris;
3. Use isotonic solution to prevent damage to organelles;
4. Keep cold to reduce damage by enzymes /
5. Use buffer to prevent protein / enzyme denaturation;
6. Centrifuge at lower speed to separate heavy organelles;
7. Re-spin supernatant at higher speed to get organelle at bottom.

Question 2

Q2. Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells.

Answer 2

1. TEM use electrons and optical use light;
2. TEM allows a greater resolution;
3. So with TEM smaller organelles can be seen and in greater detail
4. TEM view only dead specimens and optical can view live specimens;
5. TEM does not show colour and optical can;
6. TEM requires thinner specimens;
7. TEM requires a more complex and time consuming preparation;
8. TEM focuses using magnets and optical uses (glass) lenses;

Question 3

Q3. Some substances can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer. Describe other ways by which substances cross this membrane.

Answer 3

By osmosis
1. From a high water potential to a low water potential
2. Through aquaporins;

By facilitated diffusion
3. Channel / carrier protein;
4. Down concentration gradient;

By active transport
5. Carrier protein / protein pumps;
6. Against concentration gradient;
7. Using ATP

By phagocytosis / endocytosis
8. Engulfing by cell surface membrane to form vesicle

By exocytosis / role of Golgi vesicles
9. Fusion of vesicle with cell surface membrane;

Question 4

Q4. Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substances can cross a cell surface membrane.

Answer 4

1. Simple and facilitated diffusion from high to low concentration, down concentration gradient;
2. Small molecules pass via phospholipid bilayer;
3. Large molecules go through proteins;
4. Water moves by osmosis from high water potential to low water potential
5. Active transport is movement from low to high concentration, against concentration gradient;
6. Active transport involves proteins ;
7. Active transport requires energy from ATP;

Question 5

Q5. Describe the behaviour of chromosomes during mitosis and explain how this results in the production of two genetically identical cells.

Answer 5

1. chromosomes shorten;
2. chromosomes have two identical chromatids
3. chromosomes move to equator
4. attach to individual spindle fibres;
5. spindle fibres contract / centromeres divide
6. chromatids (separate)
7. move to opposite poles
8. each pole receives all genetic information
9. identical copies of each chromosome;
10. nuclear envelope forms around each group of chromosomes /
    chromatids at each pole;

Question 6

Q6. When a pathogen enters the body it may be destroyed by phagocytosis.
Describe how.

Answer 6

1. Phagocyte recognises antigen;
2. (Pathogen engulfed;
3. Enclosed in phagosome;
4. Phagosome fuses with lysosome;
5. Lysosome contains enzymes;
6. Pathogen digested

Question 7

Q7. Describe how antibodies are produced in the body following a viral infection.

Answer 7

1. virus contains antigen;
2. virus engulfed by phagocyte;
3. presents antigen to B-cell;
4. cytokines activate memory B cells
5. B cells divides by mitosis;
6. to form clones
7. plasma cells produce antibodies;
8. antibodies specific to antigen;

Question 8

Q8. When a vaccine is given to a person, it leads to the production of antibodies against a disease-causing organism. Describe how.

Answer 8

1. Vaccine contains antigen from pathogen;
2. Macrophage presents antigen on its surface;
3. T cell with complementary receptor protein binds to antigen;
4. T cell stimulates B cell;
5. B cell has complementary antibody on its surface;
6. B cell secretes large amounts of antibody;
7. B cell divides to form clone all producing same antibody.

Question 9

Q9. Describe the difference between active and passive immunity.

Answer 9

1. Active involves memory cells, passive does not;
2. Active involves production of antibody by plasma cells / memory cells;
3. Passive involves antibody introduced into body from outside
4. Active long term, because antibody produced in response to antigen;
5. Passive short term, because antibody given is broken down;
6. Active can take time to develop / work, passive fast acting.

Question 10

Q10. Describe how vaccination can lead to protection against a bacterial infection.

Answer 10

1. Antigen on surface of bacteria binds to surface receptor on a B cell.
2. Activated B cell divides by mitosis and produces clone;
3. Division stimulated by T cells;
4. Plasma cells release antibodies;
5. Some B cells become memory cells;
6. Memory cells produce antibodies faster

Question 11

Q11. Describe how a population of bacteria can become resistant to antibiotics.

Answer 11

1. Mutation
2. Results in cell with allele for resistance to one antibiotic
3. This cell survives and passes the allele for resistance to offspring;
4. Process repeated with different genes conferring resistance to each of the other (two) antibiotics

Question 12

Q12. Contrast the structure of a bacterial cell and the structure of a human cell.

Answer 12

1. Bacterial cell is much smaller than a human cell;
2. Bacterial cell has a cell wall but human cell does not;
3. Bacterial cell lacks a nucleus but human cell has a nucleus;
4. Bacterial cell lacks membrane-bound organelles but human cell has membrane-bound organelles;
5. Bacterial ribosomes smaller than human ribosomes
6. Bacterial DNA is circular but human DNA is linear;
7. Bacterial DNA is ‘naked’ whereas human DNA is bound to histones / proteins