Section 2

Question 1

cell fractionation

Answer 1

1. tissue is placed in cold, buffered, isotonic solution.
   cold - reduces enzyme activity, prevents damage to organelles from digestion
   buffered - to maintain constant optimum pH - prevents enzymes being denatured
   isotonic - prevents organelles shrinking or bursting due to osmosis
2. cells are homogenised, to release the organelles from the cell
3. the homogenate is then filtered to remove any complete cells and large pieces of debris
4. the filtrate is placed in a centrifuge
   spun at low speed - heaviest organelles are forced to the bottom forming a pellet
   supernatant (fluid at the top) is poured into a new tube and placed back in the centrifuge at an increased speed
   next heaviest organelle is forced to the bottom forming another pellet

Question 2

phagocytosis

Answer 2

• phagocyte engulfs pathogen, enclosing the pathogen into a phagosome.
• lysosomes fuse with pathogen, releasing lysozymes which digest/hydrolyse the pathogen
• the phagocyte then displays the antigen from the pathogen on its cell surface membrane, it becomes an antigen-presenting cell

Question 3

cellular response

Answer 3

• pathogens and infected body cells present antigens on their surface
• a helper T cell with specific, complementary receptors will bind to the antigen on the antigen-presenting cell
• this helper T cell then divides by mitosis to form clones
• some clones differentiate into cytotoxic cells which kill infected body cells by producing perforin which punctures holes in cells infected with the pathogen
• some become helper T cells which stimulate phagocytes to engulf the pathogens, stimulate cytotoxic T cells and stimulate the specific B cell with the correct, specific, complementary antibodies to divide and form clones
• other clones differentiate into memory T cells, which remain in the blood for a long time

Question 4

humoral response

Answer 4

• a pathogen enters the blood
• a B cell with complementary antibodies will bind to the antigens and process them, then display them on their cell surface
• a helper T cell attaches to the processed antigens and stimulates the B cell to divide
• this B cell then divides by mitosis to form clones of itself
• some clones develop into plasma cells. these produce antibodies which are specific to the antigens.
  the antibodies destroy the pathogen by causing agglutination (clumping of pathogens) which makes it easier for phagocytosis to occur.
• other clones develop into memory B cells, which remain in the blood for a long time

Question 5

HIV replication

Answer 5

• attachment protein on the HIV binds to a protein receptor on helper T cells
• HIV enter the helper T cell. the HIV reverse transcriptase converts the virus’s RNA into DNA.
• the HIV DNA is moved into the helper T cell’s nucleus and is inserted into the cell’s DNA.
• the HIV DNA in the nucleus is transcribed into mRNA, using the cells enzymes. this mRNA contains instructions for making new viral proteins and the RNA to go into the new HIV
• the mRNA passes out of the nucleus through nuclear pore and uses the cell’s ribosomes to be translated into HIV proteins
• they are then assembled into new viruses and are released out of the cell

Question 6

binary fission in bacteria

Answer 6

• replication of circular DNA
• replication of plasmids
• division of cytoplasm to produce two daughter cells
• each with a single copy of circular DNA

Question 7

mitosis

Answer 7

• during prophase, chromosomes condense by supercoiling. each chromosome consists of a sister chromatid pair, joined at the centromere. the nuclear membrane breaks down and the centrioles move to opposite poles of the cell and form spindle fibres that extend across the cell
• during metaphase, the chromosomes line up at the equator. the centromere of each chromosome attaches to the spindle fibre.
• in anaphase, the centromere divides and the spindle fibres pull each chromatid to the opposite pole of the cell, meaning that at each pole of the cell there is one of each chromosome
• during telophase, the spindle fibres break down and a nuclear membrane reforms around the chromosomes a each pole. the chromosomes uncoil
• during cytokinesis, the cytoplasm divides to result in two genetically identical daughter cells.