June 2020 P2

Question 1

Explain how HIV affects the production of antibodies when AIDS develops in a person (3)

Answer 1

Less antibody produced because HIV destroys helper T cells so fewer B cells are stimulated to differentiate into plasma cells which produce antibodies

Question 2

Use your understanding of the mass flow hypothesis to explain how pressure is generated inside this phloem tube (3)

Answer 2

Sucrose actively transported into phloem

Reduces water potential

Water moves into phloem via osmosis from xylem

Question 3

Describe the processes involved in the absorption and transport of digested lipid molecules from the ileum into lymph vessels (5)

Answer 3

Micelles contain bile salts and fatty acids / monoglycerides

Make monoglycerides more soluble in water

Fatty acids / monoglycerides absorbed by diffusion

Triglycerides reformed in cells

Vesicles move to cell membrane

Question 4

Describe how the structure of a protein depends on the amino acid it contains (5)

Answer 4

Structure determined by position of amino acid / R group

Primary structure is the sequence of amino acids

Secondary structure due to hydrogen bonds between amino acids forming alpha helices and beta pleated sheets

Tertiary structure formed by hydrogen bonds, ionic bonds, disulphide bonds interactions

Creates specific proteins

Quaternary structure contains more than 1 polypeptide chain