Paper 3 Mark Schemes

Question 1

Describe how ultrafiltration occurs in a glomerulus.

Answer 1

1. High blood/hydrostatic pressure;
2. Two named small substances pass out eg
   water, glucose, ions, urea;
3. (Through small) gaps/pores/fenestrations in
   (capillary) endothelium;
4. (And) through (capillary) basement membrane;

Question 2

Glucose and water are reabsorbed by the proximal convoluted tubule of a nephron

Answer 2

Glucose by facilitated diffusion and active
transport and water down a water potential gradient

Question 3

How medulla thickness affects urine concentration

Answer 3

1. Thicker medulla means a longer loop (of
   Henle);
2. (The longer the loop of Henle means) increase
   in sodium ion concentration (in medulla)
   OR
   (The longer the loop of Henle means) sodium
   ion gradient maintained for longer (in medulla)
   OR
   (The longer the loop of Henle means) more
   sodium ions are moved out (into medulla);
3. (Therefore) water potential gradient maintained
   (for longer), so more water (re)absorbed (from
   loop and collecting duct);
   OR
   More water is (re)absorbed from the loop (of
   Henle) / collecting duct by osmosis;

Question 4

Describe the role of saprobionts in the nitrogen cycle

Answer 4

1. (They use enzymes to) decompose
   proteins/DNA/RNA/urea;
2. Producing/releasing ammonia/ammonium
   compounds/ammonium ions;

Question 5

The mitochondrial DNA (mtDNA) control region is an area of mtDNA that is
non-coding. This region stimulates the synthesis of both mtDNA and mitochondrial
messenger RNA.
Use this information to suggest two reasons why the mutation at
nucleotide position 16 519 could lead to the differences seen in Figure 5.

Answer 5

In Group C:
1. Less mitochondrial replication/production;
2. Less transcription (of genes) for mitochondrial
proteins/CS