The table shows the concentrations of some substances in different parts of a nephron.
| Substance | Blood plasma in glomerulus / g per dm³ | Filtrate in Bowman's capsule / g per dm³ | Urine / g per dm³ |
|---|---|---|---|
| glucose | 1.0 | 1.0 | 0.0 |
| protein | 80 | 0 | 0 |
| urea | 0.3 | 0.3 | 20 |
(a) Explain why there is no protein in the filtrate. [1]
(b) Explain why glucose is found in the filtrate but not in the urine. [3]
(c) Each day 180 dm³ of filtrate is formed and 1.5 dm³ of urine is produced. Assume the filtrate and the urine are almost entirely water. Calculate the percentage of water in the filtrate that is reabsorbed. Give your answer to 1 decimal place. [2]
- (a) protein molecules are too large to pass through (the walls of the glomerulus / basement membrane)(1)
- (b) glucose molecules are small so are filtered (ultrafiltration) from the glomerulus into the Bowman's capsule(1)
- (b) all glucose is (selectively) reabsorbed (back into the blood); reject 'some glucose is reabsorbed'(1)
- (b) in the proximal (convoluted) tubule / first coiled tubule (by active transport); OR (selectively reabsorbed because) glucose is needed by the body (for respiration) — either idea for this mark(1)
- (c) (180 − 1.5) ÷ 180 × 100; accept 100 − (1.5 ÷ 180 × 100)(1)
- (c) 99.2 (%); 99.17 (or 99.16…) with no rounding scores 1 mark only; correct answer with no working scores 2(1)
- Answer: (a) proteins too large to be filtered (b) small glucose filtered, then all selectively reabsorbed in proximal tubule (c) 99.2%
- Examiner tip: Say that ALL the glucose is reabsorbed and say WHERE: the proximal convoluted tubule.