▶️ Answer/Explanation
a.
- Water molecules are polar, meaning the oxygen atom has a slight negative charge (δ−) and the hydrogen atoms have a slight positive charge (δ+).
- Because of this charge difference, the positive hydrogen of one water molecule is attracted to the negative oxygen of another.
- This attraction forms a hydrogen bond between different water molecules, not within the same molecule.
b.
- Water moves through xylem vessels from roots to leaves.
- Water is pulled up the xylem under tension caused by transpiration (loss of water vapor from leaves).
- Transpiration happens when water evaporates from the mesophyll cell walls inside leaves and exits through stomata.
- Water molecules stick together by cohesion (due to hydrogen bonding), creating a continuous column of water (transpiration stream) that does not break as it is pulled upwards.
c.
- Osmoregulation is the control of water and solute (salt) concentrations in the blood.
- The nephron is the functional unit of the kidney responsible for osmoregulation.
- Ultrafiltration occurs in the glomerulus, where blood pressure forces water and small molecules into Bowman’s capsule, forming filtrate.
- The loop of Henle creates a high solute concentration (hypertonic) in the medulla, which helps reabsorb water.
- Water is reabsorbed by osmosis from the filtrate in the collecting duct.
- The hypothalamus in the brain monitors blood solute levels and signals the pituitary gland to release ADH (antidiuretic hormone) when blood is too concentrated (hypertonic).
- ADH increases the permeability of the collecting duct to water by causing more aquaporins (water channels) to be inserted into the duct walls.
- This causes more water to be reabsorbed, producing concentrated urine (less water lost).
- When blood is too dilute (hypotonic), less or no ADH is secreted, so less water is reabsorbed, resulting in dilute urine (more water lost).
Markscheme
a.
1. water (molecules) are polar/dipolar/have partially positive and negative poles/have \(\delta+\) and \(\delta-\);
2. attraction/bonding between positive and negative (poles);
3. hydrogen bond formed between hydrogen and oxygen; Reject if \(\mathrm{H}\) and \(\mathrm{O}\) in same molecule.
4. bond/attraction between different water molecules/intermolecular;
Marks can be awarded in an annotated diagram.
Reject answers stating or implying that there are whole positive or negative charges for mpa.
b.
1. water moved/transported in xylem vessels;
2. transported under tension/suction/pulled up (in xylem vessels);
3. transpiration/loss of water (vapour) generates pulling forces/low pressure/tension;
4. tension/pull generated when water evaporates from cell walls (in mesophyll);
5. transpiration is loss of water vapour from leaf (surface)/stomata;
6. cohesivity/cohesion in water due to hydrogen bonding/attractions between water molecules;
7. cohesion/WTTE so chain/column of water (molecules) doesn’t break/remains continuous;
8. transpiration stream is a column of/flow of water in xylem from roots to leaves;
Do not award marks for absorption of water by roots.
c.
1. osmoregulation is regulation of water and solute/salt balance/solute concentrations;
2. nephron (is the functional unit of the kidney/osmoregulates);
3. ultrafiltration in glomerulus / glomerular filtrate collected by Bowman’s capsule;
4. loop of Henle establishes/maintains hypertonic conditions in medulla;
5. osmosis/reabsorption of water (from filtrate) in the collecting duct;
6. brain/hypothalamus monitors blood solute concentration / pituitary secretes ADH;
7. ADH secreted when solute concentration of blood is too high/hypertonic/when dehydrated;
8. ADH increases permeability of collecting duct to water;
9. ADH causes more aquaporins (in membranes of collecting duct wall cells);
10. more water reabsorbed resulting in more concentrated/hypertonic urine/less volume of urine;
11. less/no ADH secreted when solute concentration (of blood) is too low/hypotonic;
12. less water reabsorbed resulting in dilute/hypotonic urine/large volume of urine;
Reject ‘water balance’ and ‘water concentration’ for mpa.
▶️ Answer/Explanation
Role of ADH in human osmoregulation:
- ADH is secreted by the pituitary gland when blood plasma becomes too concentrated (hypertonic).
- It acts on the collecting ducts of the kidneys, increasing their permeability to water.
- ADH stimulates insertion of aquaporins (water channels) into the membranes of collecting duct cells.
- This allows more water to be reabsorbed from the filtrate back into the bloodstream by osmosis.
- As a result, less water is lost in urine, producing small volume of concentrated urine.
- When blood plasma is dilute (hypotonic), ADH secretion decreases, reducing water reabsorption.
- This causes production of large volume of dilute urine, helping to remove excess water.
Markscheme
Role of ADH in Osmoregulation:
1. Secreted when blood/plasma is hypertonic/too concentrated/water content too low.
2. Makes walls of collecting duct/distal convoluted tubule more permeable to water.
3. Increases the number of aquaporins in membranes of collecting duct cells.
4. Enhances water reabsorption from filtrate/urine, returning more water to the blood.
5. Results in a small volume of concentrated urine being excreted.
[Max 3 Marks]
▶️ Answer/Explanation
a.
- X: Cortex
- Y: Ureter
b.
- Osmoregulators maintain a constant internal solute concentration regardless of the external environment, whereas osmoconformers have internal solute concentrations that vary with the external environment.
- In osmoregulators, the internal environment is independent of external solute concentration; in osmoconformers, it is equal to or dependent on the external solute concentration.
- Examples:
- Osmoregulators: birds, mammals, freshwater fish
- Osmoconformers: jellyfish, starfish, squid, sharks
Markscheme
a.
X: cortex \(\checkmark\)
Y: ureter \(\sqrt{ }\)
b.
1. Concentration constant inside osmoregulators versus variable inside osmoconformers \(\checkmark\).
2. Internal solute concentration can differ from the external environment in osmoregulators versus same/equal in osmoconformers.
OR
Osmoregulators are independent of the external environment in solute concentration versus osmoconformers are tied to it \(\checkmark\).
3. Examples: birds/mammals/humans/freshwater fish are osmoregulators versus starfish/mussels/crabs/jellyfish/sea squirts/squid/sharks are osmoconformers.