Outline how hydrogen bonds form in water.

Describe the processes that cause water to move from the roots of plants to their leaves.

Explain the role of the kidney in osmoregulation.

a. water (molecules) are polar/dipolar/have partially positive and negative poles/have δ+ and δ-;

b. attraction/bonding between positive and negative (poles);

c. hydrogen bond formed between hydrogen and oxygen; Reject if H and O in same molecule.

d. 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.

a. water moved/transported in xylem vessels;

b. transported under tension/suction/pulled up (in xylem vessels);

c. transpiration/loss of water (vapour) generates pulling forces/low pressure/tension;

d. tension/pull generated when water evaporates from cell walls (in mesophyll);

e. transpiration is loss of water vapour from leaf (surface)/stomata;

f. cohesivity/cohesion in water due to hydrogen bonding/attractions between water molecules;

g. cohesion/WTTE so chain/column of water (molecules) doesn’t break/remains continuous;

h. 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.

a. osmoregulation is regulation of water and solute/salt balance/solute concentrations;

b. nephron (is the functional unit of the kidney/osmoregulates);

c. ultrafiltration in glomerulus / glomerular filtrate collected by Bowman’s capsule;

d. loop of Henle establishes/maintains hypertonic conditions in medulla;

e. osmosis/reabsorption of water (from filtrate) in the collecting duct;

f. brain/hypothalamus monitors blood solute concentration / pituitary secretes ADH;

g. ADH secreted when solute concentration of blood is too high/hypertonic/when dehydrated;

h. ADH increases permeability of collecting duct to water;

i. ADH causes more aquaporins (in membranes of collecting duct wall cells);

j. more water reabsorbed resulting in more concentrated/hypertonic urine/less volume of urine;

k. less/no ADH secreted when solute concentration (of blood) is too low/hypotonic;

l. less water reabsorbed resulting in dilute/hypotonic urine/large volume of urine;

Reject ‘water balance’ and ‘water concentration’ for mpa.

Answers were mostly good here, with many candidates scoring full marks. A very common misconception was that hydrogen bonds are strong – a single hydrogen bond is a weak interaction and it is only because there are many hydrogen bonds in water that they collectively exert large cohesive forces. A few candidates thought that hydrogen bonds were within rather than between water molecules.

This was answered quite well. There were some traditional areas of confusion, with cohesion and adhesion either muddled up or treated as the same thing. Some candidates thought high pressure caused by roots and low pressure caused by leaves could exist at the same time in xylem. Few responses referred to water being transported in the vessels of xylem. In some answers water was said to evaporate from stomata, instead of the moist, blotting-paper-like cells walls of the mesophyll. Some answers included details of how water is absorbed into roots, which was outside the scope of the question. Despite these common faults, many candidates described clearly how tension is generated in xylem and how cohesive columns of water can be pulled up to leaves.

There was some excellent knowledge of kidney function, frequently going way beyond the question’s requirements. In some cases, candidates just described all processes occurring in the kidney instead of actually answering the question. This generally led to the loss of the extra clarity mark.