Questions on water and inorganic ions

Question 1

State two properties of water that result from hydrogen bonding between its molecules, and state one biological consequence of each.

Accepted Answer

B1 any one property: high specific heat capacity, high latent heat of vaporisation, cohesion and high surface tension, or ice being less dense than liquid water B1 a consequence of that property, for example that the temperature of a cell or a habitat changes slowly B1 a second property from the same list B1 a consequence of the second property, for example that evaporation of sweat removes a large amount of heat

Question 2

Water is at its densest at 4 °C rather than at its freezing point. Explain what happens to water molecules as the water cools from 4 °C to 0 °C and then freezes, and explain the consequence for organisms in a pond.

Accepted Answer

B1 as liquid water cools further below 4 °C it expands slightly rather than continuing to contract, though it remains a liquid rather than a rigid lattice until it actually freezes B1 when it does freeze, the hydrogen bonds lock the molecules into an open lattice that holds them further apart than in the liquid B1 ice is therefore less dense than liquid water and floats, so a pond freezes from the surface downwards rather than from the bottom up B1 the layer of ice insulates the liquid water beneath it, so fish and invertebrates can survive the winter in liquid water under the ice

Question 3

Describe how a hydrogen bond forms between two water molecules.

Accepted Answer

B1 the oxygen atom attracts the shared electrons of each covalent bond more strongly than the hydrogen atoms do, so the oxygen is slightly negative and the hydrogens are slightly positive B1 the molecule is therefore polar B1 a slightly positive hydrogen atom of one molecule is attracted to the slightly negative oxygen atom of a neighbouring molecule, and that attraction is the hydrogen bond

Question 4

Explain why the temperature of a large lake changes more slowly during a hot day than the temperature of the air above it.

Accepted Answer

B1 water has a high specific heat capacity, so a large amount of energy is needed to raise the temperature of a given mass of it B1 much of the energy absorbed goes into breaking hydrogen bonds between the water molecules rather than into increasing their kinetic energy B1 so the temperature of the lake rises slowly, and organisms living in it are not exposed to rapid changes in temperature

Question 5

Explain how sweating cools a mammal, and explain why only a small volume of sweat is needed to remove a large amount of heat.

Accepted Answer

B1 water in sweat evaporates from the surface of the skin, and the energy needed for this is taken from the body B1 evaporation requires the hydrogen bonds between water molecules to be broken B1 so water has a high latent heat of vaporisation, and a small mass of water removes a large amount of heat as it evaporates

Question 6

In winter a pond freezes at the surface while the water below stays liquid. Suggest how this is explained by the structure of ice, and suggest what it means for the animals living in the pond.

Accepted Answer

B1 when water freezes, the hydrogen bonds lock the molecules into an open lattice that holds them further apart than in the liquid B1 so ice is less dense than liquid water and floats, forming a layer at the surface B1 the ice insulates the water beneath it, so the water stays liquid and fish and invertebrates survive the winter in it

Question 7

Explain why sodium chloride dissolves readily in water but a triglyceride does not.

Accepted Answer

B1 water molecules are polar, with a slightly negative oxygen end and slightly positive hydrogen ends B1 the slightly negative ends are attracted to the sodium ions and the slightly positive ends to the chloride ions, so the ions are separated from the crystal and held in solution B1 a triglyceride is non-polar and carries no charge, so no such attraction forms and it is not separated by water

Question 8

Give the role of iron(II) ions, phosphate ions and calcium ions in a named structure or process.

Accepted Answer

B1 iron(II) ions are part of the haem group of haemoglobin, where one oxygen molecule binds to each B1 phosphate ions join one nucleotide to the next in the backbone of DNA and RNA; also accepted are their presence in ATP or in the hydrophilic head of a phospholipid B1 calcium ions trigger the release of neurotransmitter at a synapse; also accepted are their role in muscle contraction or in the middle lamella between plant cells

Question 9

A crop grown in soil low in nitrate has pale leaves and grows poorly. Explain why a shortage of nitrate ions has these effects.

Accepted Answer

B1 nitrate ions are absorbed by the roots and supply the nitrogen the plant needs B1 nitrogen is required to make amino acids, and therefore proteins including enzymes, so growth is reduced B1 nitrogen is also required to make chlorophyll, so less is produced and the leaves are pale, which reduces the rate of photosynthesis

Question 10

Describe what happens to a column of water in a xylem vessel as water evaporates from a leaf, and describe the property of water responsible.

Accepted Answer

B1 water molecules hydrogen bond to each other, and that mutual attraction between molecules is cohesion B1 as water evaporates from a leaf, the whole column of water in the xylem is pulled upwards behind it rather than the column separating B1 without sufficient cohesion the column would break apart under tension instead of being pulled up as one continuous thread

Question 11

State what is meant by surface tension in water, state the property of water molecules responsible for it, and state one organism that depends on it.

Accepted Answer

B1 surface tension is the tendency of the water surface to behave as though covered by a thin skin, resisting being broken B1 it results from hydrogen bonding, since surface molecules are pulled sideways and downwards by neighbouring molecules but have none above them pulling upwards B1 a pond skater is supported by it, and a great many small organisms live on or immediately under the surface film it creates

Question 12

State the role of water in the light-dependent reactions of photosynthesis, and state the role of water in the final stage of aerobic respiration.

Accepted Answer

B1 in the light-dependent reactions, water is split, providing electrons that replace those lost from chlorophyll, and releasing oxygen as a by-product B1 in aerobic respiration, water is formed at the end of the electron transport chain, when oxygen accepts electrons and protons B1 water is therefore a reactant in one process and a product of the other

Question 13

Give the role of sodium ions in the co-transport of glucose across the ileum epithelium, and give one other named role of sodium ions in the body.

Accepted Answer

B1 sodium ions are actively pumped out of the epithelial cell into the blood, maintaining a lower sodium concentration inside the cell than in the gut lumen B1 sodium ions then diffuse back into the cell down that concentration gradient through a co-transporter protein, which carries glucose or an amino acid into the cell at the same time B1 another named role is accepted: sodium ions moving across the axon membrane, alongside potassium ions, to generate an action potential in a neurone

Question 14

Explain how hydrogencarbonate ions in the blood act as a buffer against changes in blood pH.

Accepted Answer

B1 hydrogencarbonate ions can combine with excess hydrogen ions in the blood, removing them from solution and so resisting a fall in pH B1 if the blood becomes too alkaline, carbonic acid formed from dissolved carbon dioxide dissociates into hydrogen ions and hydrogencarbonate ions, opposing the rise B1 by resisting changes in the concentration of free hydrogen ions, hydrogencarbonate keeps blood pH within the narrow range enzymes in the blood require to function normally

Question 15

Give the role of chloride ions in the chloride shift, and give one other named role of chloride ions.

Accepted Answer

B1 as hydrogencarbonate ions move out of a red blood cell into the plasma, chloride ions move into the red blood cell to balance the charge, and this exchange is the chloride shift B1 another named role is accepted: chloride ions are a cofactor amylase requires before its active site works, or chloride ions are part of the hydrochloric acid secreted in gastric juice