AQA A-level Biology: Water and Inorganic Ions (7402)

This guide teaches sections 3.1.7 Water and 3.1.8 Inorganic ions of Topic 1, Biological molecules, in the AQA AS and A-level Biology specification (7402 is the A-level), for first teaching 2015. Both sections are AS content, part of Topic 1. They have no required practical of their own, but the skills below (rates, percentages, reading tables) are used throughout the course.

Use it with the revision notes and the practice questions. For the whole topic see the Biological molecules study guide. Course hub: AQA A-level Biology. Checklist: AQA A-level Biology checklist. To find weak spots, try the free diagnostics.

What the two sections cover

Section	What you must be able to do
3.1.7	Explain water as a metabolite in condensation and hydrolysis reactions
3.1.7	Explain water as a solvent in which metabolic reactions occur
3.1.7	Link relatively high heat capacity to buffering changes in temperature
3.1.7	Link relatively large latent heat of vaporisation to cooling with little loss of water
3.1.7	Link strong cohesion to columns of water in plant transport cells and to surface tension
3.1.8	State that ions occur in solution in cytoplasm and body fluids, some at high and some at very low concentrations
3.1.8	Recognise the roles of hydrogen ions (pH), iron ions (haemoglobin), sodium ions (co-transport of glucose and amino acids) and phosphate ions (DNA and ATP)

Why water behaves as it does

A water molecule has an oxygen atom sharing electrons with two hydrogen atoms. Oxygen attracts the shared electrons more strongly, so oxygen is slightly negative (δ−) and each hydrogen slightly positive (δ+). The molecule is polar. The δ− oxygen of one molecule is attracted to a δ+ hydrogen of another. This attraction is a hydrogen bond. Each one is weak, but a body of water has vast numbers of them, and they break and re-form constantly. Every property below comes from that.

3.1.7 Water

Water as a metabolite

A metabolite is a substance involved in metabolism. Water is a reactant in some reactions and a product of others.

• Hydrolysis uses a water molecule to break a bond, for example a glycosidic, ester or peptide bond.
• Condensation joins two molecules and releases a water molecule.
• Water is also used in photosynthesis and produced in aerobic respiration, covered later in the course.

A link to calculation: the number of water molecules used or released equals the number of bonds broken or formed, not the number of monomers.

Water as a solvent

A solvent dissolves a solute. Water molecules cluster around ions and around polar molecules, forming hydrogen bonds or electrostatic attractions with them, so they separate and dissolve. Non-polar substances such as triglycerides cannot do this and do not dissolve.

Why it matters: metabolic reactions occur in solution in the cytoplasm, because dissolved reactants can move and meet enzymes. Dissolved substances are also carried in blood plasma, tissue fluid and xylem sap.

High heat capacity: buffering temperature

Heat capacity here means the energy needed to raise the temperature of a given mass of a substance by 1 °C. Water’s is relatively high because much of the energy supplied is used to disrupt hydrogen bonds rather than to make molecules move faster. Equally, a lot of energy is released as water cools.

Result: water buffers temperature change. Cells, blood and aquatic habitats warm and cool slowly, which keeps enzyme-controlled reactions steadier.

The relationship (rearranged as needed):

energy = mass × specific heat capacity × temperature change

Worked example 1. Assume a person’s body water has a mass of 42 kg. During hard exercise it absorbs 500 kJ of heat. Take the specific heat capacity of water as 4.18 J g⁻¹ °C⁻¹. Calculate the temperature rise, and compare it with a liquid of specific heat capacity 2.1 J g⁻¹ °C⁻¹.

• Convert units: 42 kg = 42 000 g and 500 kJ = 500 000 J.
• Temperature change = energy ÷ (mass × specific heat capacity) = 500 000 ÷ (42 000 × 4.18) = 500 000 ÷ 175 560.
• Rise in water = 2.85 °C (to 3 significant figures).
• For the other liquid: 500 000 ÷ (42 000 × 2.1) = 5.67 °C, almost twice as large.

Conclusion: with water the same heat gain gives about half the rise, so enzymes are less likely to leave their working range.

Large latent heat of vaporisation: cooling by evaporation

Latent heat of vaporisation is the energy absorbed when a liquid becomes a gas at constant temperature. For water it is relatively large because hydrogen bonds must be broken for a molecule to escape the liquid. The molecules that escape carry this energy away, so the remaining liquid and the surface beneath it are cooled.

Because the value is large, a small mass of water carries away a large quantity of heat: a cooling effect with little loss of water. Examples are sweating, panting, and evaporation from leaf mesophyll cell surfaces during transpiration.

energy removed = mass evaporated × latent heat of vaporisation

Worked example 2. A group of leaves loses 3.6 g of water per hour by evaporation. Take the latent heat of vaporisation as 2.4 kJ g⁻¹.

• Energy removed per hour = 3.6 × 2.4 = 8.64 kJ.
• Per second: 8 640 J ÷ 3 600 s = 2.4 J s⁻¹ (2.4 W).

Worked example 3 (why “little loss of water”). An animal must lose 90 kJ by evaporation.

• Mass evaporated = 90 ÷ 2.4 = 37.5 g.
• To remove 90 kJ by simply warming water by 1 °C would need 90 000 ÷ 4.18 = 21 531 g, about 21.5 kg.

Evaporation is therefore vastly more economical with water. Only water that actually evaporates cools the body; sweat that drips off does not.

Cohesion and surface tension

Cohesion is the attraction between molecules of the same substance. In water it comes from hydrogen bonding, and it is strong.

• In plants, strong cohesion holds water molecules together as a continuous column in the xylem, the tube-like transport cells. As water leaves the leaf, the column is pulled up the stem as one connected chain instead of breaking. This is developed later as the cohesion-tension theory in section 3.3.4.2.
• At an air–water boundary, molecules at the surface are pulled inwards and sideways by neighbours but not upwards by air. The surface behaves like a stretched skin. This is surface tension. Small animals such as pond skaters can stand on it.

Worked example 4 (a potometer). A shoot is fitted to a potometer whose capillary tube has a radius of 0.6 mm. The water column moves 30 mm in 10 minutes as the shoot takes up water.

• Volume = πr²l = π × 0.6² × 30 = 33.9 mm³ (3 significant figures).
• Rate = 33.9 ÷ 10 = 3.39 mm³ min⁻¹.
• Convert to an hour: 3.39 × 60 = 204 mm³, or 0.204 cm³.

The column moves as one unit because of cohesion. If an air bubble breaks it, uptake can stop.

3.1.8 Inorganic ions

Inorganic ions are charged particles. They occur in solution in cytoplasm and body fluids. Some are at high concentrations (sodium ions in blood plasma, for example) and others at very low concentrations (iron ions). Each type has a specific role depending on its properties, such as charge.

Hydrogen ions and pH

pH measures hydrogen ion concentration: pH = −log₁₀[H⁺]. A lower pH means a higher H⁺ concentration. One pH unit is a tenfold change, so pH 5 has 100 times the H⁺ concentration of pH 7.

H⁺ ions interact with the R groups of amino acids in proteins. A change in H⁺ concentration alters R-group charges, disrupts ionic and hydrogen bonds, and changes tertiary structure. In an enzyme this changes the shape of the active site, so substrate no longer fits and the enzyme may be denatured. Hence enzymes have an optimum pH, and organisms regulate their internal pH.

Worked example 5. An enzyme’s initial rate of reaction (arbitrary units) was measured at several pH values.

pH	4	5	6	7	8	9
Rate	2	11	26	30	19	6

• Optimum is near pH 7, the highest rate.
• Rate at pH 5 as a percentage of the maximum = 11 ÷ 30 × 100 = 36.7%.
• Percentage decrease from pH 7 to pH 9 = (30 − 6) ÷ 30 × 100 = 80%.
• The pH 9 sample has 100 times less H⁺ than pH 7. The loss of activity happens because the active site changes shape.

Data show only the pH values tested, so the true optimum could lie between 6 and 8.

Iron ions and haemoglobin

Haemoglobin is a protein with four polypeptide chains. Each carries a haem group containing one Fe²⁺ ion. Each Fe²⁺ binds one O₂ reversibly, giving a capacity of four O₂ per haemoglobin molecule. Oxygen combines with it to form oxyhaemoglobin in the lungs and releasing oxygen in respiring tissues. Without the iron there is nowhere for oxygen to bind, which is why a shortage of dietary iron can lead to anaemia.

Worked example 6. A red blood cell contains about 270 million haemoglobin molecules (data supplied). Work out how many O₂ molecules all its haemoglobin could hold when fully loaded.

• Each haemoglobin has four haem groups, each binding one O₂.
• 270 000 000 × 4 = 1.08 × 10⁹ oxygen molecules.

Sodium ions and co-transport

Co-transport moves two substances across a membrane together through one carrier protein. In the ileum it absorbs glucose and amino acids.

1. A sodium-potassium pump in the basal membrane of an epithelial cell uses ATP to move Na⁺ out of the cell into the blood. Na⁺ concentration inside the cell falls.
2. This creates a concentration gradient: Na⁺ is higher in the ileum lumen than in the cell.
3. Na⁺ diffuses into the cell through a co-transporter protein in the cell-surface membrane, bringing glucose or an amino acid with it.
4. Glucose concentration inside the cell rises above that in the blood, so glucose moves into the blood by facilitated diffusion through a carrier protein.

ATP is used for the pump, not directly for the co-transporter. A poison that stops the pump (or lack of ATP) reduces glucose absorption, because the Na⁺ gradient is lost.

Phosphate ions in DNA and ATP

A phosphate group (derived from the phosphate ion) is part of every nucleotide.

• DNA and RNA: each nucleotide has a pentose, a phosphate group and an organic base. A condensation reaction links the phosphate of one nucleotide to the pentose of the next, forming a phosphodiester bond, so a sugar-phosphate backbone forms.
• ATP: a nucleotide derivative of ribose, adenine and three phosphate groups. ATP hydrolase catalyses ATP + H₂O → ADP + Pi. The inorganic phosphate (Pi) can phosphorylate other compounds, often making them more reactive. ATP is resynthesised by condensation of ADP and Pi.

Worked example 7. A double-stranded DNA fragment is 24 base pairs long.

• Nucleotides = 24 × 2 = 48, so 48 phosphate groups.
• Each strand has 23 phosphodiester bonds, so 2 × 23 = 46 bonds.
• Each bond forms by condensation, so 46 water molecules are released.

Common errors

• Mixing up the two reactions. Condensation releases water; hydrolysis uses it.
• Writing “water has a high temperature” for heat capacity. It is the energy needed to change temperature that is high.
• Saying evaporation cools because water “absorbs heat” with no mention of the energy carried away by escaping molecules.
• Confusing cohesion (water to water) with adhesion (water to another surface).
• Describing pH as “the amount of acid”. It is a measure of H⁺ concentration on a log scale.
• Saying co-transport uses ATP directly. The pump does.

Official syllabus

AQA, AS and A-level Biology specification (7401 and 7402), first teaching 2015, sections 3.1.7 and 3.1.8.