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

These notes condense sections 3.1.7 Water and 3.1.8 Inorganic ions of Topic 1 in the AQA AS and A-level Biology specification (7402 is the A-level), for first teaching 2015. For full explanations and longer worked examples read the study guide, then test yourself with the practice questions. Wider context: Biological molecules revision notes. Course hub: AQA A-level Biology. Checklist: AQA A-level Biology checklist. Find gaps first with the free diagnostics.

The one idea behind 3.1.7

Water is polar: δ− oxygen, δ+ hydrogen. Neighbouring molecules are joined by hydrogen bonds. Each bond is weak, but there are huge numbers, so together they give water its biological properties.

3.1.7 Water: property, reason, role

Property	Reason	Biological role
Metabolite	Reactant in hydrolysis; product of condensation	Builds and breaks polymers; also used in photosynthesis
Solvent	Polar water molecules surround ions and polar molecules	Reactions in solution; transport in blood, tissue fluid and xylem
Relatively high heat capacity	Energy goes into disrupting hydrogen bonds, so temperature rises slowly	Buffers temperature change in cells, blood and habitats
Relatively large latent heat of vaporisation	Hydrogen bonds must be broken for water to evaporate; escaping molecules take energy	Cooling with little water loss (sweating, panting, transpiration)
Strong cohesion	Hydrogen bonding between water molecules	Continuous columns in xylem; surface tension at an air–water boundary

Definitions to recite

• Metabolite: a substance taking part in, or made by, a metabolic reaction.
• Heat capacity (specific): energy needed to raise the temperature of a given mass by 1 °C.
• Latent heat of vaporisation: energy needed to turn a liquid into a gas at constant temperature.
• Cohesion: attraction between molecules of the same kind.
• Surface tension: the skin-like surface where water meets air, caused by cohesion.

Explaining each property in steps

Cooling by evaporation.

1. Water molecules at the surface need energy to escape the liquid.
2. They must break hydrogen bonds with neighbours, which takes a lot of energy.
3. The escaping molecules carry that energy away.
4. The surface left behind is cooler, and a small mass of water has removed a large quantity of heat.

Temperature buffering.

1. Heat supplied first disrupts hydrogen bonds.
2. Less energy is left to increase molecular movement.
3. So temperature rises slowly (and falls slowly).
4. Enzymes in cells and aquatic organisms meet smaller temperature swings.

Water in the xylem.

1. Hydrogen bonds make water molecules cohesive.
2. Water leaving the leaf pulls on the column below.
3. The column stays unbroken and moves as one chain through the tube-like transport cells.

Calculation methods

Quantity	Relationship	Reminder
Heating or cooling	energy = mass × specific heat capacity × temperature change	Match units: g with J g⁻¹ °C⁻¹
Evaporation	energy = mass evaporated × latent heat of vaporisation	Same units for mass and latent heat
Volume in a tube	volume = π × radius² × length	Use the radius, not the diameter
Percentage change	(new − old) ÷ old × 100	Divide by the starting value

Reminder 1. 200 g of water absorbs 8.36 kJ. Specific heat capacity is 4.18 J g⁻¹ °C⁻¹. Temperature rise = 8 360 ÷ (200 × 4.18) = 8 360 ÷ 836 = 10 °C.

Reminder 2. 15 g of water evaporates. Latent heat is 2.4 kJ g⁻¹. Energy removed = 15 × 2.4 = 36 kJ.

Reminder 3. A polypeptide of 60 amino acids has 59 peptide bonds. Condensation released 59 water molecules and complete hydrolysis uses 59.

3.1.8 Inorganic ions

Ions occur in solution in cytoplasm and body fluids. Some are at high concentrations, others at very low concentrations. Each type has a role that depends on its properties.

Ion	Where	Role
H⁺	Cytoplasm, body fluids	Concentration sets pH; affects R-group charges, bonding and enzyme shape
Fe²⁺	Haem group of haemoglobin	Binds oxygen reversibly, one O₂ per Fe²⁺
Na⁺	Ileum epithelial cells	Co-transport of glucose and amino acids
Phosphate	DNA, RNA, ATP	Backbone link (phosphodiester bond); Pi released by ATP hydrolysis

Hydrogen ions and pH

• pH = −log₁₀[H⁺]. Low pH = high [H⁺].
• Each unit is a factor of 10. pH 6 has 100 times the H⁺ of pH 8.
• Away from the optimum pH, ionic and hydrogen bonds in the tertiary structure change, the active site changes shape, and the enzyme can denature.

Iron ions and haemoglobin

• Four polypeptides, four haem groups, four Fe²⁺ ions.
• One haemoglobin binds up to four O₂ molecules.
• Binding is reversible: loading in the lungs, unloading in respiring tissues.
• Too little dietary iron gives too little haemoglobin, so less oxygen is carried.

Sodium ions and co-transport (ileum)

1. Sodium-potassium pump uses ATP to remove Na⁺ from the cell into the blood.
2. Inside the cell Na⁺ concentration is low, so Na⁺ diffuses in from the lumen.
3. The co-transporter protein carries glucose (or an amino acid) in with Na⁺.
4. Glucose builds up inside the cell and leaves into the blood by facilitated diffusion.

Phosphate ions

• DNA and RNA nucleotides each contain a phosphate group.
• A condensation reaction joins the phosphate of one nucleotide to the pentose of the next: a phosphodiester bond.
• ATP has ribose, adenine and three phosphate groups. ATP + H₂O → ADP + Pi (ATP hydrolase). Pi can phosphorylate other compounds, often making them more reactive.
• ATP is resynthesised by condensation of ADP and Pi.

Reminder 4. A single DNA strand of 15 nucleotides has 14 phosphodiester bonds, so 14 water molecules were released as it formed.

Data reminder: reading a pH table

pH	5	6	7	8
Rate (units)	14	33	38	12

• Highest rate at pH 7, so the optimum is near 7 (between 6 and 8 cannot be ruled out).
• Rate at pH 6 as a percentage of the maximum = 33 ÷ 38 × 100 = 86.8%.
• Percentage decrease from pH 7 to pH 8 = (38 − 12) ÷ 38 × 100 = 68.4%.
• Describe first (quote figures), then explain (H⁺, R groups, active site shape).

Answer frames worth rehearsing

Explain why water is useful as a coolant in organisms. Hydrogen bonds must be broken for water to evaporate; this needs a lot of energy; escaping molecules take it away; so cooling occurs with little loss of water.

Explain how sodium ions help glucose enter an epithelial cell. The pump lowers Na⁺ inside the cell; Na⁺ moves in down its gradient through a co-transporter; glucose is carried in with it against its own gradient.

Explain the importance of iron in haemoglobin. Each haem group contains Fe²⁺; Fe²⁺ binds one O₂ reversibly; so each haemoglobin carries up to four O₂ and releases them in tissues.

Must-know distinctions

Pair	Difference
Hydrolysis and condensation	Hydrolysis uses water; condensation releases water
Heat capacity and latent heat	Temperature rise versus change of state
Cohesion and surface tension	Cohesion is the attraction; surface tension is the result at a surface
Hydrogen bond and covalent bond	Between molecules, weak; within a molecule, strong
Active transport and co-transport	Active uses ATP at a carrier; co-transport uses a gradient set up by the pump

Quick self-test

1. What is a metabolite? Give one reaction in which water is released.
2. 120 g of water absorbs 2.508 kJ. Calculate the temperature rise (4.18 J g⁻¹ °C⁻¹).
3. Calculate the energy removed when 25 g of water evaporates (2.4 kJ g⁻¹).
4. Which has more H⁺, pH 6 or pH 8, and by what factor?
5. A sample contains 5 000 haemoglobin molecules. How many O₂ molecules can they carry at most?
6. How many phosphodiester bonds are in a single strand of 40 nucleotides?
7. Write the word equation for hydrolysis of ATP and name the enzyme.
8. Why does glucose absorption in the ileum slow if the sodium-potassium pump is inhibited?
9. Explain why a small insect can stand on water.
10. An enzyme’s rate falls from 40 to 25 units when the pH changes. Calculate the percentage decrease.

Answers

1. A substance taking part in or produced by metabolism; condensation releases water (for example forming a glycosidic bond).
2. 2 508 ÷ (120 × 4.18) = 2 508 ÷ 501.6 = 5.0 °C.
3. 25 × 2.4 = 60 kJ.
4. pH 6, with 100 times the H⁺ concentration.
5. 5 000 × 4 = 20 000 molecules.
6. 40 − 1 = 39.
7. ATP + water → ADP + inorganic phosphate (Pi); ATP hydrolase.
8. Less Na⁺ leaves the cell, so the Na⁺ gradient is lost, so less co-transport of glucose.
9. Strong cohesion between water molecules at the surface produces surface tension, a skin that supports a light animal.
10. (40 − 25) ÷ 40 × 100 = 37.5%.

Where marks are usually lost

• Writing “heat capacity” and giving no reason involving hydrogen bonds.
• Saying evaporation cools “because heat is lost” with no mention of the energy taken by escaping molecules.
• Forgetting that the cooling effect only occurs if the water evaporates.
• Mixing grams with kilograms or kJ with J in energy calculations.
• Using the diameter instead of the radius in a volume calculation.
• Describing pH as “acidity” without linking it to H⁺ concentration and protein shape.
• Saying iron “carries” oxygen when it is the Fe²⁺ in haem that binds it, reversibly.
• Saying co-transport uses ATP directly.
• Leaving out the pump when explaining why glucose enters against its gradient.

Official syllabus

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