3. Exchange and Transport — AQA A Level Biology 7402 Revision Notes & Topical Questions

Section 1 of 27Surface area to volume ratio

Surface area

Surface area is the area exposed to the environment; volume is the internal volume.

• As organisms get larger, volume increases faster than surface area, so the SA:V ratio falls.

Calculation

• Calculation for a sphere SA = 4πr² and V = 4/3 πr³.
• Toad egg of diameter 1.794 mm: r = 0.897 mm.
• SA = 10.11 mm²; V = 3.023 mm³.
• Ratio = 10.11 ÷ 3.023 = 3.4 : 1.

Shapes to know

• Shapes to know cube, cuboid, cylinder and sphere.

Agar block practical

• Cut agar with indicator (purple with dilute NaOH and universal indicator) into cubes of 0.5, 1 and 2 cm.
• Calculate each SA:V.
• Place them in the same volume and concentration of dilute hydrochloric acid.
• Measure time for complete colour change or distance travelled in a set time.
• Convert to rateshigher SA:V gives faster diffusion.

Section 2 of 27Why larger organisms need exchange adaptations, and metabolic rate

Single-celled organisms

Single-celled organisms have a high SA:V ratio and short diffusion distance, so simple diffusion is enough.

Large organisms

Large organisms have a lower SA:V ratio and longer diffusion distances,

• so they need specialised systems such as lungs, the digestive system and leaves.
• Do not confuse total surface area with SA:V ratio: an elephant has more surface area than an amoeba, but a lower ratio.

Metabolic rate

• Metabolic rate small animals with a high SA:V lose heat faster, so they need a higher metabolic rate per unit mass.
• An elephant has a higher total BMR than a mouse but a lower BMR per gram.

Section 3 of 27Gas exchange in insects, fish and leaves

Organism	Structure	Adaptations
Single-celled	Cell surface	High SA:V ratio; short diffusion distance
Insect	Spiracles, tracheae and tracheoles	Tracheoles end in muscle fibres; diffusion down gradients; active insects contract abdominal muscles to pump air
Fish	Gills: arches, filaments and lamellae	Large area, one-cell-thick lamellae, counter-current flow keeps a gradient along the whole capillary
Dicotyledonous leaf	Stomata, spongy mesophyll	Thin, flat leaf; air spaces give large surface; guard cells control stomata

Gas exchange in insects, fish and leaves

Counter-current

• Counter-current blood and water flow in opposite directions,
• so water with the highest oxygen meets the most oxygenated blood and water with the least meets the most deoxygenated blood.

Exchange surface features

• Exchange surface features large surface area, short diffusion distance, steep concentration gradient.

Section 4 of 27Gas exchange versus water loss

Compromise

• Compromise features that favour gas exchange also allow water loss.

Insects

• Insects waterproof exoskeleton;
  • spiracles can close and are surrounded by hairs that reduce water vapour diffusion.

Xerophytes

• Xerophytes few stomata, stomata in pits, hairs around stomata, needle-shaped or rolled leaves with small surface area and a thick waxy cuticle.

Cacti

• Cacti leaves reduced to spines and a thick cuticle on the stem.

Marram grass

• Marram grass rolled leaves, deep grooves protecting stomata, no stomata on the exposed surface, thick cuticle and hairs inside to trap water vapour.

Section 5 of 27Human gas exchange system and dissection

Human gas exchange system and dissection

Structures

• Structures trachea, bronchi, bronchioles, alveoli and two lungs.
  • (windpipe, supported by C-shaped cartilage rings)

Lung dissection

• Lung dissection scalpel, scissors, forceps and pins;
  • cut through cartilage to see trachea, bronchi and bronchioles;
  • spongy texture shows air sacs.

Fish gills

• Fish gills under the operculum; gill arch and feathery filaments are visible, lamellae need a microscope.

Insect tracheae

• Insect tracheae pin the insect, cut along the abdomen, add saline; tracheae appear as silvery tubes.

Safety and ethics

• Safety and ethics lab coat, gloves and eye protection, blades cutting away from the body, specimens from reputable sources, correct disposal.

Microscopy of exchange surfaces

• Microscopy of exchange surfaces alveolar walls one flattened cell thick with blood vessels between alveoli;
  • fish gill arch, filaments and lamellae;
  • stained leaf sections showing cuticle, epidermis, palisade and spongy mesophyll, stomata and guard cells;
  • SEM for spiracles.

Section 6 of 27Alveolar epithelium and measuring gas exchange

Alveolar epithelium

Alveolar epithelium is an efficient exchange surface.

• Large surface area from many alveoli.
• Walls one flattened cell thick for a short diffusion distance.
• Steep gradient maintained by blood flow in adjacent capillaries and ventilation.

Wording

• Wording refer to alveolar walls, not cell walls.

Respirometer

• Respirometer measures oxygen consumption as pressure falls and a coloured bead of liquid moves;
  • a three-way tap resets the bead and a manometer shows pressure difference.

Section 7 of 27Ventilation and pulmonary ventilation rate

Ventilation and pulmonary ventilation rate

Inhalation (active)

• Diaphragm contracts and flattens; external intercostal muscles contract.
• Ribcage moves up and out, chest volume increases, pressure falls.
• Air moves in down a pressure gradient.

Exhalation at rest (passive)

• Diaphragm relaxes and rises; external intercostals relax.
• Elastic recoil of lungs reduces chest volume and raises pressure.
• Air moves out.

Forced exhalation

• Forced exhalation internal intercostal muscles contract to pull ribs down and abdominal muscles push the diaphragm up;
  • the intercostal muscles are antagonistic.

Pulmonary ventilation rate

Pulmonary ventilation rate tidal volume × breathing rate.

• Tidal volumevolume of one normal breath.
• ExamplePVR 7.4 dm³ min^-1 at 12 breaths per minute gives tidal volume 7.4 ÷ 12 = 0.62 dm³.

Ventilation is not respiration

• Ventilation is not respiration breathing moves air, respiration is a chemical reaction releasing energy.

Section 8 of 27Lung disease, risk factors and correlation

1Lung cancer

• Lung cancer tumours block airways or press on vessels; COPD damages airways and alveoli, reducing airflow and gas exchange.
  • (chronic bronchitis and emphysema)

2Emphysema example

• Emphysema example fewer, larger alveoli reduce surface area, so less diffusion of oxygen (breathlessness), less respiration and less energy (fatigue).

3Smoking and pollution data

• Smoking and pollution data smoking and pollution are linked to lung cancer, COPD and asthma.

4Odds ratio (OR)

• OR 1 means no association, above 1 increased risk, below 1 decreased risk.
• Example
  • hospitalisation in the past year 3.26 (strongest);
    • age 60+ 1.05;
    • other conditions 1.5;
    • male 0.92;
    • current smoker 0.64 (surprising, possibly due to small sample, no statistics or uncontrolled variables).

5Evaluating risk factor studies

• Evaluating risk factor studies control of other variables, sample size, demographic match, correlation versus causation and statistical tests.
  • (diet, activity, genetics)

6Correlation and causation

• Correlation and causation correlation is an association; causation needs repeated results, correct timing, a mechanism and controlled trials.
  • (positive or negative)
• Say "correlated with", not "causes".

Section 9 of 27Digestion and digestive enzymes

Digestion

Digestion hydrolyses large molecules to small ones: proteins to amino acids, carbohydrates to simple sugars, lipids to glycerol and fatty acids.

Digestive organs

• Digestive organs mouth, stomach (protease, hydrochloric acid), liver (bile salts), pancreas (amylase, protease, lipase), duodenum (neutralised by bile), ileum (final digestion and absorption through villi).
  • (salivary amylase)

Carbohydrates

• Carbohydrates amylase hydrolyses starch to maltose; membrane-bound maltase on ileum cells hydrolyses maltose to glucose.
  • (saliva, pancreas)

Lipids

• Lipids bile salts emulsify lipids into small droplets; lipase hydrolyses them to monoglycerides and fatty acids.
  • (not chemical digestion)

Proteins

• Endopeptidases hydrolyse peptide bonds within polypeptides.
• Exopeptidases hydrolyse at the ends to release amino acids.
• Membrane-bound dipeptidases hydrolyse dipeptides to amino acids.

Section 10 of 27Practicals on digestive enzymes

1

Put drops of iodine on a spotting tile.

2

Mix 2 cm³ amylase, 1 cm³ buffer and 2 cm³ starch; start the stopwatch.

3

Every 10 seconds drop a sample onto fresh iodine, cleaning the glass rod between drops.

4

Stop when iodine stays orange-brown; repeat at different pH values.

Amylase and pH

• Amylase and pH keep volumes and concentrations constant;
  • the shorter the time, the faster the digestion;
  • a colorimeter is less subjective.

Bile salts and lipase

• Bile salts and lipase milk (lipid) with phenolphthalein and sodium carbonate is pink;
  • lipase makes fatty acids that lower pH until it turns colourless.
• Compare the time with and without bile salts or washing-up liquid.
• Control variablesvolume and concentration of each solution, milk fat content and stirring.

Wording

• Wording say volume and concentration, not amount.

Section 11 of 27Absorption of digestion products

Absorption of digestion products

Co-transport of amino acids and glucose

• The sodium-potassium pump actively moves Na⁺ out of the epithelial cell into the blood.
• Na⁺ diffuses from the lumen into the cell with an amino acid or glucose via a co-transporter.
• The solute then diffuses into the blood by facilitated diffusion.
• Overall, the process is active because energy builds the sodium gradient.

Lipids

• Lipids monoglycerides and fatty acids join bile salts to form micelles that carry them to the cell membrane.
• Contents diffuse across the bilayer as they are non-polar.
• Inside, they recombine to triglycerides in the ER and are packaged into chylomicrons for the blood.

Visking tubing model

• Visking tubing model tubing is a partially permeable cellulose membrane.
• Fill with starch and amylase and suspend in water.
• Glucose appears outside but starch does not because it is too large.
• Limitationsno carrier proteins or active transport, no villi, and water does not flow like blood to maintain gradients.

Section 12 of 27Mass transport and haemoglobin structure

Mass transport and haemoglobin structure

Mass transport

Mass transport is the efficient movement of substances over large distances by specialised systems using pressure changes,

• because diffusion alone is too slow.
• Do not confuse low SA:V ratio (needs gas exchange systems) with large distances (needs mass transport).

Haemoglobin

Haemoglobin is a quaternary protein of four polypeptide chains (two α and two β) joined by disulfide bonds.

• Hydrophobic R groups face inwards and hydrophilic ones outwards, so it is soluble.
• Each chain has a haem group with Fe²⁺ which binds one O₂.
• Hb + 4O₂ ⇌ HbO₈ (oxyhaemoglobin), so four O₂ molecules (eight oxygen atoms) per molecule.

Red blood cell adaptations

• Red blood cell adaptations no nucleus, biconcave, flexible, and about the same diameter as capillaries to slow flow.

Calculation

• 150 g of haemoglobin per dm³ carrying 1.4 cm³ per gram carries 150 × 1.4 = 210 cm³ of oxygen.

Section 13 of 27Oxyhaemoglobin dissociation curve and the Bohr effect

Curve

• Curve percentage saturation against pO₂ is S-shaped because of cooperative binding.
• Shallow startthe first O₂ binds slowly.
• Steep middlehaemoglobin changes shape, making further binding easier.
• Plateaufew remaining sites.

Affinity

• Affinity high pO₂ in lungs gives loading; low pO₂ in respiring tissue gives unloading.
• A steep curve means a small change in pO₂ gives a large change in saturation.

Bohr effect

• Bohr effect high pCO₂ reduces haemoglobin's affinity,
  • so more oxygen is released in active tissue; the curve shifts right and saturation at a given pO₂ is lower.

Section 14 of 27Adaptations in haemoglobin

Group	Haemoglobin affinity	Curve shift	Reason
Species at high altitude (llama)	Higher	Left	Takes up oxygen at low pO2
Fetus	Higher than adult	Left	Obtains oxygen from maternal blood at the placenta where mother's Hb is dissociating
Small, highly active animals	Lower	Right	Release oxygen easily to tissues with high demand

Reading graphs

• Reading graphs at any pO₂, the curve with the highest saturation has the highest affinity.

After birth

• After birth adult haemoglobin replaces fetal haemoglobin.

Section 15 of 27The mammalian circulation

• Closed double circulation
  • blood stays in vessels and passes through the heart twice per circuit; insects have an open system.
  • Left side
    • systemic circulation.
    • Right side: pulmonary circulation.

Vessel	Carries
Coronary arteries	Oxygenated blood to heart muscle
Aorta	Oxygenated blood from the heart to the body
Vena cava	Deoxygenated blood from the body to the heart
Pulmonary artery	Deoxygenated blood from the heart to the lungs
Pulmonary vein	Oxygenated blood from lungs to heart (the only vein with oxygenated blood)
Renal artery and vein	Blood to and from the kidneys

Section 16 of 27Heart structure and the cardiac cycle

1Chambers

• Chambers two thin-walled atria and two ventricles;
  • a septum separates left (oxygenated) and right (deoxygenated);
  • the left ventricle wall is thickest as it pumps further at higher pressure.

2Valves

• Valves atrioventricular valves and semilunar valves in the aorta and pulmonary artery open when pressure behind exceeds pressure in front.
  • (tricuspid on the right, bicuspid on the left)

3Cardiac cycle

• Cardiac cycle systole is contraction; diastole is relaxation.
• Smaller volume means higher pressure.

Stage	Events	AV valves	SL valves
Atrial systole	Atria contract and force blood into relaxed ventricles	Open	Closed
Ventricular systole	Ventricle pressure exceeds atrial, AV valves shut; pressure exceeds aorta, SL valves open	Closed	Open
Diastole	Pressure falls, SL valves close; atria fill and blood flows passively into ventricles	Open	Closed

1Heart rate from a graph

• Heart rate from a graph one cycle takes 0.7 s, so 60 ÷ 0.7 = 85.7 bpm.

2Cardiac output

Cardiac output heart rate × stroke volume.

• 60 bpm × 77 cm³ = 4620 cm³ = 4.62 dm³.
• Stroke volume = 4.3 dm³ ÷ 72 = 59.7 cm³.

Section 17 of 27Practicals on heart rate and heart dissection

Practicals on heart rate and heart dissection

Exercise and pulse

• Exercise and pulse record resting heart rate, exercise at set intensity for a set time, record immediately afterwards, rest for 5 minutes and repeat at higher intensities with several people.
• Limitationsquantify intensity, record at the same moment after exercise, avoid fatigue.

Daphnia and caffeine

• Daphnia and caffeine transparent water fleas under a microscope on a cavity slide.
• Prepare caffeine concentrations (serial dilution) and a distilled water control.
• Count heartbeats for 20 seconds (dot on paper) and multiply by 3.
• Repeat with at least three Daphnia per concentration.
• Ethicshandle gently, short examination, return promptly, avoid extremes.

Heart dissection (pig or sheep)

• Front identified by coronary arteries between ventricles.
• Observe coronary arteries, atria and major vessels.
• Cut vertically through ventricles and atria, open and pin back.
• Identify ventricle walls (compare thickness), atria, septum, AV valves and vessel entrances.
• Limitationsdead tissue differs from live, vessels may be removed, valves are small, one specimen is unreliable.

Section 18 of 27Blood vessels, capillaries and tissue fluid

Vessel	Structure	Function
Artery	Thick walls with smooth muscle and elastic fibres, narrow lumen, folded endothelium	Carry blood from heart at high pressure; stretch and recoil
Arteriole	Many muscle cells, fewer elastic fibres	Contract to adjust blood flow to organs
Vein	Thin walls, large lumen, valves	Return blood at low pressure; valves stop backflow
Capillary	Wall one cell thick with pores; lumen about one red cell wide	Short diffusion distance; slow flow gives time for exchange

Tissue fluid

• Tissue fluid plasma leaks from capillaries, with fewer large proteins, and exchange between blood and cells occurs through it.

Formation and return

• Arterial endhydrostatic pressure exceeds osmotic pull, so water and small molecules leave.
• Venous endhydrostatic pressure has fallen and dissolved proteins lower water potential, so fluid is drawn back by osmosis.
• Excess is drained by lymphatic vessels.

Oedema

• Oedema high blood pressure or low blood protein leaves extra fluid in tissues.

Section 19 of 27Cardiovascular disease data

CVD

CVD includes coronary heart disease, stroke and congenital heart disease.

Risk factors

• Risk factors high blood pressure, smoking, stress, diet high in cholesterol and saturated fat, genetics, age and sex.
  • (men more at risk)

Interpreting data

• Interpreting data in a study of 523 non-smoking partners of smokers, relative risk rose with cigarettes smoked per day,
  • from 1.00 at 0 to 1.31 at 20 or more.
• This shows a correlation, not causation.
• Validity is limited by uncontrolled age, diet, sex and exercise and the lack of statistical tests.

Conflicting evidence

• Conflicting evidence judge sample representativeness and size, statistics (error bars), controls such as placebo, replication, control of other variables and bias or financial interests.

Section 20 of 27Transpiration and the xylem

Xylem

• Xylem hollow tubes with no end walls and lignin waterproofing and strength;
  • they carry water and minerals from roots to leaves.

Transpiration stream

• Water vapour diffuses out through stomata.
• This lowers water potential in the leaf air spaces, so water evaporates from mesophyll cell walls.
• Mesophyll cells draw water from the xylem by osmosis.
• Cohesion between water molecules and adhesion to xylem walls pull a continuous water column up under tension.

Terms

• Terms transpiration is water loss from leaves; the transpiration stream is the upward flow in the xylem.

Section 21 of 27Potometer practical and environmental factors

Factor	Effect on transpiration	Explanation
High wind	Increased	Removes vapour, steepening the water potential gradient
High humidity	Decreased	Reduces the gradient
High light	Increased	Stomata open for photosynthesis
High temperature	Increased	More kinetic energy so faster evaporation

1

Cut a shoot underwater at a slant (larger surface for uptake).

2

Assemble the potometer underwater and seal joints with petroleum jelly.

3

Dry the leaves.

4

Allow one air bubble to form and place the tube end in a beaker of water.

5

Set the environmental factor and wait 5 minutes.

6

Record the bubble start, wait 15 minutes, record the end, and reset the bubble.

7

Repeat three times and calculate rate = distance ÷ time (or volume ÷ time).

1Methods

• Methods fan for airflow, plastic bag for humidity, lamp distance for light, cold and warm rooms for temperature.

2A potometer measures water uptake

, not transpiration itself, as some water is used in photosynthesis.

Section 22 of 27Phloem and translocation

Assimilates

(sucrose, amino acids, hormones) move in phloem by translocation from a source to a sink.

• Sourcesphotosynthesising leaves or storage organs in early spring.
• Sinksgrowing and storage regions.
• Name specific cells, for example leaf palisade cells are a source and a growing root tip is a sink.

Phloem cells

• Phloem cells sieve tube elements with reduced contents and sieve plates;
  • companion cells with many mitochondria to supply ATP for loading.

Translocation is active

, dependent on ATP.

Section 23 of 27Mass flow hypothesis

1

At the source, companion cells use ATP to pump H⁺ out into the cell wall.

2

H⁺ returns down its gradient through a co-transporter carrying sucrose.

3

Sucrose enters sieve tubes through plasmodesmata.

4

The lowered water potential draws water in from the xylem by osmosis.

5

Hydrostatic pressure rises at the source, creating a gradient to the sink.

6

At the sink, sucrose is unloaded, raising water potential so water leaves by osmosis, maintaining the gradient.

• Mass flow, not diffusion
  • bulk movement of sap down a pressure gradient.

Section 24 of 27Evaluating mass flow: tracers and ringing

Evidence for	Evidence against
Sap oozes out of punctured phloem, showing pressure	Amino acids move more slowly than sucrose
Sap near a source has more sucrose than sap near a sink	Different substances move in opposite directions in the same tube
Metabolic inhibitors stop translocation (needs ATP)	Sieve plates would obstruct bulk flow
Ringing the bark causes a sugar-rich bulge above the ring

1Tracers

• ¹⁴CO₂ is absorbed and made into radioactive sucrose, which can be traced.

2Ringing

• Ringing removing a ring of outer tissue removes the phloem but leaves the xylem.
• A control without a ring shows normal transport.
• No radioactive sucrose is found beyond the ring, showing that phloem carries sucrose.
• Movement occurs both up and down from the source.

3Exam tip

• Exam tip when asked to evaluate, give evidence both for and against.

Section 25 of 27Worked examples

1. Explain why a fish gill is more efficient with counter-current flow than with parallel flow. [3]3 marks

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2. Calculate the SA:V ratio of a cube of side 3 cm. [2]2 marks

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3. Explain how the Bohr effect helps oxygen delivery during exercise. [3]3 marks

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4. Describe how tissue fluid is formed and returned to the blood. [4]4 marks

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5. Describe the evidence from ringing experiments that sucrose is carried in the phloem. [3]3 marks

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Section 26 of 27Key terms

Term	Meaning
SA:V ratio	Surface area divided by volume; falls as an organism grows.
Counter-current flow	Flow of two fluids in opposite directions that maintains a gradient.
Pulmonary ventilation rate	Volume of air breathed per minute: tidal volume × breathing rate.
Micelle	A tiny sphere of bile salts carrying fatty acids and monoglycerides to the gut wall.
Cooperative binding	Binding of one oxygen making further binding to haemoglobin easier.
Bohr effect	Reduced haemoglobin affinity for oxygen at high carbon dioxide concentration.
Cardiac output	Heart rate multiplied by stroke volume.
Cohesion-tension theory	Water is pulled up the xylem by cohesion and adhesion under tension.
Translocation	Movement of assimilates in the phloem from source to sink.

Section 27 of 27Exam technique

1

Saying alveoli have cell walls — they have walls one cell thick.

2

Saying ventilation is respiration — one is breathing, the other a chemical reaction.

3

Saying bile digests lipids chemically — it emulsifies; lipase digests.

4

Describing translocation as diffusion — it is mass flow driven by a pressure gradient with active loading.

5

Saying the pulmonary vein is deoxygenated — it carries oxygenated blood.