Edexcel International GCSE Biology 4BI1: Transport – Revision Notes

These notes condense sub-topic (h) Transport, points 2.51–2.69 of Topic 2 in the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3. The course is untiered. Points 2.55B–2.58B, 2.63B and 2.64B are bold B statements and are examined on Paper 2 only. For full explanations and worked examples, read the Transport study guide first; then test yourself with the Transport practice questions.

Course links: Edexcel IGCSE Biology hub · printable checklist · cell structure revision notes · free diagnostics.

2.51–2.52 Why transport systems exist

• Unicellular organisms: large surface area to volume ratio + short diffusion distance → diffusion across the membrane is fast enough.
• Multicellular organisms: small surface area to volume ratio + long distances to inner cells → diffusion too slow → need a transport system linking exchange surfaces to all cells.
• Bigger organism → volume grows faster than surface area → ratio falls.

Quick reminder. For a cube of side L, SA:V = 6L² / L³ = 6/L. Doubling the side halves the ratio.

2.53–2.54 Xylem and phloem

	Xylem	Phloem
Carries	Water and mineral ions	Sucrose and amino acids
From → to	Roots → stem, leaves, rest of plant	Leaves → growing regions, fruits, seeds, storage organs
Direction	Upwards only	Up and down
Cells	Dead, hollow, no end walls, lignified	Living

2.55B–2.58B Water uptake and transpiration (Paper 2 only)

• Root hair cells: long extension → large surface area. Cell sap more concentrated than soil water → water enters by osmosis across the partially permeable membrane.
• Transpiration = the evaporation of water from the surface of a plant. Water evaporates from mesophyll cell surfaces into air spaces; water vapour diffuses out through stomata.
• Loss from leaves pulls water up the xylem (the transpiration stream).

Factor increases	Rate	Key word in the reason
Humidity	Falls	Smaller water vapour concentration gradient
Wind speed	Rises	Water vapour blown away, gradient kept steep
Temperature	Rises	More kinetic energy: faster evaporation and diffusion
Light intensity	Rises	More stomata open

Method in steps: potometer practical

1. Cut shoot under water; assemble under water; seal joints with petroleum jelly.
2. Let it settle; record bubble movement in a fixed time; repeat and find a mean.
3. Change one factor (fan, lamp, plastic bag, temperature); keep the same shoot and all else constant.
4. Rate = distance ÷ time, or volume ÷ time where volume = πr² × distance.

The potometer measures water uptake, which is close to, but not exactly, the transpiration rate: a little water is used in the plant’s cells.

2.59–2.61 Blood and plasma

Component	Job
Red blood cells	Carry oxygen
White blood cells	Defend against pathogens
Platelets	Clotting
Plasma	Transports substances and heat

Plasma carries: carbon dioxide, digested food (glucose, amino acids), urea, hormones, heat energy. Memory aid: list all five in every answer that asks for them.

Red blood cell adaptations:

• Biconcave shape → large surface area to volume ratio → fast oxygen diffusion.
• No nucleus → more space for haemoglobin.
• Haemoglobin + oxygen → oxyhaemoglobin in the lungs; oxygen released in respiring tissues.

2.62–2.64B Defence

• Phagocytes: ingest and digest pathogens (phagocytosis). Not specific.
• Lymphocytes: release antibodies specific to the antigens on one pathogen.
• Vaccination (Paper 2 only): dead/weakened pathogen or its antigens → lymphocytes make antibodies and memory cells → on later infection, antibodies made sooner, faster, in greater quantity → no illness.
• Clotting (Paper 2 only): platelets → soluble fibrinogen converted to insoluble fibrin → mesh traps red blood cells → clot → stops blood loss and entry of micro-organisms.

2.65–2.67 The heart

Route (learn it as one chain): vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.

• Right side: deoxygenated blood. Left side: oxygenated blood. Septum separates them.
• Valves prevent backflow.
• Left ventricle wall thicker → higher pressure to pump round the whole body.
• Coronary arteries supply the heart muscle.

Heart rate rises:

• Exercise: more respiration → more CO₂ → detected by brain → nerve impulses → heart beats faster and stronger → more O₂ and glucose to muscles, CO₂ removed faster.
• Adrenaline (adrenal glands): increases heart rate for “fight or flight”.

Coronary heart disease: fatty deposits narrow coronary arteries → less oxygen to heart muscle → heart attack. Risk factors: smoking, diet high in saturated fat and salt, lack of exercise, obesity, high blood pressure, stress, genetics, age, being male.

2.68–2.69 Vessels and circulation

	Artery	Vein	Capillary
Wall	Thick, muscular, elastic	Thin	One cell thick
Lumen	Narrow	Wide	Very narrow
Valves	No	Yes	No
Pressure	High	Low	Lower than in arteries
Direction	Away from heart	Towards heart	Between arteries and veins

Double circulation: blood passes through the heart twice per circuit (heart → lungs → heart → body → heart).

Organ	In	Out
Lungs	Pulmonary artery	Pulmonary vein
Liver	Hepatic artery (+ hepatic portal vein from gut)	Hepatic vein
Kidneys	Renal artery	Renal vein
Whole body to/from heart	Aorta (out of heart)	Vena cava (into heart)

Small worked reminders

Reminder 1: an “explain” answer on transpiration (Paper 2 only). Explain why a plant loses water faster on a windy day. Build the answer in three links: wind carries water vapour away from the leaf surface (1) → this keeps the concentration of water vapour outside the leaf low, so the gradient between the air spaces and the air stays steep (2) → water vapour diffuses out through the stomata faster (3). A bare “wind increases transpiration” scores at most one mark.

Reminder 2: reading an antibody graph (Paper 2 only). A graph shows antibody concentration after a first and a second exposure to the same pathogen. Describe what you see before you explain it: after the second exposure, antibodies appear sooner (shorter delay), rise faster (steeper curve) and reach a higher peak. Then explain: memory cells made after the first exposure respond straight away. Quote values from the axes with units when you describe.

Reminder 3: tracing a molecule. Trace an oxygen molecule from the lungs to a leg muscle. Alveolus → capillary → plasma → into a red blood cell, where it joins haemoglobin → pulmonary vein → left atrium → left ventricle → aorta → artery to the leg → capillary → released from oxyhaemoglobin → diffuses into a muscle cell. Name every chamber and vessel in order; a missing step costs a mark.

Reminder 4: structure–function pairs. Examiners reward a feature linked to its job. Write “capillary walls are one cell thick, so the diffusion distance is short”, not just “thin walls”. The same pattern works for veins (valves so no backflow at low pressure) and arteries (elastic tissue so the wall can stretch and recoil with each surge of high-pressure blood).

Must-know distinctions

• Xylem vs phloem: water and ions upwards vs sucrose and amino acids both ways.
• Transpiration vs translocation: water evaporating vs food moving in phloem.
• Phagocyte vs lymphocyte: ingests vs releases antibodies.
• Antibody vs antigen: protein made by the body vs molecule on the pathogen.
• Artery vs vein: away, high pressure, thick wall vs towards, low pressure, valves.
• Pulmonary artery vs pulmonary vein: the only artery carrying deoxygenated blood vs the only vein carrying oxygenated blood.

Quick self-test

1. Calculate the surface area to volume ratio of a cube of side 3 cm.
2. Name the two substances transported in phloem.
3. Give the direction of flow in xylem.
4. Define transpiration. (Paper 2 only)
5. State the effect of increasing humidity on the rate of transpiration. (Paper 2 only)
6. A potometer bubble moves 25 mm in 5 minutes. Calculate the rate of movement.
7. The capillary in that potometer has radius 0.5 mm. Calculate the volume of water taken up in the first 4 minutes, to 1 d.p.
8. Name five things carried in plasma.
9. Explain why red blood cells have no nucleus.
10. Name the blood vessel that takes blood from the heart to the lungs.
11. A student counts 22 beats in 20 seconds. Calculate the heart rate in beats per minute.
12. State three ways in which antibody production differs after vaccination. (Paper 2 only)

Answers

1. SA = 6 × 9 = 54 cm²; V = 27 cm³; ratio 2 : 1.
2. Sucrose and amino acids.
3. Upwards, from roots to leaves.
4. The evaporation of water from the surface of a plant.
5. Rate decreases.
6. 25 ÷ 5 = 5 mm per minute.
7. Distance in 4 minutes = 20 mm; volume = π × 0.5² × 20 = 15.7 mm³.
8. Carbon dioxide, digested food, urea, hormones, heat energy.
9. More space for haemoglobin, so more oxygen carried.
10. Pulmonary artery.
11. 22 × 3 = 66 beats per minute.
12. Antibodies made sooner, faster and in greater quantity.

Where marks are usually lost

• Writing “water loss” for transpiration instead of evaporation of water.
• Saying stomata “let water out” without saying water vapour diffuses out.
• Giving a factor’s effect on transpiration with no reason, when the question says “explain”.
• Saying xylem carries “food” or “nutrients”; say water and mineral ions.
• Listing only two or three plasma substances when the question wants more.
• Writing “antibodies eat the pathogen”; phagocytes ingest, antibodies are proteins.
• Saying a vaccine contains antibodies. It contains a harmless form of the pathogen or its antigens.
• Explaining the thick left ventricle by “it pumps more blood” instead of higher pressure.
• Swapping the hepatic vein and the hepatic portal vein.
• In heart rate calculations, forgetting to scale a 15-second or 20-second count to one minute.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1), Specification, Issue 3, September 2024, Pearson Education Limited: Topic 2, sub-topic (h) Transport, points 2.51–2.69.