Transport

What you'll learn

This revision guide covers transport systems in both animals and plants, focusing on how substances move around organisms. You'll master the structure and function of the human circulatory system, blood composition, and the mechanisms of water and mineral transport in plants. These topics are essential for Paper 1 and Paper 2 of your Pearson Edexcel International IGCSE Biology exam.

Key terms and definitions

Double circulatory system — a blood system with two separate circuits: one between the heart and lungs (pulmonary circulation) and one between the heart and the rest of the body (systemic circulation)

Plasma — the pale yellow liquid component of blood that transports dissolved substances including glucose, amino acids, carbon dioxide, urea, hormones and heat

Haemoglobin — the red oxygen-carrying pigment found in red blood cells that combines reversibly with oxygen to form oxyhaemoglobin

Phagocytosis — the process by which white blood cells engulf and digest pathogens or foreign material

Transpiration — the loss of water vapour from plant leaves through stomata, creating a pulling force that draws water up through the xylem

Translocation — the transport of dissolved sugars from leaves to other parts of the plant through phloem tissue

Lymphocyte — a type of white blood cell that produces antibodies to destroy specific pathogens

Xylem — plant tissue consisting of dead cells forming hollow tubes that transport water and mineral ions from roots to leaves

Core concepts

The human circulatory system

The human circulatory system is a double circulatory system, meaning blood passes through the heart twice during one complete circuit of the body. This arrangement provides several advantages:

• Blood can be pumped to the lungs at lower pressure (preventing damage to delicate lung capillaries)
• Blood returns to the heart to be pumped at high pressure to the rest of the body
• This ensures efficient oxygen delivery to all tissues

The heart has four chambers:

• Right atrium (receives deoxygenated blood from the body via vena cava)
• Right ventricle (pumps deoxygenated blood to the lungs via pulmonary artery)
• Left atrium (receives oxygenated blood from the lungs via pulmonary vein)
• Left ventricle (pumps oxygenated blood to the body via aorta)

The left ventricle has a thicker muscular wall than the right ventricle because it must generate higher pressure to pump blood around the entire body, whereas the right ventricle only pumps blood to the nearby lungs.

Valves prevent backflow of blood:

• Atrioventricular valves between atria and ventricles
• Semilunar valves in the aorta and pulmonary artery

Blood vessels and their functions

The circulatory system contains three types of blood vessels, each adapted for specific functions:

Arteries carry blood away from the heart:

• Thick muscular walls to withstand high pressure
• Elastic tissue allows walls to stretch and recoil with each pulse
• Small lumen (internal space) relative to wall thickness
• Carry oxygenated blood (except pulmonary artery)

Veins carry blood back to the heart:

• Thinner walls (lower pressure)
• Larger lumen relative to wall thickness
• Contain valves to prevent backflow
• Carry deoxygenated blood (except pulmonary vein)

Capillaries are the site of substance exchange:

• Walls one cell thick for short diffusion distance
• Permeable walls allow exchange of materials
• Very narrow lumen (red blood cells pass through in single file)
• Form extensive networks to increase surface area

Blood flow in arteries creates a pulse that can be felt where arteries pass close to the skin surface, such as the wrist or neck. Capillaries connect arteries to veins, forming a continuous network.

Composition and functions of blood

Blood consists of plasma and blood cells. Each component has specific roles:

Plasma functions:

• Transports dissolved glucose from the small intestine to all cells for respiration
• Carries amino acids from the small intestine to cells for protein synthesis
• Transports carbon dioxide from respiring cells to the lungs
• Carries urea from the liver to the kidneys for excretion
• Distributes heat energy throughout the body
• Transports hormones from endocrine glands to target organs

Red blood cells (erythrocytes):

• Biconcave disc shape increases surface area for gas exchange
• No nucleus provides more space for haemoglobin
• Contain haemoglobin which binds reversibly with oxygen
• In the lungs: haemoglobin + oxygen → oxyhaemoglobin
• In respiring tissues: oxyhaemoglobin → haemoglobin + oxygen

White blood cells (leucocytes):

• Larger than red blood cells and contain a nucleus
• Two main types: phagocytes and lymphocytes
• Phagocytes engulf pathogens through phagocytosis
• Lymphocytes produce specific antibodies that destroy particular pathogens
• Can change shape to squeeze through capillary walls

Platelets:

• Small cell fragments without a nucleus
• Essential for blood clotting
• Convert soluble fibrinogen into insoluble fibrin threads at wound sites
• Fibrin forms a mesh that traps red blood cells, forming a clot

Coronary heart disease

The coronary arteries supply the heart muscle with oxygenated blood. Coronary heart disease occurs when these arteries become narrowed or blocked.

Causes:

• Build-up of fatty deposits (plaques) in coronary artery walls
• Plaques contain cholesterol and reduce blood flow
• Risk factors include high-fat diet, smoking, lack of exercise, genetic factors, stress and age

Consequences:

• Reduced oxygen supply to heart muscle
• Heart muscle cells cannot respire aerobically efficiently
• Angina (chest pain) during exercise
• Complete blockage causes heart attack (myocardial infarction)
• Heart muscle cells die if deprived of oxygen

Treatments:

• Lifestyle changes: exercise, balanced diet, stopping smoking
• Medications: statins to lower cholesterol, anticoagulants to prevent clots
• Surgical interventions: stents to keep arteries open, bypass surgery using veins from legs

Transport in plants - xylem

Plants have two separate transport systems. Xylem transports water and mineral ions upwards from roots to leaves.

Xylem structure:

• Made of dead cells with no end walls, forming continuous hollow tubes
• Cell walls strengthened with lignin (waterproof substance)
• Lignin provides support to the plant stem
• No cytoplasm, providing an unobstructed pathway for water flow

Water uptake and movement:

• Root hair cells have large surface area for absorption
• Water enters by osmosis (from dilute solution in soil to more concentrated solution in root hair cell)
• Water moves across root cells by osmosis until it reaches xylem vessels
• Water travels up xylem in the transpiration stream

Transport in plants - phloem and transpiration

Phloem structure and function:

• Made of living cells joined end to end
• Cell walls between cells have pores (sieve plates)
• Transports dissolved sugars (mainly sucrose) from leaves to all parts of the plant
• Movement occurs in both upward and downward directions
• Process called translocation requires energy from respiration

Transpiration process:

• Water evaporates from mesophyll cells inside the leaf
• Water vapour diffuses out through stomata
• Creates a "pull" effect drawing more water up the xylem
• Continuous column of water maintained by cohesion between water molecules

Factors affecting transpiration rate:

Temperature:

• Higher temperature increases kinetic energy of water molecules
• Faster evaporation and diffusion
• Rate increases with temperature

Humidity:

• High humidity reduces concentration gradient between leaf and air
• Lower transpiration rate in humid conditions
• Higher rate in dry conditions

Wind speed:

• Wind removes water vapour from around stomata
• Maintains steep concentration gradient
• Increases transpiration rate

Light intensity:

• Light causes stomata to open for photosynthesis
• More stomata open means increased transpiration
• Most transpiration occurs during daylight

Adaptations to reduce water loss:

• Waxy cuticle on leaf surface (waterproof layer)
• Stomata mainly on lower leaf surface (cooler, more humid)
• Some plants have sunken stomata or rolled leaves
• Cacti have reduced leaves (spines) and thick stems for water storage

Worked examples

Example 1: Comparing blood vessels

Question: The table shows information about three blood vessels.

Blood vessel	Wall thickness (mm)	Lumen diameter (mm)	Valves present
A	1.0	4.0	No
B	0.001	0.008	No
C	0.5	6.0	Yes

(a) Identify blood vessel B. (1 mark) (b) Explain how the structure of blood vessel A is related to its function. (3 marks)

Mark scheme answer:

(a) Capillary (1 mark)

(b)

• Thick/muscular wall (1 mark) withstands/maintains high pressure (1 mark)
• Elastic tissue (1 mark) allows artery to stretch and recoil / maintains blood flow (1 mark)
• Small lumen (1 mark) maintains high pressure (1 mark)

(Any 3 marks from the points above)

Example 2: Investigating transpiration

Question: A student investigated the effect of wind speed on transpiration rate using a potometer. She used a fan at different speeds to vary wind speed.

Explain why increasing wind speed increases the rate of transpiration. (3 marks)

Mark scheme answer:

• Wind/air movement removes water vapour (1 mark) from around the stomata/leaf surface (1 mark)
• This maintains/increases the concentration/diffusion gradient (1 mark) between the inside of the leaf and the outside air (1 mark)
• So water vapour diffuses out faster/more rapidly (1 mark)

(Maximum 3 marks)

Example 3: Heart structure and function

Question: (a) Explain why the left ventricle has a thicker muscle wall than the right ventricle. (2 marks) (b) Describe the role of valves in the heart. (2 marks)

Mark scheme answer:

(a)

• Left ventricle pumps blood around the whole body/systemic circulation (1 mark)
• Needs to generate higher pressure (1 mark)
• Right ventricle only pumps blood to the lungs (1 mark)

(Any 2 marks)

(b)

• Prevent backflow/backward flow of blood (1 mark)
• Ensure blood flows in one direction only (1 mark)

Common mistakes and how to avoid them

• Confusing arteries and veins by oxygen content: Remember that arteries carry blood away from the heart (not necessarily oxygenated). The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. Focus on direction of flow, not oxygen content.

• Saying red blood cells "carry oxygen": Be precise—red blood cells contain haemoglobin, and it is the haemoglobin that combines with oxygen to form oxyhaemoglobin. Red blood cells transport haemoglobin.

• Mixing up xylem and phloem: Create a clear distinction—xylem (dead cells, lignin, water and minerals upwards) versus phloem (living cells, sieve plates, sugars in both directions). Never say xylem transports food.

• Stating transpiration is "to cool the plant": Transpiration is the loss of water vapour through stomata. Cooling is a consequence, not the purpose. The primary function of stomata is gas exchange for photosynthesis.

• Describing blood clotting incorrectly: Platelets do not "form the clot" themselves. They release chemicals that convert soluble fibrinogen protein into insoluble fibrin threads, which trap red blood cells to form the clot.

• Forgetting that the heart is a double pump: When describing circulation, clearly state that blood passes through the heart twice in one complete circuit—once through the right side (to lungs) and once through the left side (to body).

Exam technique for "Transport"

• Command word awareness: "Explain" requires reasons (3+ marks usually)—give the mechanism or cause-and-effect. "Describe" needs features or changes (no reasons needed). "State" or "Name" requires simple factual recall (1 mark each point).

• Comparative questions: When asked to compare structures (like arteries vs veins), make direct comparisons in the same sentence: "Arteries have thicker walls than veins" scores marks, but "Arteries have thick walls. Veins have thin walls" may not score full marks.

• Use mark allocations strategically: A 3-mark question needs three distinct points. If asked to explain how capillaries are adapted (3 marks), give three separate structural features with their functions, not three similar points about thin walls.

• Draw clear, labelled diagrams when asked: For heart diagrams, label all four chambers, major blood vessels (with direction arrows), and valves. Use a ruler for neat lines and ensure labels don't overlap.

Quick revision summary

Transport systems move essential substances around organisms. The human double circulatory system pumps blood through the heart twice per circuit—to lungs and to body. Arteries have thick muscular walls for high pressure; veins have valves preventing backflow; capillaries have thin walls enabling exchange. Blood contains plasma (transporting dissolved substances), red blood cells (carrying haemoglobin for oxygen transport), white blood cells (fighting disease), and platelets (blood clotting). Plants use xylem (dead cells transporting water upwards) and phloem (living cells translocating sugars). Transpiration—water loss through stomata—is affected by temperature, humidity, wind and light.