Edexcel International GCSE Biology 4BI1: Transport – Study Guide

This study guide teaches sub-topic (h) Transport (specification points 2.51–2.69) of Topic 2, Structures and functions in living organisms, in the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3. The qualification is untiered. Points with a B reference (2.55B–2.58B, 2.63B and 2.64B) are bold in the specification and are examined on Paper 2 only; the rest can appear on either paper. A calculator may be used on both papers.

Use it with the Transport revision notes and the Transport practice questions. Cell structure, which this topic builds on, is in the cell structure and biological molecules guide. The full course is on the Edexcel IGCSE Biology hub, and the printable checklist lists every point.

What this sub-topic covers

Spec point	What you must be able to do	Paper
2.51–2.52	Why single cells can rely on diffusion; why multicellular organisms need a transport system	1 and 2
2.53–2.54	Role of phloem (sucrose, amino acids) and xylem (water, mineral ions)	1 and 2
2.55B–2.58B	Water uptake by root hair cells; transpiration; factors affecting its rate; transpiration practical	Paper 2 only
2.59–2.61	Composition of blood; role of plasma; adaptations of red blood cells	1 and 2
2.62	Phagocytes and lymphocytes in the immune response	1 and 2
2.63B–2.64B	Vaccination and memory cells; platelets and clotting	Paper 2 only
2.65–2.67	Structure and function of the heart; heart rate in exercise and with adrenaline; coronary heart disease risk factors	1 and 2
2.68–2.69	Arteries, veins and capillaries; the general plan of the circulation	1 and 2

Why transport systems are needed (2.51–2.52)

A single-celled organism exchanges oxygen, nutrients and carbon dioxide across its cell membrane by diffusion. That works because:

• it has a large surface area to volume ratio, so there is plenty of membrane for each unit of volume that needs supplying
• the diffusion distance is short: no part of the cytoplasm is far from the surface.

As an organism gets bigger, its volume rises faster than its surface area, so the ratio falls, and inner cells are far from the surface. Diffusion alone would be too slow. Multicellular organisms therefore need a transport system that carries substances in bulk between exchange surfaces (lungs, gut, leaves, roots) and every cell. In plants this is the xylem and phloem; in humans it is the circulatory system.

Worked example 1: surface area to volume ratio

Model a single cell as a cube of side 20 µm and a small multicellular organism as a cube of side 2 mm (2000 µm). Compare their surface area to volume ratios.

Cell:      SA = 6 × 20² = 2400 µm²      V = 20³ = 8000 µm³
           SA:V = 2400 / 8000 = 0.3 : 1
Organism:  SA = 6 × 2000² = 24 000 000 µm²   V = 2000³ = 8 000 000 000 µm³
           SA:V = 0.003 : 1

The cell’s ratio is 100 times larger, and the organism’s centre is 1000 µm from its surface instead of 10 µm.

Transport in flowering plants

Phloem (2.53)

Phloem carries sucrose and amino acids made in the leaves to other parts of the plant: growing shoot and root tips, developing fruits and seeds, and storage organs. Movement in phloem can be up or down the plant, depending on where the food is needed. This movement of food is called translocation.

Xylem (2.54)

Xylem carries water and mineral ions from the roots up the stem to the leaves and other parts. Xylem vessels are dead, hollow tubes with no end walls and lignin-strengthened walls. Flow in xylem is one way: upwards.

Water uptake by root hair cells (2.55B, Paper 2 only)

Root hair cells have long, thin extensions that give a large surface area for absorption. Their cell sap is more concentrated than the soil water, so water moves into the root hair cell by osmosis, from the dilute soil solution, across the partially permeable cell membrane, into the more concentrated cell sap. The water then passes from cell to cell across the root cortex to the xylem.

Transpiration (2.56B, Paper 2 only)

Transpiration is the evaporation of water from the surface of a plant. Most of it happens in the leaves: water evaporates from the surfaces of mesophyll cells into the air spaces, then water vapour diffuses out through the stomata. Losing water from the leaves pulls more water up the xylem from the roots, forming the transpiration stream that also carries mineral ions.

Factors affecting the rate of transpiration (2.57B, Paper 2 only)

Change	Effect on rate	Reason
Humidity increases	Decreases	Less difference in water vapour concentration between the leaf air spaces and the air outside, so slower diffusion
Wind speed increases	Increases	Moving air carries water vapour away from the leaf surface, keeping the concentration gradient steep
Temperature increases	Increases	Water molecules have more kinetic energy, so evaporation and diffusion are faster
Light intensity increases	Increases	More stomata open (for photosynthesis), so more water vapour can diffuse out

Practical: environmental factors and transpiration (2.58B, Paper 2 only)

A potometer measures how fast a leafy shoot takes up water, which is close to its rate of transpiration.

1. Cut the shoot under water and fit it to the potometer under water, so no air enters the xylem.
2. Seal the joints with petroleum jelly so the apparatus is airtight and watertight.
3. Dry the leaves gently, introduce an air bubble into the capillary tube and let the shoot settle for a few minutes.
4. Record how far the bubble moves in a set time. Reset it with the reservoir and repeat to get a mean.
5. Change one factor: a fan at set distances (wind speed), a lamp at set distances (light intensity), a clear plastic bag over the shoot (humidity), or a different room temperature. Keep the other factors and the same shoot constant.

Worked example 2: rate of water uptake

The bubble in a potometer moves 42 mm in 6 minutes. The capillary tube has a radius of 0.5 mm. Calculate the volume of water taken up per minute.

Volume = π r² × distance = π × 0.5² × 42 = 32.99 mm³
Rate   = 32.99 / 6 = 5.50 mm³ per minute

Blood (2.59–2.64B)

Composition (2.59) and plasma (2.60)

Blood is made of red blood cells, white blood cells, platelets and plasma. Plasma is the liquid part. It transports:

• carbon dioxide (dissolved) from respiring cells to the lungs
• digested food such as glucose and amino acids from the small intestine to cells
• urea from the liver to the kidneys
• hormones from endocrine glands to target organs
• heat energy from active organs such as muscles and the liver to the rest of the body and the skin.

Red blood cells (2.61)

Red blood cells carry oxygen. Each adaptation has a reason:

• Biconcave disc shape: a large surface area to volume ratio for fast diffusion of oxygen in and out.
• No nucleus: more room for haemoglobin, so more oxygen carried per cell.
• Haemoglobin: combines with oxygen in the lungs to form oxyhaemoglobin, and releases it in respiring tissues.

White blood cells and the immune response (2.62)

• Phagocytes move to infected areas and ingest pathogens (phagocytosis), then digest them with enzymes.
• Lymphocytes release antibodies. Each antibody is specific to one pathogen: it fits the antigens on that pathogen’s surface. Antibodies cause pathogens to clump together or mark them for destruction by phagocytes.

Vaccination (2.63B, Paper 2 only)

A vaccine contains a dead or weakened form of a pathogen, or its antigens. These cannot cause the disease, but lymphocytes still respond by making antibodies, and some become memory cells. If the real pathogen enters later, memory cells allow antibody production to occur sooner, faster and in greater quantity. The pathogen is destroyed before it causes symptoms, so the person is immune.

Platelets and clotting (2.64B, Paper 2 only)

Platelets are fragments of cells. At a cut, they trigger a chain of reactions that turns the soluble plasma protein fibrinogen into insoluble fibrin threads. The threads form a mesh that traps red blood cells, forming a clot, which dries into a scab. The clot prevents blood loss and stops micro-organisms entering the body.

The heart (2.65–2.67)

Structure and function (2.65)

The heart is a muscular pump with four chambers. The right side pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the body. The septum keeps the two sides apart.

Route of blood: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body.

• Valves between atria and ventricles, and at the bases of the aorta and pulmonary artery, prevent backflow.
• The left ventricle wall is thicker than the right, because it pumps blood at higher pressure all round the body, while the right ventricle pumps only to the nearby lungs.
• Coronary arteries supply the heart muscle itself with oxygen and glucose.

Heart rate in exercise and with adrenaline (2.66)

During exercise, muscles respire faster, needing more oxygen and glucose and producing more carbon dioxide. The brain detects the rise in carbon dioxide in the blood and sends nerve impulses to the heart, which beats faster and more strongly. More blood reaches the muscles each minute, delivering oxygen and glucose and removing carbon dioxide.

Adrenaline, released by the adrenal glands in fright or stress, also increases heart rate, delivering more oxygen and glucose to muscles for “fight or flight”.

Worked example 3: heart rate

A student counts 17 pulse beats in 15 seconds at rest and 31 beats in 15 seconds just after exercise. Calculate both heart rates and the percentage increase.

Rest:     17 × 4 = 68 beats per minute
Exercise: 31 × 4 = 124 beats per minute
% increase = (124 − 68) / 68 × 100 = 82.4%

Coronary heart disease (2.67)

Fatty deposits build up in the walls of the coronary arteries and narrow them. Less blood, and so less oxygen, reaches the heart muscle, which cannot respire aerobically; if an artery is blocked, part of the muscle dies (a heart attack). Factors that increase the risk:

• smoking (carbon monoxide and nicotine damage the circulatory system and raise blood pressure)
• a diet high in saturated fat (raises blood cholesterol) and high in salt
• lack of exercise and being overweight
• high blood pressure and stress
• genetic factors (family history), increasing age and being male.

Blood vessels and circulation (2.68–2.69)

Vessel	Structure	Link to function
Artery	Thick wall with muscle and elastic tissue; narrow lumen	Carries blood away from the heart at high pressure; wall stretches and recoils
Vein	Thin wall; wide lumen; valves	Carries blood back to the heart at low pressure; valves prevent backflow
Capillary	Wall one cell thick; very narrow	Short diffusion distance for exchange of substances with tissue cells

Humans have a double circulation: blood passes through the heart twice on each complete circuit, once to the lungs and once to the body. Vessels you must name:

• Heart: vena cava (in), aorta (out); coronary arteries supply the heart muscle.
• Lungs: pulmonary artery (to), pulmonary vein (from).
• Liver: hepatic artery (to), hepatic vein (from); the hepatic portal vein brings blood from the gut to the liver.
• Kidneys: renal artery (to), renal vein (from).

Common errors

• Saying xylem carries sugar, or that phloem only moves downwards.
• Defining transpiration as “water loss” without the word evaporation.
• Listing red blood cell features without explaining each one: shape gives surface area, no nucleus gives room for haemoglobin.
• Saying antibodies “kill” or “eat” pathogens; phagocytes ingest them.
• Saying vaccination gives you antibodies directly: it causes lymphocytes to make antibodies and memory cells.
• Saying the left ventricle is thicker “because it pumps more blood”. Both sides pump the same volume; the left pumps at higher pressure over a longer circuit.
• Mixing up pulmonary artery and pulmonary vein.

Next steps

Condense this into the revision notes, then try the practice questions. Take a free diagnostic to find your weakest topics.

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.