What Is Transport? A Visual Guide for Edexcel IGCSE Biology Students

Transport in living organisms: what it means and why it matters

Transport in biology refers to the movement of substances around an organism to where they are needed. In small, unicellular organisms such as amoeba, diffusion alone is sufficient because every part of the cell is close to the external environment. Multicellular organisms, however, face a different problem. Their cells are arranged in layers and tissues, many of them far from the body surface. Diffusion across those distances would be far too slow to sustain life. That is why multicellular organisms need dedicated transport systems: the circulatory system in animals and the vascular system in plants.

If you are studying for the Pearson Edexcel IGCSE Biology exam, the topic of transport is one you will meet repeatedly. It connects to respiration (delivering oxygen, removing carbon dioxide), nutrition (distributing digested food), excretion (carrying urea to the kidneys) and coordination (transporting hormones). Understanding it well pays dividends across the entire 4BI1 specification. These edexcel igcse biology notes walk through every part of the topic, with worked examples and self-check questions so you can test yourself as you go.

Key facts at a glance

Transport in plants: xylem and phloem

Plants have two distinct transport tissues. Xylem carries water and dissolved mineral ions upward from the roots to the leaves and other aerial parts. Phloem transports dissolved sugars (mainly sucrose) and amino acids from the leaves, where they are made during photosynthesis, to the rest of the plant, including growing tips, storage organs and roots. This movement through phloem is called translocation.

How water enters the roots

Water is absorbed from the soil by root hair cells. These cells have a large surface area because they project outward into the spaces between soil particles. The concentration of dissolved substances inside the root hair cell is higher than in the surrounding soil water, so water enters by osmosis, moving from a region of higher water concentration (dilute soil solution) to a region of lower water concentration (more concentrated cell sap). Once inside the root, water passes from cell to cell by osmosis until it reaches the xylem vessels in the centre of the root.

Transpiration and its control

Transpiration is the evaporation of water from the surface of a plant, mainly through the stomata on the underside of leaves. As water evaporates from the leaf cells, it creates a pull that draws more water up through the xylem. This transpiration stream is the main force moving water through the plant.

The rate of transpiration is affected by four environmental factors:

• Humidity - high humidity reduces the concentration gradient between the leaf interior and the surrounding air, slowing evaporation and reducing transpiration rate
• Wind speed - wind removes water vapour from around the leaf surface, maintaining a steep concentration gradient and increasing transpiration
• Temperature - higher temperatures give water molecules more kinetic energy, increasing the rate of evaporation from leaf cells
• Light intensity - in bright light, stomata open wider to allow carbon dioxide in for photosynthesis, which also lets more water vapour escape

Blood: composition and functions

Blood is the transport medium in humans. It consists of four main components:

Red blood cells and oxygen transport

Red blood cells are adapted for carrying oxygen in several ways. Their biconcave disc shape gives them a large surface area to volume ratio, allowing rapid diffusion of oxygen in and out. They have no nucleus, which leaves more room inside for haemoglobin. Haemoglobin is the protein that binds oxygen in the lungs (where oxygen concentration is high) to form oxyhaemoglobin, and releases it in the tissues (where oxygen concentration is low).

The immune system

White blood cells defend the body in two main ways. Phagocytes engulf and digest pathogens by surrounding them with their cytoplasm. Lymphocytes produce antibodies that are specific to antigens on the surface of the invading pathogen. Each type of antibody fits one type of antigen, like a lock and key. Once the pathogen is destroyed, some lymphocytes remain as memory cells. If the same pathogen enters the body again, these memory cells can produce antibodies much more quickly, in greater quantity, preventing the disease from developing. This is the basis of vaccination: a weakened or inactive form of a pathogen is introduced so the body produces memory cells without suffering the disease.

Platelets and clotting

When a blood vessel is damaged, platelets accumulate at the wound. They trigger a chain of reactions that converts the soluble plasma protein fibrinogen into insoluble fibrin threads. These threads form a mesh across the wound, trapping red blood cells and forming a clot. The clot serves two purposes: it prevents further blood loss and blocks microorganisms from entering the body.

The heart: structure and function

The heart is a muscular organ that pumps blood around the body. It has four chambers: two atria (upper chambers) that receive blood and two ventricles (lower chambers) that pump blood out. The right side of the heart deals with deoxygenated blood; the left side deals with oxygenated blood. The two sides are separated by the septum, which prevents mixing.

The pathway of blood through the heart follows this sequence:

1. Deoxygenated blood from the body enters the right atrium through the vena cava
2. It passes through the right atrioventricular valve into the right ventricle
3. The right ventricle contracts and pushes blood into the pulmonary artery, which carries it to the lungs
4. In the lungs, carbon dioxide diffuses out of the blood and oxygen diffuses in
5. Oxygenated blood returns to the left atrium via the pulmonary vein
6. It passes through the left atrioventricular valve into the left ventricle
7. The left ventricle contracts with the greatest force and pushes blood into the aorta, which distributes it to the rest of the body

The left ventricle wall is thicker than the right because it must generate enough pressure to push blood all the way around the body, whereas the right ventricle only needs to push blood to the nearby lungs.

Valves throughout the heart and veins prevent the backflow of blood, ensuring it moves in one direction only.

Heart rate and exercise

During exercise, the muscles need more oxygen and glucose for aerobic respiration and produce more carbon dioxide as a waste product. The heart rate increases to deliver blood more quickly to the muscles and remove waste faster. The hormone adrenaline, released by the adrenal glands, also increases heart rate and prepares the body for physical activity. This is part of the "fight or flight" response.

Coronary heart disease

The heart muscle itself needs a blood supply, which is provided by the coronary arteries. If fatty deposits (including cholesterol) build up inside the walls of these arteries, the lumen narrows and less blood reaches the heart muscle. This is coronary heart disease. Risk factors include a diet high in saturated fat, smoking, lack of exercise, stress and genetic predisposition. If a coronary artery becomes completely blocked, part of the heart muscle is starved of oxygen and dies. This is a heart attack.

Blood vessels: arteries, veins and capillaries

The thick elastic walls of arteries allow them to stretch and recoil with each heartbeat, smoothing the pulsing flow. Veins have valves because the blood pressure in them is too low to prevent backflow on its own. Capillaries have walls only one cell thick so substances can diffuse quickly between the blood and the surrounding tissue cells.

The double circulation

Humans have a double circulatory system, meaning the blood passes through the heart twice during one complete circuit of the body:

• Pulmonary circulation: right ventricle to lungs to left atrium. This is where gas exchange occurs: the blood picks up oxygen and releases carbon dioxide.
• Systemic circulation: left ventricle to body organs to right atrium. This delivers oxygenated blood to the tissues and collects deoxygenated blood along with waste products.

The advantage of a double circulation is that blood passes through the heart twice per circuit, so it can be pumped at high pressure to the body after being re-pressurised following the drop in pressure that occurs in the lungs' capillary beds.

Important blood vessel routes to remember for the edexcel exam include: the hepatic artery and hepatic vein serving the liver, the renal artery and renal vein serving the kidneys, and the hepatic portal vein carrying digested food from the small intestine to the liver for processing.

Worked example: interpreting a transpiration experiment

Self-check questions

1. State two ways in which red blood cells are adapted for transporting oxygen.
2. Explain why the left ventricle has a thicker wall than the right ventricle.
3. Name the blood vessel that carries blood from the small intestine to the liver.
4. Describe how vaccination leads to long-term immunity against a specific pathogen.
5. A student covers the underside of a leaf with petroleum jelly. Predict the effect on the transpiration rate and explain your reasoning.
6. Compare the structure of an artery and a vein, and link each structural difference to its function.

For answers to these questions, review your edexcel igcse biology notes on transport and check each point against the specification. The definition of each term should be precise: for example, the exam-standard definition of transpiration is "the evaporation of water from the surface of a plant." Answering with that exact phrasing, rather than a loose paraphrase, is what picks up the mark.

Edexcel igcse transport is one of the most commonly examined areas across both Paper 1 and Paper 2 of the Pearson Edexcel IGCSE Biology qualification. Once transport explained at this level is practised with past paper questions, it becomes a reliable source of marks. The edexcel igcse biology definition of each structure and process should be memorised word for word: examiners award marks for precise phrasing, not loose paraphrases. If you are looking for more practice questions on what is transport igcse level, the Green Bridge CBT platform has topic-filtered questions and worked solutions that follow the edexcel igcse biology explained format used in official mark schemes.