Key Stage 4 science · Biology – Transport systems
GCSE circulatory system and transport in plants
A single-celled organism can take in what it needs straight through its surface, but a person or a tree cannot. At GCSE, students learn why multicellular organisms need transport systems, how the structure of the human heart, blood vessels and blood suits their jobs, and how plants move water and sugars through xylem and phloem. It is a topic where learning the structure and explaining the function go hand in hand.
Transport systems, explained
The need for transport comes down to size. A tiny organism has a large surface area compared with its volume, so oxygen and food reach every part by diffusion. As an organism gets bigger, its surface area to volume ratio falls and the distance from the surface to the centre grows, so diffusion alone is far too slow. Multicellular organisms therefore have exchange surfaces, such as the lungs and the small intestine, and a transport system to carry substances between those surfaces and every cell.
In humans the transport system is the circulatory system, which is a double circulation. The right side of the heart pumps deoxygenated blood to the lungs, where it picks up oxygen, and the blood returns to the left side. The left side then pumps oxygenated blood at higher pressure around the rest of the body. The heart has four chambers: two thin-walled atria that receive blood and two ventricles that pump it out. The left ventricle has the thickest wall because it pushes blood furthest. Valves stop blood flowing backwards, and the heart muscle has its own blood supply through the coronary arteries.
Each type of blood vessel is shaped for its job. Arteries carry blood away from the heart under high pressure, so they have thick, elastic, muscular walls. Veins carry blood back at low pressure, so they have thinner walls, a wider channel (the lumen) and valves to stop blood flowing backwards. Capillaries link the two; their walls are one cell thick so oxygen, glucose and carbon dioxide can pass quickly between blood and body cells. Blood itself is a tissue: plasma carries dissolved substances, red blood cells carry oxygen using haemoglobin, white blood cells defend the body and platelets help clots form.
Plants have two separate transport tissues. Xylem carries water and mineral ions upwards from the roots to the leaves in one direction, and most water leaves through the stomata in a process called transpiration. Phloem carries dissolved sugars, made in photosynthesis, to growing parts and storage organs, and it can carry them in either direction. This movement of sugars is called translocation. Students are expected to link the structure of each tissue, for example the hollow, strengthened cells of xylem, to its function.
Words your child will hear
- Double circulation
- A system where blood passes through the heart twice on each complete trip around the body: once to the lungs and once to the rest of the body.
- Atrium and ventricle
- The upper chambers of the heart that receive blood (atria) and the lower chambers that pump it out (ventricles).
- Capillary
- A very narrow blood vessel with walls one cell thick, where substances are exchanged with the tissues.
- Haemoglobin
- The red protein in red blood cells that binds to oxygen.
- Xylem
- Plant tissue that carries water and mineral ions from the roots up to the leaves.
- Phloem
- Plant tissue that carries dissolved sugars to where they are used or stored.
- Transpiration
- The loss of water vapour from a plant, mainly through the stomata in the leaves.
What the National Curriculum says
The programme of study for Key Stage 4 says:
- Students should be taught about the need for transport systems in multicellular organisms, including plants
- Students should be taught about the relationship between the structure and functions of the human circulatory system
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
A worked example: tracing the circulation
Describe the path taken by a drop of blood from the moment it returns to the heart from the body until it is next pumped around the body.
- Blood from the body is deoxygenated. It returns through the vena cava and enters the right atrium.
- The right atrium contracts and pushes blood through a valve into the right ventricle. The ventricle contracts and pumps it out through the pulmonary artery towards the lungs.
- In the lungs the blood flows through capillaries next to the air sacs, where it gains oxygen and loses carbon dioxide.
- Oxygenated blood returns through the pulmonary vein into the left atrium, then passes through a valve into the left ventricle.
- The left ventricle has a thick muscular wall. It contracts strongly and sends blood out through the aorta to the whole body, and some of it goes to the heart muscle itself through the coronary arteries.
Vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta, body. This shows the blood passes through the heart twice per circuit, which is why the circulation is called double.
Common mistakes and how to help
“Arteries carry oxygenated blood and veins carry deoxygenated blood”
It is true for most of the body, so it is often taught as a rule. The pulmonary vessels break it: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.
What helps: Define them by direction instead: arteries carry blood away from the heart and veins carry blood towards it.
Labelling the heart left and right from the viewer’s side
Diagrams show the heart as if looking at someone facing you, so the “left” side of the page is the person’s right.
What helps: Remember the diagram shows the person facing you, so their left is on your right. The left ventricle has the thicker wall, which is a quick check that you have it the right way round.
Blood is blue inside veins
Veins look bluish through the skin, and diagrams colour deoxygenated blood blue.
What helps: Blood is always red. It is a brighter red when oxygenated and darker when deoxygenated. The blue is only a diagram convention.
Mixing up xylem and phloem, or thinking plants “suck up” water like a straw
The names are easy to confuse, and the pull on the water as it evaporates from the leaves is hard to picture.
What helps: Link xylem with water and mineral ions going up, and phloem with sugars going to wherever they are needed. Water evaporates from the leaves and the column of water in the xylem is pulled up behind it.
How to help at home
- Feel the pump. Find a pulse in the wrist or neck, count it for 30 seconds at rest and again after a minute of brisk movement. Ask what the heart is doing and why the body wants blood to move faster.
- Draw the double loop. Ask for a quick sketch of the circulation with two loops joined at the heart, labelling the chambers, valves and main vessels from memory. Use a different colour for oxygenated blood. It is a good check that the pathway is secure.
- Dye a stem. Stand a stick of celery or a white flower in a glass of water with a few drops of food colouring. After a few hours or overnight the colour travels up the stem. An adult can then cut across the stem so you can look for the coloured dots, which are the xylem.
- Use everyday models. Compare the circulatory system with a pump and pipes, a delivery network or a river system. Then ask where the analogy fails. Spotting the limits of a model is exactly what examiners ask for.
Related topics
Updated .