Overview
What you’ll learn
- Explain why large organisms need a transport system.
- State that arteries carry blood away from the heart, veins carry it back, and capillaries exchange substances.
- State that xylem transports water and minerals, and phloem transports food.
- Describe diffusion, and osmosis as the net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
- State the roles of plasma, red blood cells, white blood cells and platelets.
- Discuss drugs and the ethics of heart transplants without describing surgery.
In 30 seconds
- 1A transport system is needed when distances are too large for diffusion to supply every cell quickly.
- 2Arteries carry blood away from the heart. Veins carry blood towards the heart. Capillaries are where substances are exchanged.
- 3You do not need the structure of the heart or of blood-vessel walls.
- 4In plants, xylem carries water and mineral salts. Phloem carries food such as sugars.
- 5Diffusion is the net movement of particles from a higher concentration to a lower concentration.
- 6Osmosis is the net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
- 7Blood is made of plasma, red blood cells, white blood cells and platelets. Plasma carries dissolved food. Red blood cells carry oxygen.
- 8Medicines travel in the blood. Misused drugs can harm the body. A heart transplant raises questions of consent, risk and fairness.
Notes · 1 of 6
Why transport is needed
One cell can rely on diffusion. A human cannot.
In a very small organism, substances can reach the inside by diffusion because the distance is short. In a human or a tree, many cells are far from the surface. Diffusion would be too slow to bring oxygen and food, or to remove waste, so a transport system moves substances in bulk.
Blood is the transport fluid in humans. It carries oxygen, digested food and waste. The heart pumps the blood. You do not need to learn the chambers of the heart or the thickness of vessel walls.
Notes · 2 of 6
Arteries, veins and capillaries
Learn the direction of flow and where exchange happens.
| Vessel | Job |
|---|---|
| Artery | Carries blood away from the heart |
| Vein | Carries blood towards the heart |
| Capillary | Exchange of substances with nearby cells |
Oxygen and digested food leave the blood at capillaries and enter cells. Carbon dioxide and other wastes move the other way. The direction follows the concentration difference: diffusion from higher to lower concentration.
Notes · 3 of 6
What blood is made of
Four parts, four jobs. You do not need the structure of a blood cell.
Blood is the transport fluid. It is a mixture of a liquid and cells. Learn the job of each part. Do not learn the shape of a red blood cell or the names of blood groups.
| Part | What it is | Job in transport or repair |
|---|---|---|
| Plasma | The liquid | Carries dissolved food, carbon dioxide, and the cells themselves |
| Red blood cell | A cell | Carries oxygen |
| White blood cell | A cell | Defends the body against disease |
| Platelet | A fragment in the blood | Helps the blood clot at a cut |
Notes · 4 of 6
Xylem and phloem
Water and food travel in different tissues.
Xylem carries water and mineral salts from the roots to the stem and leaves. Phloem carries sugars and other food, often from the leaves to the rest of the plant. Do not swap the two.
Leaves need water for photosynthesis. Other parts of the plant need the food the leaves make. Without these tissues a large plant could not supply its cells.
Notes · 5 of 6
Diffusion and osmosis
Particles spread from where they are more concentrated.
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Oxygen diffuses from blood in a capillary into a respiring muscle cell, where the concentration is lower.
Osmosis is the net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane. Root cell membranes let water molecules through more freely than larger particles. If the soil solution is more dilute than the cell contents, water enters the root. Mineral salts are also taken up and then moved in the xylem.
Notes · 6 of 6
Drugs and heart transplants
Science describes what is possible. Ethics asks whether it should be done.
A medicine is a drug used to treat or prevent illness. After it is absorbed it is carried in the blood. Some drugs are harmful if they are misused: they can damage organs, including the heart and blood system. Never take a medicine that was not given to you.
A heart transplant can save a life when a heart has failed. Ethical questions include whether the donor gave consent, how great the risk is, and whether the chance of a transplant is shared fairly. You do not need to describe the operation.
Revise
Key terms
The words examiners expect you to use precisely.
- Diffusion
- Net movement of particles from a higher concentration to a lower concentration.
- Osmosis
- The net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
- Plasma
- The liquid part of blood. It carries dissolved food, carbon dioxide and the blood cells.
- Red blood cell
- A blood cell that carries oxygen.
- White blood cell
- A blood cell that defends the body against disease.
- Platelet
- A fragment in blood that helps the blood to clot at a cut.
Show 8 more key termsShow fewer key terms
- Artery
- A blood vessel that carries blood away from the heart.
- Vein
- A blood vessel that carries blood towards the heart.
- Capillary
- A tiny blood vessel where substances are exchanged with cells.
- Xylem
- Plant tissue that transports water and mineral salts.
- Phloem
- Plant tissue that transports food such as sugars.
- Transport system
- A set of tissues that moves substances around a large organism.
- Drug
- A substance that changes how the body works. Medicines are drugs used to treat illness.
- Heart transplant
- An operation that replaces a failing heart. It raises questions of consent, risk and fairness.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying plasma carries oxygen and red blood cells carry dissolved food.
Those jobs are the other way round.
Instead: Red blood cells carry oxygen. Plasma carries dissolved food. White blood cells defend against disease. Platelets help clotting.
Mistake: Saying arteries carry blood towards the heart.
That is the job of veins.
Instead: Arteries carry blood away from the heart. Veins carry it back.
Show 5 more mistakesShow fewer mistakes
Mistake: Saying xylem carries food.
Food travels in the phloem.
Instead: Xylem: water and minerals. Phloem: food.
Mistake: Answering a question about the job of an artery by describing how thick its wall is.
The job asked for is the direction of the blood, not the shape of the wall.
Instead: Arteries carry blood away from the heart. Veins carry it back. Capillaries exchange substances.
Mistake: Saying diffusion is movement from low concentration to high concentration.
That is the opposite direction.
Instead: Net movement is from higher concentration to lower concentration.
Mistake: Saying a one-celled organism needs the same transport system as a human.
The distances are tiny.
Instead: Large size is the reason a transport system is needed.
Mistake: Describing how a transplant operation is carried out.
The question is about ethics, not a method.
Instead: Discuss consent, risk and fair access.
Check yourself
Quick quiz
10 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Explain the need for transport
Large organisms need a transport system because
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Practise
Topical questions
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Question paper
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10 multiple-choice questions and structured questions with answer lines. Chapter 7.
Mark scheme
PROChapter 7 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
What do arteries, veins and capillaries do?
Arteries carry blood away from the heart. Veins carry blood back to the heart. Capillaries are where substances are exchanged with cells.
What is the difference between xylem and phloem?
Xylem carries water and mineral salts. Phloem carries food such as sugars.
What are the four parts of blood?
Plasma carries dissolved food and the cells. Red blood cells carry oxygen. White blood cells defend against disease. Platelets help the blood to clot.
What is diffusion?
Diffusion is the net movement of particles from a higher concentration to a lower concentration. Osmosis is the net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
Why do large organisms need blood or xylem?
Cells are too far from the outside for diffusion alone to supply them quickly.
Is transport in the Sec 2 EOY?
Most schools that follow the usual G2/G3 scheme teach this chapter in Sec 2 and include it in the Sec 2 end-of-year paper. A few schools move a topic across the two years, so check your school’s assessment outline.
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Chapter 8: Human Sexual Reproductive System