Biology · Lesson 8

Transport in plants & animals; circulatory system

JAMBWAECNECO

What the syllabus asks

JAMB: Transport in plants and animals: structure and functions of transport tissues; mammalian circulatory system; transpiration and its factors

WAEC: Transport: need for transport, media of transport (blood, lymph, cell sap), transport in plants (xylem, phloem, transpiration, absorption of water and mineral salts, translocation), transport in animals (blood and its components, blood vessels, the mammalian heart, double circulation, lymphatic system)

NECO: Transport: need for transport, media of transport (blood, lymph, cell sap), transport in plants (xylem, phloem, transpiration, absorption of water and mineral salts, translocation), transport in animals (blood and its components, blood vessels, the mammalian heart, double circulation, lymphatic system)

What you will learn

Explain why multicellular organisms need transport systems.
Describe the structure and functions of xylem and phloem.
Explain how water and mineral salts are absorbed and moved up a plant.
Describe transpiration, its types, its importance and the factors that affect it.
State the components of blood and their functions.
Compare arteries, veins and capillaries.
Describe the structure of the mammalian heart and trace the path of blood in double circulation.
Explain the formation of tissue fluid and lymph, and the role of the lymphatic system.

Key points

Large organisms need transport because their surface area to volume ratio is small and diffusion is slow.
Xylem: dead, lignified, carries water and salts upward. Phloem: living sieve tubes and companion cells, carries sucrose both ways.
Water enters root hairs by osmosis; minerals often by active transport.
Water rises by transpiration pull with cohesion, helped by capillarity and root pressure.
Transpiration rises with heat, wind, light; falls with humidity and dry soil.
Blood = plasma + red cells (oxygen) + white cells (defence) + platelets (clotting).
Arteries: away from heart, thick, high pressure. Veins: to heart, valves. Capillaries: one cell thick.
Heart: 4 chambers; left ventricle thickest; valves give one-way flow; double circulation.
Tissue fluid forms from plasma; excess drains as lymph to the veins.

Start here: Why living things need transport

Think about an amoeba in a pond. It is a single cell, very small, and every part of it is close to the water around it. Oxygen diffuses in and carbon dioxide diffuses out across its surface quickly enough to keep it alive. It needs no blood, no heart and no veins.

Now think about yourself. A cell in your big toe is more than a metre from your lungs. If oxygen had to diffuse that far, it would take years to arrive. Long before then, the cell would be dead. This is the basic reason why large organisms have a transport system: a way of carrying materials quickly over long distances by mass flow.

The need for transport increases as organisms become larger, more active and more complex, because their surface area to volume ratio falls and diffusion alone becomes too slow.

Surface area to volume ratio, made simple

Imagine cubes of agege bread. A 1 cm cube has surface area 6 cm² and volume 1 cm³, a ratio of 6 : 1. A 2 cm cube has surface area 24 cm² and volume 8 cm³, a ratio of 3 : 1. A 4 cm cube has surface area 96 cm² and volume 64 cm³, a ratio of 1.5 : 1. As the cube gets bigger, it has less outside surface for every unit of inside. A big organism therefore cannot rely on its body surface to supply all its cells.

What must be transported?

Oxygen from the lungs or leaves to respiring cells.
Food (glucose, amino acids, sucrose) from where it is absorbed or made to where it is used or stored.
Water and mineral salts from the soil to the leaves of plants.
Waste products such as carbon dioxide and urea to the organs that remove them.
Hormones from glands to target organs.
Heat, to keep the body temperature even.
Antibodies and white cells to fight infection.

Media of transport

The liquid that carries substances is called the medium of transport. In mammals it is blood and lymph. In plants it is the cell sap, water in xylem and the sugary sap in phloem. In very simple animals like hydra, water from the surroundings does the job.

💡 Exam tip: WAEC often asks "Why is a transport system necessary in higher organisms?" A full answer mentions size, the small surface area to volume ratio, the long distances involved and the slowness of diffusion.

Try it yourself. Find the surface area to volume ratio of a 3 cm cube.

Show solution

Area = 6 × 9 = 54 cm²; volume = 27 cm³; ratio = 2 : 1.

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Practice questions

1. Which blood vessel carries oxygenated blood from the lungs to the heart?

Pulmonary vein
Pulmonary artery
Aorta
Vena cava

Show answer

A: Pulmonary vein
The pulmonary vein is the only vein that carries oxygenated blood.

2. Which blood cells help in clotting?

Erythrocytes
Lymphocytes
Phagocytes
Platelets

Show answer

D: Platelets
Platelets (thrombocytes) start clotting.

3. Water is transported in flowering plants through the

cambium
cortex
xylem
phloem

Show answer

C: xylem
Xylem carries water and minerals up; phloem carries food (translocation).

4. The loss of water vapour from the leaves of a plant is called

respiration
transpiration
guttation
translocation

Show answer

B: transpiration
Transpiration is mostly through stomata. Guttation is loss of liquid water.

5. A person with blood group O can donate blood to people of

all blood groups
group O only
group AB only
groups A and B only

Show answer

A: all blood groups
O has no A or B antigens, so it is the universal donor.

Key words

Xylem: Plant tissue of dead, lignified vessels that conducts water and mineral salts upward.
Phloem: Living plant tissue (sieve tubes and companion cells) that carries manufactured food.
Transpiration: Loss of water vapour from the aerial parts of a plant, mainly through stomata.
Translocation: Movement of manufactured food in the phloem.
Root pressure: Pressure in the root that pushes water up the xylem.
Plasma: The liquid part of blood, about 90% water, carrying dissolved substances.
Haemoglobin: Iron-containing red pigment in red blood cells that carries oxygen.
Erythrocyte: Red blood cell.
Leucocyte: White blood cell.
Thrombocyte: Platelet; a cell fragment involved in clotting.
Artery: Thick-walled vessel carrying blood away from the heart.
Vein: Thin-walled vessel with valves carrying blood to the heart.

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