Transport: Class 9 Biology

Key points

Large organisms need a transport system because their cells are far from the surroundings, and diffusion alone is too slow. Transport carries food, water, oxygen, carbon dioxide, hormones and wastes between parts of the body.

In plants, water and minerals are taken up by the roots. Root hairs are long outgrowths of root epidermal cells that give a large surface area. Water enters the root hair by osmosis, since the cell sap has a lower water potential than the soil water. Mineral ions enter by active transport and diffusion. Water then moves across the cortex to the xylem.

Xylem is a tissue of dead, hollow tubes (vessels and tracheids) with walls strengthened by lignin. It carries water and minerals from the roots upwards to the stem and leaves, and it gives support. Phloem is a living tissue made of sieve tubes with perforated end walls and companion cells. It carries sugars and amino acids from the leaves to all parts of the plant.

Transpiration is the loss of water vapour from the aerial parts of the plant, mostly through the stomata of the leaves. Water evaporates from the surface of mesophyll cells into the air spaces and diffuses out through the open stomata. Each stoma is surrounded by two guard cells, which open it when turgid and close it when flaccid.

Factors that increase the rate of transpiration:

• High temperature, because evaporation is faster.
• Low humidity, since the concentration gradient of water vapour is steeper.
• Wind, which carries away the water vapour near the leaf.
• Strong light, which opens the stomata wider.
• A large leaf surface area and more stomata.

Transpiration is useful because it pulls water up the plant, brings minerals, cools the leaf and keeps cells turgid. The cohesion-tension theory explains the rise of water. Water evaporating from leaves creates a pull (tension). Water molecules stick together (cohesion) and also stick to the walls of the xylem (adhesion), so a continuous column of water is pulled up from the roots to the leaves. Root pressure also pushes water up a little.

Translocation is the movement of organic food, mainly sucrose, in the phloem, from sources (such as leaves) to sinks (growing parts, roots, fruits and storage organs). It needs energy and works in both directions in the plant. Ringing (removing a ring of bark) shows that food travels in the phloem, as sugar collects above the cut.

Blood, the transport medium in humans, has four parts:

• Plasma: the liquid part, about 55 percent, mostly water. It carries digested food, carbon dioxide, urea, hormones and heat.
• Red blood cells (erythrocytes): biconcave discs with no nucleus, full of haemoglobin, which carries oxygen as oxyhaemoglobin.
• White blood cells (leucocytes): have a nucleus and defend the body by engulfing germs (phagocytes) and by making antibodies (lymphocytes).
• Platelets: cell fragments that help blood to clot.

The heart is a muscular pump with four chambers: two thin walled atria on top and two thick walled ventricles below. The left ventricle has the thickest wall, since it pumps blood round the whole body. Valves (bicuspid on the left, tricuspid on the right, and semilunar valves at the arteries) prevent backflow. The right side carries deoxygenated blood to the lungs, and the left side carries oxygenated blood to the body. This is a double circulation.

Blood vessels:

• Arteries: thick, elastic, muscular walls, narrow lumen, carry blood away from the heart at high pressure (mostly oxygenated, except the pulmonary artery).
• Veins: thinner walls, wide lumen, valves, carry blood back to the heart at low pressure (mostly deoxygenated, except the pulmonary vein).
• Capillaries: very thin walls of one cell, where exchange of substances between blood and tissues takes place.

Blood flow: body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body. A normal resting heart beats about 72 times per minute, and the pulse is felt at the wrist. Blood pressure is measured with a sphygmomanometer, and a healthy adult has about 120/80 mm Hg.

Cardiovascular diseases are the leading cause of death. Atherosclerosis is the build up of fatty deposits in the artery walls, which narrows them. A blood clot in a coronary artery causes a heart attack. High blood pressure (hypertension) can cause stroke. The risk is reduced by exercise, a diet low in fat and salt, no smoking and control of body weight.

Study tip: Practise drawing the human heart and the path of blood with arrows and clear labels. For plant transport, link each structure to its function, such as thin wall to osmosis in root hairs. Learn the table of differences between artery, vein and capillary, as it is often asked.

Definitions and flashcards

Say the meaning first, then open the card to check.

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Common mistakes

• Saying that xylem carries food and phloem carries water. It is the reverse.
• Forgetting that the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.
• Mixing left and right on the heart diagram. The left side of the diagram is the right side of the heart if the person is facing you.
• Confusing transpiration with evaporation only. Transpiration is water loss from living plants, mainly through stomata.
• Saying that the walls of the ventricles are equal. The left ventricle is thicker than the right.

Practise this chapter

Practice questions and timed test Printable worksheet