Transport in Plants & Humans

Why transport systems matter

Small organisms can rely on diffusion alone, but plants and humans are too large and too active for that. Substances would take far too long to reach every cell. Instead, both have specialised transport systems that move materials quickly over long distances.

Key terms Xylem – tissue that carries water and dissolved mineral ions from roots to leaves.

Phloem – tissue that carries dissolved sucrose and amino acids around the plant.

Translocation – the movement of sucrose and amino acids in the phloem.

Transpiration – the loss of water vapour from a plant's leaves by evaporation and diffusion.

Transport in plants: xylem and phloem

Plants have two separate transport tissues running through the roots, stem and leaves.

Xylem vessels are made of dead cells joined end to end with their end walls removed, forming a continuous hollow tube. The walls are strengthened with lignin, which also helps support the plant.

Phloem is made of living cells. Translocation moves sucrose (made in the leaves by photosynthesis) and amino acids to wherever they are needed, such as growing tips, or to storage organs like roots.

Root hair cells and water uptake

Water enters the roots through root hair cells. Each has a long, thin extension that gives a very large surface area for absorbing water and mineral ions from the soil.

Water moves into the root hair cell by osmosis, down a water potential gradient (the soil solution is more dilute than the cell's cytoplasm). Mineral ions are usually taken up by active transport, which needs energy from respiration because the ions move against their concentration gradient.

Exam tip Be precise: water moves in by osmosis, but mineral ions move in by active transport. Mixing these up is one of the most common mistakes in this topic.

Transpiration and the transpiration stream

Water evaporates from the surfaces of cells inside the leaf and the vapour diffuses out through the stomata (tiny pores, mostly on the lower leaf surface). This loss of water is transpiration.

As water is lost from the leaves, more water is pulled up through the xylem to replace it. This continuous flow from roots to leaves is the transpiration stream. It is useful because it delivers water for photosynthesis and support, carries mineral ions, and cools the plant.

Factors affecting the rate of transpiration

Four main environmental factors change how fast a plant transpires.

Light – brighter light makes stomata open wider for photosynthesis, so more water vapour escapes. Transpiration increases.
Temperature – higher temperature gives water molecules more energy, so they evaporate and diffuse faster. Transpiration increases.
Humidity – in humid air there is already a lot of water vapour outside the leaf, so the concentration gradient is smaller and diffusion is slower. Transpiration decreases.
Air movement (wind) – moving air sweeps away water vapour from around the leaf, keeping the gradient steep. Transpiration increases.

Watch out High humidity lowers the rate of transpiration. Students often assume "more water in the air = more water lost", but the opposite is true because diffusion depends on a concentration gradient.

The potometer

A potometer measures the rate at which a plant takes up water, which is a good estimate of the rate of transpiration.

A leafy shoot is sealed into a tube of water connected to a thin capillary tube with an air bubble. As the shoot transpires, it pulls water up and the bubble moves along the scale. By timing how far the bubble travels, you can compare rates. To test a single factor (for example, wind from a fan), you change only that factor and keep everything else the same.

Transport in humans: the double circulation

Humans have a double circulatory system: in one complete journey, blood passes through the heart twice.

1. Pulmonary circuit – the heart pumps deoxygenated blood to the lungs, where it picks up oxygen, then returns to the heart.
2. Systemic circuit – the heart pumps oxygenated blood to the rest of the body, where it gives up oxygen, then returns to the heart.

A double circulation keeps blood pressure high so blood can be delivered quickly to the body's tissues.

The heart

The heart is a muscular pump with four chambers: two atria (upper) and two ventricles (lower). Valves stop blood flowing backwards.

How the heart pumps, in order:

1. Deoxygenated blood from the body enters the right atrium through the vena cava.
2. It passes into the right ventricle, which pumps it through the pulmonary artery to the lungs.
3. Oxygenated blood returns from the lungs via the pulmonary vein into the left atrium.
4. It passes into the left ventricle, which pumps it out through the aorta to the whole body.

The left ventricle has a thicker, more muscular wall than the right because it must pump blood at high pressure all the way around the body. The coronary arteries branch off the aorta to supply the heart muscle itself with oxygen and glucose.

Watch out The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood. These are the exceptions to the usual rule that arteries carry oxygenated blood and veins carry deoxygenated blood.

Blood vessels

The thick elastic walls of arteries stretch and recoil to withstand the high pressure of blood leaving the heart. Veins have valves because blood pressure is low, and the valves stop blood flowing backwards. Capillaries' thin walls give a short diffusion distance for fast exchange between blood and tissues.

The blood

Blood is made of cells and fragments suspended in a liquid called plasma.

Red blood cells – carry oxygen. They contain the red pigment haemoglobin, which joins with oxygen in the lungs to form oxyhaemoglobin, then releases it at the tissues. They have no nucleus and a biconcave shape, giving more room and surface area for oxygen.
White blood cells – defend against disease. Phagocytes engulf and digest pathogens; lymphocytes produce antibodies.
Platelets – tiny cell fragments that help blood to clot.
Plasma – the straw-coloured liquid that carries blood cells, dissolved nutrients (glucose, amino acids), carbon dioxide, urea, hormones and heat.

When a blood vessel is cut, platelets trigger a series of reactions that form a mesh of fibres, producing a clot. The clot seals the wound, stops blood loss and blocks the entry of pathogens.

Key terms Haemoglobin – the red pigment in red blood cells that carries oxygen.

Oxyhaemoglobin – the substance formed when haemoglobin combines with oxygen.

Effect of exercise on heart rate

During exercise, muscles respire faster and so need more oxygen and glucose, and produce more carbon dioxide that must be removed.

To meet this demand, the heart rate increases. The heart beats faster (and more strongly), so blood is pumped more quickly to deliver oxygen and glucose to the muscles and carry away carbon dioxide. After exercise stops, the heart rate stays raised for a while to "pay back" the oxygen debt, then gradually returns to its resting rate. Fitter people generally have a lower resting heart rate and recover more quickly.

Exam tip When explaining the effect of exercise, link each step: more activity → muscles respire more → need more oxygen and glucose → heart rate rises → faster delivery of oxygen and removal of carbon dioxide.