The diffusion of water from a less concentrated solution to a more concentrated solution through a partially permeable membrane.

Osmosis is important to plants. They gain water by osmosis through their roots. Water moves into plant cells by osmosis, making them turgid or stiff so that they are able to hold the plant upright.

What is a partially permeable membrane?

Some membranes in plant and animal cells allow certain particles to pass through them but not others. They are partially permeable membranes.

What is active transport?

Substances are sometimes absorbed against a concentration gradient. Particles move across a cell membrane from an area of low concentration to an area of high concentration. This requires energy from respiration and is called active transport.

How are human cells specialised to increase the rate of exchanging materials?


How are plant cells specialised to increase the rate of exchanging materials?

Leaf cells – carbon dioxide (needed for photosynthesis) enters leaf by diffusion, oxygen (by-product of photosynthesis) exits leaf by diffusion

Surface area of leaf increased by flat shape.

Internal air spaces give increased surface area for gas exchange.



Plants have stomata to obtain carbon dioxide from the atmosphere. Plants lose water vapour from the surface of their leaves – this is called transpiration.

The rate of transpiration is increased in hot, dry and windy conditions. Plants living in these conditions have a thick waxy layer to reduce water loss.

Most transpiration is through the stomata. The size of the stomata is controlled by guard cells. If plants lose water faster than it can be replaced by the roots, the stomata can close up to prevent wilting and to stop photosynthesis from slowing. They would normally only close in the dark when no carbon dioxide is needed for photosynthesis.

Root hair cells –Plants absorb water from the soil by osmosis through their root hair cells. Osmosis is the movement of water from a high water concentration to a low water concentration through a partially permeable membrane. Plants use water for several vital processes including photosynthesis and transporting minerals.

The long shape/large surface area, thin walls and proximity to the xylem cells used for transporting water up the plant means the root hair cells are specially adapted to gaining water from the soil.

You need to bear in mind:

Ÿ the direction of movement of water

  • the effect of water movement on the volume and therefore the pressure of the different solutions
  • that particles move in both directions through the membrane. Changing the pressure or the concentration on one side of the membrane will change the movement of the particles until equilibrium is reached.


How are dissolved materials transported around the body?

The heart pumps blood around the body. Blood flows from the heart to the organs through arteries and returns through veins. In the organs, blood flows through capillaries.

Substances needed by cells in the body tissues pass out of the blood, and substances produced by the cells pass into the blood through the walls of the capillaries.

There are two separate circulation systems, one to the lungs and one to all the other organs of the body.

The blood is a liquid tissue consisting of:

  • Plasma
  • Red blood cells
  • White blood cells
  • Platelets

Blood plasma transports:

  • carbon dioxide from the organs to the lungs
  • soluble products of digestion from the small intestine to other organs
  • urea from the liver to the kidneys
  • hormones


Red blood cells transport oxygen from the lungs to the organs. Red blood cells have no nucleus. They are packed with a red pigment called haemoglobin.

In the lungs oxygen combines with oxygen to form oxyhaemoglobin. In other organs oxyhaemoglobin splits up into haemoglobin and oxygen.

White blood cells ingest and destroy pathogens, produce antibodies to destroy pathogens, and produce antitoxins that neutralise the toxins released by pathogens.

Platelets are irregularly shaped bodies. They help to form clots to stop bleeding.



How does exercise affect the exhanges taking place within the body?

During exercise heart rate increases, rate and depth of breathing increases and arteries supplying the muscles dilate.

Blood flow to the muscles is increased (as well as the supply of sugar and oxygen), and the rate of removal of carbon dioxide is increased.

Energy released through respiration is used to enable muscles to contract. Respiration happens in mitochondria in cells. Glycogen stores in the muscles are used during exercise.

Poisonous lactic acid builds up in the muscles and causes painful cramps. When exercise stops there is an oxygen debt – you must keep breathing in order to get oxygen to the muscles and oxidise the lactic acid into carbon dioxide and water.