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Transport in cells

In this chapter you will learn:

  1. What is Diffusion?

  2. Factors affecting diffusion

  3. Diffusion in living organisms

  4. Osmosis

  5. Osmosis in living cell

  6. Osmosis in plant cell

  7. Active transport

  8. Active transport in plants

  9. Active transport in animals

  10. Comparison of diffusion, osmosis and active transport


In diffusion, particles move from an area of higher concentration to an area of lower concentration.

Diffusion occurs in gases and liquids, because the particles are free to move.

Diffusion in solids is impossible because the particles cannot move freely from one position to another.

Factors affecting diffusion

  • The larger the particles the slower the rate of diffusion.

  • The bigger the concentration difference and the faster the rate of diffusion.

  • The greater the surface area, the faster the rate of diffusion.

  • In the higher temperature, the particles gain more kinetic energy and move faster and so diffusion is faster.

Diffusion in living organisms

The thin cell membranes allow the diffusion of small molecules in and out of cells.

When blood reaches the cells the molecules of glucose and oxygen diffuse out of the blood and into the cells.

When cells use up the glucose and oxygen they produce waste chemicals and carbon dioxide.

Carbon dioxide will then naturally diffuse out of the cell to the region of lower concentration.


Osmosis can be defined as the spontaneous movement of water molecules from an area of high concentration, to an area of lower concentration through a partially permeable membrane.

Osmosis refers to the movement of water molecules only. The concentrated solution has low concentration of water molecules. A dilute solution has high concentration of water molecules.

Osmosis can happen in either direction, that means water can move in either direction depending on the relative concentration of dissolved substance.

Water will diffuse from a high water potential to a low water potential.

Osmosis in living cell

The cell membrane of animal is partially permeable.

Animal cells, which do not have cell walls, can respond adversely to change in the surrounding water pressure.

In pure water, the plant cells take up water by osmosis and become turgid.

In a more concentrated solution, the cell contents lose water by osmosis, and become flaccid.

Cells have dilute solutions of sugars, amino acids and mineral ions.

Animal cells also take in and lose water by osmosis. Animal cells do not have a cell wall, therefore they change size and shape when put into solutions of different concentration.

In animals, the concentration of body fluids must be kept within optimal limits. Cells will not function efficiently if they lose or gain too much water by osmosis.

Osmosis in plant cell

Root hairs of the plant take in water from the soil by osmosis.

Plant cells have a strong cell wall made from cellulose fibres that strengthens and supports the plant cell.

The cell wall is fully permeable to all molecules and stops the cell bursting if they gain too much water by osmosis.

When a plant has enough water, the water passes into the cells by osmosis and the vacuole fills and swells up and push against the cell wall and the cell becomes turgid.

When too much water moves out of a plant cell by osmosis, the vacuole starts to shrink and the plant cell becomes flaccid.

Active transport

In active transport molecules move against a concentration gradient.

This process requires the use of energy that comes from respiration.

Active transport in plants

In active transport the root hairs of plants allows plants to absorb mineral ions, which are necessary for healthy growth. The concentration of nitrates is higher in plant root cell compared to the soil surrounding it.

The concentration of minerals is usually lower in the soil than in the root hair.

Plants absorb mineral ions by active transport. Root hair cells are adapted for taking up water and mineral ions by having a large surface area that increase the rate of absorption.

Active transport in animals

The process of active transport takes place in humans during digestion of food in the small intestine.

Active transport is required to absorb nutrients such as amino acids and sugars like glucose from the gut.

When the glucose concentration in the intestine is lower than in the intestinal cells, movement of glucose involves active transport.

Comparison of diffusion, osmosis and active transport

Process Substances moved Energy required
Diffusion Carbon dioxide, oxygen, water, food, waste substance No energy needed
Osmosis Water No energy needed
Active transport Mineral ions into plant roots, glucose from the gut into intestinal cells, from where it moves into the blood Energy needed from respiration