Transport systems in cells

In a nutshell

Multicellular organisms require specialised transport systems to enable the exchange of substances with their environment. Substances can be transported via three processes: diffusion, active transport, and osmosis.

Diffusion

Definition

Diffusion is the movement of particles from an area of high concentration to an area of low concentration.

In diffusion, particles move down a concentration gradient, which is the difference in concentration. The greater the difference in concentration, the faster the rate of diffusion. Increasing the temperature will also increase the diffusion rate, as the particles will have more energy to move around faster. Diffusion can occur in both solutions and gases.

Cell membranes

The cell membrane enables substances to diffuse into and out of a cell. Due to the size of the membrane, only very small molecules can fit through.

Examples

Oxygen, glucose and water. 

These particles move both into and out of the cell, but there is an overall net movement from the side where there is a lot of them, to the side where there are fewer of them. Increasing the surface area of the membrane will result in a faster rate of diffusion, as more particles can pass through at a time.

Active transport

Definition

Active transport is the movement of particles across a membrane against a concentration gradient, using energy from ATP.

Active transport works oppositely to diffusion. In this process, particles move from an area of high concentration to an area of low concentration. ATP released from respiration is used to provide energy for active transport.

Active transport in animals

In the digestive system, active transport is used to transport nutrients, like glucose, from the gut into the blood when there is a lower concentration of nutrients. This is important as it prevents animals from starving.

Active transport in plants

Plant roots have millions of root hair cells which give the roots a large surface area. This allows plants to absorb water and mineral ions from the soil, which is essential for healthy growth. Active transport enables minerals to be transported into the cell, as the concentration of minerals is usually higher in the root hair cells than in the surrounding soil.

Osmosis

Definition

Osmosis is the net movement of water molecules from an area of high water concentration to an area of low water concentration across a partially permeable membrane.

Osmosis is very similar to diffusion, however only involves water molecules. Water moves across a partially permeable membrane, which is a membrane with small holes in it.

Water potential

Definition

Water potential is the likelihood of water molecules diffusing out of or into a solution.

A solution with a high water potential will have a high concentration of water molecules. A solution with a low water potential will have a low concentration of water molecules.

Note: Pure water has the highest water potential of all solutions.

Turgid and flaccid plant cells

The surrounding soil will have a high water potential when a plant is watered. The water flows into the cells by osmosis, until they become turgid. This causes the cell contents to push against the cell wall which creates turgor pressure that supports the plant tissues. However, without water in the soil, the plant will begin to wilt as they lose water. The cells become flaccid and eventually plasmolysed. Due to the inelastic cell wall, the plant won’t completely lose its shape. 

Animal cells don’t have a cell wall, therefore can burst if surrounded by a solution with a high water potential.

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