Topic 3 - Movement across membranes

Diffusion

Diffusion

Diffusion is the net movement of molecules from a region of their higher concentration, to a region of their lower concentration down a concentration gradient, as a result of their random movement.

Where does the energy for diffusion come from?

Diffusion is a passive process so the energy comes from the kinetic energy of the random movement of molecules and ions.

Why is diffusion important?

Diffusion is necessary for several important biological processes like gas exchange in the lungs, absorption of food molecules, and gas exchange in the leaves of plants.

Factors that affect the rate of diffusion

The distance it has to diffuse, the size of the concentration gradient, the surface area available for diffusion, and the temperature.

What are the some examples of substances that move in and out of cells by diffusion through the cell membrane?

Water, carbon dioxide, and oxygen.

How does the distance affect the rate of diffusion?

The smaller the distance the faster the diffusion.

How does the size of the concentration gradient affect the rate of diffusion?

The larger the difference in concentration the faster the diffusion.

How does the surface area available affect the rate of diffusion?

The larger the surface area, the more space the molecules have and the quicker they go.

How does the temperature affect the rate of diffusion?

Increasing temperature means increasing the kinetic energy of the molecules which means that the molecules move faster.

How does the size affect the rate of diffusion?

Smaller molecules diffuse faster.

Osmosis

Osmosis is the diffusion of water molecules from a region of higher water potential (less solute) to a region of lower water potential (more solute), through a partially permeable membrane.

Importance of water potential and osmosis

Water potential and osmosis are important for controlling how organisms take in or lose water.

How are plants supported by the turgor pressure within cells, in terms of water pressure acting against an inelastic cell wall.

When plant cells have absorbed a maximum amount of water by osmosis (into the vacuole) they become rigid, due to the pressure of water pressing outwards on the cell wall. As a result, the stems and leaves are supported.

What would happen to a plant cell if you put it in concentrated salt solution

The plant cell membrane would pull away from the cell wall. The cell would become flaccid/plasmolysed. This is due to the water molecules moving out of the plant cell by osmosis to a region of high water potential (inside the cell) to a region of low water potential (outside the cell).

What would happen to a plant cell if you put it in pure water

The plant cell would become turgid (expand) due to water molecules passing into the vacuole by osmosis from a region of high water potential (outside the cell) to a region of low water potential (inside the vacuole).

Define active transport

Movement of ions in or out of a cell through the cell membrane, from a region of their lower concentration to a region of their higher concentration against a concentration gradient, using energy released during respiration.

Importance of active transport

It is important in scenarios where substances need to be transported against their concentration gradient.

Examples of active transport

Mineral ions are transported from the soil into the plant from low concentration to high concentration. In animals, glucose is reabsorbed back into the blood from the kidney by active transport.

How do proteins move across membranes in active transport

In active transport, molecules or ions are moved across a membrane using protein carriers. These carriers allow the molecules to move from a lower concentration to a higher concentration, which goes against the normal flow.