Topic 2.2 Transport Across Cell Membranes - 2.2.3 Diffusion

True statements - facilitated diffusion

- Aquaporins are channel proteins for water - Facilitated diffusion is a passive process (! not an active process) - Facilitated diffusion occurs down a concentration gradient (! not up a concentration gradient) - Carrier proteins undergo a change in shape when a substance binds (! not channel proteins)

Examples of factors affecting the rate of diffusion

- Concentration gradient - Temperature - Membrane surface area

Factors that will create the highest rate of diffusion

- Large membrane surface area - High concentration gradient - High temperature

Describe what kind of substances are able to pass through a cell membrane through simple diffusion.

- Small - Lipid - Soluble

What two proteins allow facilitated diffusion?

1. Channel proteins 2. Carrier proteins

Facilitated Diffusion

In facilitated diffusion, molecules diffuse through carrier proteins or channel proteins spanning across the membrane.

Diffusion - Urea

Urea, a waste product of cells, diffuses from the cells and into the blood for excretion by the kidneys.

Diffusion - e.g. Perfume

When you spray perfume, the smell diffuses across the room from the area of high concentration (where you just sprayed it) to the area of low concentration (the other side of the room).

Explain how does a carrier protein allow movement across a cell membrane through facilitated diffusion?

A molecule / substance / particle, specific to this protein Binds with it Causing a change in shape, which then releases it on the other side of the membrane

Respiration

A process that uses up oxygen and produces carbon dioxide.

Substances that can diffuse across cel membrane

Carbon dioxide (! not DNA polymerase) Urea (! not Collagen) Oxygen (! not Glucose)

Diffusion

Carbon dioxide and urea can move in and out of cells across cell membranes via the process of diffusion.

Diffusion - Carbon dioxide & Oxygen

Carbon dioxide diffuses from the bloodstream into the lungs during cell respiration. Oxygen diffuses from the lungs and into the bloodstream.

Diffusion

Diffusion describes the passive movement of particles in fluids (liquids and gases). Molecules move randomly but tend to move into into any space available until it is evenly distributed. So, the net movement of particles will be towards the area of lower concentration or down a concentration gradient. Substances such as CO2, O2 and urea can move in and out of cells across cell membranes via diffusion.

What is diffusion?

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

Facilitated Diffusion - Carrier proteins

Facilitated diffusion also takes place through carrier proteins. When a substance binds to carrier proteins, there is a change in the shape of the carrier protein. This change in shape moves the substance from the outside of the cell to the inside.

Facilitated diffusion

Facilitated diffusion describes the net movement of particles down a concentration gradient (i.e. from a region of high concentration to a region of lower concentration). Facilitated diffusion is a passive process so it does NOT use energy.

Facilitated Diffusion - Channel proteins

Facilitated diffusion takes place through channel proteins. Channel proteins allow polar compounds to avoid the non-polar central layer of the plasma membrane. The non-polar layer would slow or prevent entry into the cell. Aquaporins are channel proteins that allow water to pass through the membrane.

True statement

Molecules that are too large to move through the membrane and polar molecules that are repelled by the hydrophobic part of the membrane move by facilitated diffusion.

In which state of substance does diffusion NOT happen?

Solids

Facilitated Diffusion - Large or polar molecules

Some materials are too large to move through the membrane. Other materials are polar molecules that are repelled by the hydrophobic part of the membrane. These materials move across the membrane by facilitated diffusion.

Factors Affecting Diffusion - Concentration gradient

The concentration gradient is linked to the difference in concentration between two areas. The bigger the difference in concentration between two areas, the greater the concentration gradient and the faster the rate of diffusion.

Factors Affecting Diffusion - Channel or carrier proteins

The higher the concentration gradient, the faster the rate of facilitated diffusion until an equilibrium is reached and the rate will level off. The number of channel or carrier proteins affects the rate of faciltated diffusion. The more proteins, the faster the rate.

Factors Affecting Diffusion - Temperature

The higher the temperature, the faster the rate of diffusion.

Factors Affecting Diffusion - Membrane surface area

The larger the surface area of the membrane that a substance is diffusing through (e.g. the membrane around a cell), the faster the rate of diffusion.

What would happen to the rate of diffusion if the concentration got steeper and the temperature rose?

The speed of diffusion would increase

Factors Affecting Diffusion - Thickness of membrane

The thicker the exchange surface, the slower the rate of diffusion.