# Diffusion & Facilitated Diffusion ## Diffusion - **Diffusion** is a type of transport that occurs across cell membranes - Can be defined as:

The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration

- Molecules or ions move **down a concentration gradient** - The random movement is caused by the natural **kinetic energy** of the molecules or ions ### Diffusion across membranes - As a result of diffusion, molecules or ions tend to reach an **equilibrium situation** (given sufficient time), where they are evenly spread within a given volume of space - The rate at which a substance diffuses across a membrane depends on several factors ## Factors Affecting the Rate of Diffusion ### 'Steepness' of concentration gradient - This is the difference in the concentration of the substance on the two sides of the surface - If there are more molecules on one side of a membrane than on the other, at any one moment more molecules will randomly move across the membrane from that side than from the other - A greater difference in concentration means a greater difference in the number of molecules passing in the two directions and therefore a faster rate of diffusion - **Example:** Think of a crowded train platform where people spread out naturally into emptier carriages - the bigger the difference between crowded and empty areas, the faster people redistribute The relationship between concentration gradient and diffusion rate can be visualized:

### Temperature - Molecules and ions have more kinetic energy at higher temperatures - They move faster, resulting in a higher rate of diffusion - **Example:** A drop of food colouring will spread through hot water much more rapidly than through cold water The effect of temperature on diffusion rate:

### Surface area - The greater the surface area across which diffusion is taking place, the greater the number of molecules or ions that can cross it at any one moment and therefore the faster diffusion occurs - The surface area of cell membranes can be increased by folding (e.g. microvilli in the small intestine or cristae in mitochondria) - As a cell increases in size, the surface area to volume ratio decreases which slows the rate substances can diffuse through a cell as the distance required becomes too great - **Example:** Crumpling paper creates many more edges and surfaces than a flat sheet - more area means more opportunities for substances to cross The relationship between surface area and diffusion rate is directly proportional:

### Properties of molecules or ions - **Large molecules** diffuse more slowly than smaller ones as they require more energy to move - **Uncharged and non-polar molecules** diffuse directly across the phospholipid bilayer - **Non-polar molecules** diffuse more quickly than polar ones as they are soluble in the non-polar phospholipid bilayer - **Example:** Small oxygen molecules ($\text{O}_2$) pass through membranes far more easily than large protein molecules

The greater the surface area to volume ratio, the faster the rate of diffusion

## Facilitated Diffusion - Certain substances **cannot diffuse through the phospholipid bilayer** of cell membranes - These include: - Large polar molecules such as glucose and amino acids - Ions such as sodium ions ($\text{Na}^+$) and chloride ions ($\text{Cl}^-$) - These substances can only cross the phospholipid bilayer **with the help of certain proteins** - This form of diffusion is known as **facilitated diffusion**

Facilitated diffusion is the movement of molecules or ions down a concentration gradient through specific transport proteins in the membrane

### Types of proteins in facilitated diffusion There are two types of proteins that enable facilitated diffusion: - **Channel proteins** - **Carrier proteins** They are **highly specific** (they only allow one type of molecule or ion to pass through) ### Channel Proteins - Channel proteins are **water-filled pores** - They allow **charged substances** (e.g. ions) to diffuse through the cell membrane - The diffusion of these ions does not occur freely, most channel proteins are **'gated'**, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore - This allows the channel protein to **control the exchange of ions** - **Example:** Sodium channels in nerve cells can open and close rapidly, like gates controlling the flow of sodium ions during nerve impulse transmission

Channel proteins can be gated, allowing cells to control when ions pass through

### Carrier Proteins - Unlike channel proteins which have a fixed shape, **carrier proteins can switch between two shapes** - This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape - The direction of movement of molecules diffusing across the membrane depends on their **relative concentration on each side of the membrane** - Net diffusion of molecules or ions into or out of a cell will occur **down a concentration gradient** (from an area containing many of that specific molecule to an area containing less of that molecule) - **Example:** The glucose transporter protein changes shape like a revolving door, allowing glucose molecules to enter cells when blood glucose levels are high ### Key differences from simple diffusion | Feature | Simple Diffusion | Facilitated Diffusion | |---------|------------------|----------------------| | Membrane crossing | Directly through phospholipid bilayer | Through transport proteins | | Substances involved | Small, non-polar molecules (e.g. $\text{O}_2$, $\text{CO}_2$) | Large polar molecules and ions (e.g. glucose, $\text{Na}^+$) | | Proteins required | No | Yes (channel or carrier proteins) | | Specificity | Non-specific | Highly specific to particular molecules | | Rate limitation | Limited by concentration gradient and membrane permeability | Also limited by number of available transport proteins |

Facilitated diffusion is still a passive process - no energy input is required as movement is down a concentration gradient

## Summary - Both simple diffusion and facilitated diffusion are **passive processes** - They do not require energy from ATP - Movement occurs **down a concentration gradient** (from high to low concentration) - The key difference is whether transport proteins are needed: - **Simple diffusion:** small, non-polar molecules pass directly through the lipid bilayer - **Facilitated diffusion:** large polar molecules and ions pass through specific channel or carrier proteins