Passive Transport
Simple diffusion
- Simple diffusion is a type of transportation that involves particles passing between phospholipids in the plasma membrane
- It 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
- The molecules or ions move down a concentration gradient
- The random movement is caused by the natural kinetic energy of the molecules or ions
- As a result of diffusion, molecules or ions tend to reach an equilibrium (given sufficient time), where they are evenly spread within a given volume of space
Diffusion across the cell membrane
- The rate at which a substance diffuses across a membrane depends on several factors:
- 'Steepness' of the concentration gradient - the greater the difference the higher the rate of diffusion
- Temperature - the higher the temperature the higher the rate of diffusion
- Surface area - the greater the surface area the higher the rate of diffusion
- Properties of the molecules or ions
- Large molecules diffuse more slowly as they require more energy to move
- Uncharged molecules (e.g. oxygen) diffuse faster as they move directly across the phospholipid bilayer
- Non-polar molecules diffuse more quickly as they are soluble in the non-polar phospholipid bilayer
- Although polar molecules cannot easily pass through the hydrophobic part of the membrane, smaller polar molecules (e.g. urea) can diffuse at low rates
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 (Na+) and chloride ions (Cl-)
- These substances can only cross the phospholipid bilayer with the help of certain proteins
- This form of diffusion is known as facilitated diffusion
- There are two types of proteins that enable facilitated diffusion:
- Channel proteins
- Carrier proteins (these can also be used during active transport)
- 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 (eg. 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
A channel protein (open and closed)
Carrier proteins
- Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes
- Initially, the binding site of the carrier protein is open to one side of the membrane
- When the carrier protein switches shape it opens to the other side of the membrane
- The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
- During facilitated diffusion, the 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)
A carrier protein changing shape during facilitated diffusion
Osmosis
- All cells are surrounded by a cell membrane which is partially permeable
- Water can move in and out of cells by osmosis
- Osmosis is the diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane
- In doing this, water is moving down its concentration gradient
- A dilute solution has a high concentration of water molecules and a concentrated solution has a low concentration of water molecules
- In doing this, water is moving down its concentration gradient
- The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)
Osmosis and the partially permeable membrane
- The term osmolarity can be used to describe the solute concentration of a solution; a solution with high osmolarity has a high solute concentration and a solution with low osmolarity has a low solute concentration.
- Water will move from a solution of low osmolarity to a solution of high osmolarity across a partially permeable membrane
- Osmosis can also be described as the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane
- Water potential describes the tendency of water to move out of a solution; this term is used to avoid confusion between water concentration and solute concentration of a solution
How osmosis works. The water moves from the region of higher water potential (dilute solution) to the region of lower water potential (concentrated solution).
Examiner Tip
Remember that the movement of molecules from high concentration to low concentration is diffusion. If this movement requires the aid of a protein (for example because the molecule is charged and cannot pass directly through the phospholipid bilayer) this is facilitated diffusion, and if it involves the movement of water across a partially permeable membrane it is osmosis.