Transport Across Membranes: Diffusion | A Level Biology | OCR, AQA, Edexcel
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Simple Diffusion is the net movement of particles down a concentration gradient, from a high concentration to low concentration.
Particles move down a concentration gradient. A concentration gradient is the difference in the concentrations of a substance between two regions, divided by the distance between the two regions. Diffusion follows a concentration gradient.
Diffusion occurs until dynamic equilibrium. A particular substance will diffuse from a region of higher concentration to a region of lower concentration until the concentrations of the substance in both regions is equivalent. This is known as dynamic equilibrium.
Diffusion is passive. Diffusion does not require energy. Particles continuously move around randomly, and particles tend to spread out until equilibrium is reached.
The membrane has to be very thin. The membrane has to be thin in order to provide the shortest path length, or distance for the substances to diffuse across.
There is an upper limit on cell size. This size limit on the membrane places an upper limit on the size of the cell. Diffusion is a critical biological process, and to ensure that it readily occurs, cells have to be small enough to minimise the diffusion path length.
Non-polar molecules can simply diffuse across a membrane. Non-polar (lipid-based) molecules can simply pass through the phospholipid bilayer (as long as they are small). This is due to the hydrophobic and non-polar nature of the hydrophobic tails of the phospholipid bilayer.
Very small molecules can also simply diffuse. Very small molecules and ions (e.g. CO2) are small enough to pass through the bilayer in between the phospholipid molecules.
Facilitated diffusion refers to the transport of polar molecules and ions into and out of a cell. It involves the use of carrier and channel proteins.
Polar molecules cannot pass by simple diffusion. Polar molecules cannot cross the hydrophobic barrier, so instead travel via facilitated diffusion.
Polar molecules instead diffuse by facilitated diffusion. Polar and hydrophilic molecules travel through carrier and channel proteins – they are ‘facilitated’ and assisted across.
Transport proteins are adapted to their role. The channel and carrier structurally designed to interact with polar molecules:
Many of these channel proteins have pores that are filled with water (attract hydrophilic molecules)
Additionally, the proteins typically are polar in this pore region. Through this, they can readily interact with polar molecules and allow them to travel through these pores in order to either enter or exit a cell.
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