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Net movement of particles
From an area of high concentration to an area of low concentration, down a concentration gradient.
How is diffusion a passive process
No additional energy is required.
Which types of substances can cross a membrane via simple diffusion
Small and/or non-polar molecules. Some small polar molecules, such as water and carbon dioxide can also pass through slowly.
Why can’t large molecules diffuse across the phospholipid bilayer
They cannot pass between the phospholipids
Why can’t polar molecules or ions diffuse across the phospholipid bilayer
They are repelled by the non-polar fatty acid tails.
How do large and/or polar molecules and ions diffuse across the membrane
Via a specific transport protein
What are the 2 types of transport proteins
Channel and carrier
What do channel proteins provide
A hydrophilic channel or pore through the hydrophobic interior of the phospholipid bilayer.
How do gated channel proteins function
They open or close depending on the presence of another molecule (ligand) or the potential difference (voltage) across the membrane.
How do carrier proteins function
They bind to a specific polar molecule on one side of the membrane and then change shape (allostery) to release the molecule on the other side of the membrane.
When does active transport occur
When molecules need to be transported from low to high concentration, against their concentration gradient
What does active transport require
A specific carrier protein (pump) and additional energy e.g. from the hydrolysis of ATP
What is the function of bulk transport
To move large quantities of materials into or out of the cell.
What does bulk transport rely on
The fluid nature of the membrane.
How is bulk transport an active process
Requires ATP for the formation of vesicles and to move them around the cell.
What are the 2 types of bulk transport
Exocytosis and endocytosis.
Exocytosis
Bulk transport of substances out of the cell e.g. secretion of hormones or proteins.
Endocytosis
Bulk transport of substances into the cell.
What are the 2 types of endocytosis
Phagocytosis and pinocytosis.
Phagocytosis
The uptake of solid particles e.g. a neutrophil engulfing a bacterium.
Pinocytosis
The uptake of solutes in solution e.g. uptake of nutrients by egg cell.
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