The plasma membrane regulates the passage of molecules into and out of the cell.
It is capable of carrying out this function because it is selectively permeable,
meaning that it allows certain substances to pass while preventing others.
Basically, substances enter the cell in one of three ways:
passive transport, active transport or bulk transport.
Let’s look at each of these in turn.
Passive transport is the movement of substances into or out of a cell
without the expenditure of energy by the cell.
One form of passive transport is diffusion.
During diffusion, molecules move across a membrane from an area of high concentration
to an area of low concentration.
The molecules are therefore said to be moving down a concentration gradient.
This continues until equilibrium is reached
and the molecules are distributed equally.
Another form of passive transport is facilitated diffusion.
Facilitated diffusion occurs when an ion or molecule diffuses across a membrane
faster than expected, either by way of a specific channel protein
or with the assistance of carrier proteins that change shape as they pass through.
The diffusion of water across a membrane,
or osmosis, is another example of passive transport.
In many cases, specialized proteins called aquaporins
allow for the more rapid transport of water molecules.
The second type of transport, active transport,
requires the input of energy in the form of ATP.
The proteins that conduct this form of transport are often called pumps,
because they force molecules or ions to move from an area of low concentration
to an area of high concentration.
This is commonly referred to as up, or against, the concentration gradient.
One of the more common examples is the sodium-potassium pump,
which moves sodium ions back out of the cell –
and potassium ions into the cell.
Notice that the sodium potassium pump undergoes a change in shape that allows it to combine
alternately with sodium ions and potassium ions.
The third type of transport, bulk transport,
is used for molecules that are too large to be moved by transport proteins.
Instead, vesicles take them into or out of the cell.
During this process, the plasma membrane surrounds and engulfs the particle.
This is known as endocytosis.
Cells use three basic types of endocytosis
depending on the size and nature of the material to be digested:
phagocytosis, pinocytosis and receptor-mediated endocytosis.
If the material taken in is large, such as bacteria or a food particle,
the process is called phagocytosis.
Pinocytosis occurs when vesicles form around a liquid, or very small particles.
During receptor-mediated endocytosis,
molecules bind to specific receptor proteins embedded in a coated pit within the plasma membrane.
When enough molecules accumulate in the coated pit,
the pit deepens, seals, and is incorporated into the cell as a vesicle.
Exocytosis is the opposite of endocytosis.
During exocytosis, membrane bound vesicles move to the surface of the plasma membrane,
fuse with the membrane, and then release their contents to the outside of the cell.
To recap, there are three main types of cell transport:
passive transport, active transport and bulk transport.
Passive transport is the movement of substances in or out of a cell
without the expenditure of energy by the cell.
Active transport requires energy
in the form of ATP to move molecules
against their concentration gradient.
Bulk transport requires vesicle formation and metabolic energy.
Forms of bulk transport include endocytosis,
pinocytosis, receptor-mediated endocytosis, and exocytosis.