As you have already learned, cellular membranes have many functions, and control of the cell’s internal composition is one of the most important. Biological membranes allow some substances, but not others, to pass through them. This characteristic of membranes is called selective permeability. Selective permeability allows the membrane to determine what substances enter or leave a cell or organelle.
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The rate of diffusion of a substance across a membrane is influenced by the size and mass of the diffusing particles, the temperature and the density of the solution, and the size of the concentration gradient.
Osmosis is diffusion of water across a biological membrane.
Proteins that act as channels facilitate diffusion across membranes.
There are two fundamentally different processes by which substances cross biological membranes:
The processes of passive transport do not require the input of chemical energy to drive them.
The processes of active transport are driven by chemical energy (metabolic energy).
This section focuses on the passive processes by which substances cross membranes. The energy for the passive transport of a substance comes from the difference between its concentration on one side of the membrane and its concentration on the other—
Animation 6.1 Passive Transport
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