Membrane Structure and Function
Cellular membranes are fluid mosaics of lipids and proteins
The arrangement of phospholipids and proteins in biological membranes is described by the fluid mosaic model.
Phospholipids and most other membrane constituents are amphipathic molecules, which have both hydrophobic and hydrophilic regions.
Integral proteins penetrate the hydrophobic interior of the lipid bilayer, usually completely spanning the membrane as transmembrane proteins
Peripheral proteins are not embedded in the lipid bilayer at all
Membrane carbohydrates may be covalently bonded to lipids, forming glycolipids, or more commonly to proteins, forming glycoproteins.
Membrane structure results in selective permeability
The passage of water through the membrane can be greatly facilitated by channel proteins known as aquaporins.
Cell membranes are permeable to specific ions and a variety of polar molecules, which can avoid contact with the lipid bilayer by passing through transport proteins that span the membrane.
Passive transport is diffusion of a substance across a membrane with no energy investment
One result of thermal motion is diffusion, the movement of molecules of any substance to spread out in the available space.
In the absence of other forces, a substance diffuses from where it is more concentrated to where it is less concentrated, down its concentration gradient
The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen
The diffusion of water across a selectively permeable membrane is called osmosis.
Both solute concentration and membrane permeability affect tonicity, the ability of a surrounding solution to cause a cell to gain or lose water.
If a cell without a cell wall, such as an animal cell, is immersed in an environment that is isotonic to the cell, there is no net movement of water across the plasma membrane
If the cell is immersed in a solution that is hypertonic to the cell (containing nonpenetrating solutes), the cell loses water to its environment, shrivels, and probably dies.
If the cell is immersed in a solution that is hypotonic to the cell, water enters the cell faster than it leaves, and the cell swells and lyses (bursts) like an overfilled water balloon
Active transport uses energy to move solutes against their gradients
This active transport requires the cell to expend metabolic energy and enables a cell to maintain internal concentrations of small molecules that would otherwise diffuse across the membrane.
The sodium-potassium pump works this way in exchanging sodium ions (Na+) for potassium ions (K+) across the plasma membrane of animal cells
Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
The voltage across a membrane is called a membrane potential
Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane.
Special transport proteins, called electrogenic pumps, generate the voltage gradient across a membrane.
In exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane
During endocytosis, a cell brings in biological molecules and particulate matter by forming new vesicles from the plasma membrane
There are three types of endocytosis: phagocytosis (“cellular eating”), pinocytosis (“cellular drinking”), and receptor-mediated endocytosis
These lipoproteins act as ligands by binding to LDL receptors on membranes and entering the cell by endocytosis.