A cell membrane (also known as a plasma membrane) is a thin semifluid structure that separates the contents of a cell or organelle from its surroundings.
The environment inside of a cell is drastically different from that of its surroundings. Outside the cell, water-soluble ions and molecules create a harsh and toxic environment. From this, the cell must selectively absorb nutrients that are essential to its growth and function. It must also excrete toxic byproducts of its metabolism. The cell membrane subsequently has two major functions:
- It acts as a barrier, enclosing and protecting the components of a cell.
- It acts as a gate, controlling the flow of molecules in and out of the cell.
Higher-level cells known as eukaryotes contain specialized components, called organelles, that play dedicated roles in its growth and development. Each organelle is surrounded by a separate membrane whose function is similar to that of plasma membranes, but with a slightly different composition that enables the organelle to perform specific tasks.
Plasma and organelle membranes are composed primarily of lipids (fatty acids, sterols, or other water-insoluble molecules) and proteins (chains of amino acids). They differ in their proportion of lipids to proteins. For example, cell membranes of structures predominantly involved in energy production (e.g., the mitochondria) have a higher percentage of proteins, while membranes acting as insulators (e.g., the Schwann cell, which insulates some nerve fibers) have a higher proportion of lipids.
LIPIDS. A membrane is actually two layers of lipids that form a shell around the cell. This lipid bilayer is composed primarily of phosopholipids (lipids containing one or more phosphate groups), each with a hydrophilic (water-soluble) "head" and a hydrophobic (water-insoluble) "tail." The bilayer is the most stable configuration for phospholipids in a water environment, with the water-repelling tails sequestered in the middle of two layers of water-soluble heads. Thus the membrane forms a stable yet flexible configuration with a certain amount of fluidity: individual phospholipids can move rapidly across the surface of the membrane, and part to allow molecules soluble in organic media (e.g., other lipids, dissolved gases, etc.) to enter the cell.
PROTEINS. One type of protein can be loosely associated with the outside of the membrane; these are called extrinsic (or peripheral) proteins. Other proteins are tightly embedded in the membrane, and may extend from one side of the membrane to the other; these intrinsic or integral proteins are difficult to remove without destroying the membrane itself.
There are two general types of membrane proteins: transporters and receptors. Although some lipid-soluble molecules can permeate the cell membrane, many of the nutrients that a cell needs to function are too large to readily enter the cell. Transporters allow the cell to be selective in which molecules it allows into its cytoplasm. Examples of transporters are channels, which facilitate free movement of molecules across the membranes, and pumps, which require a certain amount of energy in order to transport molecules. Transport proteins also exist in organelle membranes; transport channels have been shown to exist in the organelles of yeast cells and are essential to cell viability.
A cell must be able to communicate with its surroundings if it is going to adapt to changing conditions. Receptors are transmembrane proteins that detect signals from the extracellular environment and translate those signals into a cellular response. An example of a signal is the compound epinephrine (also known as adrenaline). Receptors specific to epinephrine detect its presence in the environment and bind to the molecule. This binding induces a cascade of events in the cell, resulting in increased production of glucose used as energy.
The cell is constantly bombarded by ions and molecules of different type and size. While lipid-soluble molecules can pass readily through the membrane, water-soluble and larger particles require another mode of entry. The plasma membrane consequently has numerous means of importing or exporting substances:
- Permeation occurs when a substance moves through the membrane from a region of high concentration to a region of low concentration, a process called diffusion. Only lipid-soluble molecules and some small particles (e.g., biologically important gases such as oxygen and nitrogen) can readily permeate the cell membrane.
- Passive transport or facilitated diffusion occurs when water-soluble molecules and ions move through the membrane with the help of transporters (also called permeases).
- Active transport occurs when a substance is moved against its concentration gradient, from a low concentration to a high concentration. This process requires a higher amount of energy expended by the cell.
Role in human health
Thousands of bacteria, protozoa, and fungi cause human disease. These microbial cells also have membranes that are essential to their vitality. This vulnerability is a target of the human immune system and some types of drugs that fight microbial diseases. For example, the polymyxin class of antibiotics disrupts the cell membranes of bacteria such as Pseudomonas aeruginosa (causes respiratory tract, burn wound, ear, and eye infections). Compromising the cell membranes of such microorganisms effectively kills them.
Common diseases and disorders
Some human diseases the result of faulty membrane transport systems. An example would be type 2 (adult onset) diabetes mellitus. Excess glucose in the bloodstream, caused by eating a meal rich in carbohydrates, is usually taken up by myocytes (muscle cells) and adipocytes (fat cells). The glucose transporter GluT4 is normally present in the cell membrane in small amounts. The presence of insulin (a hormone secreted by the pancreas in response to high glucose levels) causes more GluT4 transporters to be exposed, increasing uptake of glucose into the cell. In type 2 diabetes there is resistance to the metabolic effects of insulin, either at the cell membrane or in post-receptor signaling systems. This means that little glucose can be taken up by myocytes and adipocytes, and high blood glucose levels are the result.
Cystic fibrosis, a genetic disease, causes an abnormality in the mucus normally found in the lungs, resulting in increased bacterial infections and difficulty breathing. This is caused by a defective chloride and fluid transport that decreases the water content of the mucus and causes it to be excessively thick.
Active transport— Movement of a substance against its concentration gradient, from a low concentration to a high concentration.
Eukaryote— An organism whose cells contain a true nucleus bound by a membrane.
Extrinsic proteins— Proteins that are loosely associated with the outside of a plasma membrane; also known as peripheral proteins.
Hydrophilic— Having an affinity for water or for absorbing water.
Hydrophobic— Lacking an affinity for or resistant to water.
Intrinsic proteins— Proteins that are tightly embedded in a plasma membrane, and might extend from one side of the membrane to the other.
Ions— Atoms with positive or negative electric charge.
Lipid— A molecule of composed fatty acids, sterols, or other water-insoluble molecules.
Metabolism— The physical and chemical processes occurring within a cell that are necessary for life.
Organelle— A specialized compartment of a cell that performs specific functions, such as a mitochondrion, lysosome, or ribosome.
Passive transport— Movement of a substance across a membrane without the expenditure of metabolic energy.
Permeation— Movement of a substance through a permeable membrane from a region of high concentration to a region of low concentration.
Receptor— An extracellular structure capable of binding specific substances.
Transporter— A transmembrane protein that transports different substances across the membrane.
Other disorders of membrane transport include Giddleman's syndrome and Liddle's syndrome, which can cause either hypo- or hypertension. Membrane disorders are also important causes of water and electrolyte disturbances, disorders of neural transmission, and many other syndromes.
Hazardous substances such as tetrodoxin (produced by the puffer fish), dendrotoxin (venom of the black mamba snake), and cobrotoxin (another snake venom) affect the function of different ion channels in neurons, blocking signals from the nervous system to muscles. The result may be paralysis and in some cases, death.
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