Cell Membrane and its features

Cell Membrane
Cell Membrane is a "Differentially Permeable" as welI as  "Selective Permeable"  sheath arround a ceIl that separates the cytoplasm of its cell from neighbouring cells. The membranes of the membranous organelles (called Endomembranes or Plasmamembranes) provide partitions that separate these organelles from the cytoplasm.
Common Features Of Plasma Membranes: 
Plasma Membranes show a great diversity of structure and functions. They show, however, a number of features in common. Some of the most important features are:-
1. Membranes are sheath like structures, only a few molecules thick and form °boundries
between compjirtments of different composition. For example, the membranes of mitochondria separate these organelles from the cytoplasm. Lysosome is a membranous vesicle, its membrane allows itself to maintain a different chemical composition inside the organelle.
2. Membranes are mainly composed of Lipids and Proteins. They also contain Carbohydrates in the form of Glycolipids and Glycoproteins.
3. Membrane Lipids are amphipathetic molecules (a molecule that has most of its part Hydrophobic non-ionizable and one end Hydrophylic ionizable). Stearic Acid (CI7H35COOH), for example, is amphipathetic molecule because it has 17 Carbon atoms containing hydrocarbon chain which does not ionize hence is hydrophobic. The Carboxyl (COOH) group is Hydrophylic because it ionizes as COO- group.

CH3(CH2)16  ---->  COO-
Non-Ionized            Ionized
Hydrophobic part      Hydrophylic part

Stearic Acid
The membrane lipids are arranged within the membrane to form biomolecular layer.
This lipid layer makes the membrane differentially permeable barrier because water and water soluble (ionic materials) cannot 'pass across them but fat soluble non-polar materials (oil-droplets, Fatty acis and organic solvants like Ethr, or Benzene) can move across membranes freely. Hence membranes differentiate between polar and non-polar materials.
4. Membrane Proteins help the membranes to function as selective permeable barriers. They help to regulate the movement of water and other ionic materials across the membranes. Membrane Proteins also function as molecular pumps and gates through which ionic materials enter or leave the cells. Membrane Proteins also work as enzymes and reeptors that respond to different stimuli. Some Membrane Proteins of mitochondria an chloroplasts work as members of electron transport chains that help in energy transformation.
5. The Structural Components of membranes, i.e. Lipids and Proteins are beld together by very weak hydrophobic forces.
6. The membrane Carbohydrates are present either attached to a lipid molecule and thus are called Glycolipids or attached to Proteins as Glycoproteins. Carbohydrates are present only on outer face of the membrane thus makes the membrane as asymmetric structure. Membrane Carbohydrates function as receptors that select food and reject non-food molecules react with antigens and thus stimulate the cell to produce antibodies.
7. Membranes are fluicl structures. Both Lipids and Proteins can diffuse rapidly in the plane of the membrane. Thus the membranes are regarded as two dimensional colloidal solutions of o~nted Lipids and Proteins.

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