Eukaryotic cell Plasma membrane:

Biological membranes Biological membranes include all intracellular membranes and plasma membrane . Generally, these intracellular membranes perform the following important functions: 1- Separate the cell from the outside environment (they act as barrier to isolate the cell from other environmental components). 2- Regulate the passage of materials and exchange of molecules between the cell and outside environment. 3- Participate in many chemical reactions. They contain enzymes and other molecules that are necessary for chemical changes. 4- Transmit signals and information from out-side to inside the cell in a process called cell transduction. 5- Transfer and store energy (ATP).

Structure of Biological Membranes All biological membranes including plasma membrane and intracellular membranes have a general structure. The cellular membranes are structurally similar except that the cell membrane of cell surfaces is characterized by occurrence of receptors for cell signals, protein molecules for intercellular communication and immunological differentiation

The most widely accepted model of plasma membrane structure is the fluid mosaic model of Singer & Nicolson (1972). According to this model the cell membrane is composed of a fluid lipid-bilayer with a constantly changing mosaic pattern of associated proteins .

c. Membrane carbohydrate. Membrane Lipid Bilayer The molecular composition of cell membrane includes mainly 3 components: a. Lipid bilayer b. Membrane protein c. Membrane carbohydrate.   Membrane Lipid Bilayer It is formed of two leaflets. They are composed of: 1. Phospholipid molecules. They form the most forming molecules. 2. Glycolipid. It is formed of a carbohydrate chain attaching the hydrophilic glycerol. 3. Cholesterol.

The phospholipid molecules and all lipid molecules that make the core of cell membrane are amphipathic molecules. They composed of: a. Hydrophilic (water-loving) polar head group directed toward outside, and b. Hydrophobic (water-hating) non-polar tails of fatty acid chains (one usually unsaturated) and directed toward inside. At body temperature the lipid bilayer is fluid and allows for the lipid molecules to move along the monolayer, reseals spontaneously at injury, and below 17 ᵒC the lipid becomes viscous or crystal-line in nature.

The general characteristics of the lipid bilayer can be summarized as follows: 1. Acts as a barrier between two liquid (intra-cellular and extracellular) compartments. 2. Lipid bilayer behaves as a liquid crystals, because of the ordered array of molecules (heads on the outside and the tails of fatty acid chains are directed toward inside). 3. The hydrocarbon chains are in constant motion forming fluid of lipid molecules. The molecules are free to rotate. Each molecule can move laterally within its single layer depended on two factors: 1-temperture and 2- lipid composition(such as shorter fatty acids are weaker and less rigid .Unsaturated fatty acid similarly increase membrane fluidity because the presence of double bonds .

4. At body temperature and generally over 25 ᵒ C lipid bilayer is thin fluid layer of about 3.9 nm thickness but it becomes thick viscous layer of about 4.8 nm in thickness under 17ᵒc. 5. Resists forming free ends. It is self sealing and rounds up to form closed vesicles as in formation of transport vesicles and secretory vesicles . 6. Fuses with lipid bilayer of other membranes as in exocytosis. 7. Flexible. It allows change shape without breaking. 8. Cholesterol molecules are largely hydrophobic molecules that maintain the optimal fluidity of lipid bilayer.

8. Cholesterol molecules are largely hydrophobic molecules that maintain the optimal fluidity of lipid bilayer. Cholesterol plays an important role in maintaining of the stability of cell membrane. a. In high temperature; increase the body temperature that increases the movement of membrane molecules, it restricts the movement of molecules and prevents weakening of membrane. b. In low temperature, it restricts solidification (act as spacer).

Permeability of lipid bilayer Lipid bilayer is freely permeable for: 1. Gases such as nitrogen, oxygen and carbon dioxide. 2. Hydrophobic hydrocarbon or fat soluble compounds such as steroid hormones. 3. Small polar molecules such as water, glycerol and urea.

Lipid bilayer is not permeable for: 1. Large polar molecules such as glucose (mono-saccharide) and disaccharides. 2. Charged molecules such as ions of sodium, potassium, calcium, hydrogen and magnesium and amino acids or polypeptide mole-cules.

Membrane Proteins According to the fluid mosaic model, a cellular membrane consists of a fluid bilayer of lipid molecules in which the proteins are embedded or associated much like the tiles in a mosaic picture. Types of Membrane Proteins: proteins are the major constituent of cell membrane ,(25 to 75)% of the mass of the various membranes of the cell.

Morphologically membrane proteins are classified into: a. Integral membrane proteins (firmly bound). They are amphipathic molecules with hydrophobic and hydrophilic regions. They are partially embedded in lipid bilayer or formed of transmembrane proteins .They are exposed on both sides of the membrane.  

b. Peripheral proteins (loosely associated) b. Peripheral proteins (loosely associated). They are located on inner or outer surfaces of lipid bilayer and attached to hydrophilic part of integral membrane proteins or the phospholipid molecules. Distribution of membrane protein in lipid bilayer is asymmetrically oriented. The majority of protein molecules attach to the protoplasmic layer of lipid bilayer (inner layer).

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