Plasma membrane Lecture 4

According to the functions, membrane proteins can be classified into: 1. Cell – cell adhesion proteins. 2. Cell – Extracellular matrix adhesion proteins. Integrins are example for membrane proteins that connect between extracellular matrix and the cytoskeleton. 3. Membrane transport protein. They work to transport specific molecules across the cell membrane. a. Channel proteins for passage of ions or specific molecules. b. Pumps or carrier proteins for active or mediated passive transport.

4. Membrane –bound enzymes are important to catalyze chemical reactions. 5. Receptor proteins for recognition and defense. 6. Signal receptor proteins for signal transduction to interior of the cell

Membrane Carbohydrate (cell coat or glycocalyx) Membrane carbohydrates are sugar residues attaching the membrane proteins to form the cell coat or glycocalyx on the external cell surface. There are three forms of membrane carbohydrates: 1. Glycoproteins. They are oligo or polysaccaharide chains attached to membrane proteins. 2. Glycolipids. They are oligosaccharide chains attached to lipid monolayer. 3. Proteoglycans. They are formed of a core of protein surrounded by long polysaccharide chains across the lipid bilayer.

*Membrane carbohydrates have the following functions: 1. Fill the spaces between the adjacent cells .The cell coat help in holding adjacent cells together. 2. Intercellular communication (cell contact).By recognition of chemical message by the cell surface receptors which can receive chemical message and neurotransmitter signal from other cells. 3. Cell recognition process .The cell coat enable cells to recognize cells of their own special kind .This is important in transplantation of kidney, heart and skin. 4. Mechanical strength of multicellular tissue and organs.

5. formation of cell immunity :the cell coat plays an important role in the development of cell immunity against infections. 6- production of cell Allergy : The cell coat reacts with foreign materials producing surface cell reaction which protect the body against certain harmful agents.

Ways of Transport of Molecules across the Cell Membranes 1. Transport of small molecules: There are two mechanisms for transport of small molecules: a. Passive diffusion b. Active transport. The followings are ways of transporting small molecules across the cell membrane:

1-Passive Transport of Small Moleculesa. 1-Simple passive diffusion It is defined as a physical random motion of particles from higher to lower concentration gradient. This motion of particles in passive diffusion continues till equilibrium is reached. The rate of diffusion of molecules across the biological membranes depends upon: - Size of molecules. The smaller molecules diffuse rapidly than the larger ones - Shape of molecules increases or decreases the rate of transport of molecules.

Electrical charge. Charged molecules can-not diffuse through the biological membrane, while small polar molecules are freely diffuse. - Temperature. The rate of diffusion of small molecules increases with increase of temperature. - Dialysis and osmosis are examples for simple passive diffusion. Water, gasses and glycerol are examples for molecules that can transport passively from higher concentration to lower concentration.

b. Facilitated passive diffusion It is a transport process in which the passage of specific molecules from higher to lower concentration gradient is mediated by Carrier and Channel proteins. Examples: Amino acids, organic acids and inorganic salts.

2-Active Transport of Small Molecules It is the process of pumping of molecules or ions through the cell membrane against their concentration gradient. This process requires: a. Transporter (carrier transport protein) b. Energy

Types of active transport a. Direct active transport. It uses ATP hydrolysis to carry out transport of specific molecules against their concentration gradient. b. Indirect active transport (co-transport). It is a process in which directly pumped molecule transport another specific molecule indirectly.

Membrane Transport Proteins There are two types of transport proteins: A. Channel Proteins B. Carrier proteins

Dialysis It is the process of diffusion of a solute (dissolved substances) across a selectively permeable membrane (such as hemodialysis and peritoneal dialysis). It is used for separation of substances in solution by means of their unequal diffusion through semi-permeable membrane . Hemodialysis; dialysis of the blood is used to remove toxic substance or metabolic wastes from blood stream in case of kidney failure.

Osmosis It is the diffusion of solvent (water molecules) through a semipermeable membrane from a higher concentration to a lower concentration until the concentration on both sides of the semipermeable membrane becomes equal . The force that must be applied to prevent the motion across the semipermeable membrane is called the osmotic pressure of solution. Notice: Solvent is a substance that is capable of dissolving another substance (solute).

B) Ways of Transport of Macromolecules 1. Endocytosis It is a process of internalization of macromolecules from outside into inside the cell. This process is classified into phagocytosis and pinocytosis according to the nature of the internalized materials . Engulfing or eating large foreign solid particles such as bacteria is called phagocytosis, while the process of cellular drinking of fluid containing soluble particles is called pinocytosis.

2. Exocytosis It is the reverse process of endocytosis; it means transport of a processed or synthesized cellular product from inside to outside the cell .

Mechanism of exocytosis 1. Exocytosis starts by fusion of secretory vesicle membrane with the plasma membrane. 2. An opening is formed between the lumen of secretory vesicle and outside the cell. 3. The contents are released through the opening.

Receptor-mediated endocytosis It is form of pinocytosis that is specific because it involves the use of a receptor protein shaped in such a way that a specific substance such as vitaminms , hormones can bind to it.

Fill the blank:   1-According to fluid mosaic model, the plasma membrane consists of: ………………………………………,…………………………..……………, and ……………………………………… 2. Lipid bilayer is composed of 2 leaflets formed of:..…………..…….., ……………………. and …………………….... 3-Lipid- bilayer behave as liquid crystals because ………………………………………………………………………………… 4-. cholesterol molecules in lipid bilayer are important for the cell membrane because: ………………………………………………….……………………………….  

5-The plasma membrane is covered with a strong and rigid ……………………… 5-The plasma membrane is covered with a strong and rigid ……………………….that renders mechanical protection and provides the bacteria their characteristic shape 6-The nuclear material is usually concentrated in a specific clear region of the cytoplasm, called ………………………….. 7- Some bacteria (mostly Gram negative bacilli) contain non-flagellar, extremely fine, appendages called ………………………………… 8-Bacteria reproduce asexually by ………………and …………………….formation and sexually by …………………………

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