Physiology -I PHL 215 PHL 215 Dr/ Gamal Gabr Pharmacy College Pharmacy College 1

III) Plasma membrane gate  Plasma membrane is like a gate  It allow certain substances to enter or leave the cell. 2

Plasma membrane is a thin, flexible outer barrier that separates the interior of the cell from the outside environment. o Thickness: o Thickness: 8–10 nanometers (nm).  1 nm = 1/ mm Plasma membrane is a selectively permeable membrane = It controls the movement of substances in and out of cells. o What chemical materials are plasma membranes made of? () It made of proteins floating in a double layer of phospholipids, which are fluid at body temperature (like vegetable oil). 3

 Composition of plasma membrane The plasma membrane is composed of different types of macromolecules. The most common molecules are: ❶ Lipids ❷ Proteins ❸ Carbohydrates.LipidsProteinsCarbohydrates 72 % 25% 3 % Phospholipids: Phospholipids: 55% Cholesterol: Cholesterol: 13% Glycolipids: Glycolipids: 4% 4

Fluid-mosaic model The fluid-mosaic model describes the plasma membrane of animal cells.  Fluid: Because the plasma membrane has the consistency of vegetable oil at body temperature.  So, the molecules of lipids and proteins can move side-to-side within the same layer.  Mosaic: Because proteins and other substances such as cholesterol are distributes in the bilayer, giving the membrane the look of a mosaic. o What factors determine fluidity of the cell membrane? Temperature, lipid composition and cholesterol content 5

❶ Membrane lipids Lipid molecules = 3 major classes: Phospholipids (55%) + Cholesterol (13%) + Glycolipids (4%). Phospholipids  So, the major lipid molecules are Phospholipids. 6

 Phospholipids o Each phospholipid molecule has: a) Phosphorylated glycerol head hydrophilic water soluble a) Phosphorylated glycerol head that is attracted to water. i.e. hydrophilic OR water soluble. b) 2 fatty acid tails hydrophobicfat soluble b) 2 fatty acid tails that repels water. i.e. hydrophobic OR fat solubleNOTES: o Hydrophilic means o Hydrophilic means water loving = Water soluble OR Polar. o Hydrophobic means o Hydrophobic means water fearing = Fat soluble OR nonpolar. amphipathic. mphipathic o Molecules having both hydrophilic and hydrophobic properties are called amphipathic. Accordingly, the phospholipid molecules are considered as amphipathic molecules.  Which part of a phospholipid molecule is polar and thus considered hydrophilic? the phosphate glycerol head  Which part of a phospholipid molecule is nonpolar and thus is considered hydrophobic? the 2 fatty acid tails Phosphorylated glycerol head Two fatty acid tails 7

 Phospholipids Lipids bilayer …… Why? hydrophilic 1- Heads of the phospholipid molecules are hydrophilic, So attracted to water and directed into: A- ICF (inner surface of the membrane) B- ECF (outer surface of the membrane). hydrophobic 2- Tails of the phospholipid molecules are hydrophobic, So repelled by water and arranged tail to tail forming a hydrophobic region in the center of the membrane. bilayer The result = Lipid bilayer 8

 Phospholipids Functions: (The lipid bilayer) 1- Forms a barrier so protects the cell 2- Selectively permeable, so controls the movement of substances into and out of the cell: a) Hydrophobic substances as oxygen, carbon dioxide, and alcohol can diffuse easily. b) Hydrophilic substances as ions, glucose, and urea can not diffuse easily. 9

❷ Membrane Proteins The major protein molecules are Glycoproteins. NOTE: At physiological temperatures, the lipid bilayer is fluid, so proteins can move side-to-side within the same layer, but can’t flip from one layer to the other 10

Membrane proteins = 2 major types: Integral proteins transmembrane proteins A- Integral proteins OR transmembrane proteins Peripheral proteins B- Peripheral proteins 11

A) Integral proteins:  They crosses the lipid bilayer of the membrane with one end outside the cell and the other end inside.  Integral proteins are not fixed, so they moves through the membrane very quickly. 12

Each integral protein has 3 types of amino acids: 1- Hydrophilic cytosolic amino acids, in contact with ICF (Inner surface). 2- Hydrophobic central amino acids in contact with the fatty acids in the centre of the membrane. 3- Hydrophilic extracellular amino acids in contact with ECF (Outer surface). Integral proteins = 3 types: 1- Transport proteins a) Channel proteins b) Carrier proteins 2- Marker proteins 3- Receptor proteins. Hydrophilic region of protein Hydrophobic region of protein 13

1- Transport proteins: They regulate the movement of substances through the plasma membrane. Types: a- Channel proteins:  Channel proteins transport water soluble substances, especially the ions or very small molecules from one side of the membrane to the other side according to concentration diffusion  this process is known as diffusion b- Carrier proteins:  Carrier proteins change their shape to transport certain molecules from one side of the membrane to the other side against concentration active transport  this process is known as active transport. 14

2- Marker proteins:  Marker proteins are specific for each organism = fingerprints.  They identify cells to the body's immune system.  They play an important role in organ transplantation. 3- Receptor proteins:  They have specific binding sites that respond to hormones, antigens and neurotransmitters.  They play an important role in intercellular communication. 15

B) Peripheral proteins: They does not penetrate the plasma membrane. Usually Usually: found on the inside of the membrane. They attach the membrane to the cytoskeleton, so keep the shape of the cell. They function as enzymes  catalyze some metabolic reactions which occur near the plasma membrane. 16

Functions of membrane proteins: Control the movement of substances through cell membranes  Diffusion & Active transport Identification of cells to the body's immune system. Act as receptors. Act as enzymes. Attach the membrane to the cytoskeleton, so keeping the shape of the cell. 17

❸ Membrane Carbohydrates: glycocalyx Carbohydrate chains attach to the outer surface of the plasma membrane of each cell forming a loose coat called glycocalyx. They attached to : Proteins  Glycoproteins OR phospholipids  Glycolipids. Glycocalyx of some cells attaches to glycocalyx of other cells, thus attaching the cells to each other. Many of the carbohydrates act as receptors, as they bind with some hormones as insulin. 18

Function of Carbohydrates: Attaches the cells to each others. Act as receptors for some hormones. 19

Functions of plasma membrane: 1- Protects the cell 2- Controls the movement of substances into and out of the cell (Selectively permeable) 3- Identification of cells to the body's immune system (marker protein). 4- Communication with other cells and the outside world (receptor protein). 5- keep the shape of the cell. 20