Plasma Membrane. Membrane Generals. Very thin layer of lipids and proteins (& small amount of CHO) Controls entry and exit of:

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1 Plasma Membrane Membrane Generals Very thin layer of lipids and proteins (& small amount of CHO) Encloses intracellular space Controls entry and exit of: Various nutrient molecules Ions Plasma (cell) membranes vary depending on the type of cell Membrane structure: Trilaminar 2 dark layers separated by a light middle layer Fluid lipid bilayer embedded with proteins Membrane composition: Phospholipids (most abundant lipids) Cholesterol Phospholipids consist of a polar (electrically charged) head 1 negative phosphate group 2 neutral fatty acid tails 1

2 Figure 3.2 (1) Page 58 Choline Phosphate - Head (polar, hydrophilic) Glycerol Fatty acid Tails (nonpolar, hydrophobic) - = Negative charge on phosphate group Polar heads and nonpolar tails: Polar heads (hydrophilic) ECF (water) Nonpolar tails (hydrophobic) Polar heads (hydrophilic) ICF (water) Figure 3.2 (2) Page 58 Figure 3.2 (3) Page 58 Intracellular fluid Structure allows for integrity & flexibility Extracellular fluid 2

3 3 major functions of the lipid bilayer: 1. Forms structure of the membrane (fence) 2. Hydrophobic interior serves as a barrier to passage of H 2 O soluble substances H 2 O is small enough to pass Able to maintain various concentrations of solutes 3. Fluidity of the membrane Additional components of the cell membrane: 1. Cholesterol Prevent fatty acid chains from crystallizing 2. Membrane proteins Vary from spanning entire membrane to only the inner or outer surface Much larger than lipid molecules & serve various functions 3. Membrane CHO Outer surface of membrane (glycoproteins & glycolipids) Glycoprotein Glycolipid Extracellular fluid Carbohydrate chain Cholesterol molecule Phospholipid molecule Channel Various membrane proteins Intracellular fluid Figure 3.3 Page 59 Dark line Light space Dark line Appearance using an electron microscope 3

4 Specific functions of membrane proteins: 1. Channel proteins proteins spanning the membrane form channels Allow water soluble substances to pass through Channels are highly specific Size Selective (Na + through Na + channels) Channels may also change in shape Specific functions of membrane proteins: 2. Carrier proteins Carry specific substances unable to cross on their own Different cells have different carriers Specific functions of membrane proteins: 3. Docking-marker acceptors Located on inner membrane t-snares that bind with v-snares of secretory vesicles ECF Exocytosis 4

5 Specific functions of membrane proteins: 4. Membrane-bound proteins Surface proteins (inner or outer) Control specific chemical reactions Enzyme regulation Specific functions of membrane proteins: 5. Receptor sites Surface proteins (outer) Allow binding of specific molecules that may activate (or inactivate) certain cellular mechanisms Specific to chemical messengers in the blood (generally hormones) Specific functions of membrane proteins: 6. Cell adhesion molecules (CAMs) Protrude from outer surface and allow cell to grasp onto connective tissue between cells Integrins (type of CAM) Span through into intracellular space Relay regulatory signals between cells 7. Cell recognition 5

6 Specific functions of membrane CHOs: 1. Self-recognition CHO extending from surface serves as a cell indicator 2. Some tissue growth involvement Sensing external environment Glycoprotein Glycolipid Extracellular fluid Carbohydrate chain Cholesterol molecule Phospholipid molecule Various membrane Channel proteins Intracellular fluid Figure 3.3 Page 59 Dark line Light space Dark line Appearance using an electron microscope Cell Adhesion Binding process of cells to form tissues & organs Held together by 3 processes: 1. Extracellular matrix 2. Cell adhesion molecules (CHO) on membrane 3. Specialized cell junctions 6

7 Components of the extracellular matrix: 1. Collagen Provide tensile strength 2. Elastin Rubberlike protein (allowing stretch & recoil) 3. Fibronectin Cell adhesion and positioning Specialized cell junctions (CAMs): 1. Desmosomes (adhering junctions) 2 major components 1. 2 pair of dense plaque (buttons) on inner surface of adjacent cells 2. Glycoprotein filaments connecting the two plaque Found in stretching tissue Skin, heart, smooth muscle Cell 1 cytosol Cytoplasm thickening (plaque) Spot desmosome Cell 2 cytosol Figure 3.4 Page 63 Intercellular filaments 20 nm Intracellular keratin filaments Interacting plasma membranes 7

8 Specialized cell junctions: 2. Tight junctions Seal off passage between cells Kiss sites Generally found in epithelial tissue (digestive system) Passage must take place through cells, not between Lumen (contains undigested food and potent digestive enzymes) Luminal border No passage between cells Selective passage through cells Tight junction Lateral border Epithelial cell lining intestine Cell 1 Cell 2 Basolateral border Figure 3.5 (1) Page 64 Blood vessel Cell 1 cytosol Cell 2 cytosol Strands of junctional proteins Tight junction Kiss site Intercellular space Figure 3.5 (2) Page 64 Interacting plasma membranes 8

9 Specialized cell junctions: 3. Gap junction Gap linked by small connecting tunnels (connexons) Allows small water soluble particles & signaling molecules to pass (cellular exchange of ions) No entrance into ECF Generally found in cardiac and smooth muscle Cell 1 cytosol Cell 2 cytosol Connexon Gap junction Diameter of channel = 1.5 nm No passage of large molecules Longitudinal section of connexon Passage of ions and small molecules Figure 3.6 Page nm Interacting plasma membranes 9