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Cell Membrane and Transport Recapitulation. Learning Outcomes (g) Describe and explain the fluid mosaic model of membrane structure, including an outline.

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Presentation on theme: "Cell Membrane and Transport Recapitulation. Learning Outcomes (g) Describe and explain the fluid mosaic model of membrane structure, including an outline."— Presentation transcript:

1 Cell Membrane and Transport Recapitulation

2 Learning Outcomes (g) Describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins. (h) Outline the roles and functions of membranes within cells and at the surface of cells. (Knowledge of osmosis, facilitated diffusion, active transport, endocytosis and exocytosis is required.)

3 Learning Outcomes (g) Describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins. (h) Outline the roles and functions of membranes within cells and at the surface of cells. (Knowledge of osmosis, facilitated diffusion, active transport, endocytosis and exocytosis is required.)

4 Fluid Mosaic Model In the fluid mosaic model of the cell membrane, the membrane is visualised as a fluid structure of a phospholipid bilayer embedded or attached with a mosaic of various proteins.

5 Components of Membrane Phospholipids Cholesterol Glycolipids Proteins Glycoproteins

6 Phospholipids

7 Phospholipids Phospholipids are amphipathic (have both hydrophobic and hydrophilic regions) In aqueous surroundings, the hydrophilic phosphate heads face the water while the hydrophobic hydrocarbon tails aggregate away from the water, forming a phospholipid bilayer with a hydrophobic core

8 Phospholipids and fluidity Temperature increases, kinetic energy of phospholipids increases, fluidity of membrane increases  fluid state Temperature decreases, kinetic energy decreases, fluidity decreases  semisolid state Longer hydrocarbon chains, more extensive hydrophobic interactions between phospholipids, fluidity decreases More unsaturated hydrocarbon tails, more kinks, tails unable to pack closely, fluidity increases More saturated hydrocarbon tails, more straight chains, tails able to pack closely, fluidity decreases Note the flow of thought in reasoning

9 Cholesterol

10 Cholesterol’s effects Found only in animal cell membranes Intercalated into both layers  enhance mechanical stability of membrane Dual effect on membrane fluidity –High temp, interferes with phospholipid movement, reduces membrane fluidity –Low temp, prevents close packing of phospholipids, increases membrane fluidity Wedged between phospholipids, fills in spaces between them, prevents passage of small polar molecules and ions, decreases membrane permeability

11 Proteins Schematic representation of the different types of interaction between monotopic membrane proteins and the cell membrane: 1. interaction by an amphipathic α-helix parallel to the membrane plane (in-plane membrane helix) 2. interaction by a hydrophic loop 3. interaction by a covalently bound membrane lipid (lipidation) 4. electrostatic or ionic interactions with membrane lipids (e.g. through a calcium ion)cell membraneα-helixionic interactions Schematic representation of transmembrane proteins: 1. a single transmembrane α-helix (bitopic membrane protein) 2. a polytopic α-helical protein 3. a transmembrane β barrel The membrane is represented in light brown.

12 Functions of membrane proteins Anchorage Recognition (usually glycoproteins) Enzymes – catalyse reactions Receptor – cell signalling Carrier – facilitated diffusion or active transport Channels – leak/gated channels (facilitated diffusion) Intercellular joining

13 Glycolipids Consist of lipids attached to oligosaccharide chains Carbohydrate groups are highly hydrophilic Occur on the side of the cell membrane facing the extracellular fluid Function to bind extracellular signal molecules, intercellular adhesion and cell- to-cell recognition

14 Glycoproteins Consists of proteins with oligosaccharide chains attached; the short carbohydrate chain is attached during post-translational modification of the protein Found in the cell membrane and cytosol. Often occurs as integral proteins Variety of functions, including as a cell attachment recognition site and receptor

15 Learning Outcomes (g) Describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins. (h) Outline the roles and functions of membranes within cells and at the surface of cells. (Knowledge of osmosis, facilitated diffusion, active transport, endocytosis and exocytosis is required.)

16 Roles and functions of membranes Within Cell –Compartmentalization – provide suitable environment (pH, localised molecules) for reactions to occur At Cell Surface –Define cell’s boundaries –Regulate transport of substances in and out of cell (partially permeable membrane) –Cell-to-cell communication (using receptor proteins, etc.) –Localisation of structures

17 Transport across membranes Major function of cell membrane: to regulate entry and exit of substances to and from cell Regulated by components of cell membrane –Phospholipids (and cholesterol) –Proteins Channel protein (provides pore) Carrier protein (changes conformation to carry substances across)

18 Transport across membranes

19 Osmosis The net movement of freely moving water molecules from a region of less negative water potential to a region of more negative water potential, through a selectively permeable membrane Highest water potential is 0 (pure water) Solutes reduce water potential because hydration shells are formed around solute particles, reducing the number of free water molecules Animal cells: Water potential = solute potential Plant cells: Water potential = solute potential + pressure potential –Pressure due to cell wall –Pressure potential is positive (pushes water out)

20 Effect of osmosis on cells

21 Simple Diffusion Diffusion is the net movement of a substance from a region of higher concentration to a region of lower concentration, i.e. down a concentration gradient Occurs for small non-polar molecules. See figure 11-1 of notes. Hydrophobic molecules diffuse directly across the hydrophobic core of the phospholipid bilayer No energy from ATP is needed No carrier proteins are needed

22 Facilitated Diffusion For larger, hydrophilic substances (ions, polar) Facilitated by a transport protein in the membrane, without which the substance would not be able to cross the hydrophobic core of the phospholipid bilayer –Carrier protein (undergoes conformational change to transport substance) –Channel protein (central hydrophilic pore) No ATP required Down a concentration gradient

23 Active transport The movement of substances from a region of lower concentration to a region of higher concentration, against the concentration gradient ATP required Primary active transport –carrier protein-mediated –Occurs for ions and small hydrophilic molecules Bulk transport –Involves invagination of the plasma membrane or the extension of pseudopodia –Occurs for large objects or macromolecules

24 Endocytosis

25 Exocytosis


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