Cell Membranes Structure and Function

Fig. 4.3, p. 52 one layer of lipids one layer of lipids lipid bilayer fluid

Main component of cell membranes Main component of cell membranes Gives the membrane its fluid properties Gives the membrane its fluid properties Two layers of phospholipids Two layers of phospholipids Lipid Bilayer

Figure 8.2 Two generations of membrane models

The detailed structure of an animal cell’s plasma membrane, in cross section

Membrane Proteins Transport proteins Transport proteins Receptor proteins Receptor proteins Recognition proteins Recognition proteins Adhesion proteins Adhesion proteins

Fig. 4.4, p. 53 EXTRACELLULAR ENVIRONMENT (cytoskeletal pro- teins beneatch the plasma membrane) ADHESION PROTEIN oligosaccharide groups phospholipid cholesterol LIPID BILAYER RECOGNITION PROTEIN RECEPTOR PROTEIN CYTOPLASM PLASMA MEMBRANE (area of enlargment) TRANSPORT PROTEINS open channel protein gated channel proten (open) active transport protein gated channel proten (closed)

The fluidity of membranes

Cell Membranes Show Selective Permeability O 2, CO 2, and other small nonpolar molecules;and H 2 O C 6 H 12 O 6, and other large, polar (water-soluble) molecules; ions such as H +, Na +, CI -, Ca ++ ; plus H 2 O hydrogen-bonded to them X

Membrane Crossing Mechanisms Diffusion across lipid bilayer Passive transport Active transport EndocytosisExocytosis

Diffusion The net movement of like molecules or ions down a concentration gradient The net movement of like molecules or ions down a concentration gradient Although molecules collide randomly, the net movement is away from the place with the most collisions (down gradient) Although molecules collide randomly, the net movement is away from the place with the most collisions (down gradient)

Factors Affecting Diffusion Rate Steepness of concentration gradient Steepness of concentration gradient Steeper gradient, faster diffusion Steeper gradient, faster diffusion Molecular size Molecular size Smaller molecules, faster diffusion Smaller molecules, faster diffusion Temperature Temperature Higher temperature, faster diffusion Higher temperature, faster diffusion Electrical or pressure gradients Electrical or pressure gradients

Concentration Gradient Means the number of molecules or ions in one region is different than the number in another region Means the number of molecules or ions in one region is different than the number in another region In the absence of other forces, a substance moves from a region where it is more concentrated to one one where it’s less concentrated - “down” gradient In the absence of other forces, a substance moves from a region where it is more concentrated to one one where it’s less concentrated - “down” gradient

For example, For example, start with a permeable membrane separating a solution with dye molecules from pure water start with a permeable membrane separating a solution with dye molecules from pure water dye molecules cross the barrier randomly. dye molecules cross the barrier randomly. The dye will cross the membrane until both solutions have equal concentrations of the dye. The dye will cross the membrane until both solutions have equal concentrations of the dye. At this dynamic equilibrium as many molecules pass one way as cross the other direction. At this dynamic equilibrium as many molecules pass one way as cross the other direction. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 8.10a

The diffusion of solutes across membranes

Osmosis A Special Case of Simple Diffusion Diffusion of water molecules across a selectively permeable membrane Diffusion of water molecules across a selectively permeable membrane Direction of net flow is determined by water concentration gradient Direction of net flow is determined by water concentration gradient Side with the most solute molecules has the lowest water concentration Side with the most solute molecules has the lowest water concentration

Tonicity Tonicity Refers to relative solute concentration of two fluids Hypertonic - having more solutes Isotonic - having same amount Hypotonic - having fewer solutes

Tonicity and Osmosis 2% sucrose water10% sucrose 2% sucrose

The water balance of living cells

The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation

Imagine that two sugar solutions differing in concentration are separated by a membrane that will allow water through, but not sugar. Imagine that two sugar solutions differing in concentration are separated by a membrane that will allow water through, but not sugar. The hypertonic solution has a lower water concentration than the hypotonic solution. The hypertonic solution has a lower water concentration than the hypotonic solution. More of the water molecules in the hypertonic solution are bound up in hydration shells around the sugar molecules, leaving fewer unbound water molecules. More of the water molecules in the hypertonic solution are bound up in hydration shells around the sugar molecules, leaving fewer unbound water molecules. Unbound water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer. Unbound water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer.

Increase in Fluid Volume compartment 1 HYPOTONIC SOLUTION membrane permeable to water but not to solutes HYPERTONIC SOLUTION compartment 2 fluid volume increases In compartment 2

Passive Transport Faciltiated Diffusion Faciltiated Diffusion Flow of solutes through the interior of passive transport proteins down their concentration gradients Flow of solutes through the interior of passive transport proteins down their concentration gradients Passive transport proteins allow solutes to move both ways Passive transport proteins allow solutes to move both ways Does not require any energy input Does not require any energy input

Span the lipid bilayer Span the lipid bilayer Interior is able to open to both sides Interior is able to open to both sides Change shape when they interact with solute Change shape when they interact with solute Play roles in active and passive transport Play roles in active and passive transport Transport Proteins

Two models for facilitated diffusion

Active Transport Net diffusion of solute is against concentration gradient Net diffusion of solute is against concentration gradient Transport protein must be activated Transport protein must be activated ATP gives up phosphate to activate protein ATP gives up phosphate to activate protein Binding of ATP changes protein shape and affinity for solute Binding of ATP changes protein shape and affinity for solute

Active Transport ATP gives up phosphate to activate protein ATP gives up phosphate to activate protein Binding of ATP changes protein shape and affinity for solute Binding of ATP changes protein shape and affinity for solute P P P P High solute concentration Low solute concentration ATP ADP

The sodium-potassium pump: a specific case of active transport

Review: passive and active transport compared

Bulk Transport – Another form of Active Transport Exocytosis Endocytosis

The three types of endocytosis in animal cells