Membrane Structure and Function Chapter 7

 The plasma membrane  Is the boundary that separates the living cell from its nonliving surroundings

 The plasma membrane exhibits selective (semi) permeability  It allows some substances to cross it more easily than others Proteoglycan complex Collagen Fibronectin. EXTRACELLULAR FLUID Micro- filaments CYTOPLASM Polysaccharide molecule Carbo- hydrates Proteoglycan molecule Core protein Integrin

 Cellular membranes are fluid mosaics of lipids and proteins Fluid Mosaic Model

 Phospholipids (fluid part)  Are the most abundant lipid in the plasma membrane  Are amphipathic, containing both hydrophobic and hydrophilic regions Fluid Mosaic Model

 Phospholipids (fluid part)  Are the most abundant lipid in the plasma membrane  Are amphipathic, containing both hydrophobic and hydrophilic regions  Proteins (mosaic part) can be  Transmembrane (integral) or  Peripheral Fluid Mosaic Model

 Review: The structure of phospholipids: Results in a bilayer arrangement found in cell membranes Hydrophilic head WATER Hydrophobic tail

 Phospholipids in the plasma membrane  Can move within the bilayer (fluid part) Lateral movement (~10 7 times per second) Flip-flop (~ once per month) Movement of phospholipids

 The type of hydrocarbon tails in phospholipids  Affects the fluidity of the plasma membrane FluidViscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails

 The steroid cholesterol  Has different effects on membrane fluidity at different temperatures Cholesterol within the animal cell membrane Cholesterol

Membrane proteins (mosaic part) are dispersed and individually inserted into the phospholipid bilayer (fluid part) Phospholipid bilayer Hydrophilic region of transmembrane protein Hydrophobic region of transmembrane protein What about peripheral proteins?

 Short carbohydrates bound to lipids (glycolipids) or proteins (glycoproteins) cover the outer surface of cells  These carbohydrates mediate cell-cell recognition Membrane Carbohydrates

Cells need to control the exchange of material with their environment Crossing the Membrane

 Polarity determines ease of passage  Hydrophobic molecules (non-polar)  Are lipid soluble and can pass through the membrane rapidly  Hydrophilic molecules (polar, ionic)  Do NOT cross the membrane rapidly  Need help of transport proteins Crossing the Membrane

 Passive transport is the movement of a substance across a membrane with no energy investment  Involves the process of simple diffusion  Or facilitated diffusion  Active transport requires an energy input Passive vs. Active Transport

 Simple Diffusion  Is the tendency for molecules to move from areas of high concentration to areas of low concentration  Small, hydrophobic molecules can move across the membrane this way Molecules of dye Membrane (cross section) Net diffusion Equilibrium (a) Passive Transport

 In facilitated diffusion  Transport proteins speed the movement of molecules across the plasma membrane that can not easily pass Facilitated Diffusion

 Channel proteins  Provide corridors that allow a specific molecule or ion to cross the membrane  If transporting ions, they are called ion channels EXTRACELLULAR FLUID Channel protein Polar or ionic Solute CYTOPLASM Facilitated Diffusion

 Carrier proteins  Undergo slight shape changes when solutes bind  Change allows movement of solute binding site across the membrane Facilitated Diffusion

 Osmosis  Is the diffusion of water across a semipermeable membrane Lower concentration of solute = Higher concentration of water Higher concentration of solute = Lower concentration of water Water moves from an area of higher water concentration to an area of lower water concentration  s Special Case of Facilitated Diffusion

Tonicity  Is the ability of a solution’s solute concentration to cause a cell to gain or lose water  In general, water moves down ITS concentration gradient from high to low solute in a solution means water solute in a solution means water

If a solution is isotonic  The concentration of solutes outside is EQUAL to the concentration inside the cell  There is no NET movement of water 20% NaCl 80% H 2 O 20% NaCl 80% H 2 O Solution (Environment) Cell

If a solution is hypertonic  The concentration of solutes outside is HIGHER than the concentration inside the cell  The cell will LOSE water. 40% NaCl 60% H 2 O 20% NaCl 80% H 2 O Solution (Environment) Cell

If a solution is hypotonic  The concentration of solutes outside is LOWER than the concentration inside the cell  The cell will GAIN water. 10% NaCl 90% H 2 O 20% NaCl 80% H 2 O Solution (Environment) Cell

5% NaCl 95% H 2 O 5% NaCl 95% H 2 O 10% NaCl 90% H 2 O 20% NaCl 80% H 2 O Isotonic Hypotonic Hypertonic Hypotonic Hypertonic Water movement A B C D Tonicity Diagram: It’s all relative!

 Water balance in cells without walls  Such as animal or protist Hypotonic solution Isotonic solution Hypertonic solution H2OH2O H2OH2O H2OH2O H2OH2O Lysed NormalShriveled/Crenation Osmosis in Cells

 Water balance in cells with walls  Such as plant cells H2OH2OH2OH2O H2OH2O H2OH2O Turgid (normal ) Flaccid Plasmolyzed Hypotonic solution Isotonic solution Hypertonic solution Osmosis in Cells

 Active transport  Moves substances AGAINST their concentration gradient from LOW to HIGH  Requires energy, usually in the form of ATP  Uses carrier proteins which act as pumps Active Transport

 Sodium Potassium Pump is an example of active transport  Cells typically have higher K + inside and lower Na +  Maintaining this concentration gradient requires ATP payment Sodium Potassium Pump

 Protein pumps can alter both the  Balance of ion concentration across the membrane AND  Balance of negative and positive charges  This combination of factors is called the electrochemical gradient  Nerve cells depend on this electrochemical gradient to send signals  Transport proteins involved are called electrogenic pumps Active Transport

 The Sodium Potassium pump is the main electrogenic pump in animals  The Proton pump is the main electrogenic pump in plants, fungi and bacteria  An important use of the Proton pump is during cellular respiration Active Transport

 Transport of large molecules across the plasma membrane occurs by exocytosis and endocytosis Movement of Macromolecules

In exocytosis  Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents In endocytosis  The cell takes in macromolecules by forming new vesicles from the plasma membrane

EXTRACELLULAR FLUID Pseudopodium CYTOPLASM “Food” or other particle Food vacuole 1 µm Pseudopodium of amoeba Bacterium Food vacuole PINOCYTOSIS 0.5 µm Plasma membrane Vesicle Three types of endocytosis PHAGOCYTOSIS

0.25 µm Receptor Mediated Endocytosis Recepto r Ligand Coated pit Coated vesicle Plasma membrane Coat protein