Chapter 5 – Plasma Membrane Structure and Function

Plasma Membrane Functions –Isolates cell’s contents from environment –Regulates exchange substances –Communicates with other cells –Creates attachments within and between other cells –Regulates biochemical reactions

Membranes Are “Fluid Mosaics” Membranes are dynamic, ever-changing structures “Fluid mosaic” model of a membrane proposed in 1972 –A lumpy, constantly shifting mosaic of “tiles” or proteins –Proteins float around in sea of phospholipids

The Phospholipid Bilayer Phospholipids are the basis of membrane structure –Polar, hydrophilic head –Two non-polar, hydrophobic tails

The Phospholipid Bilayer Cell exterior/interior face watery environments Hydrophobic and hydrophilic interactions drive phospholipids into bilayers –Double row of phospholipids –Polar heads face outward/inward –Non-polar tails driven together in membrane –Cholesterol in animal membranes keeps them flexible

The Phospholipid Bilayer Flexible, fluid membrane allows cellular shape changes Individual phospholipid molecules - not bonded to one another Some phospholipids have unsaturated fatty acids –double bonds introduce “kinks” into their “tails” and makes membrane more fluid

Membrane Proteins Form a Mosaic Proteins embedded in phospholipid bilayer –Some proteins can float/drift –Other proteins are anchored by protein filaments in cytoplasm –Many proteins have attached carbohydrates (glycoproteins)

Categories of Membrane Proteins –1. Receptor Proteins –2. Recognition Proteins –3. Enzymatic Proteins –4. Attachment (Adhesion) Proteins –5. Transport Proteins

Categories of Membrane Proteins (5) 1. Receptor Proteins –Trigger cellular responses upon binding specific molecules, e.g. hormones 2. Recognition Proteins –Serve as ID tags on surface of cell

Receptor Activation

Categories of Membrane Proteins (5) 3. Enzymes –Promote chemical reactions that synthesize or break apart biological molecules 4. Attachment (Adhesion) Proteins –Anchor cell membrane to inner cytoskeleton, to proteins outside cell, and to other cells 5. Transport Proteins –Include channel and carrier proteins –Regulate import/export of hydrophilic molecules

Movement of Molecules in Fluids Definitions relevant to substance movement –Fluid - substance that can move or change shape in response to external forces –Solute - substance that can be dissolved in a solvent –Solvent - fluid capable of dissolving a solute

Movement of Molecules in Fluids –Concentration - # of molecules in a given volume unit –Gradient - physical difference in temperature, pressure, charge, or concentration of a substance in a fluid between two adjacent regions

Movement of Molecules in Fluids Why molecules move from one place to another: –Substances move in response to a concentration gradient Molecules move from high to low concentration (diffusion) until dynamic equilibrium is reached

Movement of Molecules in Fluids Greater the concentration gradient, the faster the rate of diffusion Diffusion cannot move molecules rapidly over long distances

Movement Across Membranes Concentration gradients exist across plasma membranes of cells Two types of movement: –Passive transport –Active transport

Passive Transport –Substances move down their concentration gradients across a membrane –No energy is expended –Membrane proteins and phospholipids may limit which molecules or ions can cross, but not the direction of movement

Passive Transport Plasma membranes are selectively permeable –Different molecules move across at different locations and rates –A concentration gradient drives all 3 types of passive transport: simple diffusion facilitated diffusion osmosis

Passive Transport Simple diffusion –Lipid soluble molecules (e.g. vitamins A and E, gases) & very small molecules diffuse directly across the phospholipid bilayer

Passive Transport Facilitated diffusion –Water soluble molecules like ions, amino acids, and sugars diffuse with the aid of channel and carrier transport proteins

Passive Transport Osmosis – diffusion of water –Water diffuses from high concentration (high purity) to low concentration (low purity) across a membrane –Dissolved substances reduce the concentration of free water molecules (and hence the purity of water) in a solution

Osmosis Flow of water across a membrane depends on the concentration of water in the internal or external solutions

Passive Transport - Osmosis Comparison terms for solutions on either side of a membrane: –Isotonic –Hypertonic –Hypotonic

Passive Transport - Osmosis Isotonic solutions have equal concentrations of water and equal concentrations of dissolved substances No net water movement occurs across the membrane

Passive Transport - Osmosis –Hypertonic solution (higher osmotic strength) is one with lower water concentration or higher dissolved particle concentration Water moves across a membrane towards the hypertonic solution

Passive Transport - Osmosis –Hypotonic solution (lower osmotic strength) is one with higher water concentration or lower dissolved particle concentration Water moves across a membrane away from the hypotonic solution

Passive Transport - Osmosis Effects of osmosis –Red blood cells placed in various solutions

Passive Transport - Osmosis Fresh water protists -contractile vacuoles Water leaks in continuously b/c cytosol is hypertonic to fresh water Salts are pumped into vacuoles, making them hypertonic to cytosol Water follows by osmosis and is expelled by contraction

Active Transport –Substances are driven against their concentration gradients –Energy is expended biology4kids.com

Active Transport Cells need to move some substances against their concentration gradients Active-transport membrane proteins move molecules across using ATP –Proteins span the entire membrane –Often have a molecule binding site and an ATP binding site –Often referred to as pumps

Endocytosis Cells import large particles or substances via endocytosis Plasma membrane pinches off to form a vesicle –Types of endocytosis Pinocytosis Receptor-mediated endocytosis Phagocytosis

Pinocytosis –Pinocytosis (“cell drinking”) brings in droplet of extracellular fluid

Receptor-Mediated Endocytosis –Move specific molecules into the cell

Phagocytosis –Phagocytosis (“cell eating”) moves large particles or whole organisms into the cell

Exocytosis Vesicles join the membrane, dumping out contents in exocytosis

How Do Specialized Junctions Allow Cells to Connect and Communicate?

Desomosomes Attach cells together –Found where cells need to adhere tightly together under stresses of movement (e.g. the skin)

Tight Junctions Make cell leakproof –Found where tubes and sacs must hold contents without leaking (e.g. the urinary bladder)

Gap Junctions and Plasmodesmata Allow for communication –Gap Junctions Cell-to-cell channels allowing for passage of hormones, nutrients, and ions in animal cells –Plasmodesmata Cytoplasmic connections in plant cells

Viscous Venoms Snake and spider venoms contain phospholipases, enzymes that break down phospholipids Venoms attack cell membranes, causing cells to rupture and die Cell death destroys tissue around bite

The End