Chapter 5: Biological Membranes AP Biology Chapter 5

Plasma Membrane Functions: separates the cell's insides from outside regulates passage of materials into/out of cell transmitting signals and info. between the cell and environment participates in chemical reactions essential part of energy transfer and storage systems

Composition: - lipid bilayer - proteins - in constant motion

Membrane Proteins *Important area of research: how membrane proteins function in health/disease many are enzymes function in transport of materials/information connect cells together to form tissues

Phospholipid Bilayer Phospholipid: 2 fatty acid chains linked to a glycerol molecule - nonpolar, hydrophobic ends (fatty acids) - hydrophilic ends (phosphate group) - hydrophobic "tails" turn to inside of membrane - hydrophilic "heads" turn to outer ends of membrane - amphipathic - have distinct hydrophilic and hydrophobic regions

Phospholipid Bilayer Hydrogen bonds form between the phospholipid "heads" and the watery environment inside and outside of the cell Hydrophobic interactions force the "tails" to face inward Phospholipids are not bonded to each other, which makes the double layer fluid

Fluid Mosaic Model Cell membrane consists of a fluid bilayer of phospholipid molecules in which proteins are embedded - like the tiles in a mosaic picture NOT static - proteins can move p.107 diagram of fluid mosaic model

Membrane Proteins Functions: Transport of small molecules Enzymes Information transfer Identification tags - allow for cell-cell recognition

Membrane Proteins amphipathic Integral Proteins - firmly bound to the membrane, usually do not extend all the way through amphipathic hydrophilic regions extend out of the cell or into cytoplasm - hydrophobic regions interact with fatty acid tails of the phospholipids

Membrane Proteins Transmembrane Proteins - extend through the membrane, also amphipathic

Membrane Proteins Peripheral Proteins - not embedded in the lipid bilayer located on the inner or outer surfaces of the plasma membrane can be removed from the membrane without disrupting the structure

Cell Membrane is Selectively Permeable Most membranes are permeable to small molecules and lipid-soluble or polar molecules Water molecules may pass through the lipid bilayer gases: such as O2 and CO2

Transport Across Membranes Passive Transport - does not require energy, moves with the concentration gradient Active Transport - requires ATP, moves against the concentration gradient

Simple Diffusion Process based on random motion Particles move down concentration gradient -from an area of high concentration to low concentration can occur rapidly occurs until equilibrium is reached

Types of Diffusion: Osmosis - diffusion of water across a selectively permeable membrane p.114 Figure 5-11 Dialysis - diffusion of a solute across a selectively permeable membrane p.113 Figure 5-10

High H2O potential Low solute concentration Low H2O potential High solute concentration

Osmotic Pressure the tendency of water to move into a solution by osmosis solution with high solute concentration, low water, has a high osmotic pressure solution with a low solute concentration, high water, low osmotic pressure

Isotonic - equal solute concentration ex. blood plasma isotonic to blood cells Hypertonic - higher solute concentration - if a cell is placed in a hypertonic environment, water will leave the cell, the cell shrinks - plasmolysis occurs: plasma membrane separates from cell wall Hypotonic - lower solute concentration - a cell placed in a hypotonic environment will gain water, swell, and possibly burst

Turgor Pressure Turgor Pressure - internal pressure of cells with cell walls Plants, Algae, and Bacteria Enables them to withstand a low solute concentration outside the cell Cell is hypertonic to environment Water moves into cell, cell swells, building pressure -> turgor pressure against cell wall Cell does not burst b/c of cell wall, resist stretching and water molecules must stop moving into the cell

Carrier-Mediated Transport Membrane proteins move ions or molecules across a membrane 2 types: 1. facilitated diffusion (passive) 2. carrier-mediated active transport

Carrier-Mediated Transport Facilitated diffusion - with concentration gradient, requires transport protein energy comes from concentration gradient ex. glucose permease - transports glucose into red blood cells

Carrier-Mediated Active Transport Carrier-mediated Active Transport - movement of solutes across membrane against concentration gradient particles must be "pumped" from region or low conc. to region of high conc. requires energy source - ATP, and transport protein ex. sodium-potassium pump: in all animal cells, pump sodium ions out of cell and potassium ions into cell

Other Types of Active Transport large particles such as food, cell parts requires ATP Endocytosis and Exocytosis Active Transport video clip

Exocytosis Cell ejects waste products or hormones by the fusion of a vesicle with the plasma membrane Vesicle releases contents from the cell

Endocytosis Materials are taken into the cell Types: phagocytosis, pinocytosis Endocytosis Video clip

Phagocytosis "cell eating" cell ingests large solid particles such as bacteria and food ex. protists, white blood cells plasma membrane folds enclose particle, forms a vacuole, fuses, then enters the cell and fuses with lysosomes

Pinocytosis "cell drinking" cell takes in dissolved materials droplets of fluid are trapped by folds in the membrane pinch off into the cytosol as tiny vesicles liquid is slowly transferred into the cytosol vesicles become smaller, then disappear