Cell membrane AP Biology Phospholipids Phosphate head u

Cell membrane AP Biology

Phospholipids § Phosphate head u “attracted to water” hydrophilic § Fatty acid tails u Phosphate hydrophobic § Arranged as a bilayer Fatty acid “repelled by water” Aaaah, one of those structure–function examples AP Biology

Arranged as a Phospholipid bilayer § Serves as a cellular barrier / border sugar polar hydrophilic heads nonpolar hydrophobic tails H 2 O salt impermeable to polar molecules polar hydrophilic heads waste AP Biology lipids

AP Biology

Cell membrane defines cell § Cell membrane separates living cell from aqueous environment u thin barrier = 8 nm thick § Controls traffic in & out of the cell u allows some substances to cross more easily than others § hydrophobic (nonpolar) vs. hydrophilic (polar) AP Biology

Permeability to polar molecules? § Membrane becomes semi-permeable via protein channels u specific channels allow specific material across cell membrane inside cell NH AP Biology 3 salt H 2 O aa sugar outside cell

Cell membrane is more than lipids… § Transmembrane proteins embedded in phospholipid bilayer u create semi-permeabe channels lipid bilayer membrane AP Biology protein channels in lipid bilyer membrane

Many Functions of Membrane Proteins “Channel” Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Cell adhesion Attachment to the cytoskeleton “Antigen” AP Biology Cell surface identity marker

Membrane Proteins § Proteins determine membrane’s specific functions u cell membrane & organelle membranes each have unique collections of proteins § Classes of membrane proteins: u peripheral proteins § loosely bound to surface of membrane § ex: cell surface identity marker (antigens) u integral proteins § penetrate lipid bilayer, usually across whole membrane § transmembrane protein § ex: transport proteins w channels, permeases (pumps) AP Biology

Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Peripheral protein AP Biology Cytoplasm Transmembrane proteins Filaments of cytoskeleton 1972, S. J. Singer & G. Nicolson proposed Fluid Mosaic Model

Membrane carbohydrates § Play a key role in cell-cell recognition u ability of a cell to distinguish one cell from another § antigens important in organ & tissue development u basis for rejection of foreign cells by immune system u AP Biology

Movement across the Cell Membrane AP Biology 2007 -2008

Diffusion § 2 nd Law of Thermodynamics governs biological systems u universe tends towards disorder (entropy) § Diffusion u AP Biology movement from HIGH LOW concentration

Simple Diffusion § Move from HIGH to LOW concentration “passive transport” u no energy needed u AP Biology diffusion movement of water osmosis

Facilitated Diffusion § Diffusion through protein channels u u channels move specific molecules across cell membrane facilitated = with help no energy needed open channel = fast transport HIGH LOW AP Biology “The Bouncer”

Getting through cell membrane § Passive Transport u Simple diffusion § diffusion of nonpolar, hydrophobic molecules w lipids w HIGH LOW concentration gradient u Facilitated transport § diffusion of polar, hydrophilic molecules § through a protein channel w HIGH LOW concentration gradient § Active transport u diffusion against concentration gradient § LOW HIGH u u AP Biology uses a protein pump requires ATP

Transport summary simple diffusion facilitated diffusion active transport AP Biology ATP

How about large molecules? § Moving large molecules into & out of cell through vesicles & vacuoles u endocytosis u § phagocytosis = “cellular eating” § pinocytosis = “cellular drinking” u AP Biology exocytosis

Endocytosis phagocytosis fuse with lysosome for digestion pinocytosis non-specific process receptor-mediated endocytosis triggered by molecular signal AP Biology

Osmosis is just diffusion of water § Water is very important to life, § so we talk about water separately Diffusion of water from HIGH concentration of water to LOW concentration of water u AP Biology across a semi-permeable membrane

Concentration of water § Direction of osmosis is determined by comparing total solute concentrations u Hypertonic - more solute, less water u Hypotonic - less solute, more water u Isotonic - equal solute, equal water AP Biology hypotonic hypertonic net movement of water

Managing water balance § Cell survival depends on balancing water uptake & loss AP Biology freshwater balanced saltwater

Do you understand Osmosis… . 05 M . 03 M Cell (compared to beaker) hypertonic or hypotonic Beaker (compared to cell) hypertonic or hypotonic AP Biology Which way does the water flow? in or out of cell