Membrane Structure Function Fluid mosaic model Membrane is

Membrane. Structure & Function Fluid mosaic model

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 Cytoplasm Transmembrane proteins Filaments of cytoskeleton

Phospholipids Phosphate head l “attracted to water” hydrophilic Fatty acid tails l Phosphate hydrophobic Arranged as a bilayer Fatty acid “repelled by water”

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 lipids

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

Proteins domains anchor molecule Within membrane l Polar areas of protein nonpolar amino acids • hydrophobic On outer surfaces of membrane in fluid l polar amino acids • hydrophilic Nonpolar areas of protein

H+ H+ Examples Retinal chromophore NH 2 aquaporin = water channel in bacteria H 2 O Porin monomer b-pleated sheets Bacterial outer membrane Nonpolar (hydrophobic) a-helices in the cell membrane COOH H+ + H Cytoplasm proton pump channel in photosynthetic bacteria H 2 O function through conformational change = protein changes shape

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

Membrane Proteins Classes of membrane proteins: l peripheral proteins • loosely bound to surface of membrane • ex: cell surface identity marker (antigens) l integral proteins • penetrate lipid bilayer, usually across membrane • transmembrane protein • ex: transport proteins – channels, permeases (pumps)

Membrane carbohydrates Play a key role in cell-cell recognition l ability of a cell to distinguish one cell from another • antigens l basis for rejection of foreign cells by immune system

Movement across the Cell Membrane 2007 -2008

Simple Diffusion Move from HIGH to LOW concentration l l l “passive transport” no energy needed Small, nonpolar molecules movement of water diffusion osmosis

Facilitated Diffusion through protein channels l l Large and/or polar molecules (like water) no energy needed facilitated = with help open channel = fast transport HIGH LOW “The Bouncer”

Active Transport Cells may need to move molecules against concentration gradient (low to high) l l protein “pump” “costs” energy = ATP conformational change LOW ATP HIGH “The Doorman”

Transport summary simple diffusion facilitated diffusion active transport ATP

How about large molecules? Moving large molecules into & out of cell l l through vesicles & vacuoles Endocytosis~ in • phagocytosis = “cellular eating” l Exocytosis ~out exocytosis

The Special Case of Water Movement of water across the cell membrane 2007 -2008

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

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

Pumping water out Contractile vacuole in Paramecium ATP