Membrane Structure and Function Chapter 7 Biology – Campbell Reece

Membrane Structure Made of phospholipids Hydrophilic (phosphate) head Hydrophobic (fatty acid) tails Fluid mosaic model – the membrane is a fluid structure with proteins embedded in or attached to the phospholipid bilayer

Membranes are fluid…

Membranes are mosaics… A membrane is a collage of different proteins embedded in the lipid bilayer Integral Proteins – proteins that completely span the membrane (transmembrane proteins) Peripheral Proteins – are not embedded at all, but are loosely bound to the surface

Membrane Protein Functions Transport Enzymatic activity Signal transduction Intercellular joining Cell-cell recognition (membrane carbohydrates do this as well) Attachment to the cytoskeleton and extracellular matrix

Fluid Mosaic Model

Synthesis & Sidedness of Membranes Membranes have distinct inside and outside faces Synthesis of membrane components: 1.Proteins and lipids are made in the ER 2.In the Golgi apparatus, the glycoproteins are further modified and lipids acquire carbohydrates to become glycolipids 3.The products are transported in vesicles to the plasma membrane 4.The vesicles fuse with the membrane, releasing the contents

Permeability of the Membrane Selectively permeable – will only allow certain things across Hydrophobic molecules cross easily Ions and polar molecules cannot cross without help Small molecules may be able to pass easily Transport proteins – help substances move across the membrane Channel proteins (ex. aquaporins) Carrier proteins

Passive Transport (Diffusion) Does not require an input of energy Diffusion – movement of a substance from an area of higher concentration to an area of lower concentration Substances diffuse down the concentration gradient Diffusion will continue until equilibrium is reached

Diffusion

Passive Transport (Osmosis) Osmosis is the diffusion of water Solute concentration determines which direction the water will move Hypertonic – higher concentration of solute Hypotonic – lower concentration of solute Isotonic – equal concentration of solute

Water Balance

Osmoregulation – the control of water balance Ex. Contractile vacuole in a Paramecium Turgid – very firm (healthy state for plants) Flaccid – limp (plant is wilted) Plasmolysis – the plasma membrane pulls away from the cell wall (usually lethal)

Facilitated Diffusion Transport proteins help move substances (such as polar molecules and ions) across the membrane Still moves from higher to lower The transport protein is specific to the solute it transports Some are channel proteins such as aquaporins and gated channels Some are carrier proteins (ex. glucose transporter)

Facilitated Diffusion

Active Transport Requires an input of energy (ATP) Moves substance from an area of lower concentration to an area of higher concentration Moves substances up the concentration gradient Specific proteins are involved

Sodium-Potassium Pump

Transport of Large Molecules Through the use of vesicles Exocytosis – movement out of the cell (“exits”) Endocytosis – movement into the cell (“enters”) Phagocytosis – “Cell eating” Pinocytosis – “Cell drinking” Receptor-mediated – endocytosis of a specific substance

Exocytosis and Endocytosis