Membrane Structure & Function

Terms Selective Permeability Fluidity of membranes

Membrane Proteins Determine most of the membrane’s specific functions Two types: – Integral Proteins: often transmembrane; hydrophilic areas in a hydrophobic membrane – Peripheral Proteins: not embedded but are appendages bound to membrane

Functions of Membrane Proteins Transport – some are a selectively permeable channel; some work as a pump that changes shape to transport with the help of ATP Enzymatic Activity – active site of the enzyme exposed to cytoplasm Signal Transduction – chemical signal attaches to protein, which changes shape to communicate inside the cell

Functions of Membrane Proteins (2) Cell-cell Reception: recognize another cell’s glycoproteins in their ECM to communicate messages Intercellular Joining: form junctions with other cells Attachment to Cytoskeleton and ECM

Glycoproteins Carbohydrate chains attached to membrane proteins as identity tags – Blood types are determined by glycoproteins on RBC’s

Membrane Synthesis By vesicles from the Endomembrane System

Transport Proteins More specified type of membrane protein Transport hydrophilic substances which could not naturally pass through Types: – Channel proteins – hydrophilic channel used as a tunnel – Carrier proteins – lock and change shape around a substance to carry it across membrane Aquaporins – channel proteins that facilitate passage of water molecules Two types of transport: Passive and Active

Passive Transport Diffusion of a substance across a membrane – Cell doesn’t have to expend energy Diffusion – tendency for molecules of any substance to spread out evenly into the available space. – Each molecules moves randomly, but diffusion of a population of molecules could be directional – Move from more populated area to less populated This movement is called a concentration gradient – Each molecule has its own concentration gradient

Effect of Osmosis on Water Balance

Cells Without Walls (Animal) Solute concentration and membrane permeability must be considered Tonicity – the ability of a solution to cause a cell to gain or lose water – Depends on how many solutes cannot cross the membrane – Water will go where the higher nonpermeable solute is located Hypertonic Hypotonic

Cells with Walls Walls help maintain water balance Cells will still swell with water, but only to a point, then a cell becomes turgid – pressure opposing further water intake If there is no swelling of the cell, it becomes flaccid – no tendency for water to enter

Facilitated Diffusion Passive transport aided by proteins

Active Transport Molecular movement across a membrane that requires a pump doing work ATP releases phosphate, which attaches to transport protein, causing it to change shape to move the molecule across the membrane – Ex: Sodium-Potassium Pump

Sodium-Potassium Pump Necessary because inside of a cell is negative compared to the outside, so membrane potential (voltage across a membrane) favors cations moving into the cell and anions moving out. Pumps 3 Na as it pumps 2 K, giving one “extra” positive voltage built up, which can be later used for energy

Cotransport