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