Membrane Structure and Function

Chapter 7

ObjectivesDescribe the fluid mosaic model of cell

membranes and the roles of proteins in the membranes

Define the terms diffusion, osmosis, facilitated diffusion, active transport, exocytosis, endocytosis, isotonic, hypotonic and hypertonic

Membrane Function

Organizes chemical activities of cell separates cells from outside environment controls passage of molecules across

membranes partitions organelle function in eukaryotes provides reaction surfaces and organizes

enzymes and their substrates

Membrane Structure

Phospholipids of membranes form bilayers phospholipids have polar “head” and nonpolar

“tail” form stable bilayer in water with heads out and

tails in hydrophobic interior forms barrier to

hydrophilic molecules

Membrane Structure

Membrane is fluid mosaic of lipids and proteins proteins embedded in phospholipid bilayer

individual molecules free to move laterally

some proteins extend through both sides of bilayer

cholesterol helps stabilize animal cell membranes at different temperatures

lipids in membrane are not fixed lipids can move in membrane - semi-fluid nature of

membrane two major classes of proteins in membrane

integral – transmembrane

peripheral - loosely associated with membrane surface

Amphipathic: ___________________

membrane also shows “sidedness” interior - attachment to cytoskeletonexterior - carbohydrates, extracellular matrix

An integral protein could be a transporter

A. True

B. False

Membrane Function

Proteins make membrane a mosaic of function identification tags-glycoproteins enzymes receptors-trigger cell activity when molecular messenger

binds cell junctions transporters

Membrane Function

Diffusion across a membrane diffusion is tendency of molecules to spread out

spontaneously from area of high concentration to area of low concentration

passive diffusion across membranes occurs when molecules diffuse down concentration gradient at equilibrium molecules diffuse back and forth-no net gain

or loss

Different molecules diffuse independently of each other

Ex. Water, Sodium, Chloride

Membrane Function

Osmosis is diffusion of water (passive) if cell membrane permeable to water but not solute

separates area of high solute concentration (hypertonic) from area of low concentration (hypotonic), water diffuses from hypotonic area to hypertonic area until concentrations are equal

direction of osmosis is determined by differences in relative concentrations

Gradient = concentration difference

Isotonic

Isotonic = solutions of equal solute concentration (no osmosis) are isotonic

Hypotonic = more solvent relative to solute

Hypertonic = more solute relative to solvent

Membrane Function

Water balance between cells and surroundings critical cell membranes semi-permeable

cells in isotonic solution do not change size -no osmosis

cells in hypotonic solution gain water

cells in hypertonic solutions lose water

The cell contains 80% water, the beaker 10% salt. What will happen to the cell?

A cell that contains 22% solute is in a beaker that contains 22% solvent. Which of the following is correct?

A. The cell is hypotonic to the solution in the beaker

B. The cell is hypertonic to the cell in the beaker

C. The cell is isotonic to the solution in the beaker

Membrane Function

Specific proteins facilitate diffusion across membranes facilitated diffusion occurs when protein

pore in membrane allows solute to diffuse down concentration gradient

no energy required rate depends on number of transport

proteins and strength of gradient

Membrane Function

Cells expend energy for active transport

transport protein involved in moving solute against concentration gradient

energy from ATP-mediated phosphorylation changes protein shape and moves solute molecule across membrane

active transport of two solutes in opposite directions often coupled, but not always

Transporters have complex subcategories

Uniports- One Substance in One Direction

Symport- Two Substances in Same Direction

Antiports- Two Substances in Opposite Directions

Aquaporins – Rapid water transport channels

Electrogenic Pumps

Sometimes the transporters pump ions that cause an electrical gradient to from across the membrane

An example: This occurs in your nerve cells

Electrogenic pumps, like the Na+-K+ pump, generate voltage across membranes.

The resulting voltage, or membrane potential, is energy that can be used to drive the transport of ions against a chemical gradient

Co-transport

Membrane proteins co-transport two solutes by coupling the downhill diffusion of one solute with the uphill diffusion of the other

Membrane Function

Exocytosis and endocytosis transport large molecules

exocytosis: membrane-bound vesicles containing large molecules fuse with plasma membrane and release contents outside cell

endocytosis: plasma membrane surrounds materials outside cell, closes around materials, and forms membrane-bound vesicles

Types of Endocytosis

Three important types of endocytosis are:phagocytosispinocytosisreceptor-mediated endocytosis

Signal Transduction Across Membranes

Proteins embedded in the cell membrane can carry messages and cause a chain reaction of signals that “tell” you cells its time to do an activity or make a chemical in short supply

You will learn more about this in other classes