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