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Membrane Structure and Function
Chapter 7

Objectives
 Describe
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
 some
molecules free to move laterally
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 cytoskeleton
 exterior - carbohydrates, extracellular matrix
An integral protein could be a transporter
A.
B.
True
False
Membrane Function

Proteins make membrane a mosaic of function
 identification
tags-glycoproteins
 enzymes
 receptors-trigger
binds
 cell junctions
 transporters
cell activity when molecular messenger
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.
B.
C.
The cell is hypotonic to the solution in the
beaker
The cell is hypertonic to the cell in the
beaker
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:
phagocytosis
pinocytosis
receptor-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