Cell Ultrastructure
Membranes
Plasma membrane
Outermost layer of cytoplasm
Serves as a barrier to prevent cell
contains from mixing with the
surroundings
Allows molecules to enter and leave
Contains protein sensors that allow cell to
interact with environment
Allows cell growth and movement
Many other functions as well
Some Membrane Functions
Membrane Distribution
Prokaryotes have a single membrane - the
plasma membrane
Eukaryotes
Plasma membrane
Many internal membranes that enclose
organelles
Prokaryotes and Eukaryotes
Membranes in a Eukaryotic Cell
Membrane composition
1890s - thought membranes largely lipid
1930s - membranes determined to be
lipid AND protein
Various models developed over the years
Today biologists accept the fluid mosaic
model of membrane structure
lipid bilayer with scattered proteins
Fluid Mosaic Model
Membrane Lipids
Phospholipids
Lipid Bilayer
In water a phospholipid bilayer will form
a closed spherical vesicle
Lipid Bilayer is Fluid
Lipid molecules in
each monolayer can
rotate
move laterally change position with
neighbor
Fluidity due to
unsaturated FA chains
and cholesterol
Flip-flop does not
occur
Cholesterol
Cholesterol and unsaturated FA
prevent packing of phospholipids
Components of Model
Protein Properties
Integral Proteins
Integral Proteins
have a hydrophobic
region
Nonpolar side chains
immersed in lipid
bilayer
Various Protein Shapes
Examples of Plasma Membrane Proteins
and Their Functions
Functional Class
Transporters
Protein Example
Na+ pump
Linkers
Integrins
Receptors
Platelet-derived
growth factor
(PDGF) receptor
Enzymes
Adenylate cyclase
Specific Function
Actively pumps Na+ out of cells and
K+ in
Link intracellular actin filaments to
extracellular matrix proteins
Binds extracellular PDGF and, as a
consequence, generates intracellular
signals that cause the cell to grow and
divide
Catalyzes the production of
intracellular cyclic AMP in response to
extracellular signals
Membrane Proteins
Membrane Proteins
Glycocalyx
Membrane Asymmetry
Cell fusion experiment proves
membrane is fluid
Membranes Are Selective
Barriers
Membranes have a variety of functions
but the most basic function is that of a
permeability barrier
Membranes serve as selective barriers
that regulate the passage of materials into
and out of cells (or organelles)
Any substance that enters or leaves a cell
must pass through the membrane
Membrane Barriers
For a material to cross a membrane
The membrane must be permeable to that
substance
OR the material is transported across the
membrane
Membrane Permeability
If a membrane is permeable to a
substance that means that the substance
is able to diffuse across or through the
membrane
Kinetic Energy
All particles (all matter in the universe)
possess a certain amount of kinetic
energy - results in random motion of
particles (Brownian Movement)
Largely as a result of temperature (temp
is a measure of average kinetic energy)
Because particles are in constant motion,
there are several effects - diffusion is one
Diffusion
Net movement of particles from a region
of higher concentration of the substance
to a region of lower concentration
Net movement because there is always
some in all directions because of the
random movement
Particles of a substance tend to become
distributed within the available space
Diffusion
Diffusion is fastest in gases
Slower in liquids
Infinitely slow in solids
Net effect is always from a region of high
concentration to lower concentration as
long as temperature and pressure are
equal
Or from a region of high free energy to lower
free energy
Diffusion
Diffusion and Osmosis
A
B
A
B
Membrane is permeable to water but not to glucose molecules
Osmosis is the diffusion of water across a differentially
permeable membrane
Measuring Osmotic Pressure
Osmosis in Animal Cells
Area outside cell is Hypotonic or Hypo-osmotic
Osmosis in Animal Cells
Equal concentrations of solutes
inside and outside
Concentration of solutes lower inside the
cell - water will leave
Isotonic or Iso-osmotic
concentrations
Area outside is hypertonic or
hyperosmotic - Plasmolyzed
Plant Cells
Iso-osmotic
Hypo-osmotic
Hyperosmotic