Transcript Membrane Structure and Function Chapter 7 TEM of Phospholipid Bilayer Membrane Structure • Basic fabric of membranes is a phospholipid bi-layer • Phospholipids are amphipathic, so.

Membrane Structure and
Function
Chapter 7
TEM of Phospholipid Bilayer
Membrane Structure
• Basic fabric of membranes
is a phospholipid bi-layer
• Phospholipids are
amphipathic, so the center
of the bi-layer is
hydrophobic and the
outsides are hydrophilic
• Proteins are found in the
layer – the hydrophobic
region of proteins are found
in the center of the bi-layer,
with the hydrophilic regions
protruding on both sides
• Proteins may be integral or
peripheral
Hydrophobic region of protein
Hydrophilic regions of protein
The fluid mosaic model
Membranes have
the consistency of
cooking oil!
(Protein + Oligosaccharide = Glycoprotein)
Cholesterol
(Oligosaccharide added in the Golgi body)
(Part of cytoskeleton)
The fluidity of membranes
• The phospholipids of
membranes are
constantly drifting moving laterally
• Sometimes the
phospholipids flip-flop
• The embedded proteins
or surface proteins also
drift
• Some proteins are held
in place by the
cytoskeleton
The fluidity of membranes, cont’d.
• Membranes remain fluid when temperature decreases - up to a certain
critical temperature, after which they solidify
• The more the concentration of unsaturated hydrocarbons in the
phospholipid tails, the longer the membrane stays fluid (Because of
kinks in the tails, they cannot pack closely)
• Cholesterol is a common component of animal membranes – it keeps
the membrane fluid at low temperatures, but reduces fluidity at
moderate temperatures.
Evidence of membrane protein drift
When mouse and human cells were fused, their phospholipid bi-layers,
along with their membrane proteins intermingled within one hour –
creating a chimeric plasma membrane.
Membrane Proteins
• Integral proteins are either completely embedded
(transmembrane), or partially embedded in the bilayer
• Peripheral proteins are not embedded in the membrane,
they are attached to the surface of the bilayer or to integral
proteins
A transmembrane protein
Functions of the membrane proteins
Types of membrane proteins – and their roles
Passive transport vs. Active transport
For example: Enzymes embedded in the inner membrane of mitochondria
play a role in cellular respiration
For example: insulin binding to membrane proteins, which starts a signaling
pathway that stimulates cells to take up more glucose from the bloodstream
Types of membrane proteins – and their roles, cont’d.
For example: cells of the immune system need to bind to glycoproteins
on cell surfaces, in order to decide if the cell belongs to the body or is
foreign
Integrins are an example of cell surface receptor proteins that adhere to and
interact with the ECM. Integrins also coordinate activities inside and outside
cells via signal transduction.
Traffic Across Membranes
• The phospholipid bilayer is selectively permeable – it
allows only certain substances across – depending on
SIZE and/or POLARITY
• Because it is hydrophobic in the center, it does not
allow ions and polar molecules across – even small
ions like H+, Na+ or OH- cannot cross membranes
• For the same reason, it does allow nonpolar
molecules like O2, CO2 (Diffusion and osmosis)
• Large molecules whether polar or nonpolar cannot
cross over (most sugars, proteins, amino acids, lipids,
etc.)
• Membrane proteins help transport molecules that
cannot cross the bilayer on their own
Selective
permeability
Electrostatic Gradient
• The interior of cells is negatively charged compared to
the outside
• This creates a voltage across the membrane, which is
called the membrane potential
• For this reason, anions will automatically move outside
the cell (drawn by the + charges) and cations will be
drawn to the inside (drawn by the neg- charges) – ions
however, need to pass through membrane proteins.
• This difference in charge is called the electrostatic
gradient
• The membrane potential of a resting cell is about
-70 mV (It can range from -50 to -200 mV)
Electrostatic Gradient, cont’d.
Concentration Gradient
• Molecules introduced to
a new environment, will
move away from their
initial location, creating a
concentration gradient –
their concentration
becomes exceedingly
lower as you move away
from the introduction site
Electrostatic gradient + Chemical gradient =
Electrochemical gradient
PASSIVE TRANSPORT
• Passive transport is the movement of
molecules down their electrochemical
gradient
• Passive transport requires no energy
expenditure on the part of the cell. “Free”
energy is used – the energy of the system
• Examples of passive transport:
– Diffusion
– Osmosis
– Facilitated diffusion (Protein channels
involved)
Diffusion
• Molecules have the natural tendency (due to random
molecular motion) of moving from an area where they
are highly concentrated, to an area where their
concentration is low – they move down their
concentration gradient+
•Once the molecules
are evenly dispersed
in the environment,
they reach a state of
equilibrium – they
continue to move, but
it is equal in every
direction – so no net
change
Low free energy –
stable system
High free energy
Diffusion
• Diffusion is passive transport
• It is the random movement of molecules
from and area of high concentration to an
area of low concentration
• Diffusion requires NO energy
• In diffusion, molecules move along their
concentration gradient
Osmosis
• Osmosis is passive transport
• It is the random movement of WATER
molecules from an area of high water
concentration to an area of low water
concentration
• Osmosis requires NO energy
• In osmosis, molecules move along their
concentration gradient
Osmosis
• The diffusion of water molecules
• The tendency of water molecules (due to random
molecular motion) to move from an area where
their concentration is high (higher free energy), to
an area where their concentration is lower (lower
free energy) – until equilibrium is reached (no net
movement of water)
• Movement of water molecules is down their
concentration gradient
Low solute
High solute
Isotonic Solution
solute and solvent balanced
(Also a form of Passive Transport)
(of water molecules)
As solute concentration
increases, “free” water
concentration decreases
– so water potential
decreases
Water then moves from an area
of high water potential to an
area of low water potential
 Inside the cell is lower,
because of solutes in the
cytosol
Water molecules always move
from an area of higher water
potential to an area of lower
water potential, so water rushes
into the “cell” from the outside
(Net movement is inwards)
 Inside the cell is higher than
the outside, because the outside
has more solute particles
Water will therefore move out of
the cell to an area of lower  (Net
movement is outwards)
 Is equal on both sides,
so no net movement
Is the “cell” hypertonic, hypotonic or isotonic with respect to its environment?
Water Potential
Osmosis & Plant cells
Plants & water potential
• Plants can use the
potential energy in
water to perform
work.
• Tomato plant
regains turgor
pressure – cell
pushes against wall
due to uptake of
water
Plants & water potential
• The combined effects of
1.) solute concentration
2.) physical pressure (cell wall)
can be measured as Water Potential 
• = psi
• is measured in megapascals (MPa)
• 1 Mpa = 10 atmospheres of pressure
Calculating Water Potential

•
=
P
+
S
Or
Water =
Potential
pressure +
potential
solute
potential
Solute Potential  S
• Solute potential is also called the osmotic
potential because solutes affect the direction of
osmosis.
•  S of any solution at atmospheric pressure is
always negative – why?
• Answer = less free water molecules to do work
Solute Potential  S
• Solutes bind water
molecules reducing
the number of free
water molecules 
lowers waters
ability to do work.
Pressure Potential P
•P is the physical pressure on a solution.
•
P can be negative  transpiration in the
xylem tissue of a plant (water tension)
•P can be positive  water in living plant
cells is under positive pressure (turgid)
Standard for measuring

• Pure water is the standard.
• Pure water in an open container has a
water potential of zero at one
atmosphere of pressure.
Water Potential: an artificial model
• (a) addition of
solutes on right side
reduces water
potential. S = -0.23
• Water flows from
“hypo” to “hyper”
• Or from hi  on left
to lo  on right
Water Potential: an artificial model
• (b) adding +0.23 pressure with plunger  no net
flow of water
• (c) applying +0.30 pressure increases water
potential solution now has  of +0.07
• Water moves right to left
Water Potential: an artificial model
• (d) negative
pressure or tension
using plunger
decreases water
potential on the left.
• Water moves from
right to left
Water relations in plant cells
• (b) Flaccid cell in pure water  Water
potential is into cell cell becomes turgid
Water relations in plant cells
• (a) Flaccid cell placed in hypertonic solution
Water potential is out of cell  plasmolysis
Calculating Solute potential
• Need solute concentration
• Use the equation
 S = - iCRT
i = # particles molecule makes in water
C = Molar concentration
R = pressure constant 0.0831 liter bar
mole oK
T = temperature in degrees Kelvin
= 273 + oC
Solve for water potential
(literal equation)
• Knowing solute potential, water
potential can be calculated by inserting
values into the water potential
equation.

=
P
+
S
In an open container, P = 0
Hints & reminders
1. Remember water always moves from [hi]
to [lo].
2. Water moves from hypo  hypertonic.
3. [Solute] is related to osmotic pressure.
Pressure is related to pressure potential.
4. Pressure raises water potential.
5. When working problems, use zero for
pressure potential in animal cells & open
beakers.
6. 1 bar of pressure = 1 atmosphere
Water and the Bilayer
• Although water is a polar molecule, some
water molecules ARE able to “sneak” past
the phospholipids via osmosis.
• But the majority of the water molecules are
prevented from passing the hydrophobic
tails of the lipid bilayer
• So water has to use Aquaporins, a special
class of integral transmembrane channel
proteins
Aquaporins
• More than 10 different mammalian
aquaporins have been identified to date,
and additional members are suspected to
exist.
• Some aquaporins transport solute-free
water across cell membranes; they appear
to be exclusive water channels and do not
permeate membranes to ions or other
small molecules.
• Other aquaporins transport water and
other small polar molecules and ions.
More about Auaporins
In the absence of aquaporins, cells do
not swell osmotically!
Plasmolysis
When a cell is placed in a hypertonic environment – more solute outside
than inside:
- Water potential is greater inside
- Water will move from where water potential is greater, to where it is
lower
- Water will move out of the cell, causing plasma membrane to collapse
(low pressure potential)
- Cell wall will keep cell from losing its shape – animal cell loses shape
Facilitated Diffusion
• Ions and small polar molecules use facilitated
diffusion
• Integral membrane channel proteins –
1. open channel – (water uses this method -aquaporins)
2. gated channel
3. carrier proteins – (glucose uses this method)
• Requires no cellular energy (ATP, GTP, etc.)
• Specific channel proteins for specific ions –
“lock-key” system
• Diffusion is down concentration gradient
Facilitated Diffusion
• Ions and small polar (Hydrophilic)
molecules use facilitated diffusion
• Membrane channel proteins are used
• Requires no cellular energy (ATP, GTP,
etc.)
• Diffusion is down concentration gradient
Facilitated
Diffusion, Cont’d.
Facilitated Diffusion, Cont’d.
ACTIVE TRANSPORT
•
•
•
•
Uses cellular energy (ATP, GTP, etc.)
Uses integral membrane proteins
Specific proteins for specific molecules
Molecules can be moved against their
electrochemical gradient
• Ion pumps – like the Na+ / K+ pump and the Proton pump (H+)
are an example of active transport
• Concentration of Na+ has to be higher outside
the cell whereas that of K+ has to be higher
inside the cell – so active transport is used to
maintain these concentrations (pumping against
electrochemical gradient)
ACTIVE TRANSPORT
•
•
•
•
Uses cellular energy (ATP, GTP, etc.)
Uses membrane proteins
Specific proteins for specific molecules
Molecules can be moved against their
concentration gradient – from a low
concentration to a high concentration.
Active Transport Cont’d.
1.
Na+ binds to the
transport protein
at specific
binding sites
3.
Phosphorylation
causes
conformational
change in protein,
which moves the
Na+ out of the
cell
2.
Na+ binding
causes ATP to
phosphorylate
protein
4.
6.
When K+ exits
its binding site, it
causes the
release of the
inorganic
phosphate group
When Na+ exits
the binding site,
the binding site
for K+ is made
accessible and
K+ binds to sites
5.
When K+ binds, it causes another
conformational change, which
moves K+ into cell
Active Transport Cont’d.
Proton Pumps helps move H+ against their gradient (out of cell) – this build-up of
H+ outside the cell is VERY important, because it is a high-energy/ unstable
system that can be used to energize other cellular processes
Endocytosis = Phagocytosis + Pinocytosis
Pinocytic vesicle forming
Endocytosis is
active transport –
needs energy
expenditure
A lymphocyte
attacking E.coli
TEM of lymphocyte – E.coli being ingested
SEM of stained prep.
THE END