Transcript Ch. 8 Honors PP

CHAPTER 8
MEMBRANE STRUCTURE AND
FUNCTION
STRUCTURE OF
MEMBRANES
Ingredients of cell membranes are lipids
and proteins (some carbohydrates also)
PHOSPHOLIPIDS
- Are AMPHIPATHIC- having both a
hydrophobic and hydrophilic region
- Proteins are also amphipathic
HOW ARE LIPIDS AND PROTEINS
ARRANGED?
FLUID MOSAIC MODEL
- Membrane is a fluid structure with
proteins embedded in or attached to a
double layer of phospholipids
- This model was developed in 1972 by
Singer and Nicolson (contributions by
many other scientists)
PROOF FOR FLUID
MOSAIC?
FREEZE FRACTURE- method of preparing
cells for electron microscopy
- Cell membrane is split along the middle
of the phospholipid bilayer
- Interior of the bilayer appears
cobblestoned, with proteins interspersed
in a smooth matrix
MODELS ARE CONSTANTLY BEING
REVISED
MEMBRANES ARE FLUID
Membranes are held together by weak
hydrophobic interactions
- Lipids and some proteins can drift
laterally
- They do not usually flip from one side of
the membrane to the other
- Membranes remain fluid as temperature
decreases, until a certain temperature is
reached
Temperature at which a membrane
solidifies depends on the types of lipids
it is made of:
- Membranes with unsaturated tails will
remain liquid at lower temperatures
- The kinks in the tails (double bonds) prevent
the lipids from packing close together
- Cholesterol can make membranes less fluid
by restraining the movement of
phospholipids, but also lowers the
temperature required to solidify because
phospholipids cannot pack together
PROTEINS AND
CARBOHYDRATES
2 types of proteins:
INTEGRAL- penetrate the hydrophobic
core of the lipid bilayer
- Many are transmembrane proteins that
completely span the membrane
PERIPHERAL- not embedded in the lipid
bilayer at all; loosely bound to the surface
of the membrane
- Some proteins are attached to the
cytoskeleton and the extracellular matrix to
give the cell a stronger framework
- Carbohydrates on the surface of the cell
membrane function in cell-to-cell
recognition
- Some are bonded to lipids (glycolipids) but
most are bonded to proteins
(glycoproteins)
- Surface carbohydrates vary from cell to
cell
PERMEABILITY OF
MEMBRANES
PERMEABILITY OF THE BILAYER
ITSELF AFFECTS THE SELECTIVE
PERMEABILITY OF THE MEMBRANE:
- Hydrophilic molecules such as ions and
polar molecules have difficulty crossing
the hydrophobic core of the membrane
THE PRESENCE OF TRANSPORT
PROTEINS AFFECTS THE SELECTIVE
PERMEABILITY OF THE CELL
MEMBRANE:
- Ions and polar molecules avoid the lipid
bilayer by passing through transport
proteins
- Some proteins have channels that act
like tunnels for hydrophilic molecules to
pass through
- Some transport proteins hold onto their
passengers and physically move them
across the membrane
WHAT DETERMINES THE DIRECTION
OF TRAFFIC ACROSS A MEMBRANE?
PASSIVE TRANSPORT
DIFFUSION- tendency for molecules of
any substance to spread out into the
available space
- Each molecule moves randomly, but a
population of molecules may move in a
certain direction
- A SUBSTANCE WILL DIFFUSE FROM
WHERE IT IS MORE CONCENTRATED
TO WHERE IT IS LESS
CONCENTRATED
- Any substance will diffuse down its
CONCENTRATION GRADIENT
- Diffusion is spontaneous because it
decreases free energy
- Diffusion is PASSIVE TRANSPORT
because no energy must be used in
order for it to happen
- The concentration gradient represents
potential energy and drives diffusion
In comparing 2 solutions of unequal solute
concentrations:
- The solution with the higher
concentration of solutes is HYPERTONIC
- The solution with the lower concentration
of solutes is HYPOTONIC
- Solutions with equal concentrations of
solute are ISOTONIC
OSMOSIS- diffusion of water across a
selectively permeable membrane
- Water moves from a hypotonic solution to
a hypertonic solution (in hypotonic
solution there is a greater concentration
of WATER)
DRAW A DIAGRAM DEMONSTRATING
OSMOSIS
WATER BALANCE IN
LIVING CELLS
WATER BALANCE IN CELLS WITHOUT
CELL WALLS (ANIMAL CELLS):
- In an isotonic environment, water moves
into and out of the cell at equal rates
- In a hypertonic environment, water will
leave the cell, the cell will shrivel and die
- In a hypotonic environment, water will
enter the cell and the cell will burst
Most cells live in isotonic environments so
water balance is maintained
- Animal cells living in hypertonic or
hypotonic environments must have
adaptations for OSMOREGULATION, the
control of water balance
Ex: Paramecium has a contractile vacuole
that pumps excess water out of the cell
WATER BALANCE IN CELLS WITH CELL
WALLS (PLANTS, PROKARYOTES,
FUNGI):
- As water enters these cells, the cells
swell but will not burst
- The elastic cell wall will expand only so
much before it exerts a back pressure
that opposes uptake of more water
- At this point the cell is TURGID or firm, a
healthy state for most plant cells
- If plant cells are in an isotonic solution,
water will not enter and cells become
FLACCID (limp) and the plant wilts
- If plant cells are in a hypertonic
environment water will leave the cell and
PLASMOLYSIS occurs; the cells usually
die
PASSIVE TRANSPORT
CONT.
FACILITATED DIFFUSION- diffusion with
the help of transport proteins
TRANSPORT PROTEINS BEHAVE LIKE
ENZYMES:
- Transport proteins are specific for the
solutes they transport and may have a
binding site similar to the active site of an
enzyme
- Transport proteins may become
saturated
- Transport proteins may be inhibited by
molecules that resemble the normal
solute
TRANSPORT PROTEINS
FACILITATE DIFFUSION
- Many transport proteins provide corridors
for molecules or ions to cross the
membrane- CHANNEL PROTEINS
- Some function as GATED CHANNELSsome stimulus causes them to open
- Some transport proteins undergo a
change in shape that moves the solutebinding site across the membrane
ACTIVE TRANSPORT
Some transport proteins can move solutes
against their concentration gradients from
less concentration to greater
concentration
ACTIVE TRANSPORT- movement against
the concentration gradient requiring
energy from the cell
Ex: Sodium-potassium pump- Na+ pumped
out, K+ pumped in
ION PUMPS GENERATE
VOLTAGE
All cells have voltages across their plasma
membranes
- Voltage is electrical potential energy, a
separation of opposite charges
- Voltage across a membrane is called
MEMBRANE POTENTIAL (inside of the cell
is negative compared to outside)
- ELECTROCHEMICAL GRADIENTcombination of forces acting on an ion
(concentration gradient and membrane
potential)
- The sodium-potassium pump does not
translocate Na+ and K+ one for one, but
pumps 3 sodium ions out for every 2
potassium ions it pumps in
- There is a net transfer of 1 positive
charge from inside the cell to the outside
of the cell with each crank of the pump
- This stores energy in the form of voltage
ELECTROGENIC PUMP- a transport
protein that generates voltage across a
membrane
- The sodium-potassium pump is the main
electrogenic pump of animal cells
- The PROTON PUMP is the main
electrogenic pump of plants, bacteria,
and fungi (H+ ions are pumped out of the
cell transferring positive charge to the
outside of the cell)
COTRANSPORT
In COTRANSPORT an ATP-driven pump
stores energy by concentrating a
substance on one side of the membrane
- As the substance leaks back across the
membrane through specific transport
proteins, it escorts other substances into
the cell
WATER THAT HAS BEEN PUMPED
UPHILL PERFORMS WORK AS IT
FLOWS BACK DOWN
TRANSPORT OF LARGE
MOLECULES
EXOCYTOSIS- secretion of large
molecules by the fusion of vesicles with
the plasma membrane
- A transport vesicle from the Golgi comes
into contact with the plasma membrane
- The layers of the bilayer rearrange
themselves so that the vesicle membrane
and the plasma membrane fuse
- The contents of the vesicle spill outside
the cell
ENDOCYTOSIS- cell takes in large
molecules by forming new vesicles from
the plasma membrane
- A small area of the plasma membrane
sinks inward to form a pocket
- The pocket pinches in, forming a vesicle
that contains material from outside the
cell
3 TYPES OF
ENDOCYTOSIS
SEE DIAGRAMS
1. PHAGOCYTOSIS- cell eating
2. PINOCYTOSIS- cell drinking
3. RECEPTOR-MEDIATED
ENDOCYTOSIS- very specific
- receptor sites on the surface of the
cell bind ligands, and coated pits form
vesicles (lined with proteins)