Membrane
Structure and
Function
1
Plasma Membrane
Is the boundary that
separates the living cell
from its nonliving
surroundings
Selectively Permeable (it
chooses what may cross
the membrane)
Lipid bilayer (2 layers of
phospholipids)
Contains embedded
proteins
Phospholipids
Are the most abundant lipid in the
plasma membrane
Are amphipathic, containing both
hydrophilic (head) and hydrophobic
regions (tails)
Head composed of phosphate group
attached to one carbon of glycerol
is hydrophilic
Two fatty acid tails are hydrophobic
Phospholipid Bilayer
WATER
Hydrophilic
head
Hydrophobic
tail
WATER
Hydrophilic region
of protein
Phospholipid
bilayer
Hydrophobic region of protein
Fluid Mosaic Model
http://www.youtube.com/watch?NR=1&feat
ure=fvwp&v=Rl5EmUQdkuI
A membrane is a fluid structure
with a “mosaic” of various proteins
embedded in it when viewed from
the top
Phospholipids can move laterally a
small amount and can “flex” their
tails
Membrane proteins also move
laterally making the membrane
fluid
The Fluidity of Membranes
Phospholipids in the plasma membrane
Can move within the bilayer two ways
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
The Fluidity of Membranes
The type of hydrocarbon tails in
phospholipids Affects the fluidity of
the plasma membrane
Fluid
Unsaturated hydrocarbon
tails with kinks
Viscous
Saturated hydroCarbon tails
7.5
The Fluidity of Membranes
The steroid cholesterol has different
effects on membrane fluidity
http://www.youtube.com/watch?v=0ekfkxwl
5bQ
Cholesterol
Membrane Proteins and Their Functions
A membrane is a collage of different
proteins embedded in the fluid matrix
of the lipid bilayer
Fibers of
extracellular
matrix (ECM)
Types of Membrane Proteins
Integral proteins
Penetrate the hydrophobic core of the lipid
bilayer
Are often transmembrane proteins,
completely spanning the membrane
EXTRACELLULAR
SIDE
Types of Membrane Proteins
Peripheral proteins
Are appendages loosely bound to the
surface of the membrane
Through the Cell Membrane
A cell must exchange materials
with its surroundings, a process
controlled by the plasma
membrane
Permeability of the Lipid Bilayer
Hydrophobic molecules
can pass through
the membrane
rapidly
Phospholipid
bilayer
Hydrophilic region
of protein
Hydrophillic molecules
Do NOT cross the Hydrophobic region of protein
membrane rapidly
and therefore
need help from
transport proteins
Transport Proteins
Transport proteins
Allow passage of hydrophilic
substances across the
membrane
Passive transport. Substances move spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
ATP
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse through the
lipid bilayer.
Facilitated diffusion. Many hydrophilic
substances diffuse through membranes with the
assistance of transport proteins,
either channel or carrier proteins.
1. Passive Transport
Passive transport is diffusion of a
substance across a membrane
with no energy investment
CO2, H2O, and O2 easily diffuse
across plasma membranes
Diffusion of water is known as
Osmosis
Simple Diffusion
http://www.youtube.com/watch?v=sdiJtDRJ
QEc
Diffusion
Is the tendency for molecules of any substance to
spread out evenly into the available space
Move from high to low concentration`
Down the concentration gradient
Effects of Osmosis on Water Balance
Osmosis
Is the movement of water
across a semipermeable
membrane
Is affected by the
concentration gradient of
dissolved substances
Isotonic Solutions
If a solution is isotonic
The concentration of solutes is the
same as it is inside the cell
There will be NO NET movement of
WATER
Hypertonic Solution
If a solution is hypertonic
The concentration of solutes is greater
outside the cell than it is inside the
cell
The cell will lose water
Hypotonic Solutions
If a solution is hypotonic
The concentration of solutes outside
the cell is less than it is inside the
cell
The cell will gain water
Water Balance in Cells Without
Walls
Animal cell. An animal cell fares best in an isotonic
environment unless it has special adaptations to offset
the osmotic uptake or loss of water.
How Will Water Move Across
Semi-Permeable Membrane?
Solution A has 100 molecules of glucose
per ml
Solution B has 100 molecules of
fructose per ml
How will the water molecules move?
There will be no net movement of water since the
concentration of solute in each solution is equal
How Will Water Move Across
Semi-Permeable Membrane?
Solution A has 100 molecules of
glucose per ml
Solution B has 75 molecules of
fructose per ml
How will the water molecules move?
There will be a net movement of water from
Solution B to Solution A until both solutions have
equal concentrations of solute
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
ATP
Diffusion. Hydrophobic
Facilitated diffusion. Many hydrophilic
molecules and (at a slow
substances diffuse through membranes with the
rate) very small uncharged
assistance of transport proteins,
polar molecules can diffuse through the lipid
either channel or carrier proteins.
bilayer.
Facilitated Diffusion
Facilitated diffusion
Is a type of Passive Transport Aided
by Proteins
In facilitated diffusion
Transport proteins speed the
movement of molecules across the
plasma membrane
Facilitated Diffusion & Proteins
http://www.youtube.com/watch?v=GFCcnx
gXOhY
Channel proteins
Provide corridors that allow a specific
molecule or ion to cross the membrane
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.
Facilitated Diffusion & Proteins
Carrier proteins
Undergo a subtle change in shape that
translocates the solute-binding site
across the membrane
A carrier protein alternates between two conformations, moving a solute
across the membrane as the shape of the protein changes. The protein
can transport the solute in either direction, with the net movement being
down the concentration gradient of the solute.
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
ATP
Diffusion. Hydrophobic
Facilitated diffusion. Many hydrophilic
molecules and (at a slow
substances diffuse through membranes with the
rate) very small uncharged
assistance of transport proteins,
polar molecules can diffuse through the lipid
either channel or carrier proteins.
bilayer.
2. Active Transport
Active transport
Uses energy to move solutes against
their concentration gradients
Requires energy, usually in the form
of ATP (Adenosine Triphosphate)
Proton Pump
http://www.youtube.com/watch?v=STzOiR
qzzL4
–
EXTRACELLULAR
FLUID
+
–
ATP
+
H+
H+
Proton pump
H+
–
+
H+
H+
+
–
CYTOPLASM
–
+
+
H+
Comparison of Passive & Active Transport
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
ATP
Diffusion. Hydrophobic
Facilitated diffusion. Many hydrophilic
molecules and (at a slow
substances diffuse through membranes with the
rate) very small uncharged
assistance of transport proteins,
polar molecules can diffuse through the lipid
either channel or carrier proteins.
bilayer.
Bulk Transport
Bulk transport across the plasma
membrane occurs by exocytosis
and endocytosis of large
molecules (e.g. proteins and
polysaccharides)
Exocytosis & Endocytosis
http://www.youtube.com/watch?v=K7yku3s
a4Y8
In exocytosis
Transport vesicles migrate to
the plasma membrane, fuse
with it, and release their
contents
In endocytosis
The cell takes in macromolecules
by forming new vesicles from
the plasma membrane
Endocytosis
Exocytosis