Membrane Structure and
Function
Chapter 7
A. P. Biology
Mr. Knowles
Liberty Senior High School
Plasma Membrane
• Also called the plasmalemma.
• Plasma Membrane =
Phospholipid Bilayer +
Transmembrane Proteins +
Supporting Fibers +
Glycoproteins and Glycolipids
Scientists studying the plasma membrane
– Reasoned that it must be a phospholipid
bilayer
WATER
Hydrophilic
head
Hydrophobic
tail
Figure 7.2
WATER
Phospholipid Bilayer
• Glycerol + 2 Fatty Acids +
Phosphorylated Alcohol =
Phospholipid
• Hydrophilic or Polar Region =
Phosphate
• Hydrophobic or Nonpolar region =
Fatty Acids
The Davson-Danielli sandwich model
of membrane structure
–Stated that the membrane was made
up of a phospholipid bilayer
sandwiched between two protein
layers.
–Was supported by electron
microscope pictures of membranes
In 1972, Singer and Nicolson
– Proposed that membrane proteins are dispersed and
individually inserted into the phospholipid bilayer
Hydrophobic region
of protein
Phospholipid
bilayer
Figure 7.3
Hydrophobic region of protein
Freeze-fracture studies of the plasma membrane
– Supported the fluid mosaic model of membrane structure
APPLICATION
A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior.
TECHNIQUE
Extracellular
layer
A cell is frozen and fractured with a knife. The fracture plane often follows
the hydrophobic interior of a membrane, splitting the phospholipid bilayer
into two separated layers. The membrane proteins go wholly with one of
the layers.
Proteins
Knife
Plasma
membrane
RESULTS
These SEMs show membrane proteins (the “bumps”) in the two layers,
demonstrating that proteins are embedded in the phospholipid bilayer.
Extracellular layer
Figure 7.4
Cytoplasmic layer
Cytoplasmic layer
Lipid Bilayer
• Nonpolar interior prevents passage
of water-soluble, polar
compounds.
• Only very small, uncharged
molecules like O2 and H2O can
enter through the lipid bilayer.
• Also, allows nonpolar compounds
to freely enter.
The Fluidity of Membranes
• Phospholipids in the plasma membrane
– Can move within the bilayer
Lateral movement
(~107 times per second)
(a) Movement of phospholipids
Figure 7.5 A
Flip-flop
(~ once per month)
The type of hydrocarbon tails in phospholipids
– Affects the fluidity of the plasma membrane
Fluid
Unsaturated hydrocarbon
tails with kinks
(b) Membrane fluidity
Figure 7.5 B
Viscous
Saturated hydroCarbon tails
The steroid cholesterol
– Has different effects on membrane fluidity at different
temperatures
Cholesterol
Figure 7.5
(c) Cholesterol within the animal cell membrane
•
Membrane Proteins and Their
A membrane Functions
– Is a collage of different proteins embedded in
the fluid matrix of the lipid bilayer
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
Microfilaments
of cytoskeleton
Figure 7.7
Cholesterol
Peripheral
protein
Integral
protein
EXTRACELLULAR
SIDE OF
MEMBRANE
CYTOPLASMIC SIDE
OF MEMBRANE
Lipid Bilayer is Fluid
• Fluid = Moving, Dynamic.
• Each lipid can rotate, move laterally
• Fluidity depends on temperature and
type of fatty acid used.
• Unsaturated fatty acids are more
fluid.
• Fluid Mosaic Model
Transmembrane Proteins
(Integral Proteins)
• Part of the protein that extends through the
bilayer is nonpolar (several nonpolar amino
acids in this region).
• Usually is an alpha helix or beta barrel.
• Used to anchor protein in the membrane.
• Beta-barrels = form a pore and are called a
porin protein.
Integral proteins
– Penetrate the hydrophobic core of the lipid bilayer
– Are often transmembrane proteins, completely spanning
the membrane
Extracellular Side
EXTRACELLULAR
SIDE
N-terminus
C-terminus
Figure 7.8
a Helix
CYTOPLASMIC
SIDE
• An overview of six major functions of membrane
proteins
(a)
Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some
of these proteins hydrolyze ATP as an energy source
to actively pump substances across the membrane.
ATP
(b)
(c)
Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as
a team that carries out sequential steps of a
metabolic pathway.
Enzymes
Signal transduction. A membrane protein may have
a binding site with a specific shape that fits the shape
of a chemical messenger, such as a hormone. The
external messenger (signal) may cause a
conformational change in the protein (receptor) that
relays the message to the inside of the cell.
Figure 7.9
Signal
Receptor
(d)
Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Glycoprotein
(e)
(f)
Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions (see Figure 6.31).
Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes (see Figure 6.29).
Figure 7.9
Transmembrane Proteins
• Channels = passive transport of molecules across
membrane.
• Carriers = transport of molecules against the
gradient.
• Receptors = transmit information into the cell.
• Cell Adhesion Proteins = connect cells to each
other.
• Cytoskeleton Attachment Proteins = to attach
actin.
Membrane Receptors Conduct
Signals
Integral Proteins Laterally Diffuse in
the Membrane
• Proteins in the plasma membrane
– Can drift within the bilayer
EXPERIMENT
Researchers labeled the plasma membrane proteins of a mouse
cell and a human cell with two different markers and fused the cells. Using a microscope,
they observed the markers on the hybrid cell.
RESULTS
Membrane proteins
+
Mouse cell
Human cell
Hybrid cell
CONCLUSION
Figure 7.6
Mixed
proteins
after
1 hour
The mixing of the mouse and human membrane proteins
indicates that at least some membrane proteins move sideways within the plane
of the plasma membrane.
Frey and Edidin Experiment
How important are cell
receptors?
Video: Nova- Surviving
AIDS
Who cares?
How do we use this
stuff?
Movement Across the
Membrane
• Diffusion = random motion of
molecules that causes a net movement
from areas of high concentration to
areas of low concentration.
• Osmosis = diffusion of water across a
selectively permeable membrane.
Passive Diffusion
Factors that Affect the
Direction of Diffusion
• The concentration gradient;
High  Low.
• Temperature; High heat 
Low Heat.
• Pressure; High Pressure 
Low Pressure
Factors that Affect the
Rate of Diffusion
• The steepness of the
gradient.
• The molecular weight of
the solute.
Osmosis through a Selectively
Permeable Membrane
Concentrations
• Osmotic concentrations =
concentrations of all solutes in a solution.
• If unequal concentrations…
Hyperosmotic = solution with the higher
solute concentration.
Hypoosmotic = solution with the lower
solute concentration.
Isosmotic = solutions with the same
osmotic or solute concentration.
Crenate
Plasmolyzes
Osmotic Pressure
• If a cell’s cytoplasm is hyperosmotic
to the extracellular fluid, then water
diffuses into the cell and it swells.
Pressure of the cytoplasm pushing out
against the membrane- hydrostatic
pressure.
• Osmotic pressure is the pressure
needed to stop the osmotic movement
of water across a membrane.
Osmotic Pressure
How do living things maintain
osmotic balance?
• Some oceanic eukaryotes adjust
internal [solutes]- they are isosmotic.
• Animals – circulate an isosmotic
fluid around their cells. Must
constantly monitor the fluid’s
[solute] Ex. Humans secrete
albumin into the plasma to match the
body cells.
How do living things maintain
osmotic balance?
• Protozoa- are hyperosmotic, so use
extrusion to remove excess water; may
have special organelles-contractile
vacuoles.
• Plants- are hyperosmotic, but do not
circulate an isosmotic solution; are
usually under osmotic pressure- turgor
pressure-presses the plasma membrane
against the cell wall.
• Water balance in cells with walls
(b)
Plant cell. Plant cells
are turgid (firm) and
generally healthiest in
a hypotonic environment, where the
uptake of water is
eventually balanced
by the elastic wall
pushing back on the
cell.
H2O
Turgid (normal)
Figure 7.13
H2O
H2O
Flaccid
H2O
Plasmolyzed
Turgor Pressure Regulates a
Stoma (ta)
How do animals
osmoregulate?
It’s a story about shark
homeostasis!
Video: Attack of the Mystery
Shark
Outline
I. Passive Transport (high-->low)
A. Diffusion through Lipid Bilayer
1. Nonpolar, oxygen, water.
2. Hyper- Hypo-, IsoB. Bulk Transport
1. Endocytosis
a. phagocytosis
b. pinocytosis
c. receptor-mediated
2. Exocytosis
C. Channel Proteins
1. Ions
D. Carrier Proteins
1. Ions, sugars, amino acid
2. Facilitated diffusion
How do large, polar
molecules enter cells?
Bulk Movement through
Membranes
• Endocytosis- the cytoskeleton extends
the membrane outward toward food
particles. Bulk transport into cell.
• Extended membrane encircles the
particle, fuses with itself, and contracts.
• Forms a vesicle around particle.
Three Types of Endocytosis
In phagocytosis, a cell
engulfs a particle by
Wrapping pseudopodia
around it and packaging
it within a membraneenclosed sac large
enough to be classified
as a vacuole. The
particle is digested after
the vacuole fuses with a
lysosome containing
hydrolytic enzymes.
EXTRACELLULAR
FLUID
1 µm
CYTOPLASM
Pseudopodium
PHAGOCYTOSIS
Pseudopodium
of amoeba
“Food” or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM).
PINOCYTOSIS
In pinocytosis, the cell
“gulps” droplets of
extracellular fluid into tiny
vesicles. It is not the fluid
itself that is needed by the
cell, but the molecules
dissolved in the droplet.
Because any and all
included solutes are taken
into the cell, pinocytosis
is nonspecific in the
substances it transports.
Figure 7.20
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM).
Vesicle
Three Kinds of Endocytosis
• Phagocytosis - “cell eating”- large,
amounts of organic material;white
blood cells and protists.
• Pinocytosis- “cell drinking”- liquid
material brought into cell; mammalian
ova and follicle cells.
• Receptor-mediated Endocytosis- use
receptors in the membrane for specific
transport into cell.
RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor-mediated endocytosis enables the
cell to acquire bulk quantities of specific
substances, even though those substances
may not be very concentrated in the
extracellular fluid. Embedded in the
membrane are proteins with
specific receptor sites exposed to
the extracellular fluid. The receptor
proteins are usually already clustered
in regions of the membrane called coated
pits, which are lined on their cytoplasmic
side by a fuzzy layer of coat proteins.
Extracellular substances (ligands) bind
to these receptors. When binding occurs,
the coated pit forms a vesicle containing the
ligand molecules. Notice that there are
relatively more bound molecules (purple)
inside the vesicle, other molecules
(green) are also present. After this ingested
material is liberated from the vesicle, the
receptors are recycled to the plasma
membrane by the same vesicle.
Coat protein
Receptor
Coated
vesicle
Ligand
Coated
pit
Coat
protein
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs).
Plasma
membrane
0.25 µm
Receptor-Mediated Endocytosis
• Have indentations on the plasma membrane.
• Indentations = are clathrin-coated pits.
• Pits have receptor proteins on the
extracellular side = trigger
• When receptor binds to target molecule,
clathrin proteins on the cytoplasmic side
begins endocytosis.
• Forms a clathrin -coated vesicle.
• Very specific.
Receptor-mediated
Endocytosis
Exocytosis
• The release of material from
vesicles at the cell surface.
• Examples: protists using a
contractile vacuole to release
water, gland cells secreting
hormones, neurons releasing
neurotransmitters.
Secretion
Vesicles
Exocytosis and Neurons
Problems with Bulk
Transport
• Endocytosis and Exocytosis
are energy-intensive.
• Not highly selective.
Channel \Proteins
– Provide corridors that allow a specific molecule
or ion to cross the membrane
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
(a)
A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.
Figure 7.15
Selectively Permeable
Transport
• Channels- proteins in the cell
membrane that transport specific ions
into and out of the cell.
• Water-filled pores span the membrane.
• Ions do not interact with the channel
protein.
• Diffusion is passive, [high] --> [low].
Channels are somewhat selective
Who cares about protein
channels? One in twenty-five
of you should?
The story of cystic fibrosis.
Video: Nova-Cracking the
Code
Facilitated Diffusion
Some Channels are Gated
Carrier Proteins
– Undergo a subtle change in shape that translocates the
solute-binding site across the membrane
Carrier protein
(b)
Figure 7.15
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.
Solute
Selectively Permeable
Transport
• Carriers- proteins that transport specific
ions, sugars, and amino acids into and out
of the cell.
• The proteins facilitate movement by
binding to the solute-facilitated
diffusion.
• The proteins bind to the solute on one
side of the membrane and release them on
the other.
Uniport Facilitated Diffusion
Facilitated Diffusion
• It is specific, only certain
molecules transported by a given
carrier.
• It is passive, net movement is
[high]-->[low].
• It may become saturated if all
protein carriers are occupied.
How do you move
molecules AGAINST
their concentration
gradient?
Active Transport
Active Transport
• A method of transporting specific
ions, sugars, amino acids,
nucleotides against conc. gradient.
• Involves protein carriers in the
membrane and energy (ATP).
• How cells accumulate molecules
internally.
The Sodium-Potassium Pump
– Is one type of active transport system
1 Cytoplasmic Na+ binds to
the sodium-potassium pump.
2 Na+ binding stimulates
phosphorylation by ATP.
[Na+] high
[K+] low
Na+
Na+
Na+
Na+
EXTRACELLULAR
FLUID
[Na+] low
[K+] high
Na+
CYTOPLASM
Na+
ATP
P
ADP
Na+
Na+
Na+
3 K+ is released and Na+
sites are receptive again;
the cycle repeats.
4 Phosphorylation causes the
K+
protein to change its conformation, expelling Na+ to the
outside.
P
K+
K+
K+
5 Loss of the phosphate
restores the protein’s
original conformation.
Figure 7.16
K+
P
Pi
K+
6 Extracellular K+ binds to the
protein, triggering release of the
Phosphate group.
Sodium-Potassium Pump
Another
+
+
Na /K
Pump
An Example of Active
Transport
• Sodium-Potassium Pump- Active
transport of Na+ and K+ ions.
• Normally, inside the cell:
the [Na+] is low
the [K+] is high
• The cell maintains this by actively
pumping Na+ out and K+ in.
Sodium-Potassium Pump
• The protein uses ATP as an energy
source for this movement against the
gradient [low]--> [high].
• See Fig. 6.21, p. 135 for how.
• Uses ~1/3 of all ATP in resting cell.
• This pump can transport 300 Na+
ions/second.
• All animals use it.
+
Na
Who cares about
outside of the cell?
Cotransport: active transport driven by a
concentration gradient
–
+
H+
ATP
–
H+
+
H+
Proton pump
H+
–
+
H+
–
+
Sucrose-H+
cotransporter
H+
Diffusion
of H+
H+
–
–
Figure 7.19
+
+
Sucrose
Cotransport and
Countertransport
• Many amino acids and sugars are
transported into the cell through
coupled channels.
• Their active transport is coupled with
the movement of Na+ inside the cell.
• [Na+] high --> [Na+] low into cell.
• [Amino acid] low--> [Amino acid]
high.
Show me more about the
Na+/K+ pump!
Wm. Brown Publishing
Interactive CD Animation
An Electrogenic Pump
– Is a transport protein that generates the voltage across a
membrane
–
EXTRACELLULAR
FLUID
+
–
ATP
+
H+
H+
Proton pump
H+
–
+
H+
H+
+
–
CYTOPLASM
–
Figure 7.18
+
+
H+
The Proton Pump
• A transmembrane protein that moves H+
against their concentration gradient, from
[low] --> [high] outside of cells or
organelles.
• Example: mitochondria move H+ across
the inner membrane during electron
transport.
• Energy to power this pump comes from
NADH and FADH molecules.
Proton Pump
• The proton pump moves H+ out of the
matrix, through the inner membrane.
• ATP synthase channels H+ back into the
matrix, DOWN the gradient.
PROVIDES EVERGY!
• ATP synthesis is coupled to H+
movement.
• Almost all of the energy for cells is
made this way.
Chemiosmosis
Electron Transport Chain
What does ATP synthase
really look like? The 1997
Nobel Prize in Chemistry!
Review: Passive and Active Transport Compared
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
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse through
the lipid bilayer.
Figure 7.17
Facilitated diffusion. Many hydrophilic
substances diffuse through membranes with
the assistance of transport proteins,
either channel or carrier proteins.