1
Plasma Membrane
boundary
Selectively Permeable
Fluid mosaic
bilayer
proteins
2
Phospholipids
most abundant
amphipathic
Head
“attracted to water”
Fatty acid
hydrophilic
hydrophobic
Phosphate
tails
“repelled by water”
Phospholipid Bilayer
WATER
Hydrophilic
head
Hydrophobic
tail
WATER
4
Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Peripheral
protein
Transmembrane
proteins
Cytoplasm
Filaments of
cytoskeleton
1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model
Fluid Mosaic Model
fluid structure
Phospholipids
laterally
Membrane proteins
laterall
6
Freeze-fracture
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.
7
The Fluidity of Membranes
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
8
The Fluidity of Membranes
type of hydrocarbon tails
Fluid
Unsaturated hydrocarbon
tails with kinks
Viscous
Saturated hydroCarbon tails
9
7.5
The Fluidity of Membranes
steroid cholesterol
Cholesterol
10
Membrane Proteins and Their Functions
collage of different
proteins embedded
Fibers of
extracellular
matrix (ECM)
11
Types of Membrane Proteins
Integral proteins
transmembrane proteins
Polar
areas
of protein
Nonpolar areas12of
protein
HH++
Retinal
chromophore
NH2
aquaporin =
water channel in bacteria
Porin monomerH2O
b-pleated sheets
Bacterial
outer
membrane
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
COOH
HH++
Cytoplasm
proton pump channel
in photosynthetic bacteria
H 2O
function through
conformational change =
protein changes shape
Types of Membrane Proteins
Peripheral proteins
14
(a)
(b)
(c)
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.
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.
ATP
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
15
(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
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
16
The Role of Membrane Carbohydrates
in Cell-Cell Recognition
Cell-cell recognition
“Antigen”
Membrane carbohydrates
17
Synthesis and Sidedness of Membranes
outside faces
distinct inside and
affects the movement
proteins synthesized
endomembrane system
18
Membrane proteins and lipids
ER
Golgi apparatus
ER
19
Membrane Permeability
structure
selective permeability
cell must exchange materials with
its surroundings
20
sugar
H 2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to polar molecules
polar
hydrophilic
heads
waste
lipids
Permeability of the Lipid Bilayer
Hydrophobic molecules
lipid soluble
rapidly
Polar molecules
NOT
rapidly
22
Transport Proteins
Transport proteins
substances
hydrophilic
23
Passive Transport
Passive transport
no energy
CO2, H2O, and O2
Osmosis
24
Diffusion
spread out evenly
Down
Diffusion

movement from HIGH  LOW concentration
25
Effects of Osmosis on Water Balance
Osmosis
affected
concentration gradient
of dissolved substances
tonicity
26
Water Balance of Cells Without Walls
Tonicity
KABOOM!
impact
on cells without
walls
No problem,
here
27
freshwater
Three States of Tonicity
28
Isotonic Solutions
isotonic
concentration of solutes
same
NO NET
WATER
29
Hypertonic Solution
hypertonic
concentration of solutes
greater
lose water (PLASMOLYSIS)
30
Hypotonic Solutions
hypotonic
concentration of solutes
les
gain water
31
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.
32
Water Balance of Cells with Walls
Cell Walls
Turgor pressure
turgid
hypotonic
firm,
flaccid
isotonic or hypertonic
33
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.
34
.05 M
.03 M
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell
How will the water molecules move?
There will be no net movement of water since the
concentration of solute in each solution is equal
36
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
37
How will the water molecules move?
Each molecule of NaCl will dissociate to form a Na+ ion and a
Cl- ion, making the final concentration of solutes 200 molecules per
mil. Therefore, there will be a net movement of water from
Solution A to Solution B until both solutions have equal
concentrations of solute 38
Facilitated Diffusion
Facilitated diffusion
Passive
Proteins
facilitated = with help
open channel = fast transport
HIGH
Transport proteins
LOW
39
Channel proteins
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.
40
Carrier proteins
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.41
Active Transport
Uses energy
against
ATP
LOW
conformational change
ATP
HIGH
42
sodium-potassium pump
Cytoplasmic Na+ binds to
the sodium-potassium pump.
Na+ binding stimulates
phosphorylation by ATP.
EXTRACELLULAR
[Na+] highFLUID
[K+] low
1
2
Na+
Na+
Na+
Na+
Na+
[Na+] low
[K+] high
Na+
CYTOPLASM
ATP
P
ADP
Na+
Na+
Na+
K+ is released and Na+
3
sites are receptive again;
the cycle repeats.
4
K+
P
K+
Phosphorylation causes the
protein to change its
conformation, expelling Na+ to
the outside.
K+
K+
Loss of the phosphate
5
restores the protein’s
original conformation.
P
K+
K+ P i
6
Extracellular K+ binds to the
protein, triggering release of the
Phosphate group.
43
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.
44
Maintenance of Membrane Potential by Ion Pumps
Membrane potential
electrochemical gradient
electrogenic pump
45
Proton Pump
–
EXTRACELLULAR
FLUID
+
–
ATP
+
H+
H+
Proton pump
H+
–
+
H+
H+
+
–
CYTOPLASM
–
+
+
H+
46
Cotransport
Cotransport
active transport
protein
membrane
concentration gradient
47
concentration gradient
driven by a
48
Bulk Transport
and endocytosis
exocytosis
49
Exocytosis & Endocytosis
exocytosis
Transport vesicles
exocytosis
endocytosis
forming new vesicles from the plasma
membrane
50
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
molecular
signal
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.
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.
PHAGOCYTOSIS
52
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.
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Ligand
Coated
pit
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs).
Coat
protein
Plasma
membrane
0.25 µm
53