Chapter 7 notes
Membrane Structure
and Function
Concept 7.1
Most abundant lipids in membranes are
phospholipids.
- phospholipids are amphipathic (head is
hydrophilic, tail is hydrophobic )
Phospholipids and proteins are arranged in the
“fluid mosaic model”: membrane is fluid w/
proteins embedded in or attached to the
bilayer
Concept 7.1
Hydrophilic
head
WATER
Hydrophobic
tail
WATER
Concept 7.1
Phospholipids and proteins are arranged
in the “fluid mosaic model”:
membrane is fluid w/ proteins
embedded in or attached to the bilayer
- disproved the Davson-Danielli
“sandwich” model
Concept 7.1
Phospholipid
bilayer
Hydrophobic regions
of protein
Hydrophilic
regions of protein
Concept 7.1
The membrane is fluid
-membranes are not static sheets of
molecules locked in place
-the membrane is held together
primarily by hydrophobic interactions
Concept 7.1
Lateral movement
(107 times per second)
(a) Movement of phospholipids
Flip-flop
( once per month)
Concept 7.1
Fluid
Unsaturated hydrocarbon
tails with kinks
(b) Membrane fluidity
Viscous
Saturated hydrocarbon tails
Concept 7.1
Membranes are mosaics of structure and
function
- proteins are embedded in the fluid
matrix; the lipid bilayer is the main
fabric of the membrane, but proteins
determine its specific fcn.
Concept 7.1
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
Concept 7.1
Two major types of membrane proteins:
- Integral proteins: penetrate the
hydrophobic core of the bilayer; many
are transmembrane proteins
- Peripheral proteins: appendages
loosely bound to the surface of the
membrane
Concept 7.1
N-terminus
C-terminus
 Helix
EXTRACELLULAR
SIDE
CYTOPLASMIC
SIDE
Concept 7.1
Membrane carbohydrates are important
for cell-cell recognition
- cell-cell recognition is the ability of
a cell to distinguish one type of
neighboring cell from another
- membrane carbohydrates are usually
oligosaccharides (can vary greatly)
Concept 7.1
Signaling molecule
Enzymes
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic activity
(c) Signal transduction
Concept 7.1
Glycoprotein
(d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)
Concept 7.2
Hydrophobic molecules can cross the
bilayer with ease. However, ions and
polar molecules cannot pass through
because they are hydrophilic.
- proteins play keys roles in regulating
transportation.
Concept 7.2
Transport proteins: allow hydrophilic
molecules to enter and exit the cell.
The selective permeability of a
membrane depends on the specific
transport proteins built into the
membrane.
Concept 7.3
Passive transport involves diffusion across
a membrane.
- Diffusion: the tendency for
molecules of any substance to spread
out into available space
- any substance will move down a
[gradient]. [high]  [low]
Concept 7.3
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
(a) Diffusion of one solute
Net diffusion
Equilibrium
Concept 7.3
Net diffusion
Net diffusion
(b) Diffusion of two solutes
Net diffusion
Net diffusion
Equilibrium
Equilibrium
Concept 7.3
Passive transport: diffusion of a substance
across a biological membrane. (no energy is
used)
Osmosis is the passive transport of water
- sln. w/ a higher [solute] = hypertonic
- sln. w/ a lower [solute] = hypotonic
- slns. w/ equal [solute] = isotonic
Concept 7.3
Concept 7.3
Organisms without cell walls that live in
hypertonic or hypotonic environments
must have adaptations for
osmoregulation, the control of water
balance
Concept 7.3
Organisms with cell walls
- turgid (very firm) when placed in a
hypotonic sln.
- flacid (limp) if the sln. is isotonic
- plasmolysis (shriveled) occurs when
put in a hypertonic sln.
Concept 7.3
Hypotonic solution
H2O
Isotonic solution
H2O
H2O
Hypertonic solution
H2O
(a) Animal
cell
Lysed
H2O
Normal
H2O
Shriveled
H2O
H2O
(b) Plant
cell
Turgid (normal)
Flaccid
Plasmolyzed
Concept 7.3
Facilitated diffusion: passive transport
of molecules through transport proteins
- each protein is specific for the solute it
transports
Concept 7.3
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Carrier protein
(b) A carrier protein
Solute
Concept 7.4
Active transport: movement of
molecules across a membrane against
the gradient (uses ATP)
- sodium-potassium pump: movement
of 3 Na+ for every 2 K+ ions
Concept 7.4
EXTRACELLULAR
FLUID
[Na+] high
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
1
Na+
[Na+] low
[K+] high
2
ATP
P
ADP
P
3
P
6
5
4
P
Concept 7.4
Some ion pumps generate voltage across
membranes
- membrane potential: the voltage
across a membrane
- electrogenic pump: a transport
protein that generates voltage across a
membrane (ex. Sodium-potassium
pump)
Concept 7.4
The main electrogenic pump for plants
and fungi is a proton pump which
transports H+ ion out of the cell.
Concept 7.4
–
ATP
EXTRACELLULAR
FLUID
+
–
+
H+
H+
Proton pump
H+
–
+
H+
–
+
CYTOPLASM
–
H+
+
H+
Concept 7.4
In cotransport, a ATP powered pump
can drive the transport of other solutes.
1) active transport of a substance
against a gradient
2) cotransport through a protein w/ 2nd
substance
Concept 7.4
–
ATP
+
–
H+
H+
+
H+
Proton pump
H+
–
H+
+
–
+
Sucrose-H+
cotransporter
Sucrose
–
–
H+
H+
Diffusion
of H+
H+
+
+
Sucrose
Concept 7.5
Exocytosis: the secretion of
macromolecules by the fusion of
vesicles with the plasma membrane
Endocytosis: the cell takes in
macromolecules by forming new
vessicles
- 3 types: phagocytosis, pinocytosis,
and receptor-mediated endocytosis
Concept 7.5
PHAGOCYTOSIS
EXTRACELLULAR
FLUID
1 µm
CYTOPLASM
Pseudopodium
Pseudopodium
of amoeba
“Food” or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium
via phagocytosis (TEM)