Download protein






Cell membrane = plasma membrane
The plasma membrane is a phospholipid
bilayer
The plasma membrane exhibits selective
permeability, meaning some things can get
through more easily than others.
Permeable: Things can get through
Impermeable: Things can’t get through
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Phospholipid
bilayer
Hydrophobic regions
of protein
Hydrophilic
regions of protein
Fig. 7-4
TECHNIQUE
RESULTS
Extracellular
layer
Knife
Plasma membrane
Proteins
Inside of extracellular layer
Cytoplasmic layer
Inside of cytoplasmic layer



Phospholipids in the plasma membrane can
move within the bilayer
Most of the lipids, and some proteins, drift
laterally (from side to side)
Rarely does a molecule flip-flop transversely
(upside down) across the membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-5a
Lateral movement
(107 times per second)
(a) Movement of phospholipids
Flip-flop
( once per month)
Fig. 7-6
We know membranes are fluid (liquid) due to
experiments that fused mouse cells with human
cells.
RESULTS
Membrane proteins
Mouse cell
Mixed proteins
after 1 hour
Human cell
Hybrid cell




As temperatures cool, membranes become
more solid
The temperature at which a membrane
solidifies depends on the types of fatty acid
tails on the phospholipids
Unsaturated vs. saturated fatty acids and
fluidity – which makes it more fluid?
Membranes must be fluid to work properly;
they are usually about as fluid as salad oil
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-5b
More Fluid
Unsaturated hydrocarbon
tails with kinks
(b) Membrane fluidity
Less fluid
Saturated hydrocarbon tails




The steroid cholesterol has different effects on
membrane fluidity at different temperatures
At warm temperatures (such as 37°C),
cholesterol restrains movement of
phospholipids so the membrane doesn’t get
too fluid
At cool temperatures, it maintains fluidity by
preventing tight packing
Overall, cholesterol prevents membranes from
getting too fluid or too solid.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-5c
Cholesterol
(c) Cholesterol within the animal cell membrane



A membrane is a collage of different proteins
embedded in the phospholipid bilayer
Proteins determine most of the membrane’s
specific functions
Different organisms and different organelles
have different kinds of membrane proteins.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-7
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE



Some proteins are bound to the surface of the
membrane
Others penetrate the hydrophobic core
The hydrophobic regions of these proteins
consist of one or more stretches of nonpolar
amino acids
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-8
EXTRACELLULAR
SIDE
 Helix
CYTOPLASMIC
SIDE

Six major functions of membrane proteins:
Fig. 7-9
Signaling molecule
Enzymes
ATP
(a) Transport
Receptor
Signal transduction
(b) Enzymatic activity
(c) Signal transduction
(e) Intercellular joining
(f) Attachment to the
cytoskeleton and extracellular
matrix (ECM)
Glycoprotein
(d) Cell-cell recognition



Hydrophobic (nonpolar) molecules, such as
hydrocarbons, can dissolve in the lipid bilayer
and pass through the membrane quickly
Polar molecules, such as sugars, do not cross
the membrane easily
Small molecules move through the membrane
faster than large ones
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Transport proteins are specific and allow hydrophilic
substances to move through the membrane
Fig. 7-15
EXTRACELLULAR
FLUID
Channel
protein
Solute
CYTOPLASM
Channel Proteins vs. Carrier proteins
Carrier protein
Solute

Most cells have aquaporins, channel proteins
that allow water to pass through
Image source: http://www.isisinnovation.com/licensing/6961.html



Diffusion is the tendency for molecules to
spread out evenly into the available space as
they move randomly
At equilibrium, as many molecules cross one
way as cross in the other direction
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion
_works.html
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-11
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
Net diffusion
Net diffusion
(b) Diffusion of two solutes
Net diffusion
Net diffusion
Equilibrium
Equilibrium


Substances diffuse down their concentration
gradient, the difference in concentration of a
substance from one area to another
The diffusion of a substance across a biological
membrane is a type of passive transport
because it requires no energy from the cell to
make it happen
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Osmosis = diffusion of water
High water
Low solutes
Solute
H 2O
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Low water
High solutes
Fig. 7-12
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
H2O
Selectively
permeable
membrane
Osmosis
Same concentration
of sugar




Tonicity = ability of a solution to cause a cell to
gain or lose water
Isotonic: Equal solute concentrations in cell and
solution
Hypertonic: Area with higher solute
concentration; gains water
Hypotonic: Area with lower solute
concentration; loses water
Keep track of whether the term is talking about
the cell or the solution.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-13
Why do plant and animal cells prefer different
solutions?
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
Video: Plasmolysis



Hypertonic or hypotonic environments create
osmotic problems for organisms
Osmoregulation, the control of water balance,
is necessary for life in such environments
The protist Paramecium, which is hypertonic to
its pond water environment, has a contractile
vacuole that acts as a pump
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-14
Filling vacuole
50 µm
(a) A contractile vacuole fills with fluid that enters from
a system of canals radiating throughout the cytoplasm.
Contracting vacuole
(b) When full, the vacuole and canals contract, expelling
fluid from the cell.


Facilitated diffusion = diffusion through a
protein
Examples:
◦ Aquaporins, for facilitated diffusion of water
◦ Ion channels that open or close in response to a
stimulus (gated channels)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-15
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Carrier protein
(b) A carrier protein
Solute


Carrier proteins undergo a change in shape that
moves the solute across the membrane
Some diseases are caused by malfunctions in
specific transport systems, for example the
kidney disease cystinuria
• Cystinuria is caused by mutations in 2 genes that
tell ribosomes how to make transport proteins
• People with the disease are missing these
transport proteins, which move some amino acids
out of the kidneys
• These amino acids build up and crystallize in the
kidneys, causing painful kidney stones
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings



Facilitated diffusion is still passive because the
solute moves down its concentration gradient,
no energy required
Some transport proteins can move solutes
against their concentration gradients
These types of proteins require added energy
(usually in the form of ATP), so they are forms
of active transport.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings


Active transport allows cells to maintain
concentration gradients that differ from their
surroundings
The sodium-potassium pump is one type of
active transport system
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-16-7
EXTRACELLULAR
[Na+]
FLUID
high
Na+
Na+
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLAS
M
1
Na+
[Na+] low
[K+] high
P
2
ADP
ATP
P
3
P
P
6
5
4
Fig. 7-17
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion



Membrane potential is the voltage difference
across a membrane
Voltage is created by differences in the
distribution of positive and negative ions
Most cells have a negative membrane potential
(inside of the cell is more negative than
outside)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings



An electrogenic pump is a transport protein
that generates voltage (+ and – charge) across
a membrane (contributing to the membrane
potential)
The sodium-potassium pump is the major
electrogenic pump of animal cells
The main electrogenic pump of plants, fungi,
and bacteria is a proton pump
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-18
–
ATP
EXTRACELLULAR
FLUID
+
–
+
H+
H+
Proton pump
H+
–
+
H+
H+
–
+
CYTOPLASM
–
H+
+


Large molecules, such as polysaccharides and
proteins, cross the membrane in vesicles; this
is called bulk transport
Bulk transport requires energy
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings





In exocytosis, transport vesicles migrate to the
membrane, fuse with it, and release their
contents
Many secretory cells use exocytosis to export
their products
Exo = out
cyt = cell
Exocytosis moves things out of the cell
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings


In endocytosis, the cell takes in
macromolecules by forming vesicles from the
plasma membrane
There are three types of endocytosis:
◦ Phagocytosis (“cellular eating”)
◦ Pinocytosis (“cellular drinking”)
◦ Receptor-mediated endocytosis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-20a


In phagocytosis a cell engulfs a particle or cell
in a vacuole
The vacuole fuses with a lysosome to digest the
particle
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)
Fig. 7-20b

In pinocytosis, molecules are taken up when
extracellular fluid is “gulped” into tiny vesicles
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle



In receptor-mediated endocytosis, binding of
ligands to receptors triggers vesicle formation
A ligand is any molecule that binds specifically
to a receptor site of another molecule
Receptor mediated endocytosis lets the cell be
selective about what molecules come into the
cell in vesicles.
Fig. 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand