Outline
• 5.1 Plasma Membrane Structure and
Function
• 5.2 Passive Transport Across a Membrane
• 5.3 Active Transport Across a Membrane
1
Plasma Membrane of an Animal Cell
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Plasma membrane
carbohydrate
chain
extracellular
Matrix (ECM)
glycoprotein
phospholipid
glycolipid
hydrophobic
hydrophilic
tails
heads
phospholipid
bilayer
filaments of cytoskeleton
peripheral protein
Outside cell
Inside cell
integral protein
cholesterol
2
5.1 Plasma Membrane
Structure and Function
• The plasma membrane is common to all cells
• Separates:
 Internal cytoplasm from the external environment of
the cell
• Phospholipid bilayer:
 External surface lined with hydrophilic polar heads
 Cytoplasmic surface lined with hydrophilic polar
heads
 Nonpolar, hydrophobic, fatty-acid tails sandwiched in
between
3
Plasma Membrane Structure
and Function
• Components of the Plasma Membrane
 Three components:
• Lipid component referred to as phospholipid
bilayer
• Protein molecules
– Float around like icebergs on a sea
– Membrane proteins may be peripheral or
integral
» Peripheral proteins are found on the inner
membrane surface
» Integral proteins are partially or wholly
embedded (transmembrane) in the
membrane
• Cholesterol affects the fluidity of the membrane
4
Membrane Proteins
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phospholipid
Water outside cell
integral
protein
hydrophobic
region
hydrophilic
regions
cholesterol
peripheral
proteins
Water inside cell
5
Plasma Membrane Structure
and Function
• Carbohydrate Chains
 Glycoproteins
• Proteins with attached carbohydrate chains
• Example: glycoproteins on the surface of red blood
cells ae responsible for ABO group and RH
 Glycolipids
• Lipids with attached carbohydrate chains
 These carbohydrate chains exist only on the
outside of the membrane
• Makes the membrane asymmetrical
6
Plasma Membrane of an Animal Cell
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Plasma membrane
carbohydrate
chain
extracellular
Matrix (ECM)
glycoprotein
phospholipid
glycolipid
hydrophobic
hydrophilic
tails
heads
phospholipid
bilayer
filaments of cytoskeleton
peripheral protein
Outside cell
Inside cell
integral protein
cholesterol
7
Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
Transporter
Enzyme
activity
Cell surface
receptor
Cell surface
identity marker
Cell adhesion
Attachment to the
cytoskeleton
• Types and Functions of Membrane Proteins
• Channel proteins
are embedded in the cell membrane & have a pore for materials to cross
• Carrier proteins
can change shape to move material from one side of the membrane to the
other
Cell Recognition Proteins:
-Some glycoproteins are identification flags for that identified by
other cells.
-Antigens . basis for rejection of foreign cells by
immune system
 Receptor Proteins:
• Bind with specific molecules
• Allow a cell to respond to signals from other cells
 Enzymatic Proteins:
• Carry out metabolic reactions directly
 Junction Proteins:
• Attach adjacent cells
9
Membrane Protein Diversity
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Channel Protein:
Allows a particular
molecule or ion to
cross the plasma
membrane freely.
Cystic fibrosis, an
inherited disorder,
is caused by a
faulty chloride (Cl–)
channel; a thick
mucus collects in
airways and in
pancreatic and
liver ducts.
a.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Carrier Protein:
Selectively interacts
with a specific
molecule or ion so
that it can cross the
plasma membrane.
The inability of some
persons to use
energy for sodiumpotassium (Na+–K+)
transport has been
suggested as the
cause of their obesity.
b.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cell Recognition Protein:
The MHC (major
histocompatibility
complex) glycoproteins
are different for each
person, so organ
transplants are difficult
to achieve. Cells with
foreign MHC
glycoproteins are
attacked by white blood
cells responsible for
immunity.
c.
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Receptor Protein:
Is shaped in such a
way that a specific
molecule can bind to
it. Pygmies are short,
not because they do
not produce enough
growth hormone, but
because their plasma
membrane growth
hormone receptors
are faulty and cannot
interact with growth
hormone.
d.
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Enzymatic Protein:
Catalyzes a specific
reaction. The membrane
protein, adenylate
cyclase, is involved in
ATP metabolism. Cholera
bacteria release a toxin
that interferes with the
proper functioning of
adenylate cyclase;
sodium (Na+) and water
leave intestinal cells, and
the individual may die
from severe diarrhea.
e.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Junction Proteins:
Tight junctions join
cells so that a tissue
can fulfill a function, as
when a tissue pinches
off the neural tube
during development.
Without this
cooperation between
cells, an animal
embryo would have no
nervous system.
f.
The neural tube is the embryonic structure
that develops into the brain and spinal cord:
Cell Signaling
Refer to the attached power
point on Edmodo
16
Plasma Membrane Structure
and Function
• Permeability of the Plasma Membrane
 The plasma membrane is selectively permeable
• Allows some substances to move across the membrane
• Inhibits passage of other molecules
 Small, non-charged molecules (CO2, O2, glycerol,
alcohol) freely cross the membrane by passing
through the phospholipid bilayer
• These molecules follow their concentration gradient
– Move from an area of high concentration to an area of
low concentration.
17
Plasma Membrane Structure
and Function
• Permeability of the Plasma Membrane
 Water moves across the plasma membrane
• Specialized proteins termed aquaporins speed up water
transport across the membrane
 The movement of ions and polar molecules across
the membrane is often assisted by carrier proteins
 Some molecules must move against their
concentration gradient with the expenditure of energy
Ex.Active transport
 Large particles enter or exit the cell via bulk transport
 Examples:
• Exocytosis
• Endocytosis
18
Getting through cell
membrane
• Passive transport
 No energy needed
 Movement down concentration gradient
• Active transport
 Movement against concentration gradient
• low  high
 requires ATP
5.2 Passive Transport Across
a Membrane
• A solution consists of:
 A solvent (liquid), and
 A solute (dissolved solid)
• Diffusion
 Net movement of molecules down a concentration
gradient
 Molecules move both ways along gradient, but net
movement is from high to low concentration
 Equilibrium:
• When NET movement stops
• Solute concentration is uniform – no gradient
20
Process of Diffusion
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time
time
crystal
dye
a. Crystal of dye is placed in water
b. Diffusion of water and dye molecules
c. Equal distribution of molecules results
21
Facilitated diffusion
• Move from HIGH to LOW concentration
through a protein channel
 passive transport
 no energy needed
 facilitated = with help
2005-2006
The aquaporin channel protein
• There is some diffusion of water
directly across the bi-lipid
membrane.
• Aquaporins: Integral membrane
proteins that form water selective
channels – allows water to diffuse
faster
• Alters the rate of water flow
across the plant cell membrane –
NOT direction
• Found in specialized cells in the
kidney.
Active transport
• Cells may need molecules to move
against concentration situation
 need to pump against concentration
 protein pump
 requires energy
• ATP
Na+/K+ pump
in nerve cell
membranes
Transport summary
2005-2006
Gas Exchange in Lungs
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highO2
concentration
lowO2
concentration
O2
O2
O2
O2
O2
O2
O2
O2
O2
oxygen
O2
O2
O2
O2
alveolus
O2
bronchiole
capillary
26
Osmosis is diffusion of water
• Diffusion of water from
high concentration of water to
low concentration of water
 across a semi-permeable membrane
Why does
the water
level rise on
one side?
Passive Transport Across a
Membrane
• Osmosis:
 Special case of diffusion
 Focuses on solvent (water) movement rather than
solute
 Diffusion of water across a selectively permeable
membrane
• Solute concentration on one side is high, but water
concentration is low
• Solute concentration on other side is low, but water
concentration is high
 Water can diffuse both ways across membrane but
the solute cannot
 Net movement of water is toward low water (high
solute) concentration
• Osmotic pressure is the pressure that develops
due to osmosis
28
Concentration of water
• Direction of osmosis is determined by
comparing total solute concentrations
 Hypertonic - more solute, less water
 Hypotonic - less solute, more water
 Isotonic - equal solute, equal water
water
hypotonic
hypertonic
net movement of water
Passive Transport Across a
Membrane
• Isotonic Solutions
 Solute and water concentrations are equal on
both sides of membrane
 No net gain or loss of water by the cell
• Hypotonic Solutions
 Concentration of solute in the solution is lower
than inside the cell
 Cells placed in a hypotonic solution will swell
• Causes turgor pressure in plants
• May cause animal cells to lyse (rupture)
30
Passive Transport Across a
Membrane
• Hypertonic Solutions
 Concentration of solute is higher in the
solution than inside the cell
 Cells placed in a hypertonic solution will
shrink
• Crenation in animal cells
• Plasmolysis in plant cells
31
Osmosis in Animal and Plant Cells
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Animal
cells
plasma
membrane
nucleus
In an isotonic solution, there is no net In a hypotonic solution, water
In a hypertonic solution, water
movement of water.
mainly enters the cell, which may mainly leaves the cell, which
burst (lysis).
shrivels (crenation).
Plant
cells
cell
wall
central
vacuole
nucleus
plasma
membrane
chloroplast
In an isotonic solution, there is no In a hypotonic solution, vacuoles
net movement of water.
fill with water, turgor pressure
develops, and chloroplasts are
seen next to the cell wall.
In a hypertonic solution, vacuoles
lose water, the cytoplasm shrinks
(plasmolysis), and chloroplasts
are seen in the center of the cell. 32
Osmosis and tonicity
• The human body is able to keep cells in
osmotic balance through a variety of
mechanisms.
•
This is observed between red blood cells and
plasma, where solute concentration of blood
plasma (extracellular) is similar to the solute
concentration in the cytosol (intracellular).
• Plasma is isotonic to red blood cells.
•
If red blood cells are placed in a solution with
lower solute concentration than is found in the
cells, water moves into the cells by osmosis,
causing the cells to swell and eventually burst
(hemolysis); such a solution is hypotonic to the
cells, while cell cytosol is hypertonic to the
plasma.
Copyright: OpenStax Biology for AP Courses, OpenStax,
and Rice University
(credit: modification of work by Mariana Ruiz Villareal)
Red blood cell in hypertonic solution
• Hypertonic Solutions
• In a hypertonic solution, the prefix hyperrefers to the extracellular fluid having a
higher osmolarity than the cell’s cytoplasm
• Therefore, the fluid contains less water than
the cell does. Because the cell has a
relatively higher concentration of water,
water will leave the cell.
• Solution has more solute (so LESS water)
than the cell.
From the wikimedia free licensed media
file repository
Red blood cell in hypotonic solution
• Hypotonic Solutions
• In a hypotonic solution, the prefix
hypo- means that the extracellular
fluid has a lower concentration of
solutes, or a lower osmolarity, than the
cell cytoplasm.
• The extracellular fluid has lower
osmolarity than the fluid inside the
cell, and water enters the cell.
• Solution has less solute (so MORE
water) than the cell.
From the wikimedia free licensed media file
repository
Red blood cell in Isotonic solution
• Isotonic Solution
• Water is the same inside the cell and
outside
• An Isotonic solution has the same
solute than the cell. So no osmotic
flow
• Water freely moves in and out of the
cell, so there is NO net movement of
water molecules between the cell
and the solution.
• All Biochemical activity is normal.
From the wikimedia free licensed media
file repository
Active transport
• Cells may need molecules to move
against concentration situation
 need to pump against concentration
 protein pump
 requires energy
• ATP
Na+/K+ pump
in nerve cell
membranes
How about large molecules?
• Moving large molecules into & out of cell
requires ATP!
 through vesicles & vacuoles
 endocytosis
• phagocytosis = “cellular eating”
• pinocytosis = “cellular drinking”
• receptor-mediated
endocytosis
 exocytosis
exocytosis
2005-2006
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
ligand signal
2005-2006
• Macromolecules are transported into or out of
the cell inside vesicles via bulk transport
 Exocytosis – Vesicles fuse with plasma membrane
and secrete contents
 Endocytosis – Cells engulf substances into a pouch
which becomes a vesicle
• Phagocytosis – Large, solid material is taken in by
endocytosis
• Pinocytosis – Vesicles form around a liquid or very small
particles
• Receptor-Mediated Endocytosis– Specific form of
pinocytosis using receptor proteins and a coated pit
40
Endocytosis
• In endocytosis
 The cell takes in macromolecules by forming
new vesicles from the plasma membrane
Narrated animation of endo/exocytosis
http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/
• 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.
PHAGOCYTOSIS
EXTRACELLULAR
CYTOPLASM
FLUID
Pseudopodium
1 µm
Pseudopodium
of amoeba
“Food” or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM).
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
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM).
Vesicle
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.
Summary of active
transport processes
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
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