Chapter 7
Membrane Structure and Function
AP Biology
Overview: Life at the Edge
• The plasma membrane separates the living cell from its
surroundings
– It controls traffic into and out of the cell
– It also exhibits SELECTIVE PERMEABILITY
(allows some substance to cross it more easily than
others)
• In this chapter, we will learn how cell membranes control
the passage of substances
Concept 7.1:
Cellular membranes are fluid
mosaics of lipids and proteins
•
Lipids and proteins are the main ingredients of membranes, though carbohydrates
are also important
–
Of all the lipids that make up membranes, phospholipids are most abundant
•
–
Phospholipids are AMPHIPATHIC molecules – have BOTH hydrophilic
and hydrophobic region
How are phospholipids and proteins arranged in membranes
of cells?
•
The currently accepted model is called the
FLUID MOSAIC MODEL
– This model states that the membrane is a FLUID
structure with a “MOSAIC” (many different)
of proteins stuck in or
attached to the bilayer
of phospholipids
The Sandwich Model
•
Since 1925, we have reasoned that cell membranes must be phospholipid bilayers made of
protein and lipids (via chemical analysis)
–
This is the only molecular arrangement that shields the hydrophobic tails of
phospholipids from water, while exposing the hydrophilic heads to water
–
At this time, however, scientists were unsure of where proteins were located
•
Scientists had observed that a pure phospholipid bilayer is less attracted to water
than the surface of a cell membrane
•
Taking this observation in to account, the first model of the cell membrane was
proposed in 1935 by Hugh Davson and James Danielli
–
This model, known as the SANDWICH MODEL, proposed that the
membrane was coated on both sides with hydrophilic proteins
Problems With the Sandwich Model
•
Later studies found problems with the sandwich model, particularly:
–
1) All membranes of cells are not identical
•
The plasma membrane is 7-8 nm thick but the mitochondria’s inner membrane is
only 6 nm thick
•
Membranes look different under a microscope
– Plasma membrane appears as a 3-layered structure in electron micrographs
–
•
Membranes do not have identical composition
–
–
Mitochondrial membranes look like a row of beads
Mitochondrial membranes have a higher percentage of proteins and different
kinds of phospholipids and other lipids
2) Membrane proteins are not very soluble in water because they are amphipathic (have
hydrophobic regions, along with hydrophilic regions)
•
So if these proteins were layered on surface of membrane, even their hydrophobic
parts would be exposed to water
The Fluid Mosaic Model
•
In 1972, S.J. Singer and G. Nicolson proposed that membrane proteins are dispersed and
individually inserted into the phospholipid bilayer with their hydrophilic regions protruding
–
His molecular arrangement maximizes contact of hydrophilic regions of proteins and
phospholipids with water in the cytosol and extracellular fluid
•
It also provides the hydrophobic portions of these proteins and phospholipids with
a nonaqueous environment
–
Thus, in this fluid mosaic model, as it was eventually named , the membrane is a
mosaic of protein molecules
bobbing in a fluid bilayer of
phospholipids
Proof for the Fluid Mosaic Model: Freeze-Fracture
•
A method of preparing cells for electron microscopy called freeze-fracture has
provided proof for the fluid mosaic model
–
This technique involves freezing a cell and then fracturing it with a knife
•
The fracture plane often follows the hydrophobic interior of a membrane,
splitting the phospholipid bilayer into 2 separated layers
– The membrane proteins remain whole, staying with one of the 2 layers
(like pulling apart a chunky peanut butter sandwich)
–
Under the electron microscope, these membrane layers appear cobblestoned,
with protein particles interspersed in a smooth matrix
•
Some proteins travel with one layer or the other (like peanut chunks in the
sandwich)
The Fluidity of Membranes
•
Membranes are FLUID structures, since they are held together primarily by weak
hydrophobic interactions
–
Most of the lipids and some proteins in a membrane can drift laterally (side-to-side) in
the plane of the membrane
•
–
This is similar to people elbowing their way through a crowded room
Lipids and proteins don’t usually flip flop transversely across the membrane (from one
layer to the other), however
•
To do this, the hydrophilic portion of the molecule would have to cross the
hydrophobic core of the
membrane
Lateral Movement of Phospholipids and Proteins
•
The lateral movement of phospholipids within the membrane is rapid
–
Adjacent phospholipids switch positions ~107 times per second
•
•
This means that a phospholipid can move ~2 µm in 1 second (the length of a
bacterial cell)
Proteins are much larger than lipids so they move more slowly than phospholipids
–
Some membrane proteins seem to move in a highly directed manner
•
–
Scientists hypothesize that these membrane proteins may be driven along
cytoskeletal fibers by motor proteins connected to the membrane proteins’
cytoplasmic regions
Other membrane proteins
seem to be held firmly in
place by their attachment
to the cytoskeleton
Experiment: Do Membrane Proteins Move?
•
A classic experiment by David Frye and Michael Edidin proved that proteins do , in fact, drift
laterally along membranes
–
Experiment: Plasma membrane proteins of a mouse cell and a human cell were labeled
with 2 different markers (red vs. blue) and then fused together
•
–
Markers on the hybrid cells were then observed with a microscope
Conclusion: There was some mixing of mouse and human membrane proteins
•
This indicates that at least some membrane proteins move sideways within the
plane of the plasma membrane
Membrane Fluidity and Temperature
•
Membranes switch from a fluid to solid state as temperature decreases (like bacon grease
forms lard when cooled)
–
The composition of a membrane (what kinds of lipids it contains) determine at what
temperature it solidifies
•
If phospholipids have unsaturated hydrocarbon tails (kinks due to double bonds),
they remain fluid to a lower temperature
•
–
Recall: unsaturated fats are liquid at room temperature
These kinks prevent close packing of phospholipid molecules, making the
membrane more
fluid
Cholesterol and Membrane Fluidity
•
Other components of the membrane can also affect its fluidity:
–
The steroid cholesterol (another type of lipid) is wedged between the phospholipid
molecules in animal cell plasma membranes
•
Cholesterol has different effects of membrane fluidity at different temperatures:
–
At higher temperatures (37o C – human body temperature), cholesterol makes
the membrane less fluid by restraining phospholipid movement
– At cooler temperatures, it maintains fluidity by preventing tight packing of
phospholipids
•
•
This lowers the temperature required for the membrane to solidify
We can thus think of cholesterol as a “temperature buffer”
–
It resists changes in
membrane fluidity at
different temperatures,
ensuring the membrane
does not become too
fluid or too solid
Membrane Fluidity and Function
•
Membranes must be fluid to work properly (about as fluid as salad oil)
–
When a membrane solidifies, its permeability to different molecules can
change
–
It can also inactivate certain membrane enzymes
• Ex) Some enzymes need to be able to move laterally within the membrane
to function
–
To remain fluid, the lipid composition of cell membranes can change as an
adjustment to changing temperature
• Ex) In many cold-tolerant plants, the percentage of unsaturated
phospholipids increases in autumn to keep the membranes from
solidifying during winter
Membrane Proteins and Their Functions
•
Now we will discuss the MOSAIC part of the fluid mosaic model
–
Recall: membranes have many different proteins embedded in their fluid matrix
•
Ex) More than 50 different kinds of proteins have been found in RBC plasma
membranes
–
Though phospholipids make up the
main fabric of the membrane,
proteins determine most of the
membrane’s functions
•
Thus, different cells have
different membrane proteins
•
Different membranes in the same cell also have different proteins
Integral Proteins
•
There are 2 major types of membrane proteins:
–
1) Integral proteins – penetrate the hydrophobic core
of the membrane
•
Some integral proteins go all the way through the
membrane and are thus called
TRANSMEMBRANE PROTEINS
–
•
Others only extend part of the way through
the membrane
The hydrophobic regions of
integral proteins are made up
of nonpolar amino acids
stretches that often coil into
alpha helices
–
Hydrophilic parts of
integral proteins are left
exposed on either surface
of the membrane
Peripheral Proteins
•
There are 2 major types of membrane proteins:
–
2) Peripheral proteins – not embedded in the membrane but are loosely bound to the
surface of the membrane
•
These proteins often bind to the exposed parts of integral proteins
•
Many membrane proteins are also held in place by the cytoskeleton (on the
cytoplasmic side) or attached
to fibers of the extracellular
matrix (ECM) on the other side
Functions of Membrane Proteins
• There are six major functions of membrane proteins:
– Transport
– Enzymatic activity
– Signal transduction
– Cell-cell recognition
– Intercellular joining
– Attachment to the cytoskeleton and extracellular matrix
(ECM)
Functions of Membrane Proteins: Transport
• Some transmembrane proteins have hydrophilic channels in them
– These channels are usually selective for a particular solute (see
left)
– Other transport proteins move substances
from one side to the other by changing
shape (see right)
• Some must use ATP to actively
pump substances across the membrane
Functions of Membrane Proteins: Enzymatic Activity
• Some enzymes are built directly into the membrane
• The active sites of these enzymes are exposed to aqueous
solution
– Teams of enzymes can also work together to carry out a sequence
of steps in a metabolic pathway
(see illustration)
Functions of Membrane Proteins: Signal Transduction
•
Some membrane proteins act as receptors
–
These proteins have binding site with a specific shape that fits the shape of a
chemical messenger, called a signaling molecule
• Many signaling molecules are hormones
– Ex) Insulin, external signaling molecule,
causes the cell to take up glucose from
blood
Functions of Membrane Proteins: Cell-Cell Recognition
• Some glycoproteins act like ID tags
– These glycoproteins allow membrane proteins on other cells to
recognize each other
– Immune system cells can also pick
out foreign cells in this way
Functions of Membrane Proteins: Intercellular Joining
• Membrane proteins of cells next to one another can join these cells
using junctions
– Ex) Gap junctions allow
communication between cells
of a tissue (heart muscle)
Functions of Membrane Proteins: Attachment to
Cytoskeleton & ECM
•
•
Microfilaments can attach to membrane proteins, which helps to:
–
Maintain cell shape
–
Keep membrane proteins from moving
Proteins can also bind to ECM molecules and coordinate
changes inside the cell
–
This also allows communication between cells
The Role of Membrane Carbohydrates in Cell-Cell Recognition
•
Cells need to be able to recognize each other
•
–
Ex) Cell recognition allows cells to be sorted into tissues and organs during
embryonic development
Cells recognize each other by binding to surface molecules like carbohydrates on each
others’ plasma membranes
•
Some membrane carbohydrates have lipids bound to them
–
•
Most membrane carbohydrates are bound to proteins
–
–
These molecules are called glycolipids
These molecules are called glycoproteins
Cells recognize each other because these carbohydrates are all different
•
They vary from species to species, members of same species, and from one cell
type to another
–
Ex) A, B, AB, and O blood types reflect differences in carbohydrates found
on the surface of RBCs
Synthesis and Sidedness of Membranes
•
Membranes have distinct inside and outside faces
–
Their 2 lipid layers may differ in specific lipid composition
–
Each protein has directional orientation in the membrane
• When a vesicle fuses with the plasma membrane, the outside of the vesicle
becomes continuous with the cytoplasmic (inner) layer of the plasma
membrane
– Thus, molecules that start out on the inside face of the ER end up on
the outside face of the plasma membrane
Synthesis and Sidedness of Membranes
•
•
1) The synthesis of membranes begins as membrane proteins and lipids are made in ER
–
Carbohydrates (green) are added to proteins (purple), making glycoproteins
–
The carbohydrate portions may then be modified
2) Glycoproteins are transferred to the GA for further
modification of carbohydrates
–
Lipids from the ER acquire carbohydrates,
making glycolipids
•
3) Transmembrane proteins (purple dumbells),
glycoproteins, and secretory proteins (purple circles)
are transported from the GA to the plasma membrane
in vesicles
•
4) Vesicles fuse with the plasma membrane and
release their secretory proteins from the cell
–
Fusion of vesicles positions carbohydrates of
glycoproteins and glycolipids on the outside
of the plasma membrane
Concept Check 7.1
• 1) The carbohydrates attached to some proteins and lipids
of the plasma membrane are added as the membrane is
made and refined in the ER and Golgi apparatus; the new
membrane then forms transport vesicles that travel to the
cell surface. On which side of the vesicle membrane are
the carbohydrates?
• 2) How would you expect the saturation levels of
membrane phospholipid fatty acids to differ in plants
adapted to cold environments and plants adapted to hot
environments?
Concept 7.2:
Membrane structure results in
selective permeability
• It is essential that cells be able to regulate transport across
their boundaries
• This process is controlled by the plasma membrane
– Although a steady traffic of small ions and molecules
move across the plasma membrane in both directions,
these substances do not cross the barrier
indiscriminately
• The plasma membrane is selectively permeable
The Permeability of the Lipid Bilayer
• Because nonpolar molecules are hydrophobic, they can
easily cross through the lipid bilayer without the aid of
membrane proteins
– Hydrophilic ions and polar molecules (glucose, water) ,
however, can’t pass through as easily
• This is due to the hydrophobic core of the plasma
membrane
Transport Proteins: Channel Proteins
•
Hydrophilic ions and polar molecules cross the lipid bilayer by passing through
transport proteins that span the membrane (like a bridge)
–
Transport proteins called channel proteins have a hydrophilic channel that
polar or charged molecules can pass through
• Ex) Channel proteins called aquaporins help water molecules pass
through the plasma
membrane
– 3 billion water molecules can pass through each aquaporin per second
– Water molecules can only pass through in a single file
– The channel fits 10 water molecules at a time
Transport Proteins: Carrier Proteins
•
Another type of transport protein is called a carrier protein
–
These transport proteins hold on to their passengers and change shape
• This change in shape allows molecule to be shuttled across the plasma
membrane
– Ex) Glucose carried by blood enters RBCs through carrier proteins,
allowing it to pass through 50,000 times faster than by simple
diffusion
–
Carrier proteins are very specific for the substances they move
• Ex) Carrier proteins for glucose are so selective that they even reject
fructose (a structural isomer of glucose)
Concept Check 7.2
• 1) Two molecules that can cross a lipid bilayer without help from
membrane proteins are O2 and CO2. What properties allow this to
occur?
• 2) Why would water molecules need a transport protein to move
rapidly and in large quantities across a membrane?
• 3) Aquaporins exclude passage of hydronium ions (H3O+). But recent
research has revealed a role for some aquaporins in fat metabolism,
in which they allow the passage of glycerol, a 3-carbon alcohol (see
Figure 5.11), as well as H2O. Since H3O+ is much closer in size to
water than is glycerol, what do you suppose is the basis of this
selectivity?
Concept 7.3:
Passive transport is diffusion of a
substance across a membrane with
no energy investment
Diffusion
•
Diffusion: the movement of molecules of any substance so that they spread out evenly into
an available space
–
Though each molecule moves randomly, the movement of a population of molecules
may be directional (net movement)
–
Once there are equal amounts of molecules on either side of a membrane, a DYNAMIC
equilibrium is reached
•
Molecules still continue to
move but at equal rates in both
directions (no net movement)
Animation: Membrane Selectivity
Animation: Diffusion
Passive Transport
•
Simple rule of diffusion:
–
A substance will diffuse from where it is more concentrated to where it is less
concentrated
•
•
Thus, the substance moves down its CONCENTRATION GRADIENT
Diffusion is a spontaneous process
–
No work must be done (no energy
needed)
•
–
Ex) Dissolved oxygen (in
aqueous solution inside the body)
diffuses into cells across their
plasma membranes as cellular
respiration consumes it
Diffusion of a substance across a
biological membrane is called
PASSIVE TRANSPORT
•
Passive transport does not
require energy
Effects of Osmosis on Water Balance
•
The diffusion of water across a selectively permeable membrane is called
OSMOSIS
–
Water diffuses across a membrane from an area of lower SOLUTE
concentration to an area of higher
solute concentration
• At equilibrium, the solute
concentrations on both sides of
the membrane will be the same
Tonicity
• Tonicity: the ability of a solution to cause a cell to gain or lose water
– It depends (in part) on the concentration of solutes that cannot
cross the membrane (nonpenetrating solutes) relative to that
inside the cell
• Higher concentrations of nonpenetrating solutes in the
surrounding solution will cause water to leave the cell
• Lower concentrations of nonpentrating solutes in the
surrounding solution will cause water to enter the cell
Isotonic Solution
•
For cells in an isotonic solution (iso = same):
–
The solute concentration
in the surrounding solution
is the same as that inside
the cell
•
Thus, there is no net
movement of water
across the plasma
membrane
–
Water flows across the membrane at the same rate in both directions, creating
equal concentrations of solutes on both sides of the membrane
Hypertonic Solution
•
For cells in hypertonic solution (more nonpentrating solutes):
–
The solute concentration in the surrounding solution is greater than that inside cell
•
This causes the cell to lose water and shrivel
Hypotonic Solution
•
For cells in hypotonic solution (less nonpenetrating solutes):
–
The solute concentration in the surrounding solution is lower than that inside the cell
•
Water will therefore enter the cell faster than it leaves
–
This will cause
the cell to
swell/burst
•
Animal cells fare best in an
isotonic environment
–
Plant cells do best in a
hypertonic environment
Water Uptake and Cells Without Walls
•
Cells without walls can’t tolerate excessive uptake or loss of water
–
–
The resulting problem of water balance is automatically solved if the cell lives in
isotonic surroundings
•
Ex) Sea water is isotonic to many marine invertebrates
•
Ex) Our extracellular fluid is isotonic to the fluid inside cells
Organisms without cell walls living in
hypertonic or hypotonic environments need to
have special adaptations for
OSMOREGULATION (control of water balance)
•
Ex) The protist Paramecium is
hypertonic to its pond water environment
–
It has a contractile vacuole that
functions as a pump to force water
out of cell as fast as it enters by
osmosis
Video: Chlamydomonas
Video: Paramecium Vacuole
Water Balance of Cells with Walls
•
The cells of plants, prokaryotes, fungi, and some protists have walls
–
–
In hypotonic solutions, this wall helps maintain water balance
•
The cell wall is relatively inelastic and will expand only so much before it exerts
back enough pressure to oppose further water uptake
•
At this point, the cell is TURGID (very firm)
–
This is the healthy state
for most plant cells
–
Plants (especially those
that are not woody)
depend on the mechanical
support of these turgid
cells
If plant cells are in isotonic
solution, the cell become flaccid
(limp)
•
This condition may cause the plant to wilt
•
In a hypertonic environment, the plant cell will also lose water like an animal cell
–
As the plant cell shrivels, the plasma membrane pulls away from cell wall
•
–
This process is known as plasmolysis
Plasmolysis causes the plant to wilt , which can lead to plant death
Video: Plasmolysis
Video: Turgid Elodea
Animation: Osmosis
Facilitated Diffusion: Passive Transport Aided by Proteins
•
Polar molecules and ions cannot diffuse through membrane without the help of
transmembrane proteins
•
Diffusion that requires the help of these transport proteins is called
FACILITATED DIFFUSION
–
Proteins involved in facilitated diffusion are also referred to as channel proteins
because they provide corridors that allow specific molecules or ions to pass (like a
hallway)
•
Their passages are hydrophilic, which allows polar or charged particles to cross
the hydrophobic core
of the membrane
Types of Channel Proteins
•
There are 2 types of channel proteins:
–
1) Aquaporins: these water channel proteins help water diffuse at a much quicker rate
than it would otherwise
•
Though water is small enough to diffuse without channel proteins, it is impeded
by its polarity
–
Kidney cells, in particular, have large numbers of aquaporins, allowing them
to reclaim water from urine before excretion
•
–
If water was completely excreted in the urine, a person would otherwise
have to drink 50 gallons of water per day
2) Ion (gated) channels: channel proteins that open or close in response to an electrical
or chemical stimulus
•
If chemical, the stimulus is a substance other than the one to be transported
–
Ex) In the sodium potassium pump, neurotransmitter molecules stimulate
gated channel to open in nerve cells, allowing sodium ions to move into cell
•
Later, an electrical stimulus activates the ion channel protein so
potassium ions can rush out
Carrier Proteins
•
Another type of protein involved in facilitated diffusion is the carrier protein
–
Carrier proteins undergo a subtle change in shape that allows a specific solute
to move across the membrane during the shape change
• These changes in shape are usually triggered by the binding and release
of the transported molecule
Cystinuria
• In some inherited diseases, these specific transport systems are either
defective or missing
–
Ex) In the disease cystinuria, an individual lacks a carrier protein to
transport cysteine and other amino acids across the membranes of their
kidney cells
• Though these amino acids are still absorbed from urine, the return
of these amino acids into the blood is impeded
• This causes the development of painful stones from the
accumulation and crystallization of amino acids in the kidneys
Concept Check 7.3
• 1) How do you think a cell performing cellular
respiration rids itself of the resulting carbon dioxide?
• 2) In the supermarket, produce is often sprayed with
water. Explain why this makes the vegetables look
crisp.
• 3) If a Paramecium swims from a hypotonic
environment to an isotonic one, will its contractile
vacuole become more active or less? Why?
Concept 7.4:
Active transport uses energy to move
solutes against their gradients
Active Transport
•
Even through facilitated diffusion requires the help of specific transport proteins, it
is still considered diffusion
• This is because the solute moves down its concentration gradient (It does
NOT alter the direction of transport)
–
Some transport proteins, however, can move solutes against their
concentration gradient, from less concentrated to more concentrated
– This process, called ACTIVE TRANSPORT, requires the cell to use
energy (usually in the form of ATP)
• All of the transport proteins involved in active transport are carrier
proteins (no channel proteins)
– They must pick up the solutes and transport them rather than simply
allowing them to flow through
Animation: Active Transport
An Example of Active Transport: The Sodium-Potassium Pump
•
Active transport lets cells have different concentrations of solutes inside and outside
the cell
–
Ex) Animal cells have higher concentrations of K+ ions and lower
concentrations of Na+ ions due to the action of the sodium-potassium pump
Passive vs. Active Transport: A Review
•
Passive transport: spontaneous diffusion down a concentration gradient
–
•
No expenditure of energy by the cell
•
The rate of diffusion can be greatly
increased by membrane transport
proteins
A) Diffusion: transports hydrophobic
molecules
•
–
B) Facilitated diffusion: allows transport
of hydrophilic substances
•
•
Can also transport small uncharged
polar molecules at a very slow rate
Assisted by transport proteins (channel or carrier proteins)
Active transport: transport proteins acts as pumps
–
Move substances against their concentration gradients
–
Energy for work is usually supplied by ATP
How Ion Pumps Maintain Membrane Potential
•
Cells have VOLTAGES across their plasma membranes (separation of opposite
charges)
–
Their cytoplasm is NEGATIVELY charged relative to the extracellular fluid
• This is due to unequal distribution of anions and cations on opposite sides
of the membrane
–
Voltage across a membrane is called MEMBRANE POTENTIAL
• Membrane potential ranges from –50 to –200 millivolts (mV)
– The minus sign indicates that the inside of the cell is negative relative
to the outside
The Electrochemical Gradient
•
Because of the membrane potential (negatively charged cytoplasm relative to extracellular
fluid), the passive transport of cations into the cell and anions out of the cell is favored
–
–
Therefore, 2 forces drive the diffusion of ions across the membrane:
•
1) A chemical force, due to the ion’s concentration gradient
•
2) An electrical force , due to the effect of the membrane potential on the ion’s
movement
This combination of forces is called the ELECTROCHEMICAL GRADIENT
•
•
Ex) The concentration of Na+ inside a resting nerve cell is much lower than
outside the cell
–
Thus, the concentration gradient favors the movement of Na+ INTO the cell
–
The membrane potential also favors movement of Na+ INTO the cell
(attracted to negative interior of cell)
Electrical forces can also OPPOSE simple diffusion
–
Active transport may be necessary in these cases
Electrogenic Pumps
•
Membrane proteins involved in active transport of ions can contribute to membrane potential
–
Ex) The sodium-potassium pump moves 3 Na+ ions out of the cell for every 2 K+ ions
brought in
•
–
Each pump therefore provides a net transfer of one positive charge from
cytoplasm to the extracellular fluid
the
Transport proteins that generate voltage in this way across the membrane are called
ELECTROGENIC PUMPS
•
The sodium-potassium pump is the main electrogenic pump in animal cells
•
The main electrogenic pump of
plants, fungi, and bacteria is the
PROTON PUMP
The Proton Pump
•
The proton pump actively transport H+ ions OUT of the cell
–
This creates a negative charge INSIDE the cell relative to the more positively charged
outside
•
This stores energy that
can be used for cellular
work
–
Ex) ATP synthesis
during cellular
respiration
–
Ex) A type of
membrane traffic called COTRANSPORT
Cotransport: Coupled Transport by a Membrane Protein
•
In COTRANSPORT, the active transport of one solute indirectly drives the transport of
another solute
–
A substance that is actively transported against its concentration builds up potential
energy
•
–
This potential energy can then be used to do work as the substance diffuses back
down its concentration gradient
In this case, the energy can be used
to drive the active transport of
another solute
•
Ex) Plant cells use potential
energy built up by the
concentration of H+ outside the
cell to drive the active
transport of amino acids,
sugars, and other nutrients
Concept Check 7.4
• 1) When nerve cells establish a voltage across their
membrane with a sodium-potassium pump, does this
pump use ATP or does it produce ATP? Why?
• 2) Explain why the sodium-potassium pump in Figure
7.16 (pp. 136) would not be considered a
cotransporter.
• 3) What would happen if cells had a channel protein
allowing unregulated passage of hydrogen ions?
Concept 7.5:
Bulk transport across the plasma
membrane occurs by exocytosis and
endocytosis
Bulk Transport of Large Molecules
• Large molecules (proteins, polysaccharides) can’t cross the plasma
membrane by diffusion or through transport proteins
– Generally, these large polymers cross the membrane in bulk,
packaged within vesicles
• This bulk transport requires energy
– Types of bulk transport include:
• Exocytosis
• Endocytosis
Exocytosis
•
In the process of exocytosis, transport vesicles move biological molecules OUT of the cell by
fusing with the plasma membrane
–
Vesicles bud first from the ER and then the Golgi apparatus
•
When the plasma membrane and the vesicle come into contact, the lipids of the 2
bilayers rearrange and their membranes FUSE
–
–
The contents of the vesicle then spill outside of the cell
–
The vesicle membrane then becomes part of the plasma membrane
Many secretory cells use exocytosis to export products out of the cell
•
Ex) Cells in the pancreas make insulin and excrete it into the extracellular fluid by
exocytosis
Animation: Exocytosis
Endocytosis
•
In the process of endocytosis, the cell takes in material by forming new vesicles from the
plasma membrane
–
Events look like the reverse of exocytosis
•
A small area of plasma membrane sinks inward to form a pocket
•
As the pocket deepens, it pinches in, forming a vesicle containing material from
outside the cell
–
There are 3 types of endocytosis:
•
1) Phagocytosis – cellular eating
•
2) Pinocytosis – cellular drinking
•
3) Receptor-mediated endocytosis
Animation: Exocytosis and Endocytosis Introduction
Phagocytosis
•
Phagocytosis can be considered “cellular eating”
–
A cell engulfs a particle by wrapping pseudopodia around it
•
Pseudopodia package the particle within a membrane-enclosed sac large enough to
be called a vacuole
–
The particle is digested after the vacuole fuses with a lysosome containing hydrolytic
enzymes
Animation: Phagocytosis
Pinocytosis
•
Pinocytosis can be considered “cellular drinking”
–
The cell “gulps” droplets of extracellular fluid into tiny vesicles
• The fluid itself is not needed but rather the molecules dissolved in droplets
are required
–
Pinocytosis is a nonspecific form of transport:
• Any and all solutes are taken into the cell
Animation: Pinocytosis
Receptor-Mediated Endocytosis
•
Receptor-mediated endocytosis allows cells to acquire bulk quantities of specific substances,
even if these substances are not very concentrated in the extracellular fluid
–
Proteins in the plasma membrane have specific receptor sites exposed to the
extracellular fluid
•
–
These receptor proteins are usually clustered in regions of the membrane called
COATED PITS (called this because they are lined on their cytoplasmic side by a
fuzzy layer of coat proteins)
The specific substances that bind to the
receptors are called LIGANDS
•
When binding occurs, the coated pit forms
a vesicle containing the ligand molecules
•
After the ingested material is freed into
the cell from the vesicle, receptors are
recycled to the plasma membrane by
the same vesicle
Animation: Receptor-Mediated Endocytosis
Concept Check 7.5
• 1) As a cell grows, its plasma membrane expands.
Does this involve endocytosis or exocytosis? Explain.
• 2) To send a signal, a neuron may carry out
exocytosis of signaling molecules that are recognized
by a second neuron. In some cases, the first neuron
ends the signal by taking up the molecules by
endocytosis. Would you expect this to occur by
pinocytosis or by receptor-mediated endocytosis?
Explain.
You should now be able to:
1. Define the following terms: amphipathic
molecules, aquaporins, diffusion
2. Explain how membrane fluidity is influenced
by temperature and membrane composition
3. Distinguish between the following pairs or
sets of terms: peripheral and integral
membrane proteins; channel and carrier
proteins; osmosis, facilitated diffusion, and
active transport; hypertonic, hypotonic, and
isotonic solutions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
4. Explain how transport proteins facilitate
diffusion
5. Explain how an electrogenic pump creates
voltage across a membrane, and name two
electrogenic pumps
6. Explain how large molecules are transported
across a cell membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings