chapter 7 bio ch_ 7.pdfchapter 7 membrane structure and function ap biology . overview: life at the...
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Chapter 7
Membrane Structure and Function
AP Biology
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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
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Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
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• 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
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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
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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
– Mitochondrial membranes look like a row of beads
• Membranes do not have identical composition
– 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
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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)
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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
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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
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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)
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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
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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
– These molecules are called glycolipids
• Most membrane carbohydrates are bound to proteins
– 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
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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
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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
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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?
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Concept 7.2: Membrane structure results in
selective permeability
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• 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
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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
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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
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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)
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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?
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Concept 7.3: Passive transport is diffusion of a
substance across a membrane with no energy investment
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• 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
Diffusion
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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
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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
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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
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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
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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
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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
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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
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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
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Video: Plasmolysis
Video: Turgid Elodea
Animation: Osmosis
• 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
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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
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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
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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
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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
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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?
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Concept 7.4: Active transport uses energy to move
solutes against their gradients
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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
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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
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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
• Can also transport small uncharged
polar molecules at a very slow rate
– B) Facilitated diffusion: allows transport
of hydrophilic substances
• 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
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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
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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
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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 the
cytoplasm to the extracellular fluid
– 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
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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
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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
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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?
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Concept 7.5: Bulk transport across the plasma
membrane occurs by exocytosis and endocytosis
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• 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
Bulk Transport of Large Molecules
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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
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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
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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
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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
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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
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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.
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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
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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