Bio 6
Study Guide Chapter 7
Chapter 7: Membrane Structure and Function
Learning Objectives
1. Explain the fluid mosaic model for membrane structure.
The fluid mosaic model of a cell membrane structure consists a phospholipid
bilayer with proteins of different types moving freely within a fluid bilayer of
phospholipids. The fluid aspect of the membrane is due to the lateral and rotational
movement of phospholipids, and embedded proteins account for the mosaic aspect.
Since the membrane is held together by relatively weak hydrophobic interactions, the
lipids can move laterally.
 Cholesterol in animal cell membranes provides some rigidity to the
membrane.
 In plants, the cell wall provides support.
2. Describe the properties of phospholipids and their arrangement in cellular
membranes.
1.barrier about 8 nm thick, separates living cell from its surrounding
2. phospholipids and proteins are the main macromolecules in membranes
3. constituents are amphipathic molecules meaning they have hydrophobic
regions and hydrophilic regions
4. selectively permeable allows some substances to cross more easily than
others
5. The membrane is a mosaic of proteins dispersed within a lipid bilayer.
Note to self: Protein function in transport, signal transduction, cell
recognition, and cell adhesion.
Integral proteins contact the hydrophobic core and some (transmembrane
proteins) span the entire membrane.
Peripheral proteins are loosely bound to the surface of the membrane.
Carbohydrates are covalently bound to either lipids or proteins.
3. Describe the fluid properties of the cell membrane and explain how membrane
fluidity is influenced by membrane composition. (temperature, cholesterol,
saturation)
https://youtu.be/BWQCAsM-CF4 & (fig 7.5)
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A membrane is held together primarily by hydrophobic interactions,
which are much weaker than covalent bonds.
High temperatures=more fluidity & phospholipids spread apart more.
low temperatures= less fluidity & phospholipids are closer (cyrstalized*)
However, temperature at which a membrane solidifies depends on the types of
lipids.
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Study Guide Chapter 7
Unsaturated hydrocarbon tails (kinked) prevent packing, enhancing
membrane fluidity.
Saturated hydrocarbon tails pack together, decrease membrane fluidity.
Cholesterol at at low temperatures its higher fluidity by spacing packing
of phospholipids.
But at higher temperatures decreases membrane fluidity by reducing
phospholipid movement.
4. Describe how proteins and carbohydrates are spatially arranged in cell
membranes and how they contribute to membrane function.
The proteins are embedded in the membrane. They act as transport
molecules to move material in and out of the cell (kind of like a tunnel). These
proteins are needed due to the phospholipid bilayer. The inside of the bilayer
repels water, so this keeps all materials out of the cell unless it has been
selected to be carried in by the proteins.
The carbohydrates are located on the outside of the membrane. Cells
recognize other cells by binding to molecules, often containing carbohydrates.
5. Describe factors that affect the selective permeability of membranes.
The lipid bilayer is only one aspect of the gatekeeper system responsible for
a cell’s selective permeability.
1) Nonpolar molecules, such as hydrocarbons, CO2, and O2, are hydrophobic.
They can therefore dissolve in the lipid bilayer of the membrane and cross it easily,
without the aid of membrane protein.
2) Polar molecules such as glucose and other sugars pass only slowly through a
lipid bilayer, and even water, a very small polar molecule, do not cross rapidly.
6. Describe the locations and functions of transport proteins. (7.7****)
Since specific ions and a variety of polar molecules can’t move through cell
membranes on their own. Hydrophilic substances pass through transport proteins that
span the membrane.
One kind is, Channel proteins, function by having a hydrophilic channel that certain
molecules or atomic ions use as a tunnel through the membrane.
Ex. Aquaporins: allow water to diffuse very rapidly across the membranes.
Other transport proteins are called carrier proteins. This transport proteins shuttles a
substance from one side to the other by changing shape. Some of these proteins
hydrolyze ATP as an energy source to actively pump substances
7. Define diffusion. Explain what causes diffusion and why it is a spontaneous
process.
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Study Guide Chapter 7
Diffusion is the tendency for a substance to diffuse from where it is more
concentrated to where it is less concentrated (aka the substance will diffuse down
its concentration gradient.) No (energy) required; so it is spontaneous and therefore
each substance diffuses down its own concentration gradient, unaffected by the
concentration gradients of other substances.
8. Explain what regulates the rate of passive transport.
Concentration gradients regulate the rate of passive transport.
9. Explain why a concentration gradient across a membrane represents potential
energy.
A concentration gradient represents potential energy because of the
structure/ location of the molecules it has potential to diffuse.
10. Distinguish between hypertonic, hypotonic, and isotonic solutions.
Hypotonic- a solution which contains more solute than solvent
Hypertonic- a solution which contains more solvent than solute
Isotonic solutions- solution in which the solute and solvent are equally distributed
11. Define osmosis and predict the direction of water movement based on
differences in solute concentrations.
Water diffuses out through the permeable membrane of a cell in a process
called (osmosis). Water diffuses across a membrane from the region of lower
solute concentration to the region of high solute concentration until the solute
concentration is equal on both sides.
Ex. if the solution outside has a higher solute concentration (hypertonic)
than the cytosol; water enters the cell if the solution has a lower solute
concentration (hypotonic). If the concentrations are equal (isotonic), no net
osmosis occurs.
12. Describe how living cells with and without walls regulate the balance of water
content.
Living cells with walls will expand only so much before it exerts a back
pressure on the cell that opposes further water uptake. Cells without walls cannot
tolerate excessive uptake nor excessive loss of water so they have other
adaptations like a large contractile vacuole seen in ex. P. caudatum cell doesn’t
burst.
13. Explain how transport proteins are similar to enzymes.? Not sure
- They may have specific binding sites for the solute. Transport proteins
can become saturated when they are translocating.
-Transport proteins can be inhibited by molecules that resemble the
normal "substrate."
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Study Guide Chapter 7
14. Explain how transport proteins facilitate diffusion.
In facilitated diffusion, transport proteins speed the passive movement of
molecules across the plasma membrane. Transport proteins are transmembrane
proteins that allow passage of hydrophilic substances across the membrane.
-aquaporins, facilitate massive amounts of diffusion.
15. Explain how active transport differs from diffusion.
Active transport- uses ATP energy, Active transport moves substances
against their concentration gradient. This requires the cell to expend its own
metabolic energy, , goes to less concentration to higher. Active transport is
performed by specific proteins embedded in the membranes. Active transport
allows cells to maintain concentration gradients that differ from their surroundings.
Diffusion- uses no energy in passive transport down it’s concentration gradient. The
spontaneous movement of a substance down its concentration or electrochemical
gradient from a region where it is more concentrated to a region it is less
concentrated.
16. Describe the mechanism of the sodium-potassium pump.
Sodium potassium pump is a mechanism where ions are pushed against
their concentration gradient across cell membrane. This can be achieved by
active transport and by using cellular energy.
17. Explain what mechanism can generate a membrane potential or
electrochemical gradient.
Membrane potential uses voltage across membranes against their
concentration gradient, which requires ATP. Some membrane proteins that
actively transport ions contribute to the membrane potential.
Ex. sodium-potassium pump!. Pumps three sodium ions out of the cell for every two
potassium ions it pumps into the cell.
Electrochemical gradient uses two forces: the chemical force (the ion's
concentration gradient) and the electrical force (membrane potential) on the ion's
movement across the cell membrane
1. The concentration gradient of an ion
2. Negatively charged proteins inside the cell
3. Plasma membrane's selective permeability to various ions
4. The Na - K pump
Ex. For example, the concentration of Na+ inside a resting nerve cell is much lower than
outside it. When the
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Study Guide Chapter 7
cell is stimulated, gated channels open that facilitate Na+ diffusion. Sodium ions then
“fall” down their electrochemical gradient, driven by the concentration gradient of Na+
and by the attraction of these cations to the negative side (inside) of the membrane.
18. Describe the process of co-transport (fig 7.18)
Cotransport of two solutes occurs when a membrane protein enables the “downhill”
diffusion of one solute to drive the “uphill” transport of the other. They use an
electrochemical gradient as a means of energy. It works by binding to two molecules or
ions at a time and using the gradient of one solute's concentration to force the other
molecule or ion against its gradient.
19. Explain how large molecules are transported across the cell membrane.
Large molecules(proteins and polysaccharides) are transported across the
membrane packaged in vesicles by exocytosis and endocytosis. Require
Energy.
20. Compare pinocytosis and receptor-mediated endocytosis.
pinocytosis- A type of endocytosis in which the cell ingests extracellular fluid
and its dissolved solutes. "cellular drinking," the cell engulfs drops of fluid by
pinching in and forming vesicles that are smaller than the phagosomes formed in
phagocytosis.
receptor-mediated- substances bind to specific receptor proteins, enabling the
cell to ingest and concentrate specific substances in protein coated vesicles.
Vocabulary
selective permeability
amphiphathic
fluid mosaic model
integral proteins
peripheral proteins
transport proteins
diffusion
concentration gradient
passive transport
hypertonic
hypotonic
isotonic
osmosis
osmoregulation
turgid
plasmolysis
facilitated diffusion
gated channels
active transport
ligands
membrane potential
electrochemical gradient
electrogenic pump
proton pump
cotransport
exocytosis
phagocytosis
pinocytosis
receptor-mediated endocytosis
sodium-potassium pump
Word Roots
Amphi = dual
Co = together
aqua = water
trans = across
-pori = a small opening
electro = electricity
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-genic = producing
exo = outer
hypo = lower
pino = drink
Study Guide Chapter 7
endo = inner
hyper = exceeding
iso = same
plasm = molded
cyto = cell
-tonus = tension
phago = eat
-lyso = loosen
selective permeability – A property of biological membranes that allows them to
regulate the passage of substances across them.
amphiphathic
fluid mosaic model – The currently accepted model of cell membrane structure, which
envisions
the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of
phospholipids.
integral proteins – are embedded in the lipid bilayer;
peripheral proteins
- are attached to the membrane surface.
transport proteins – A transmembrane protein that helps a certain substance or class
of closely related substances to cross the membrane.
diffusion
- The random thermal motion of particles of liquids, gases, or solids. In
the presence
of a concentration or electrochemical gradient, diffusion results in the net movement
of a substance from a region where it is more concentrated to a region where it is less
concentrated.
concentration gradient – A region along which the density of a chemical substance
increases or decreases.
passive transport - The diffusion of a substance across a biological membrane with
no expenditure of energy.
hypertonic - Referring to a solution that, when surrounding a cell, will cause the cell
to lose water.
hypotonic - Referring to a solution that, when surrounding a cell, will cause the cell to
take up water.
isotonic – Referring to a solution that, when surrounding a cell, causes no net
movement of water into or out of the cell.
osmosis – The diffusion of free water across a selectively permeable membrane.
osmoregulation- Regulation of solute concentrations and water balance by a cell or
organism.
turgid - Swollen or distended, as in plant cells. (A walled cell becomes turgid if it has a
lower water potential than its surroundings, resulting in entry of water.)
plasmolysis- A phenomenon in walled cells in which the cytoplasm shrivels
and the plasma membrane pulls away from the cell wall; occurs when the cell loses
water to a hypertonic environment.
facilitated diffusion- The passage of molecules or ions down their electrochemical
gradient
across a biological membrane with the assistance of specific transmembrane transport
proteins, requiring no energy expenditure.
gated channels- (gated ion channel) A gated channel for a specific
ion. The opening or closing of such channels may alter a cell’s membrane potential.
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active transport- The movement of a substance across a cell membrane against its
concentration or electrochemical gradient,
mediated by specific transport proteins and requiring an expenditure of energy.
ligands- A molecule that binds specifically to another molecule, usually a larger one.
membrane potential- The difference in electrical charge (voltage) across a cell’s
plasma
membrane due to the differential distribution of ions. Membrane potential affects the
activity
of excitable cells and the transmembrane movement of all charged substances.
electrochemical gradient – The diffusion gradient of an ion, which is affected by both
the
concentration difference of an ion across a membrane (a chemical force) and the ion’s
tendency to move relative to the membrane
potential (an electrical force).
proton pump- An active transport protein in a cell membrane that uses ATP to
transport hydrogen ions out of a cell against their concentration gradient, generating a
membrane potential in the process.
cotransport- The coupling of the “downhill” diffusion of one substance to the “uphill”
transport
of another against its own concentration gradient.
exocytosis- The cellular secretion of biological molecules by the fusion
of vesicles containing them with the plasma membrane.
phagocytosis- A type of endocytosis in which large particulate substances
or small organisms are taken up by a cell. It is carried out by some protists and by
certain
immune cells of animals (in mammals, mainly macrophages, neutrophils, and dendritic
cells).
pinocytosis- A type of endocytosis in which the cell ingests extracellular fluid and its
dissolved solutes.
receptor-mediated endocytosis- The movement of specific molecules into a cell by
the infolding of vesicles containing proteins with receptor sites specific to the molecules
being taken in; enables a cell to acquire
bulk quantities of specific substances.
sodium-potassium pump- A transport protein in the plasma membrane of animal cells
that
actively transports sodium out of the cell and potassium into the cell.
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