Izzie Gall
Oct 4, 2009
Period 6
Membranes
1. What does selective permeability mean and why is that important to cells?
Selective permeability is the ability of a membrane to allow only certain chemicals and
other materials to pass into and out of a cell. This is very important to a cell because it
guarantees that ATP, glucose, and other important molecules, along with larger units
such as the organelles of the cell, will stay where the cell can manage them. Selective
permeability also helps prevent alien substances that could harm the cell from entering it.
(www.estrellamountain.edu)
2. What is an amphipathic molecule?
An amphipathic molecule is one with both hydrophilic and hydrophobic regions. The
phospholipids that form a cell’s bilayer membrane are amphipathic molecules.
3. How is the fluidity of cell membranes maintained?
Cell membranes often include cholesterol (this is most common in non-bacterial cells).
Cholesterol is embedded in the hydrophobic middle of the phospholipid bilayer,
separating the tails of the phospholipids and allowing more movement of the membrane.
Other macromolecules, including glycoproteins and integral and peripheral proteins, are
also present in the bilayer helping it maintain fluidity.
4.
Label each structure in the diagram below and briefly list its function:
Glycoprotein: aids in cell-to-cell recognition
Integral proteins: might act as channel proteins, pumping or facilitating ions, protons, or
macromolecules into/out of the cell
Cytoskeleton filaments: maintain cell shape, rigidity, and structure
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Phospholipid bilayer: separates cytoplasm of the cell from cell environment; contains
proteins and glycolipids
Cholesterol: helps maintain cell membrane fluidity
Peripheral protein: helps maintain cell membrane fluidity; also could be an enzyme
5. List the six broad functions of membrane proteins.
a. Enzymatic activity (those not fully embedded in membrane)
b. Cell-to-cell communication
c. Active and passive transport
d. Cell adhesion
e. Maintenance of structure
f. Interaction between cell and environment
6. How do glycolipids and glycoproteins help in cell-to-cell recognition?
Glycolipids and glycoproteins are specially shaped to receive other proteins, located on
the membranes of other cells. When these proteins on either membrane find their corresponding
glycoprotein or –lipid on the other, they fit together in a manner similar to enzyme-substrate fit
(i.e. close to a lock and key). This joining of several proteins between two cell membranes is
cell-to-cell recognition, and is useful in a variety of situations, from virus-cell attachment to
hormone-cell attachment to sperm-oocyte attachment.
7. Why is membrane sidedness an important concept in cell biology?
Membrane sidedness is the condition of membranes, both cell membranes and organelle
membranes, having different proteins, lipids, and other membrane contents inside and out of the
cell. These can be enzymes with different needs in and out of the cell,
8.
What is diffusion and how does a concentration gradient relate to passive transport?
Diffusion is the movement of any substance from an area of high concentration to an area
of low concentration (in other words, with a concentration gradient). Passive transport is the use
of an integral protein channel to ferry a macromolecule across a membrane along its
concentration gradient.
Why is free water concentration the “driving” force in osmosis?
Free water concentration refers to the amount of water that is not attached to a solute or
that is otherwise unable to move freely. Osmosis is a form of diffusion; it involves the movement
of water across a selectively permeable membrane. If a water molecule is bonded to a solute, it
will not be small enough to pass through the membrane. Rather, the water that flows down the
concentration gradient is the “free” water, which makes that the “driving” force of osmosis.
(http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html)
9.
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10. Why is water balance different for cells that have walls as compared to cells without
walls?
Water balance, the flow of water in and out of a system, is different between cells with
and cells without walls. Cells with walls have a restricted amount of space to expand into, so
water will only enter such cells to an extent before having to stop. Cells without walls, by
contrast, can continue to expand into their environments until they burst, since there is no rigid
structure preventing this expansion.
11.
Label the diagram below:
Not walled
Lysed: too much water enters the cell; the cell expands rapidly until the volume is too great for
the surface area and the membrane ruptures
Normal: water concentration is isotonic, at equilibrium
Shriveled: too little water in the cell; the surface area becomes greater than the volume of the cell
Walled
Turgid: straining against the cell wall (a common condition and stable)
Flaccid: water concentration is isotonic, at equilibrium
Plasmolised: too little water in the cell; cell contents recede from the wall
12. What is the relationship between ion channels, gated channels and facilitated diffusion?
Write 1 -2 sentences using those terms correctly.
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An ion channel is a type of integral transport protein. Its specific use is to allow a certain
ion of a certain charge to pass through the membrane, which helps maintain a slight voltage
across the membrane of a cell. A gated channel is a type of ion channel that only allows ions
through the membrane when activated by a ligand (see question number 16). Because the protein
channels are not actively pumping the ions, and the ions are moving on their own down a
concentration gradient, the process is a type of facilitated diffusion.
13. How is ATP specifically used in active transport?
In direct active transport, a molecule of ATP is brought to a carrier protein and undergoes
hydrolysis, breaking into a Pi and an ADP molecule. The protein directly harnesses the energy
released from this hydrolysis reaction and uses it to pump ions or protons across a membrane
against the concentration gradient.
Indirect active transport requires direct active transport to occur first. Once enough ions
have been pumped against their concentration gradient, some ions are allowed to diffuse with the
concentration gradient. The protein in question harnesses the energy released by the more natural
flow of the ions and uses it to pump another protein or ion across the membrane.
14. Define and contrast the following terms: membrane potential, electrochemical gradient,
electrogenic pump and proton pump.
Define
Membrane potential: voltage discrepancy between a cell’s interior and its environment
Electrochemical gradient: similar to a chemical gradient for electrical potential; the
membrane potential is achieved with the help of proton pumps to maintain the electrochemical
gradient
Electrogenic pump: type of ion pump that uses active transport to pump ions against
their concentration gradients, contributing to the electrochemical gradient
Proton pump: a special type of integral protein, similar to an ion pump, specially
equipped to transport H+ protons across a membrane and up their concentration gradient,
contributing to the electrochemical gradient
Contrast
Electrogenic pumps propel ions across membranes against their gradients; proton pumps
do this with H+ protons. This pumping creates the cell’s electrochemical gradient, which helps
define its membrane potential.
15. What is cotransport and why is it an advantage in living systems?
Cotransport is the process of transporting two substances across a membrane
simultaneously. If they are being transported the same direction, the process is called symport; if
they are being transported in opposite directions, antiport. Either one or both of the substances
that is being transported is being sent across the membrane actively, that is, against its
concentration gradient.
This is important in living systems because certain gradients need to be maintained and
cotransport allows more transportation to occur with fewer proteins and thus less space being
taken up. In the specific case of the K+ / Na+ pump, cotransport is very useful for maintaining cell
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membrane potential. In order to function properly, a cell needs to have a higher concentration of
potassium ions, and a lower concentration of sodium ions, than the outside environment.
16. What is a ligand?
A ligand is a substance that binds to membrane proteins such as gated channels (see
question number 12) to signal the opening of the gate and allow ions to pass through, for
example. Similarly to a substrate binding onto an enzyme, the association of a ligand and a
protein is reversible.
17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated
endocytosis.
Phagocytosis is the ingestion of solids, such as bacteria, damaged organelles, and
macromolecules, using vesicles. Phagocytosis engulfs whole particles that are later broken down
in the lysosomes. It is a specific process, engulfing only the particles meant to be later digested.
Pinocytosis is similar, in some ways, to phagocytosis: it is the ingestion of liquids (rather
than solids) using vesicles. The biggest difference between pino- and phagocytosis is that
pinocytosis is very unspecific. The vesicle pinched off the membrane could contain any number
of solutes or other materials that were present in the surrounding environment. In addition,
pinocytosis requires the aid of ATP.
Receptor-mediated endocytosis, like phagocytosis, describes the ingestion of molecules
for later digestion. Unlike pino- or phagocytosis, receptor-mediated endocytosis is initiated by a
ligand. Once a ligand binds to the plasma membrane, a signal is sent to the membrane, which
then is coated with a certain amount of clathrin, a protein used to coat vesicles. Once part of the
membrane is thus coated, the ligand, the protein receptor, and nearby objects are contained in a
vesicle and internalized.
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