Rahwa Netsanet
10/4/09
Membrane reading guide
1. What does selective permeability mean and why is that important to cells?
Selective permeability means that a cell’s membrane is thin or porous enough to
let certain molecules pass based on their size. This is especially important when
it comes to evening out the concentration of solutions within and out of the cell.
A selectively permeable cell will allow the molecules of a solute, if they are small
enough, to pass through to the inside or outside, going down their concentration
gradient (from high to low concentration), so the cell can reach equilibrium in
regards to the particular solution. This applies to water quantities too, and a
cell’s even concentration of water is vital for living organisms since it makes up
some 70% of the body.
2. What is an amphipathic molecule?
An amphipathic molecule is an important transport molecule in the body which
has both a hydrophilic (polar) end and a hydrophilic (non-polar) end. This makes
it vital for the body so it can transport hydrophobic substances like lipids (fat)
around in the blood vessels easily. It bonds to the fat cells so they can be moved
through the blood vessels, otherwise the hydrophobic lipids would cause an
obstruction in the vessels.
3. How is the fluidity of a cell’s membrane maintained?
Cells are able to regulate the fluidity of their plasma membranes to meet their
particular needs by synthesizing more of certain types of molecules, such as
those with specific kinds of bonds that keep them fluid at lower temperatures.
The presence of cholesterol and glycolipids, which are found in most cell
membranes, can also affect molecular dynamics and inhibit phase transitions.
4. Label the diagram below – for each structure – briefly list its function:
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extracellular matrix – matrix of fibers outside the cell of animals that usually provides
structural support for the cell along with other functions. It makes up part of the
connective tissue in animals.
carbohydrate – work in cellular recognition, or cell to cell recognition, letting the cell
“know” what it can or cannot interact with.
glycoprotein – this is used in cellular recognition also, when cell membranes bump,
the cells use their glycoproteins to identify what type of cell is touching them.
cytoskeleton – dynamic structure within cytoplasm of all cells that aids in
maintenance of cell shape, protection of cell, enables cellular motion, plays a role in
intracellular transport and cell division.
cholesterol – cholesterol comes between phospholipids to add flexibility to the cell’s
membrane, a cell functions better with a more fluid, less rigid, membrane.
glycolipid – though not common, a cell may use glycolipids as “anchors”.
peripheral protein – regulate cell signaling, “id”-ing, among other important cell
functions.
integral protein – transport channels for substances entering and leaving cell.
5. List the six broad functions of membrane proteins.
Transport, enzymatic activity, signal transduction, cell-cell recognition,
intercellular joining, attachment to the cytoskeleton and extracellular matrix
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6. How do glycolipids and glycoproteins help in cell to cell recognition?
Most glycolipids are covalently bonded to glycoproteins. Carbohydrates on the
external side of the membrane vary from one another, and even from those on
the same cell, or the same type of cell in one individual. This diversity of
molecules and their location on the cell’s surface distinguish one cell from
another, so that an adjacent cell can identify it using the glycolipid/glycoprotein
embedded in the cell membrane.
7. Why is membrane sidedness an important concept in cell biology?
Membrane sidedness is an important concept because it explains which face of
the cell membrane substances will end up on after they are moved from their
original starting place. For example, molecules that begin on the inside face of
the ER will end up on the outside face of the plasma, or cell, membrane.
8. What is diffusion and how does a concentration gradient relate to passive
transport? Why is free water concentration the “driving” force in osmosis?
Diffusion is the tendency of the molecules of any substance to evenly distribute
themselves over an available space, going down their concentration gradient
(from high to low concentration). The concentration gradient is essential to
passive transport because a semi-permeable cell membrane will allow small
molecules to simply pass through the small pores of the membrane to where
there is more available room. The free water molecules are the molecules that
can freely move through the semi permeable membrane to even out the
concentration of a solute on either side of the membrane. Water molecules that
are attracted or clustered around the solute molecules cannot move through the
membrane with their “baggage”, making free water molecules specifically the
actively important part, or “driving” force, of osmosis.
9. Why is water balance different for cells that have walls as compared to cells
without walls?
Cells with walls exert a pressure onto the cell so water uptake in a hypotonic
environment decreases, while those without walls simply expand and possibly
lyse. In a hypertonic environment, cells with walls react with plasmolysis, where
the plasma membrane detaches from the cell wall, and cells without walls shrivel.
10.
Label the diagram below:
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11. What is the relationship between ion channels, gated channels and facilitated
diffusion?
Ion channels, some of which are gated channels, help the transport of
substances through the semi permeable membrane that cannot diffuse by itself
when they are stimulated (by electric or chemicals).
12. How is ATP specifically used in active transport?
One way ATP can power active transport is by transferring its terminal phosphate
group directly to the transport protein, which may reconfigure the protein to allow
it to transport to the substance it is attached to through the plasma membrane.
13. Define and contrast the following terms: membrane potential, electrochemical
gradient, electrogenic pump and proton pump.
The membrane potential is the voltage across a membrane. It acts like an
energy source that affects the movement and traffic of all the electrically charged
substances across a membrane. The electrochemical gradient is the result of
the effect of the membrane potential on its ion movement, specifically driving the
concentration of ions on either side of the membrane. An electrogenic pump is a
transport protein that generates electrical voltages across the membrane, found
in animal cells. A proton pump generates voltage by movement on hydrogen
ions (protons) across the membrane out of the cell. The membrane potential
seems to be the accumulated result of the active work of electrogenic pumps and
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proton pumps, and also is affected and affects the electrochemical gradient of a
substance. In contrast, electrochemical gradient does not store, but uses
electrochemical energy to move ions through the cell membrane. The
electrogenic pump produces electrical energy though pumping positive ions of
different substances through the cell while the proton pump’s function only
includes the pumping of one positive ion, the hydrogen ion, which is just a proton,
through the cell membrane.
14. What is cotransport and why is an advantage in living systems?
When a single ATP-powered pump that transports a specific solute can indirectly
drive the active transport of several other solutes. This function has played a role
in helping the rehydration of people in countries where dehydration and death of
the symptoms that come with that is high. The process of contransportation has
shown how rehydration is possible.
15. What is a ligand?
“Ligand” is a general term for any molecule that binds itself specifically to the
active site of another molecule.
16. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated
endocytosis.
Phagocytosis is when a vesicle takes in solid substances, no one substance in
particular, from outside the cell for lysosomes to digest. Pinocytosis is when a
vesicle takes in some of the extracellular fluid so the substances in the fluid, no
one substance in particular, can be used for the cell. Receptor-mediated
endocytosis is when the cell uses membrane proteins that bind with ligands to
attract specific molecules from outside the cell, then form a vesicle to bring it in.
The difference is that phagocytosis brings in unspecific solid substances, while
pinocytosis brings in unspecific fluid substances, and receptor-mediated
endocytosis brings in specific molecules from outside the cell.
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