1. The phospholipid bilayer forms a viscous fluid in which various globular proteins sluggishly move about. This bilayer forms continuous sheet. Since the tendency of the fatty acids to associate with oe another and exclude water, small holes or rips in a membrane seal themselves spontaneously.. This property helps membranes fuse when a cell membrane is ruptured by a sharp object.. Even though the phospholipid bilayer stabilizes the entire structure, the membrane is fluid (phospholipid molecules can move pass one another thus allowing materials to move laterally within the membrane. The phospholipid layer is the gray round structures with tails in the figure below. Some membrane proteins (orange) are relatively free to migrate about.
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2. The correct answer is "c". Water molecules are polar and can not
pass through the hydrophobic portion of the membrane; therefore, for these
molecules to pass through they must move through pores (openings), or
intrinsic channeled proteins.
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3. The correct answer is "a", true. The phospholipid molecules contain
two ends with opposite solubility properties. The phosphorus-containing
head is hydrophilic and the two fatty acid chains are hydrophobic.
When placed in water these molecules will mix together (no
covalent or ionic bonding) so that the nonpolar potions will mix orienting
the polar head heads to the outside to dissolve in the water. The hydrophobic
tails will mix together in the interior of the bilayer. See figure below.
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4. This statement is false. The phospholipids are not covalently bonded
together. To some extent the cholesterol molecules and the cytoskeleton
may anchor part of the membrane to partially stabilize its structure.
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5. The correct answer is "d". Proteins provide little support in stabilizing
the bilayer. Proteins in the membrane function in regulating specific
molecules which may cross the membrane, acting as triggers for external
molecules may come into contact and controlling a cellular
response, catalyzing chemical reactions on the membrane surface.
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6. The correct answer is B (green). These are membrane carbohydrates
which may act as identification tags. They are crucial in recognizing specific
molecules. the carbohydrates are attached to lipid or protein molecules,
mostly on the outside of the membrane, where they protrude into the environment,
away from the cell.
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7. The correct answer is C. These are intrinsic proteins with
a channel or a carrier protein.
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8. The correct answer is D (gray). This phospholipid portion of the
membrane will prevent polar molecules from passing across the membrane
but will allow nonpolar ethanol to pass through.
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9. The correct answer is E. Cholesterol straightens and provide some
structure in the phospholipid bilayer.
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10. The correct answer is F. The cytoskeleton consists
of protein microfilaments or tubules which will anchor the membrane to
various structures (organelles) in the cytoplasm. This will provide structure
for the membrane and position the organelles at specific locations within
the cell.
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11. This is the wrong answer. Selectively permeability is not due to
the fluid nature of protein and proteins located in the bilayer are not
stationary.
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12. Leukemia is a type of cancer where phagocytotic white blood cells
are greatly increased in the circulatory system. These cells will
engulf various structure they come into contact with, including white blood
cells. With this increased number they will engulf the individual's
red blood cells at a faster rate than they can be formed causing the individual
to become anemic.
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13. The correct answer is "d". Receptor proteins on the surface of
the cell acts as receptors which recognize and trigger phagocytosis of
specific molecules.
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14. The correct answer is "b". The process being described here is
phagocytosis. Large structures are engulfed during this process.
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15. The correct answer is "a". The process being described here is
pinocytosis. Usually only solutions are engulfed during this process.
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16. The contents of the vesicle is being expelled to the outside.
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17. Desmosome are like spot welds between two cell membranes holding
the two
cells together. Gaps would appear between
the spot welds.
Tight junctions are like a stream of
weld between two cells holding them
together and preventing
leakage between the cells on the outside surface.
Material may get through a tight junction
only by entering the cells themselves.
Gap Junctions are similar to plasmodesmata
in plant cells but consist of protein.
They are intrinsic membrane proteins
of two adjacent animal cells aligned with
one another with a channel passing through
both proteins. Gap junctions allow
chemical substances or electrical
signals to pass form cell to cell..
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18. The correct answer is "a". Plasmodesmata are cytoplasmic strands
through pores located in the cellulose cell walls of two adjacent
plant cells.
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19. The correct answer is "a".
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Created by the Center for Learning Technologies, Academic Technology Services.
Last modified October 22, 1997.