A8 fiffAnswers
Q2: Why can’t ions (such as Na+) cross the plasma membrane
without the help of a transport protein?
A2: The electrical charges on ions make them hydrophilic, so they
cannot cross the lipid portion of the bilayer, which is hydrophobic,
without the help of a transport protein.
Q3: If no energy were available to the cell, what forms of transport
would not be able to occur? What forms of transport could occur?
(Hint: Look ahead at Figures 4.5 and 4.6.)
A3: No form of active transport is possible without the input
of energy. Any form of passive transport, including diffusion
and osmosis, could occur even in the absence of an energy
source.
Figure 4.5
Q1: Is the dye at equilibrium in any of these glasses? Describe how
the first glass will look when the dye is at equilibrium with the
water.
A1: Yes, the dye is at equilibrium in the third glass. If the first
glass reaches equilibrium, it will look like the third glass.
Q2: Will diffusion mix the molecules of dye evenly through the
water, or is it necessary to shake the container to get a uniform
mixture?
A2: Diffusion is sufficient to mix the dye thoroughly, but it is a very
slow process and you would probably get tired of waiting.
Q3: Will diffusion mix the dye faster in hot water than in
cold water? Why or why not? (Hint: Review the discussion of
the behavior of water molecules at different temperatures in
Chapter 3.)
A3: Diffusion is faster at higher temperatures because the water
molecules have more energy, form and break hydrogen bonds at a
higher rate, and hence move around more rapidly.
Figure 4.6
Q1: What would the second diagram look like if the pores in the
semipermeable membrane were too small to allow water molecules
to pass through?
A1: The second diagram would look the same as the first diagram,
since neither the sugar molecules nor the water molecules would
be able to pass through the membrane.
Q2: What would the second diagram look like if the pores were
large enough to let both water molecules and sugar molecules
through?
A2: If the pores were large enough for sugar molecules to
pass through, some sugar molecules would diffuse down the
concentration gradient of sugar from the left side to the right
while water molecules were diffusing from right to left down the
concentration gradient of water. At equilibrium, the concentration
of sugar and water would be the same on both sides of the
membrane, and the depth of the solution would also be the same
on both sides.
Q3: The fluid in an IV bag is isotonic to blood. What change would
you see in the red blood cells of a patient if a bag of hypertonic
solution was used in error?
A3: A hypertonic solution would be more dilute than blood, so an
IV of hypertonic solution would dilute the blood—that is, increase
the concentration of water in the blood. Now the red blood
cells would have a lower concentration of water than the blood
surrounding them, so water would move by osmosis into the red
blood cells, causing them to swell.Figure 4.7
Q1: If endocytosis itself is nonspecific, how does receptor-
mediated endocytosis bring only certain molecules into a cell?A1: The receptor protein embedded in the plasma membrane
attracts and holds only specific molecules. When they attach, the
plasma membrane bulges inward to engulf the molecule.Q2: What sorts of molecules could be moved by endocytosis or
exocytosis, but not by diffusion?A2: Endocytosis and exocytosis can move molecules that are too
large to pass through the plasma membrane through diffusion.Q3: How does the fluid that enters a cell via pinocytosis differ from
the fluid that enters by osmosis?A3: The fluid that fills the inward bulge of a cell during
pinocytosis may contain molecules that are in solution in the
fluid outside the cell, whereas osmosis allows only water
molecules to enter a cell.Figure 4.8
Q1: What structures do prokaryotic and eukaryotic cells have in
common?A1: A plasma membrane, ribosomes, and DNA.Q2: What cellular processes occur in both prokaryotic and
eukaryotic cells?A2: Both prokaryotic and eukaryotic cells regulate their internal
concentration and the movement of substances in and out. Both
require a supply of energy and carry out metabolic processes. Both
respond to changes in their internal state and to conditions in the
environment around them.Q3: Both plants and animals are eukaryotes, but there are
differences in their cellular structure. What are those differences?A3: Plants have a cell wall and chloroplasts, while animals do not.
Most animal cells also do not contain vacuoles.CHAPTER 5
END-OF-CHAPTER ANSWERS
- c
- a
- c
- b
- d
- opposite, catabolism, produces