Answers to Review Questions - Chapter 5
1. What molecules are responsible for the physical properties of a cell membrane?
The molecules primarily responsible for the physical properties of the cell
membrane are the amphipathic phospholipids composing the lipid bilayer. Also, the
cholesterol molecules, the proteins (peripheral and integral), and surface molecules,
such as glycoproteins, add even more complex properties.
2. How do the hydrophilic and hydrophobic regions of the protein affect its
orientation?
Transmembrane proteins are usually folded into an alpha helix and tend to wind
their way back and forth within the cell membrane. They are three-dimensionally
folded to have their hydrophilic portions at both free edges of the lipid bilayer, with
the hydrophobic portion in between. This matches the hydrophilic outer and inner
layers of the lipid bilayer, with the hydrophobic tails between them.
3. What is the pathway used by cells to place carbohydrates on plasma membrane
proteins? How does this pathway result in the carbohydrate groups being exposed
on only one side of the lipid bilayer?
Plasma membrane proteins with carbohydrates attached to them originate on the
ribosomes on the rough ER. Carbohydrates are added when the protein is embedded
in the membrane of the ER; therefore the carbohydrate is attached on the inner
portion. From there, this portion buds, and the resultant vesicle passes to the Golgi
complex for possible further modification. Then it passes (again, with the
carbohydrate on the inner surface of the vesicle) to the plasma membrane. The
membrane of the vesicle bearing this protein-carbohydrate complex fuses with the
plasma membrane, and in turning inside out, presents the carbohydrate on the outer
surface of the cell membrane.
4. Give three important functions performed by the plasma membrane of a cell.
The plasma membrane functions as the boundary of the cell, facilitates in
transport of materials in and out of the cell (either passively or often through protein
channels via active transport), aids in cell to cell recognition, and houses enzymes
that function in various ways, contain proteins that form intercellular junctions, and
also in signal transduction.
5. What is the immediate source of energy for simple diffusion? For facilitated
diffusion? For active transport?
The source of energy for simple diffusion comes from the random motion of
molecules (all atoms/molecules at temperatures above absolute zero have kinetic
energy: energy of motion). The energy for facilitated diffusion also requires this
potential energy of the concentration gradient. The energy for active transport comes
from ATP and is a significant expenditure of energy to a cell.
6. What would happen if a plant cell were placed in a relatively a) isotonic, b)
hypertonic, or c) hypotonic environment? How would you modify your
predictions for an animal cell?
Plant cells placed in a relatively isotonic solution would not change in size; the
exchange of water molecules in and out of the cell would be equal. Plant cells placed
in a hypertonic solution would lose water molecules, and the cell membrane would
shrink away from the cell wall. This condition is called plasmolysis, and the plant
itself would wilt. Plant cells placed in a hypotonic solution would gain water,
swelling against the strong cell wall. This condition is called turgidity. Animal cells
lack the strong cell wall, so they shrink in size when placed in a hypertonic solution,
and will swell and may burst in a hypotonic solution.
7. How are exocytosis and endocytosis similar?
Exocytosis and endocytosis both involve vesicles or vacuoles to transport large
particles (solid or liquid) across the cell membrane. Exocytosis moves materials
outside the cell; endocytosis moves materials into the cell. Both of these are energyrequiring processes, and hence can be classified as types of active transport.