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 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.
4. 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.
5. 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 energy-requiring processes, and
hence can be classified as types of active transport.