58 ❯ STEP 4. Review the Knowledge You Need to Score High
Answers and Explanations
- A—Cell walls exist in plant cells and prokaryotic
cells, but not animal cells. They function to
shape and protect cells. - D—The lysosome acts like the stomach of the
cell. It contains enzymes that break down
proteins, lipids, nucleic acids, and carbohydrates.
Absence of these enzymes can lead to storage dis-
orders such as Tay-Sachs disease. - B—The mitochondrion is the power plant of the
cell. This organelle specializes in the production
of ATP and hosts the Krebs cycle and oxidative
phosphorylation. - C—The ribosome is an organelle made in the
nucleolus that serves as the host for protein syn-
thesis in the cell. It is found in both prokaryotes
and eukaryotes. - D—The fluid mosaic model says that proteins
can extend all the way through the phospholipid
bilayer of the membrane, and that these proteins
are of various sizes and lengths. - B—Answer choice B is the definition of active
transport, which requires the input of energy.
Facilitated diffusion (answer choice E), simple
diffusion (answer choices A and D), and osmosis
(answer choice C) are all passive processes that
do not require energy input.- C—Prokaryotes do not contain many organelles,
but they do contain cell walls. - D—This is the description of the smoothendo-
plasmic reticulum. We know that this is a tricky
question, but we wanted you to review the dis-
tinction between the two types of endoplasmic
reticulum. - C—Microtubules play an enormous role in cell
division. They make up the spindle apparatus
that works to pull apart the cells during mitosis
(Chapter 9). A loss of microtubules would crip-
ple the cell division process. Actin fibers (answer
choice D) are the building blocks of microfila-
ments (answer choice A), which are involved in
muscular contraction. Keratin fibers (answer
choice E) are the building blocks of intermediate
filaments (answer choice B), which function as
reinforcement for the shape and position of
organelles in the cell. - E—Lipids are the only substances listed that are
able to freely diffuse across selectively permeable
membranes.