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Answers to Review Questions - Chapter 4
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1. How does the plasma membrane help maintain homeostasis?
The plasma membrane is a selectively semi-permeable membrane that acts as a
physical boundary of the cell. Materials that pass the membrane are under control,
and are often specific to that cell type and the metabolic needs of that cell.
2. What organelles belong to the endomembrane system?
The endomembrane system includes the plasma membrane, the nuclear membrane,
the endoplasmic reticulum, the Golgi complex, the lysosomes, and assorted vesicles
and vacuoles.
3. What is the function of the nuclear envelope?
The nuclear envelope is a physical boundary; large structures are held within its
perimeter; the contents of the nucleus is referred to as nucleoplasm. The envelope,
however, has many relatively large pores, which allows many molecules to pass
through the membrane boundary. These pores are bounded by protein complexes that
regulate this passage of molecules.
4. How do the structure and function of the rough ER differ from that of the smooth
ER?
Because the membranes of the rough ER are studded with ribosomes, the function of
the rough ER is the production of proteins. The amino acid sequence is formed on
the ribosomes themselves; the protein chain is further modified within the lumen of
the rough ER, where it attains secondary, tertiary, and if applicable, quaternary
structure. Other molecules such as carbohydrates may be added to the protein
molecules.
Smooth ER is involved in the metabolism of phospholipids, steroids, and fatty acids
in the synthesis of lipids and detoxification of drugs and other toxic compounds.
5. What are the functions of the Golgi complex?
The Golgi complex functions in protein modification. It packages proteins destined
for secretion by the cell, and the lipid membrane covering the vesicles budding from
the Golgi ultimately becomes part of the cell membrane, and act as storage
compartments, or are shuttled to other parts of the cell. Proteins may have other types
of molecules added to them (such as carbohydrates), which aid in diverse functions
such as cellular signaling.
6. What sequence of events must take place for a protein to be manufactured and
then secreted from the cell?
A protein is first assembled on the ribosome, then is moved into the lumen of the
rough endoplasmic reticulum. It attains its 3 dimensional shape here, and may have
other types of molecules chemically bonded to it. These proteins may then pass to
other structures of the cell when they are enclosed in vesicles that bud from the RER.
Further modification may occur in the smooth ER and/or the Golgi complex.
Ultimately, these membrane bound vesicles containing these proteins may stay within
the cytoplasm as storage vesicles, and may migrate to the plasma membrane, fuse
with it, and expel the contents, or form other organelles with specialized functions,
such as the lysosomes.
7. What are the main functions of the cytoskeleton?
The functions of the cytoskeleton include giving shape and structure to the cell, to
allow movement of cellular components or of the cell itself, and in transportation of
materials within the cell.
8. How are microfilaments and microtubules similar? How are they different?
Microtubules are hollow cylinders of –tubulin and -tubulin. They are
approximately 25 nm in diameter, and function in the structure of the cytoplasm and
in the movement of chromosomes during cell division. They are the major
components of cilia and flagella. Microtubule-associated proteins (MAPs) are
classified into structural MAPs and structural MAPs. Microfilaments are composed
of two strands of actin, are about 7 nm in diameter, form mechanical support for
cellular structure, and are associated with myosin in muscle cells to effect contraction.
Microvilli are supported by microfilaments.
9. How are cilia and flagella similar? How are they different?
Both cilia and flagella are supported by microtubules in a characteristic 9 x 2
arrangement, and at their bases, in the basal body, with a 9 x 3 structure. Classically,
the term cilia has been used for numerous short projections, and flagella for longer,
sparse projections, but their internal structure is the same. Both are covered by the
plasma membrane. Their functions include movement of the cell (in single-celled
organisms) and movement of materials over the surface of the cell. Their mechanical
dynamics, however, are similar on an ultrastructural level, but cilia move like oars,
flagella like whips, creating different pushes and pulls.