Cellular Structure, Proteins, and Metabolism
55
VI. Mitochondria are the major cell sites that consume oxygen
and produce carbon dioxide in chemical processes that transfer
energy to ATP, which can then provide energy for cell functions.
VII. Lysosomes digest particulate matter that enters the cell.
VIII. Peroxisomes use oxygen to remove hydrogen from organic
molecules and in the process form hydrogen peroxide.
IX. Vaults are cytoplasmic structures made of protein and RNA,
and may be involved in cytoplasmic-nuclear transport.
X. The cytoplasm contains a network of three types of fi laments
that form the cytoskeleton: (1) microfi laments,
(2) intermediate fi laments, and (3) microtubules.
SECTION A KEY TERMS
SECTION A REVIEW QUESTIONS
1. Identify the location of cytoplasm, cytosol, and intracellular
fl uid within a cell.
2. Identify the classes of organic molecules found in cell
membranes.
3. Describe the orientation of the phospholipid molecules in a
membrane.
4. Which plasma membrane components are responsible for
membrane fl
uidity?
5. Describe the location and characteristics of integral and
peripheral membrane proteins.
6. Describe the structure and function of the three types of
junctions found between cells.
7. What function does the nucleolus perform?
8. Describe the location and function of ribosomes.
9. Contrast the structure and functions of the rough and smooth
endoplasmic reticulum.
10. What function does the Golgi apparatus perform?
11. What functions do endosomes perform?
12. Describe the structure and primary function of mitochondria.
13. What functions do lysosomes and peroxisomes perform?
14. List the three types of fi laments associated with the
cytoskeleton. Identify the structures in cells that are composed
of microtubules.
phospholipid
46
plasma membrane
45
prokaryotic cell
45
ribosome
50
secretory vesicle
52
tight junction
48
transmembrane protein
47
tubulin
54
vault
53
actin
53
cadherin
48
cell organelle
45
centriole
54
centrosome
54
chromatin
50
chromosome
50
cilia
54
cristae
52
cytoplasm
45
cytoskeleton
53
cytosol
45
desmosome
48
endoplasmic reticulum
51
endosome
52
eukaryotic cell
45
fl uid-mosaic model
48
gap junction
48
glycocalyx
47
Golgi apparatus
51
integral membrane protein
47
integrin
48
intermediate fi lament
53
intracellular fl
uid
45
lysosome
52
matrix
52
microfi lament
53
microtubule
54
mitochondrion
52
nuclear envelope
50
nuclear pore
50
nucleolus
50
nucleus
45
peripheral membrane
protein
47
peroxisome
53
SECTION B
Proteins
Genetic Code
The importance of proteins in physiology cannot be overstated.
Proteins are involved in all physiological processes, from cell
signaling to tissue remodeling to organ function. This section
describes how cells synthesize, degrade, and, in some cases,
secrete proteins. We begin with an overview of the genetic
basis of protein synthesis.
As noted previously, the nucleus of cells contains DNA,
which directs the synthesis of all proteins in the body.
Molecules of DNA contain information, coded in the
sequence of nucleotides, for protein synthesis. A sequence of
DNA nucleotides containing the information that specifi es the
amino acid sequence of a single polypeptide chain is known
as a
gene.
A gene is thus a unit of hereditary information. A
single molecule of DNA contains many genes.
The total genetic information coded in the DNA of a
typical cell in an organism is known as its
genome.
The human
genome contains roughly 30,000 to 40,000 genes. Recently,
scientists determined the nucleotide sequence of the entire
human genome (approximately 3 billion nucleotides). This is
only a fi rst step, however, because the function and regulation
of most genes in the human genome remain unknown.
It is easy to misunderstand the relationship between genes,
DNA molecules, and chromosomes. In all human cells other
than the eggs or sperm, there are 46 separate DNA molecules
in the cell nucleus, each molecule containing many genes. Each
DNA molecule is packaged into a single chromosome com-
posed of DNA and proteins, so there are 46 chromosomes in
each cell. A chromosome contains not only its DNA molecule,
but also a special class of proteins called
histones.
The cell’s
nucleus is a marvel of packaging. The very long DNA mole-
cules, with lengths a thousand times greater than the diameter
of the nucleus, fi t into the nucleus by coiling around clusters
of histones at frequent intervals to form complexes known as
nucleosomes.
There are about 25 million of these complexes
on the chromosomes, resembling beads on a string.
Although DNA contains the information specifying the
amino acid sequences in proteins, it does not itself participate
directly in the assembly of protein molecules. Most of a cell’s
DNA is in the nucleus, whereas most protein synthesis occurs
in the cytoplasm. The transfer of information from DNA to
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