100
UNIT ONE
If the cell is developmentally programmed to divide, it
must amass important biochemicals and duplicate much of
its contents so that two cells can form from one. For example,
the cell must replicate DNA and synthesize and assemble the
parts of membranes, ribosomes, lysosomes, peroxisomes,
and mitochondria.
Interphase is divided into phases based on the sequence
of activities. DNA is replicated during S phase (S stands for
synthesis) and is bracketed by two G phases, G
1
and G
2
(G
stands for gap or growth). Structures other than DNA are
synthesized during the G phases. Cellular growth occurs
then, too (see ±
g. 3.35).
Mitosis
Mitosis
is a form of cell division that occurs in somatic (non-
sex) cells and produces two daughter cells from an original
cell
(fig. 3.36)
. These new cells are genetically identical,
each with the full complement of 46 chromosomes. In con-
trast is
meiosis,
a second form of cell division that occurs
only in the cells that give rise to sex cells (sperm and eggs).
Meiosis halves the chromosome number. In this way, when
3.4
THE CELL CYCLE
The series of changes that a cell undergoes, from the time it
forms until it divides, is called the
cell cycle
(F
g. 3.35)
. This
cycle may seem straightforward—a newly formed cell grows
for a time, and then divides in half to form two new cells,
called daughter cells, which, in turn, may grow and divide.
The speci± c events of the cycle are quite complex. For ease
of study, the cell cycle is considered in distinct stages: inter-
phase, mitosis, cytoplasmic division, and differentiation.
The actions of several types of proteins form “check-
points” that control the cell cycle. One particularly impor-
tant checkpoint determines a cell’s fate, whether it will: (a)
continue in the cell cycle and divide; (b) stay specialized and
alive, yet not divide; or (c) die.
Interphase
Once thought to be a time of rest,
interphase
is actually a
very active period. During interphase, the cell grows and
maintains its routine functions as well as its contributions to
the internal environment.
TABLE
3.3
|
Movements Into and Out of the Cell
Process
Characteristics
Source of
Energy
Example
I. Passive (Physical) Processes
A. Simple dif
usion
Molecules move through the phospholipid bilayer From regions oF
higher concentration toward regions oF lower concentration.
Molecular
motion
Exchange oF oxygen and carbon
dioxide in the lungs
B.
±acilitated
dif
usion
Molecules or ions move across the membrane through channels or
by carrier molecules From a region oF higher concentration to one oF
lower concentration.
Molecular
motion
Movement oF glucose through a cell
membrane
C. Osmosis
Water molecules move through a selectively permeable membrane
toward the solution with more impermeant solute (greater osmotic
pressure).
Molecular
motion
Distilled water entering a cell
D. ±iltration
Smaller molecules are Forced through porous membranes From
regions oF higher pressure to regions oF lower pressure.
Hydrostatic
pressure
Molecules leaving blood capillaries
II. Active (Physiological) Processes
A. Active transport
Carrier molecules transport molecules or ions through membranes
From regions oF lower concentration toward regions oF higher
concentration.
Cellular
energy
Movement oF various ions and amino
acids through membranes
B. Endocytosis
1. Pinocytosis
Membrane engulFs droplets oF liquid From surroundings.
Cellular
energy
Membrane-Forming vesicles containing
large particles dissolved in water
2. Phagocytosis
Membrane engulFs solid particles From surroundings.
Cellular
energy
White blood cell membrane engul²
ng
bacterial cell
3. Receptor-
mediated
endocytosis
Membrane engulFs selected molecules combined with receptor
proteins.
Cellular
energy
Cell removing cholesterol-containing
LDL particles From its surroundings
C. Exocytosis
Vesicles Fuse with membrane and release contents outside oF the cell.
Cellular
energy
Protein secretion, neurotransmitter
release
D. Transcytosis
Combines receptor-mediated endocytosis and exocytosis to Ferry
particles through a cell.
Cellular
energy
HIV crossing a cell layer
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