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,
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
stands for gap or growth). Structures other than DNA are
synthesized during the G phases. Cellular growth occurs
then, too (see ±
is a form of cell division that occurs in somatic (non-
sex) cells and produces two daughter cells from an original
. These new cells are genetically identical,
each with the full complement of 46 chromosomes. In con-
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
THE CELL CYCLE
The series of changes that a cell undergoes, from the time it
forms until it divides, is called the
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.
Once thought to be a time of rest,
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.
Movements Into and Out of the Cell
I. Passive (Physical) Processes
A. Simple dif
Molecules move through the phospholipid bilayer From regions oF
higher concentration toward regions oF lower concentration.
Exchange oF oxygen and carbon
dioxide in the lungs
Molecules or ions move across the membrane through channels or
by carrier molecules From a region oF higher concentration to one oF
Movement oF glucose through a cell
Water molecules move through a selectively permeable membrane
toward the solution with more impermeant solute (greater osmotic
Distilled water entering a cell
Smaller molecules are Forced through porous membranes From
regions oF higher pressure to regions oF lower 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
Movement oF various ions and amino
acids through membranes
Membrane engulFs droplets oF liquid From surroundings.
Membrane-Forming vesicles containing
large particles dissolved in water
Membrane engulFs solid particles From surroundings.
White blood cell membrane engul²
Membrane engulFs selected molecules combined with receptor
Cell removing cholesterol-containing
LDL particles From its surroundings
Vesicles Fuse with membrane and release contents outside oF the cell.
Protein secretion, neurotransmitter
Combines receptor-mediated endocytosis and exocytosis to Ferry
particles through a cell.
HIV crossing a cell layer