110
UNIT ONE
b. Diffusion is movement of atoms, molecules, or ions
from regions of higher concentration toward regions of
lower concentration (down a concentration gradient).
c. It exchanges oxygen and carbon dioxide in the body.
d. The most important factors determining the rate
of diffusion in the body include distance, the
concentration gradient, and temperature.
2. Facilitated diffusion
a. Facilitated diffusion uses protein channels or carrier
molecules in the cell membrane.
b. This process moves substances such as ions,
sugars, and amino acids from regions of higher
concentration to regions of lower concentration.
3. Osmosis
a. Osmosis is a process in which water molecules
move through a selectively permeable membrane
toward the solution with greater osmotic pressure.
b. Osmotic pressure increases as the number of
impermeant solute particles dissolved in a solution
increases.
c. A solution is isotonic when it contains the same
concentration of dissolved particles as the cell
contents.
d. Cells lose water when placed in hypertonic solutions
and gain water when placed in hypotonic solutions.
4. Filtration
a. In ±
ltration, molecules move through a membrane
from regions of higher hydrostatic pressure toward
regions of lower hydrostatic pressure.
b. Blood pressure ±
lters water and dissolved
substances through porous capillary walls.
5. Active transport
a. Active transport moves molecules or ions from
regions of lower concentration to regions of higher
concentration.
b. It requires cellular energy and carrier molecules in
the cell membrane.
6. Endocytosis
a. In pinocytosis, a cell membrane engulfs tiny
droplets of liquid.
b. In phagocytosis, a cell membrane engulfs solid
particles.
c. In receptor-mediated endocytosis, receptor
proteins combine with speci±
c molecules in the
cell surroundings. The membrane engulfs the
combinations.
7. Exocytosis
a. Exocytosis is the reverse of endocytosis.
b. In exocytosis, vesicles containing secretions fuse
with the cell membrane, releasing the substances to
the outside.
8. Transcytosis
a. Transcytosis combines endocytosis and exocytosis.
b. In transcytosis, a substance or particle crosses a cell.
c. Transcytosis is speci± c.
3.4
THE CELL CYCLE (PAGE 100)
1. The cell cycle includes interphase, mitosis, cytoplasmic
division, and differentiation.
2. Interphase
a. Interphase is the stage when a cell grows, DNA
replicates, and new organelles form.
b. It terminates when the cell begins mitosis.
g. Proteins provide the special functions of the
membrane, as receptors, cell surface markers of
self, transporters, enzymes, and cellular adhesion
molecules.
h. Cell adhesion molecules oversee some cell
interactions and movements.
4. Cytoplasm
a. Cytoplasm contains networks of membranes and
organelles suspended in fl
uid.
b. Ribosomes are structures of protein and RNA that
function in protein synthesis.
c. Endoplasmic reticulum is composed of connected
membranous sacs, canals, and vesicles that
provide a tubular communication system and an
attachment for ribosomes; it also functions in the
synthesis of proteins and lipids.
d. Vesicles are membranous sacs containing
substances that recently entered or were produced
in the cell.
e. The Golgi apparatus is a stack of fl
attened,
membranous sacs that package glycoproteins for
secretion.
f. Mitochondria are membranous sacs containing
enzymes that catalyze the reactions that release
energy from nutrient molecules and change it into
a usable form.
g. Lysosomes are membranous sacs containing
digestive enzymes that destroy debris and worn-
out organelles.
h. Peroxisomes are membranous, enzyme-containing
vesicles.
i. The centrosome is a nonmembranous structure
consisting of two centrioles that aid in the
distribution of chromosomes during cell division.
j. Cilia and fl
agella are motile extensions on some
cell surfaces.
(1) Cilia are tiny, hairlike structures that wave,
moving fl
uids across cell surfaces.
(2) Flagella are longer extensions.
k. Micro± laments and microtubules are threadlike
structures built of proteins that aid cellular
movements and support and stabilize the cytoplasm.
l. Cytoplasm may contain nonliving cellular products,
such as nutrients and pigments, called inclusions.
5. Cell nucleus
a. The nucleus is enclosed in a double-layered
nuclear envelope that has nuclear pores that
control movement of substances between the
nucleus and cytoplasm.
b. A nucleolus is a dense body of protein and RNA
where ribosome synthesis occurs.
c. Chromatin is composed of loosely coiled ±
bers of
protein and DNA that condense into chromosomes
during cell division.
3.3
MOVEMENTS INTO AND OUT OF THE
CELL (PAGE 90)
Movement of substances into and out of the cell may use
physical or physiological processes.
1. Diffusion
a. Diffusion is due to the random movement of
atoms, molecules, or ions in air or liquid solution.
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