+Lecture Notes: Unit I: Organization of the Human Body
Chapter Three - The Cellular Level of Organization
I.
Introduction
A. Cytology - study of cell structure
B. Cell Physiology - study of cell function
C. Cytosol- the fluid/gel portion ofthe cell
D. Cytoplasm - cytosol plus its suspended organelles
E. Organelles - recognizable, permanent subcellular structures; each having
a distinct and characteristic shape and function
a. Cytoskeleton (Specializations: Cilia, Flagellum, Spindle Apparatus )
b. Centrosome (2 centrioles surrounded by a pericentriolar region)
c. Ribosomes
d. Endoplasmic Reticulum - SER and RER
e. Golgi Complex
f. Lysosomes
g. Peroxisomes
h. Proteasomes
i. Mitochondria
j. Nucleus
k. Nucleolus
II.
Plasma Membrane
A. Fluid mosaic model - the membrane is composed of a phospholipid bilayer interspersed
with integral proteins which pass through the membrane and peripheral proteins which
attach to either the inner or outer surface
1. Phospholipids are amphipathic having a polar "head" and a nonpolar "tail"
a. the polar heads are directed outward to interact with the water molecules
b. the non-polar tails are directed toward each other
2. Glycolipids - carbohydrates are attached to the head portion of the phospholipid
3. Glycoproteins - carbohydrates are attached to the proteins of the membrane
4. Cholesterol- steroid interspersed within the phospholipid bilayer and is involved with
maintaining the fluidity of the membrane
B. Functions of the cell membrane (aka plasmalemma)
1. Selective permeability - the membrane only allows certain substances through (proteins
serve as channels and transporters)
C. Transport across membranes
I.Simple diffusion
a. Transport from a region of higher concentration to a region of lower concentration
b. Passive, or physical, process as no cellular energy is expended
c. Membrane does not serve as a barrier
d. Example: oxygen and carbon dioxide move by simple diffusion across the respiratory
membrane (alveolar/capillary interface) of lung tissue.
2.Osmosis
a. Transport of water from a region of higher water concentration (low solute conc.) to a
region of lower water concentration (high solute cone.) through special water
channels in the plasmalemma called aquaporins
b. Passive, or physical, process as no cellular energy is expended
c. The plasmalemma serves as a barrier
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d. Types of solutions based upon osmotic potential (aka: tonicity = a measure of the
concentration of solutes in an aqueous solution)
(1) Isotonic solutions (iso = same)
a) Solute concentration on both sides of the membrane barrier are the same.
b) No net movement of water molecules across the membrane.
(2) Hypotonic solutions (hypo = less than)
a) Solute concentration on the outside of the cell's membrane is less than that
ofthe cytosol. (Therefore, the water concentration is greater outside the cell)
b) Water molecules pass from the exterior into the cell causing it to swell, and
eventually to burst (lysis). ( ie: bursting of red blood cells: hemolysis.)
(3) Hypertonic solutions (hyper = greater than)
a) Solute concentration external to the cell is less than that of the cytosol.
(Therefore, the water concentration is less outside the cell)
b) Water molecules pass out ofthe cell into the exterior causing the cell to
shrink (crenation)
3. Facilitated diffusion (transport)
a. Transport of substances from a region of higher conc. to a region of lower concentration
b. The plasmalemma serves as a barrier
c. Protein channels are required
d. Passive, or physical, process as no cellular energy is expended
4. Active transport
a. Movement of substances from a lower concentration to a higher concentration
b. The plasmalemma serves as a barrier
c. Active, or physiological, process as cellular energy is expended
d. Types of active transport:
(1) Primary active transport - substances are directly pumped across the membrane
due to the expenditure of energy ttom ATP (via ATP-ase activity)
(2) Secondary active transport - substances follow other substances after they have
been transported by means of primary active transport
(3) uniport - only one substance is transported across the membrane
(4) coupled transport - more than one substance is transported
(a) symport - substances are transported in the same direction
(b) antiport - substances are transported in opposite directions
(5) vesicular transport-formation of a vesicle as substances are brought into or leave a cell
(a) Endocytosis - substances are taken into the cell
(1) Receptor mediated endocytosis - a specific membrane protein binds to a
specific ligand and thus initiates the vesicle-formation process.
(2) Phagocytosis - large, solid objects are taken into the cell
(3) Pinocytosis - tiny droplets of material are taken up into the cell
(b) Exocytosis - substanced are taken out of the cell when secretory vesicles fuse
with the plasmalemma
III. Cytoplasm (the term which refers to the cytosol plus all the organelles except the nucleus)
A. The cytosol is 75 -90% water in composition
B. It suspends organelles and solutes (ions, glucose, amino acids, fatty acids, proteins, lipids, ATP,
gases, and waste products.
IV.Organelles
P. 3
A. Cytoskeleton - composed of three distinct, structural components:
1. Microfilaments
a. Thinnest diameter, thread-like, but solid core
b. Composed of the protein actin
c. Functions in muscle contraction, cell division, cell locomotion, and for mechanical support
in providing strength and shape of cells as well as supporting microvilli (the nonmotile,
microscopic fingerlike projections of the cell membrane)
2. Intermediate filaments
a. Thread-like with solid core, but larger diameter than microfilaments.
b. Provide strength to anchor organelles in the cell such as the nucleus
3. Microtubules
a. Largest diameter of the cytoskeletal components, more rigid, and have a hollowed center.
b. Composed of the protein tubulin
c. Makes up centrosomes, flagella, and cilia.
B. Centrosome
1. Pericentriolar area
a. A dense network of small protein fibers
b. Organizing center for the mitotic spindle apparatus
2. Centrioles
a. Paired, barrel-shaped structures (one oriented 90* to the other, in a "T" fashion)
b. Composed of nine microtubules arranged in a circular pattern.
C. Cilia and Flagella
1. Composed of microtubules
2. Cilia (singular: cilium)
a. Much smaller and more numerous than flagella
b. Function to transport substances along tissue surfaces (ie: ciliary escalator of the trachea;
movement o/the ovulated oocyte in the uterine tubes of the female reproductive system)
3. Flagella (Singular: flagellum)
a. Longer and fewer than cilia; there is only one human flagellated cell: the spermatozoan
b. The single flagellum is involved with the propulsion of the sperm cell
D. Ribosome
1. Composed ofrRNA and proteins, and consists of two sub-units: one small, one large.
2. Types of ribosomes
a. Free ribosomes
(1) Unattached to any cellular structure (free-floating in the cytosol)
(2) Synthesize proteins used inside the cell
(3) When these encounter an mRNA (copy of the gene) with a specific beginning sequence
(initiation codon), they move and bond to the rough endoplasmic reticulum to become
membrane-bound ribosomes.
b. Membrane-bound ribosomes
(l)Attach to the ER (these attached ribosomes give the ER a "rough" appearance when viewed
under the electron microscope
(2)Produce proteins which are accumulated within the ER and then are able to be transported
for insertion into membranes of the organelles, the plasma membrane or secreted by the
cell (ie: insulin from the beta cells of the pancreas)
c. Polyribosomes (aka: Polysomes)
(1 )Represents a string of many ribosomes (10 - 20) on a single mRNA all involved in the
translation ofthe same protein
(2) Greatly increases the speed at which proteins may be synthesized in the cell
E. Endoplasmic Reticulum (ER)
1. General Characteristics
P. 4
a. Network of membranes which form flattened sacs or tubules (cisterns)
b. Typically associated with the nuclear envelop
2. Types of ER
a. Rough ER (RER)
(1) Continuous with the nuclear envelope
(2) Outer surface has attached ribosomes
(3) Involved with protein synthesis and used for processing and sorting of proteins
b. Smooth ER (SER)
(1) Extends from the rought ER
(2) No ribosomes are present on the outer surface
(3) Produces phospholipids, fats and steroids
F. Golgi Complex (also called the Golgi Apparatus, Golgi Body, or just Golgi)
1. This organelle consists of a stack of 3 - 20 flattened membranous sacs (cisterns)
a. Cis face - the cistern that is directed toward the ER (lies closer to the center of the cell)
b. Trans face - the cistern that is directd toward the plasmalemma (closer to the plasmalemma)
c. Medial cisterns - cisterns in between the cis and trans cisterns.
2. Each cistern contains different enzymes used in the processing of proteins
3. Vesicles associated with the Golgi Complex
a. Transport vesicles carry proteins nom the RER to the cis face where they fuse with the cis cistern.
b. Transfer vesicles release the enzymatically modified proteins from one cistern to the next cistern
in the stack.
c. Secretory vesicles deliver the final proteins from the trans cistern to the plasma membrane where
they fuse and release their contents to the outside of the cell (exocytosis)
d. Storage vesicles remain within the cell and store proteins until needed by the cell (ie: lysosomes
and peroxisomes)
G. Lysosomes
1. Contain up to 40 different kinds of digestive (hydrolytic) enzymes
2. Recycle cellular components and organelles (autophagy)
3. Can be triggered to release contents to cause destruction of its own surrounding cell (autolysis)
which is where they get their nickname "suicide bags" of a cell.
H. Peroxisomes
1. Smaller than lysosomes
2. Contain enzymes which oxidize organic substances such as amino acids, fatty acids; and also
contain catalase, the enzyme which detoxifies hydrogen peroxide (H2O2) to H2O and O2.
Peroxisomes are especially numerous in hepatocytes.
I. Mitochondria
1. Generate ATEP for the cell ("Powerhouse" of the cell)
2. Presence of two membranes
a. Outer, smooth mitochondrial membrane
b. Inner, folded mitochondrial membrane; the folds are referred to as cristae
c. The space between the outer and inner membranes is called the intermembrane space
3. Matrix is the fluid-substance which fills the space surrounded by cristae of the inner membrane
4. Mitochondria contain their own small strand of DNA and their own ribosomes; thus they are
self-replicating and do replicate independently of cell-division.
J. Nucleus
1. Spherical or oval in shape
2. Number(s) of this organelle found in each cell
a. Most cells have a single nucleus (are uni-nucleated cells)
b. Red blood cells (RBC's) have no nucleus (are anucleated cells)
c. Skeletal muscles cells, osteoclasts, and a few other cells have more than one nucleus
(are multi-nucleated cells)
P. 5
3. Nuclear envelope is composed of a double membrane
a. Outer membrane is continuous with the RER
b. Nuclear pores are present in the nuclear envelope which allow for the passage of
the single-stranded RNA molecules out of the nucleus
c. Nucleoplasm is the name of the sol-gel substance inside the nucleus which surrounds
the nucleolus (or nucleoli)
4. Nucleoli
a. Clusters of protein, DNA nucleotides, and RNA nucleotides; usually many per nucleus
b. Responsible for assembling ribosomes from rRNA and proteins
c. Depending upon how much transcription is going on, the nucleoli may be very
diffuse in form; or if not much transcription activity occurring, very dense in form.
5. Nucleus contains the genetic information of the cell in the form of genes on the DNA
a. Genes are arranged along 46 strands called chromosomes
(1) Chromosomes are the visible form of DNA when it condenses prior to cell division
and the genes are NOT accessible for transcription when the DNA is in this form.
(2) Chromatin is the diffuse form of DNA during the interphase (non-mitotic) stage of
the cell's life cycle and the genes ARE accessible for transcription in this form.
(a) Chromatin has a "beads-on-a-string" arrangement
(b) A nucleosome is one "bead" unit consisting of DNA wrapped twice around a
core of eight proteins (histones)
b. The process of RNA synthesis (transcription) occurs here
V. Cellular Processes
A. Four basic processes occur during the "life of a cell": Membrane transport, Protein Synthesis, Specialization, and Cell Division. We have already looked at the physical (passive) and physiological (active)
transport mechanisms by which cells may move useful substances (water, electrolytes, oxygen, and nutrients into its interior) and waste substances (carbon dioxide, excess hydrogen ion, urea, etc.) out.
B. Protein Synthesis
1.Transcription (RNA synthesis)
a. Process which produces the blueprint for making a protein (gene in DNA) is copied (mRNA)
(1) The information stored within the DNA sequence ofnucleotides is "copied" as the
mRNA transcript is assembled; the functional units of mRNA are codons: sequences
of three sequential nucleotides - a base triplet)
(2) Genetic Code
(a) Specifies which codon for which amino acid of the protein is to be sequenced
(b) Universal in nature - all organisms use the same genetic code
(3) Transcription occurs within the nucleus ofthe cell
(4) Three kinds ofRNA are manufactured by the process of transcription:
(a) rRNA (ribosomal) - combines with proteins in order to give rise to ribosomes
(b) tRNA (transfer) - molecule which carries a specific amino acid to the site of and
protein synthesis and matches it up with the appropriate codon ofthe mRNA strand
tRNA molecules have a very specific 3-D shape consisting of two "prongs" on their
top side and revealing an anticodon region, a base-triplet, on their bottom.
(c) mRNA (messenger) - copy ofthe gene for use during translation of protein syn.
b. Steps of Transcription
(1) RNA polymerase travels down the DNA strand until it finds the promoter which is a
special sequence of nucleotide bases which instruct the RNA polymerase to begin
copying the gene.
(2) RNA polymerase follows the complementary base pairing rule to match a C with a
P. 6
G; a G with a C; an A with a T; and a U with an A (Remember, RNA has no T.)
(a) Only one side of the double-helix DNA molecule is used for transcription;
this is called the sense-strand and contains the correct promoter and gene
sequence upon which the mRNA transcript will be assembled.
(b) The anti-sense strand is the “other side of DNA” which does not get transcribed; it merely “hangs out” and waits to spiral around the sense-strand once
transcription is complete.
(c) Introns – portions of the gene which are initially transcribed, but which do
not code for an amino acid; therefore ultimately will be snipped
out and will not become part of the mature mRNA transcript.
(d) Exons – portions of the gene which are initially transcribed and are the
amino-acid encoding sequences; therefore these are spliced together
and become the mature mRNA transcript.
(e) Small, Nuclear Ribonucleoproteins (SNuRP’s) are responsible for “cutting
out” the introns and “splicing together” the introns to produce mature mRNA.
(3) RNA polymerase stops copying the gene when it reaches the nucleotide sequence
which serves as the terminator.
(4) Once matured, the transcript exits the nucleus through a nuclear pore ad makes its
way to a ribosome once out in the cytoplasm.
2. Translation
a. Process during which specific codons of mRNA are matched up with specific amino acids
b. Occurs in the cytoplasm of the cell; specifically, on a ribosome (either free or attached)
c. Steps of Translation
(1) mRNA binds with the small subunit of the ribosome
(2) a tRNA which contains the UAC anticodon on one end and the amino acid methionine at
the other end matches up with the start codon AUG on the mRNA strand. (Note:all proteins
start with the amino acid methionine because AUG is always the initiator codon and AUG
is the only codon specifying methionine.)
(3) Large ribosomal subunit binds with the small ribosomal subunit
(4) A second tRNA with its specific amino acid binds in the second site to the next codon
(Remember, the anticodon has to match – be complementary to – the codon)
(5) A peptide bond forms between methionine and the second amino acid
(6) The bond between the first tRNA and its amino acid is broken and the tRNA leaves the
ribosome, goes back in the cytoplasm, in search of its own specific amino acid molecule
(7) The ribosome moves down one more codon and the process is repeated
(8) These processes continue until the ribosome reaches the terminator codon (one of three)
d. The mRNA strand detaches from the ribosome upon which it was just translated, and it may
be used hundreds of times to be translated on successive ribosomes (as in a polysome) until
it “wears out” or become fragmented (therefore; useless to produce its unique protein)
C. The Cell Life Cycle
1. Orderly sequence of events by which a cell lives its life; then either dies or divides into two
2. Portions of the Cell Life Cycle
a. Interphase
(1) Represents the bulk of the cell life cycle
(2) The cell is metabolically very active; protein synthesis is possible only in the phase
(3) Stages of interphase:
-
P. 7
(a) Gl phase
(1) Phase in which organelles and cytosol components are duplicated
(2) Any cell which no longer undergoes cell division (ie: skeletal myofibers, neurons)
do not continue on from here and instead are referred to as being in the GO state
(b) S phase - DNA replication occurs
(1) The helical, double-stranded DNA is unwound (each side separates from the other)
(2) The two single strands which are separated from each other form replication forks
(3) DNA polymerase places the complementary nucleotide across from each strand
(that is, A-T and T-A; C-G and G-C)
(c) G2 phase - enzymes and cellular components necessary for cell division are mfg'd
b. Mitotic phase
1. Prophase-nuclear envelope disappears; chromatin condenses to form visible chromosomes
(these are replicated chromosomes each consisting of two identical sister chromatids
joined at the centromere); replicated centrioles begin migrating to opposite poles
2. Metaphase - chromosomes line up at the equator (metaphase plate) of the cell and
centrioles have formed a full mitotic spindle apparatus complete with
kinethochore and non-kinetochore microtubules and aster rays
3. Anaphase - Sister chromatids of each chromosomes are pulled to the opposite poles via
kinetochore mitotic spindle fibers emanating from the centrioles and attach to
the kinetochore of each chromatid (Remember, each chromatid, now separate,
is referred to as a chromosome - now a non-replicated chromosome)
4. Telophase - Chromosomes (non-replicated chromosomes) at opposite poles of the cell revert to chromatin (the chromosomes "relax"), nucleoli reappear and the
nuclear envelope reappears.
5. Cytokinesis - refers to the division of the cytoplasm (cytosol and organelles) of the
parent cell via formation of a cleavage furrow starting at the outer perimeter of the cell and proceeding inward towards the center. The non
kinetochore microtubules of the mitotic spindle apparatus are often the last
remnants to be "dissolved" as the two new daughter cells separate.
3. Meiosis
a. Occurs in gamete-producing cells of the sex organs (male testes and female ovaries)
b. Two divisions occur (I a reduction division and II an equatorial division) with no S phase
inbetween which gives rise to one-half the normal number of chromosomes (46 23)
c. This is the process by which gametogenesis occurs: spermatogenesis (sperm cell formation) and oogenesis (oocyte formation).