4.1 Cellular
Level of Organization
A. Cell Theory
1. All organisms, both unicellular and multicellular, are made up of cells.
2. Cells are the smallest
units of living matter and structural and functional units of all organisms.
3. In 1830’s, Mathias Schleiden
(plants) and Theodore Schwann (animals) declared organisms were made of cells.
4. Cells are capable of
self-reproduction; Rudolf Virchow declared cells come only from preexisting
cells.
B. Cell Size
1. Cells range in size from a frog’s egg (one millimeter) down to one
micrometer.
2. Cells need surface area of plasma
membrane large enough to adequately exchange materials.
3. Surface-area-to-volume
ratio requires that cells be small.
a. As cells
get larger in volume, relative surface area actually decreases.
b. Limits
how large actively metabolizing cells can become.
c. Cells
needing greater surface area use modifications such as folding, microvilli,
etc.
C. Microscopy of Today
1. Bright-field microscope uses light rays focused by glass
lenses.
2. Transmission electron
microscope (TEM) uses electrons passing through specimen;
focused by magnets.
3. Scanning electron
microscope (SEM) uses electrons scanned across metal-coated
specimen.
4. Magnification is
function of wavelengths; shorter wavelengths of electrons allow greater
magnification.
5. Resolution is
minimum distance between two objects before they are seen as one larger object.
6. Immunofluorescence
microscopy uses fluorescent antibodies to review proteins in cells.
7. Confocal microscopy
uses laser beam to focus on shallow plane; forms series of optical sections.
8. Video-enhanced contrast
microscopy accentuates the light and dark regions and may use a
computer
to contrast
regions with false colors.
9. Bright-field, phase
contrast, differential interference and darkfield are different types
of light microscopy
that improve
ability to see various features.
4.2
Bacterial Cells
A. Bacteria Are Prokaryotic Cells
1. Bacteria belong to the domain Bacteria.
2. Most are between 1-10 micrometers
in diameter, just visible with light microscopes.
3. Structures:
a. Cell wall
is composed of peptidoglycan.
b. Bacteria
may be surrounded by a capsule and/or gelatinous sheath called a slime
layer.
c. Motile
bacteria usually have flagella, which rotate like propellers to
move through fluid medium.
d. Fimbriae
are short appendages that help them attach to an appropriate surface.
e. Plasma
membrane is the outermost membrane; regulates the entrance and exit of
molecules.
f. Cytoplasm
consists of cytosol, a semifluid medium.
g. Ribosomes
are granular inclusions that coordinate synthesis of proteins.
h. Nucleoid
contains most genes in a circular DNA molecule.
i. Plasmids
are small accessory rings of DNA aside from the nucleoid.
j. Thylakoids
are flattened discs with light-sensitive pigment molecules.
4. Although prokaryotes are
relatively simple, they are also metabolically diverse.
4.3 Eukaryotic
Cells
A. Eukaryotic Cells
1. Include larger cells of organisms (belonging to kingdoms Fungi, Animalia,
Plantae and Protista)
2. Membrane-bounded nucleus
houses DNA in threadlike structures called chromatin.
3. Similar to prokaryotic cells,
eukaryotic cells have a plasma membrane and cytoplasm including ribosomes.
4. Eukaryotic cells
are more complex than prokaryotic cells, have organelles,
including a true nucleus, and an
organized
lattice of protein filaments called the cytoskeleton.
B. Evolution of the Eukaryotic Cell
1. Invagination of the plasma membrane might explain origination of nuclear
envelope and Golgi apparatus.
2. Laboratory observations indicate
amoeba infected with bacteria become dependent on them.
3. Lynn Margulis proposes
mitochondria are aerobic heterotrophic bacteria; chloroplasts are
cyanobacteria.
4. Prokaryotes enter cell by endocytosis;
this establishes symbiotic relationship where they utilize oxygen
and
synthesize food.
5. Evidence for this endosymbiotic
hypothesis includes the following:
a.
Mitochondria and chloroplasts are similar to bacteria in size and structure.
b. Both
bounded by double membrane: outer derived from engulfing vesicle, inner from
plasma membrane
of prokaryote.
c.
Mitochondria and chloroplasts contain a limited amount of genetic material and
divide by splitting;
their DNA is circular loop similar to bacterial DNA.
d. Although
most proteins within them are produced by eukaryotic host, they have their own
ribosomes
to produce own proteins, and ribosomes resemble bacterial ribosomes.
e. The RNA
base sequence of their ribosomes suggests an eubacterial origin.
6. Margulis also suggests eukaryotic
flagella are from a spirochete prokaryote.
C. The Nucleus
1. Structures
a. Nucleus
has a diameter of about 5 micrometers.
b. Chromatin
is a threadlike material that coils into chromosomes just before cell division
occurs;
contains DNA, protein, and some RNA.
c. Chromosomes
are rod-like structures formed during cell division; coiled or folded
chromatin.
d. Nucleoplasm
is semifluid medium of nucleus; has a different pH from cytosol.
e. Nucleoli
are dark-staining spherical bodies in nucleus; sites where rRNA joins proteins
to form ribosomes.
f. Nuclear
envelope is a double membrane that separates nucleoplasm from
cytoplasm.
g. Nuclear
pores (100nm) permit passage of proteins into nucleus and ribosomal
subunits.
h. The
nucleus is the site of DNA and determines characteristics of the cell by coding
for proteins.
D. Ribosomes Are Sites of Protein Synthesis
1. Ribosomes of eukaryotic cells are 20nm by 30nm; those of prokaryotic cells
are slightly smaller.
2. Ribosomes are composed of a large
and a small subunit.
3. Each subunit has its own mix of
proteins and rRNA.
4. Polyribosomes are
several ribosomes synthesizing same protein; may be attached to ER or may lie
free.
5. Ribosomes coordinate assembly of
amino acids into polypeptide chains (i.e., protein synthesis).
6. Ribosomes attached to ER depend
on an ER signal sequence to bind to a receptor protein.
E. The Endomembrane System
1. Endomembrane system is a series of intracellular membranes that
compartmentalize the cell.
2. Endoplasmic reticulum
a.
Endoplasmic Reticulum (ER) is system of membrane channels continuous
with outer membrane
of the nuclear envelope.
b. Rough
ER is studded with ribosomes on cytoplasm side; site where proteins are
synthesized and
enter the ER interior for processing and modification.
c. Smooth
ER is continuous with rough ER, but lacks ribosomes; site of various
synthetic processes,
detoxification, and storage; smooth ER forms transport vesicles.
d. A
combination of microscopy and biochemical analysis allows researchers to
determine function of
cell parts by fractionation.
3. Golgi Apparatus
a. Golgi
apparatus is named for Camilo Golgi who discovered it in 1898.
b. Golgi
apparatus consists of a stack of 3-20 slightly curved saccules.
c. Golgi
apparatus receives protein-filled vesicles that bud from the ER.
d. Vesicle
fuses with membrane of Golgi apparatus or moves to outer face after proteins
repackaged.
e. Vesicles
formed from membrane of outer face of the Golgi apparatus then move to
different locations
in cell; at plasma membrane, they discharge their contents as secretions.
4. Lysosomes
a. Lysosomes
are membrane-bound vesicles produced by Golgi apparatus and contain
digestive enzymes.
b.
Macromolecules enter a cell by vesicle formation; lysosomes fuse with vesicles
and digest contents.
c. White
blood cells that engulf bacteria use lysosomes to digest bacteria.
d.
Autodigestion occurs when lysosomes digest parts of cells.
e. Apoptosis
is programmed cell death, a normal part of development (e.g., tadpole tail
absorption,
degeneration of webbing between human fingers).
f. Missing or
inactive lysosomal enzymes cause serious childhood diseases.
g. Peroxisomes
are membrane-bounded vesicles that contain specific enzymes.
1) Peroxisomes are abundant in liver; form hydrogen peroxide that is broken
down to water and
oxygen by catalase.
2) Peroxisomes also occur in leaves where they give of CO2, that can be used in
photosynthesis
and in germinating seeds where they convert oils into sugars used as nutrients
by growing plant.
5. Vacuoles
a. A vacuole
is a large membranous sac; vesicles are smaller than vacuoles.
b. More
prominent plant cell vacuoles (usually one or two) are water filled and give
support to cell.
c. Plant
vacuoles store water, sugars, salts, pigments and toxic substances to protect
plant from herbivores.
d. Vacuoles
in protozoa include digestive vacuoles and water-regulating contractile
vacuoles.
F. Energy-Related Organelles
1. Chloroplasts are membranous organelles that serve as sites of
photosynthesis.
a. Photosynthesis
is the process by which solar energy is converted to the chemical energy of
carbohydrates: light energy + carbon dioxide + water 
carbohydrate +
oxygen.
b. Only
plants, algae, and cyanobacteria are capable of carrying on photosynthesis.
c.
Chloroplasts are about 4-6 micrometers in diameter and 1-5 micrometers in
length.
d. Chloroplasts
are a type of organelle called a plastid; plastids include amyloplasts,
which store
starch, and chromoplasts, which contain red and orange pigments.
e.
Chloroplasts are bounded by a double membrane organized into flattened sacs (thylakoids)
piled
into stacks called grana with a fluid-filled space around
thylakoids called the stroma.
f. Chlorophyll
is located within the thylakoid membranes.
g. The
stroma contains enzymes that catalyze reactions involved in synthesis of
carbohydrates.
2. Mitochondria are
membranous organelles; sites of cellular respiration.
a. Cellular
respiration is the process where chemical energy of carbohydrates is
converted to that of ATP,
the carrier of energy in cells: carbohydrate + oxygen carbon dioxide
+ water + energy.
b. Cell
energy is provided by ATP; all organisms carry on aerobic respiration and all
except bacteria
have mitochondria.
c.
Mitochondria are about 0.5-1.0 micrometers in diameter and 7 micrometers in
length.
d.
Mitochondria are bounded by a double membrane; inner membrane has folds (cristae)
that project into inner
space (matrix) with enzymes that break down carbohydrate-derived
products; ATP production occurs at cristae.
e.
Mitochondria contain ribosomes and their own DNA that specifies some proteins;
other proteins are coded
by nucleus DNA.
G. The Cytoskeleton
1. Cytoskeleton is a network of connected filaments and tubules;
extends from nucleus to plasma membrane
in
eukaryotes.
a. Electron
microscopy reveals organized cytosol; immunofluorescence microscopy identifies
protein fibers.
b. Elements
of cytoskeleton maintain cell shape and allow it and organelles to move.
c. Elements
can disassemble and reassemble in life of a cell.
2. Actin Filaments
a. Actin
filaments are long, thin fibers (about 7nm in diameter) that occur in
bundles or mesh-like networks.
b. Actin
filament consists of two chains of globular actin monomers twisted to form a
helix.
c. Actin
filaments play a structural role, forming a dense complex web just under the
plasma membrane.
d. Actin
filaments in microvilli of intestinal cells likely shorten or extend cell into
intestine.
e. In plant
cells, they form tracts along which chloroplasts circulate.
f. Actin
filaments move by interacting with myosin: myosin combines with
and splits ATP, binding to actin
and changing configuration to pull actin filament forward.
g. Similar
action accounts for pinching off cells during cell division and for amoeboid
movement.
3. Intermediate Filaments
a.
Intermediate filaments are 8-11 nm in diameter, between actin filaments and
microtubules in size.
b. They are
rope-like assemblies of fibrous polypeptides.
c. Some
support nuclear envelope, others support plasma membrane, form cell-to-cell
junctions.
4. Microtubules
a. Microtubules
are small hollow cylinders (25 nm in diameter and from 200nm-25 micrometers in
length).
b.
Microtubules are composed of a globular protein tubulin; occurs
as alpha tubulin and beta tubulin.
c. Assembly
brings these two together as dimers, and the dimers arrange themselves in rows.
d.
Regulation of microtubule assembly is under control of a microtubule-organizing
center: a centrosome.
e.
Microtubules radiate from centrosome, helping maintain shape of cells and
acting as tracks along
which organelles move.
f. Similar
to actin-myosin, motor molecules kinesin and dynein are associated with
microtubules.
g. Different
kinds of kinesin proteins specialize to move one kind of vesicle or cell
organelle.
h.
Cytoplasmic dynein is similar to the molecule dynein found in flagella.
5. Centrioles
a. Centrioles
are short cylinders with a 9 + 0 pattern of microtubule triplets.
b. In animal
cells and most protists, centrosome contains two centrioles lying at right
angles to each other.
c. Plant and
fungal cells have equivalent of a centrosome but it does not contain
centrioles.
d. Centrioles
serve as basal bodies for cilia and flagella.
6. Cilia and Flagella
a. Cilia
are short, usually numerous hairlike projections that can move in an undulating
fashion
(e.g., Paramecium, lining of human upper respiratory tract).
b. Flagella
are longer, usually fewer, whip-like projections that move in whip-like fashion
(e.g., sperm cells).
c. Both have
similar construction, but differ from prokaryotic flagella.
1) Membrane-bounded cylinders enclose a matrix containing a cylinder of nine
pairs of microtubules
encircling two single microtubules (9 + 2 pattern of microtubules).
2) Cilia and flagella move when the microtubules slide past one another.
3) Cilia and flagella have basal body at base with same arrangement of
microtubule triples as centrioles.
4) Cilia and flagella grow by the addition of tubulin dimers to their tips.