Download Finer Points of Chapter 4

Chapter 4
A Tour of the Cell
Fine Points of the Chapter
Cytology: science/study of cells
Link between Cytology and Biochemistry (form/function)
•
•
Light microscopy
– Resolving power - measure of clarity
• Resolution – minimum distance two points can be separated and
still distinguished as two separate points
– Magnification – ratio of an objects image to its real size
– Various Staining or Resolving Methods (ex. brightfield, phasecontrast, differential-interface contrast, flourescence, confocal)
Electron microscopy
•TEM - electron beam to study cell ultrastructure
•SEM - electron beam to study cell surfaces
Cell fractionation
cell separation based on mass of
organelle; organelle study
- Homogenate cells
- Ultracentrifuge - cell
fractionation; 130,000 rpm
- Form “Pellets”
- Decant “Supernatant”
- Continue
Cell size
• As cell size increases, the surface area to volume
ratio decreases
• Rates of chemical exchange may then be inadequate
for cell size
• Cell size, therefore, remains small
Cell Types: Prokaryotic
• Nucleoid: DNA
concentration
• No organelles with
membranes
• Ribosomes: protein
synthesis
• Plasma membrane (all
cells); semi-permeable
• Cytoplasm/cytosol (all
cells)
The Endosymbiotic Theory
• Mitochondria and chloroplasts were formerly
from small prokaryotes living within larger
cells (Lynn Margulis)
Peroxisomes
• Single membrane
• Produce hydrogen
peroxide in cells
• Metabolism of fatty
acids; detoxification of
alcohol (liver)
• Hydrogen peroxide
then converted to
water
Extracellular matrix (ECM)
•
•
•
Glycoproteins:
• proteins covalently bonded to
carbohydrate
Collagen (50% of protein in human
body)
•embedded in proteoglycan
(another glycoprotein-95%
carbohydrate)
Fibronectins
•bind to receptor proteins in plasma
membrane called integrins
(cell communication?)
Intercellular junctions
• PLANTS:
• Plasmodesmata:
cell wall perforations; water
and solute passage in plants
(cytoplasmic streaming)
• ANIMALS:
• Tight junctions - fusion of
neighboring cells; prevents
leakage between cells
• Desmosomes - riveted,
anchoring junction; strong
sheets of cells
• Gap junctions - cytoplasmic
channels; allows passage of
materials or current between
cells
Cilia/flagella
• Locomotive appendages
• Ultrastructure: “9+2”
•9 doublets of microtubules in a
ring
•2 single microtubules in center
•connected by radial spokes
•anchored by basal body
•dynein protein
EUKARYOTIC FLAGELLA
•
Cell Locomotion via Cilia and
Flagella
Cilia and flagella, which extend from
the plasma membrane, are composed
of microtubules, coated with plasma
membrane material. Eukaryotic cilia
and flagella have an arrangement of
microtubules, known as the 9 + 2
arrangement (9 pairs of microtubules
(doublets) around the circumference
plus 2 central microtubules). "Spokes"
radiate from the microtubules towards
the central microtubules to help
maintain the structure of the cilium or
flagellum.
•
Each of the microtubule doublets has
motor molecule "arms", the dynein
arms, which can grip and pull an
adjacent microtubule to generate the
sliding motion. (The protein of this
motor molecule is dynein.)
A bacterial flagellum has 3 basic parts: a
filament, a hook, and a basal body.
• 1) The filament is the rigid, helical structure that extends
from the cell surface. It is composed of the protein
flagellin arranged in helical chains so as to form a hollow
core. During synthesis of the flagellar filament, flagellin
molecules coming off of the ribosomes are thought to be
transported through the hollow core of the filament where
they attach to the growing tip of the filament causing it to
lengthen.
• 2) The hook is a flexible coupling between the filament and the
basal body
• 3) The basal body consists of a rod and a series of rings that anchor
the flagellum to the cell wall and the cytoplasmic membrane. Unlike
eukaryotic flagella, the bacterial flagellum has no internal fibrils and
does not flex. Instead, the basal body acts as a molecular motor,
enabling the flagellum to rotate and propell the bacterium through
the surrounding fluid. In fact, the flagellar motor rotates very rapidly. (The
motor of E. coli rotates 270 revolutions per second!)
• Flagella beating pattern
(a) Motion of flagella. A flagellum
usually undulates, its snakelike
motion driving a cell in the same
direction as the axis of the
flagellum. Propulsion of a human
sperm cell is an example of
flagellatelocomotion (LM).
Direction of swimming
Figure 6.23 A
1 µm
• Ciliary motion
(b) Motion of cilia. Cilia have a backand-forth motion that moves the
cell in a direction perpendicular
to the axis of the cilium. A dense
nap of cilia, beating at a rate of
about 40 to 60 strokes a second,
covers this Colpidium, a
freshwater protozoan (SEM).
Figure 6.23 B
15 µm
• Cilia and flagella share a common
ultrastructure
Outer microtubule
doublet
Dynein arms
0.1 µm
Central
microtubule
Outer doublets
cross-linking
proteins inside
Microtubules
Radial
spoke
Plasma
membrane
Basal body
(b)
0.5 µm
(a)
0.1 µm
Triplet
(c)
Figure 6.24 A-C
Cross section of basal body
Plasma
membrane
• The protein dynein
– Is responsible for the bending movement of
Microtubule
cilia and flagella
ATP
doublets
Dynein arm
(a) Powered by ATP, the dynein arms of one microtubule doublet
grip the adjacent doublet, push it up, release, and then grip again.
If the two microtubule doublets were not attached, they would slide
relative to each other.
Figure 6.25 A
ATP
Outer doublets
cross-linking
proteins
Anchorage
in cell
(b) In a cilium or flagellum, two adjacent doublets cannot slide far because
they are physically restrained by proteins, so they bend. (Only two of
the nine outer doublets in Figure 6.24b are shown here.)
Figure 6.25 B
1
3
2
(c) Localized, synchronized activation of many dynein arms
probably causes a bend to begin at the base of the Cilium or
flagellum and move outward toward the tip. Many successive
bends, such as the ones shown here to the left and right,
result in a wavelike motion. In this diagram, the two central
microtubules and the cross-linking proteins are not shown.
Figure 6.25 C