Chapter 4

A Tour of the Cell

Fine Points of the Chapter

Cytology: science/study of cellsLink 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, phase-contrast, 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

FlagellaCilia 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).

1 µm

Direction of swimming

Figure 6.23 A

• Ciliary motion(b) Motion of cilia. Cilia have a back- and-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

(a)

(c)

(b)

Outer microtubuledoublet

Dynein arms

Centralmicrotubule

Outer doublets cross-linkingproteins inside

Radialspoke

Plasmamembrane

Microtubules

Plasmamembrane

Basal body

0.5 µm

0.1 µm

0.1 µm

Cross section of basal body

Triplet

Figure 6.24 A-C

• The protein dynein– Is responsible for the bending movement of

cilia and flagellaMicrotubuledoublets ATP

Dynein arm

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.

(a)

Figure 6.25 A

Outer doubletscross-linkingproteins

Anchoragein cell

ATP

In a cilium or flagellum, two adjacent doublets cannot slide far because they are physically restrained by proteins, so they bend. (Only two ofthe nine outer doublets in Figure 6.24b are shown here.)

(b)

Figure 6.25 B

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.

(c)

1 3

2

Figure 6.25 C