Lecture #6: Prokaryotic Cell Structure I Flashcards
Bacteria vs. Micron Size
A bacterial cell is generally on the order of 1-10 microns. A virus is generally 1-10 nm in size. Find two objects that show the same size contrast to share in class on Tuesday. Take home a sticky note, jot them down, post them on the white board.
Side note—a eukaryotic ribosome is about the size of a bacterium, and a prokaryotic ribosome is about the size of a bacteriophage virus. Hmmm…
Cytoplasmic Volume
Bacterial cells have a large surface for diffusion. Considering a cell 1 x 3 microns, 2 microns in diameter, or 0.1 x 10 microns suggests something like 3 microns cubed in volume, with a surface to volume ratio on the order of 3:1.
Eukaryotic cells have a surface to volume ratio on the order of 0.3:1, considering a small animal cell about 20 microns in diameter.
Effective diffusion relates to a large surface. For bacteria, getting many things efficiently in and out of the cell is accomplished by diffusion alone. Due to the small internal volume, movement within the cell is efficient based on concentration gradients.
As Cell Size Increases
Increasing need for infrastructure, organization, mixing, and support.
Microtubules to stabilize the cell, to provide for movement of organelles and mix the cytoplasm, to depolymerize and repolymerize during cell division, to provide extensible scaffolding for ready expansion/contraction, surface on which to localize reactions.
Large Cells Need to be Subdivided to be Efficient
Not unlike large businesses. Work is segregated to specific departments (Marketing, Human Resources, Design, Administration, Custodial Services, Records, Manufacturing, Facilities).
In a eukaryotic cell, same departments (Proteins directed to the cell membrane advertise and identify, Chemical signals from vesicles recruit other cells and coordinate function in tissues, mRNA in the cytoplasm directs resources to product under control of the nucleus, Lysosomes degrade and recycle (clean-up), the Nucleus holds and maintains the records, ER and Golgi control modification & flow, Mitochondria/Chloroplasts convert energy.
Eukaryotes are the Masters of the Membrane
Membranes regulate what goes into and out of the cell, an organelle.
Membranes shuttle substances around in vesicles in the cell, and take-in as well as secrete larger substances. Endo-, exocytosis.
Membranes control mass flow, modification, and fate within the cell (ER, Golgi = UPS of the cell).
Membranes Within Chloroplasts, Mitochondria Order Steps in Conversion Reactions
Allow for efficiency. Products of the conversion series ordered along internal membrane systems feed into reactions in the adjacent liquid phase (between the inner and outer mitochondrial membranes, in the stroma of the chloroplast).
So the Larger Cells of Eukaryotes
Need more support.
Move and mix the cytoplasm, facilitating enzyme-substrate interaction.
Require a spindle to efficiently separate the chromosomes.
Use membranes to compartmentalize and concentrate enzymes so processes are efficient.
Use endo- and exocytosis to get larger things in and out.
Are longer lived and more is invested into their development and maintenance.
Can be specialized.
Are set up for intercellular communication within a tissue.
Prokaryotes Have an Entirely Different Strategy
Short-lived, and live to divide.
Small volumes where diffusion is immensely important.
Cytoplasm is uncluttered, lacking organelles.
Do not endo-, exocytose but do ‘drool.’ Akin to fungi.
Divide without using a spindle (Binary Fission).
Use microtubules sparingly vs. the elaborate cytoskeleton of a eukaryote.
Collectively show an range of metabolic specialization that dwarfs that of eukaryotes—individually each microbe has its own metabolic specialty.
They are the Masters of Metabolism, Masters of Multiplication.
Procaryotes Have a Nucleoid Region
No nucleus.
Long loop of double stranded DNA. Twisted, condensed ‘nucleoid’ region. Genes accessible 24/7. In Escherichia coli the length of DNA is 210 times the length of the cell.
The chromosome lacks histone proteins and does not coil as tightly or precisely as in a eukaryote where genes are locked away during cell division, and inaccessible.
The DNA in Eukaryotes is
Splayed out as chromatin during Interphase, supercoiled during cell division.
Metabolism between cell divisions, and temporary shutdown while DNA is condensed. Energy diverted to cell division only.
Prokaryotes are Structurally Simple
Uncluttered cytoplasm that lacks membrane-bound organelles.
Efficient for a limited existence.
Primitive? No, just have a wholly different plan. Don’t invest in cells same way eukaryotes do—quick and direct. Super8 Motel vs. the Ambassador.
Rigidity & Flagellation Differ
Cocci are not flagellated. They are not motile.
Rods and spirilli may have flagella. Numbers and distributions differ.
Spirochaetes are more flexible and have internal or sheathed flagella.
