Chapter 4: Cellular Structure

1. Eukaryotic Cell Structure

2. Prokaryotic Cell Structure 1. Eukaryotic Cell Structure Eukaryotic Organelles Nucleus Holds Genetic Material • DNA associated with histone proteins Chromosomes when condensed in M phase

Chromatin in G1, S & G2 when uncondensed Ribosome Assembly • assembled in nucleolus • rRNA + ribosomal prot. • carry out protein synthesis in cytoplasm *all gene expression begins in nucleus (transcription)* Endoplasmic Reticulum (ER) Rough ER (RER) • ribosomes on cytoplasmic face of ER membrane • synthesize proteins across ER membrane into lumen • beginning of the “secretory pathway” Smooth ER (SER) • no ribosomes • has membrane-associated enzymes that catalyze new lipid synthesis (also found in RER) The Golgi Complex

Proteins destined to leave ER next go to the Golgi • transported in vesicles, next stop in “secretory pathway” • undergo any necessary modifications or processing • then sent via vesicles to various destinations • e.g., plasma membrane, exterior of cell, lysosomes Mitochondria ATP production via respiration • Krebs cycle • e- transport • chemiosmosis

Mitochondrial structure is key for H+ gradient • high [H+] in the intermembrane space produced by e- transport *H+ gradient fuels ATP synthesis * in inner membr. Chloroplasts Organelle of photosynthesis: • “light” reactions occur in the thylakoids

• convert sunlight to energy in ATP and NADPH • “dark” reactions occur in stroma

• energy from ATP & NADPH used to make sugars from

CO2 and H2O Flagella & Cilia Microbial structures used for locomotion: Flagella • long & “few” • wave-like motion Cilia • short & “many” Other Organelles Lysosomes • acidic compartments for the breakdown or “digestion” of foreign or waste material Vacuoles • large storage compartments Peroxisomes • metabolize fats for heat production, degrade toxins

• H2O2 byproduct is “neutralized” by catalase Centrosomes • region containing centrioles and other proteins • “organizing center” for mitotic spindle fibers 2. Prokaryotic Cell Structure

A. Cell Shape

B. “External” Structures

C. “Internal” Structures A. Cell Shape Prokaryotic Cell Shape One convenient characteristic with which to identify and classify is their size and shape as seen in the microscope.

• the diameter of prokaryotic cells ranges from ~0.2 to 2.0 μm • prokaryotes are essentially unicellular and more or less maintain a constant shape (monomorphic)

• most prokaryotes have a spherical, rod-shaped or spiral appearance though other shapes exist as well… Spherical or “Round” Cells • spherical prokaryotes are referred to as cocci (singular = )

• different kinds of cocci exhibit characteristic “arrangements”:

diplo- = found in pairs strepto- = found in “chains” staphylo- = irregular clusters tetrad = group of 4 sarcina = cube structure “Rod-Shaped” Cells • rod-shaped prokaryotes are referred to as bacilli (singular = )

• also found in various “arrangements”:

diplo- = length-wise pairs

strepto- = length-wise chains”

cocco- = “rounded” bacilli Curved or Spiral Cells

vibrio = “curved rod”

spirillum = “twisted rod”

spirochete = “corkscrew rod” B. External Structures Prokaryotic Cell Structures Plasma Membrane

true barrier between “internal” & “external”

• a phospholipid bilayer like any other membrane though phospholipid content a bit different from eukaryotes Diffusion & Osmosis Plasma membrane is a semi-permeable barrier across which some substances can diffuse:

diffusion = movement from high to low conc.

osmosis = diffusion of water

lysis prevented by … Concentration Gradients Different concentrations of various substances (i.e., ions) inside vs outside the cell are set up & maintained by various membrane proteins:

protein pumps • move substances from lower to higher conc. • active transport – requires energy (ATP)

protein channels, transport proteins • facilitated diffusion of specific molecules

The overall result of all these gradients is a net negative charge inside the plasma membrane relative to outside. Bacterial Cell Wall The bacterial cell wall provides structure & support: • general component of cell wall is a structure called • chains of a repeating disaccharide connected by polypeptides

• structure varies among different

*protects cell from osmotic lysis* Gram-positive Cell Wall

• multi-layered peptidoglycan cell wall w/teichoic acids • NO outer membrane • Gram-positive since 1o stain/mordant trapped by the thick peptidoglycan layer (i.e., not removed by wash step) Gram-negative Cell Wall

*

• single layer of peptidoglycan • outer membrane containing (LPS) • LPS contains Lipid A, also referred to as endotoxin • Gram-negative since 1o stain/mordant lost with wash Some Features of Gram-negative vs Gram-positive Bacteria

* Bacterial (“sugar coat”) Outermost layer that surrounds the bacterium

Made of protein, polysaccharide, or both • varies greatly among bacteria • called a capsule if compact, tightly attached to cell wall • called a slime layer if loosely attached, water soluble • mediates adhesion, formation • protects from dessication, phagocytosis Bacterial Flagella

Some bacteria “get around” via 1 or more flagella: 0 = atrichous 1 = monotrichous 1 @ ea end = amphitrichous >1 @ ea end = lophotrichous “all over” = peritrichous Flagellum Structure

• consists of basal body, hook & filament • basal body anchors flagellum in PM & cell wall, rotates hook & filament to propel bacterium • different from “wave-like” motion of eukaryotic flagellum Flagella & Bacterial Motility (RUN = flagella rotate counterclockwise) Bacteria can undergo movement toward or away from something (taxis):

e.g., chemotaxis • toward or away from a chemical substance

(TUMBLE = Involves random “runs” clockwise) & “tumbles”:

• longer runs, less tumbles to move toward “good stuff”

• shorter runs, more tumbles to avoid “bad stuff” Axial Filaments “Endoflagella” found in spirochaetes.

• anchored at one end and rotate

• propel cell like a“corkscrew” Fimbriae and Pili Non-motile appendages that are chemically and functionally different than flagella. Fimbriae Pili (or singular ) • many in number (usu.) • >1 used in conjugation • involved in adhesion to specific targets C. Internal Structures Prokaryotic Ribosomes

Carry out protein synthesis (i.e., translation of mRNA). Ribosomes consist of 1 large and 1 small subunit. • both subunits are made of rRNA & ribosomal proteins • smaller, somewhat different from eukaryotic ribosomes • specifically targeted by some When conditions are bad, some Gram+ bacteria can form endospores: • inactive or “resting” cells enclosed in a highly resistant spore coat • remain dormant until conditions are good (can be 1000’s of yrs) • very resistant to heating, freezing, dessication (“active” cells are called vegetative) The Genetic Material A region called the nucleoid contains the circular bacterial chromosome (DNA + non-histone proteins):

• usually several million base pairs (bp) in size e.g. the E. coli genome is ~4 mega-bp’s (4 Mbp)

• contains all bacterial genes plus an origin of replication (Ori)

• Ori is where DNA replication starts, essential to copy the chromosome Plasmids Some bacteria have >1 extrachromosomal, non-essential circular DNA molecules called plasmids: plasmid map • much smaller than bacterial chromosome

• several kilo-base pairs (usu. 3-6 Kb)

• have own Ori so it is copied when cell divides What’s the Role of Plasmids? Plasmids generally contain genes that confer some sort of advantage for survival and reproduction:

1) genes providing protection from toxic substances • including resistance

2) genes enabling the metabolism of additional sources of energy

3) genes for toxins to kill microbial competitors, enhance pathogenicity

4) genes involved in gene transfer by conjugation Inclusions & Chromatophores Inclusions are deposits of various materials found in certain types of bacteria (e.g., magnetosomes).

Chromatophores are pigment-containing infoldings of the plasma membrane in some photosynthetic bacteria. Key Terms for Chapter 4

• coccus, bacillus, vibrio, spirillum, spirochete • diplo-, strepto-, staphylo-, tetrad, sarcinae • peptidoglycan, , LPS, endotoxin • glycocalyx, capsule, fimbriae, pili • a-, mono-, amphi-, lopho-, peritrichous flagella • chemotaxis, endospores, plasmids, nucleoid • inclusions, chromatophores, vegetative • periplasmic space ()

Relevant Chapter Questions rvw: 1, 3-7, 9, 11 MC: 1-7, 9