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Chapter 4:Cellular Structure

2. Prokaryotic Cell Structure

1. Eukaryotic Cell Structure

1. Eukaryotic Cell Structure

Eukaryotic Organelles

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Nucleus Holds Genetic Material• DNA associated with

histone proteinsChromosomes whencondensed in M phase

Chromatin in G1, S & G2when 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 cytoplasmicface of ER membrane

• synthesize proteins acrossER membrane into lumen

• beginning of the“secretory pathway”

Smooth ER (SER)• no ribosomes• has membrane-associated

enzymes that catalyzenew 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

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Mitochondria ATP productionvia respiration

• Krebs cycle

• e- transport

• chemiosmosis

Mitochondrialstructure is keyfor H+ gradient

• high [H+] in the intermembranespace producedby e- transportin inner membr.*H+ gradient fuels ATP synthesis *

Chloroplasts Organelle of photosynthesis:

• “light” reactionsoccur in thethylakoids

• convert sunlightto energy in ATPand NADPH

• “dark” reactionsoccur in stroma

• energy from ATP& NADPH used tomake sugars fromCO2 and H2O

Flagella & Cilia Microbial structures used for locomotion:Flagella• long & “few”• wave-like motion

Cilia• short & “many”

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Other OrganellesLysosomes

• 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

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Prokaryotic Cell Shape One convenient characteristic with which toidentify and classify prokaryotes is theirsize 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 moreor less maintain a constant shape (monomorphic)

• most prokaryotes have a spherical, rod-shapedor spiral appearance though other shapes existas well…

Spherical or “Round” Cells • spherical prokaryotes arereferred to as cocci(singular = coccus)

diplo- = found in pairs

• different kinds of cocciexhibit characteristic“arrangements”:

strepto- = found in “chains”

staphylo- = irregular clusters

tetrad = group of 4

sarcina = cube structure

“Rod-Shaped” Cells • rod-shaped prokaryotesare referred to as bacilli(singular = bacillus)

• also found in various“arrangements”:

diplo- = length-wise pairs

strepto- = length-wise chains”

cocco- = “rounded” bacilli

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Curved or Spiral Cells

vibrio = “curved rod”

spirillum = “twisted rod”

spirochete = “corkscrew rod”

B. External Structures

Prokaryotic Cell Structures

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Plasma Membrane true barrier between

“internal” & “external”

• a phospholipid bilayer like any other membrane thoughphospholipid content a bit different from eukaryotes

Diffusion & Osmosis Plasma membraneis a semi-permeablebarrier across which some substances can diffuse:

diffusion = movement from high to low conc.

osmosis = diffusion of water

lysis preventedby cell wall…

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 negativecharge inside the plasma membrane relative to outside.

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Bacterial Cell Wall The bacterial cell wall provides structure & support:• general component of cell wall isa structure called peptidoglycan• chains of a repeating disaccharideconnected by polypeptides

*protects cell from osmotic lysis*

• structurevariesamongdifferentbacteria

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 lipopolysaccharide (LPS)

• Gram-negative since 1o stain/mordant lost with wash

• LPS contains Lipid A, also referred to as endotoxin

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Some Features of Gram-negativevs Gram-positive Bacteria

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Bacterial Glycocalyx (“sugar coat”)Outermost layer that surroundsthe bacterium

Made of protein,polysaccharide,or both

• varies greatly among bacteria

• called a slime layer if loosely attached, water soluble• mediates adhesion, biofilm formation

• protects from dessication, phagocytosis

• called a capsule if compact, tightly attached to cell wall

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

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Flagellum Structure

• basal body anchors flagellum in PM & cell wall,rotates hook & filament to propel bacterium

• consists of basal body, hook & filament

• different from “wave-like” motion of eukaryotic flagellum

Flagella & Bacterial Motility Bacteria can undergomovement toward or away from something (taxis):

e.g., chemotaxis• toward or away froma chemical substance

Involves random “runs”& “tumbles”:

• longer runs, less tumblesto move toward “good stuff”

• shorter runs, more tumblesto avoid “bad stuff”

(RUN = flagella rotate counterclockwise)

(TUMBLE = clockwise)

Axial Filaments “Endoflagella” found in spirochaetes.

• anchored atone end androtate

• propel cell likea“corkscrew”

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Fimbriae and Pili Non-motile appendages that are chemically and functionally different than flagella.Fimbriae

• many in number (usu.)• involved in adhesionto specific targets

Pili (or singular pilus)• >1 used in conjugation

C. Internal Structures

Prokaryotic Ribosomes

Ribosomes consist of 1 large and 1 small subunit.

Carry out protein synthesis (i.e., translation of mRNA).

• both subunits are made of rRNA & ribosomal proteins• smaller, somewhat different from eukaryotic ribosomes• specifically targeted by some antibiotics

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Endospores When conditions are bad, some Gram+

bacteria can formendospores:

• inactive or “resting”cells enclosed in ahighly resistantspore coat

• remain dormant untilconditions are good(can be 1000’s of yrs)

• very resistant toheating, freezing, dessication(“active” cells are called vegetative)

The Genetic MaterialA region called the nucleoid contains the circular bacterial chromosome (DNA + non-histone proteins):

• usually several millionbase pairs (bp) in sizee.g.

the E. coli genome is ~4 mega-bp’s (4 Mbp)

• contains all bacterialgenes plus an originof replication (Ori)

• Ori is where DNAreplication starts,essential to copy the chromosome

PlasmidsSome bacteria have >1extrachromosomal,non-essential circular DNA molecules calledplasmids:

• much smaller thanbacterial chromosome

• have own Ori so it is copied when cell divides

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

plasmidmap

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What’s the Role of Plasmids?Plasmids generally contain genes that confersome sort of advantage for survival andreproduction:

1) genes providing protection from toxic substances

2) genes enabling the metabolism of additionalsources of energy

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

4) genes involved in gene transfer by conjugation

• including antibiotic resistance

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

• diplo-, strepto-, staphylo-, tetrad, sarcinae

• glycocalyx, capsule, fimbriae, pili

• peptidoglycan, teichoic acid, LPS, endotoxin

• a-, mono-, amphi-, lopho-, peritrichous flagella

• coccus, bacillus, vibrio, spirillum, spirochete

• chemotaxis, endospores, plasmids, nucleoid

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

• inclusions, chromatophores, vegetative• periplasmic space (periplasm)