1. Eukaryotic Cell Structure

Transcription

1 Chapter 4: Cellular Structure 1. Eukaryotic Cell Structure 2. Prokaryotic Cell Structure 1. Eukaryotic Cell Structure Eukaryotic Organelles 1

2 Nucleus Holds Genetic Material DNA associated with histone proteins Chromosomes when condensed in M phase Chromatin in G 1, S & G 2 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 2

3 Mitochondria ATP production via respiration Krebs cycle e - transport chemiosmosis *H + gradient fuels ATP synthesis * Mitochondrial structure is key for H + gradient high [H + ] in the intermembrane space produced by e - transport in inner membr. 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 CO 2 and H 2 O Chloroplasts Flagella & Cilia Microbial structures used for locomotion: Flagella long & few wave-like motion Cilia short & many 3

4 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 H 2 O 2 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 4

5 Prokaryotic Cell Shape One convenient characteristic with which to identify and classify prokaryotes 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 = coccus) 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 = bacillus) also found in various arrangements : diplo- = length-wise pairs strepto- = length-wise chains cocco- = rounded bacilli 5

6 Curved or Spiral Cells vibrio = curved rod spirillum = twisted rod spirochete = corkscrew rod B. External Structures Prokaryotic Cell Structures 6

7 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 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 negative charge inside the plasma membrane relative to outside. 7

8 Bacterial Cell Wall The bacterial cell wall provides structure & support: general component of cell wall is a structure called peptidoglycan chains of a repeating disaccharide connected by polypeptides structure varies among different bacteria *protects cell from osmotic lysis* Gram-positive Cell Wall multi-layered peptidoglycan cell wall w/teichoic acids NO outer membrane Gram-positive since 1 o 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) LPS contains Lipid A, also referred to as endotoxin Gram-negative since 1 o stain/mordant lost with wash * 8

9 Some Features of Gram-negative vs Gram-positive Bacteria * Bacterial Glycocalyx ( 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, biofilm formation protects from dessication, phagocytosis Bacterial Flagella Some bacteria get around via 1 or more flagella: 0 = atrichous 1 = monotrichous ea end = amphitrichous ea end = lophotrichous all over = peritrichous 9

10 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 = clockwise) Involves random runs & 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 10

11 Fimbriae and Pili Non-motile appendages that are chemically and functionally different than flagella. Fimbriae many in number (usu.) involved in adhesion to specific targets Pili (or singular pilus) >1 used in conjugation 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 antibiotics 11

12 Endospores 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) ( active cells are called vegetative) very resistant to heating, freezing, dessication 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 12

13 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 antibiotic 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, teichoic acid, LPS, endotoxin glycocalyx, capsule, fimbriae, pili a-, mono-, amphi-, lopho-, peritrichous flagella chemotaxis, endospores, plasmids, nucleoid inclusions, chromatophores, vegetative periplasmic space (periplasm) Relevant Chapter Questions rvw: 1, 3-7, 9, 11 MC: 1-7, 9 13