Eukaryotic Microorganisms: Features and Cell Envelopes

• Eukaryotic microorganisms are important in ecosystems, model systems, and industry
• Some eukaryotic microorganisms are major human pathogens
• Protists and fungi are two main groups of eukaryotic microbes

Common Features of Eukaryotic Cells

• Eukaryotic cells have membrane-delimited nuclei
• Eukaryotic cells have membrane-bound organelles for specific functions
• Eukaryotic cells have an intracytoplasmic membrane complex that transports materials
• Eukaryotic cells are structurally complex and larger than bacteria or archaea

Eukaryotic Cell Envelopes

• Cell envelopes include the plasma membrane and all external coverings
• The plasma membrane is a lipid bilayer
• Major membrane lipids include phosphoglycerides, sphingolipids, and cholesterol
• Major membrane lipids contribute to membrane strength
• Eukaryotes lack peptidoglycan in their cell walls, unlike Bacteria and Archaea
• The cell walls of photosynthetic algae have cellulose, pectin, and silica
• Fungal cell walls are composed of cellulose, chitin, or glucan

Cytoplasm of Eukaryotes

• The cytoplasm is composed of liquid, cytosol, and organelles
• The cytoskeleton is a network of interconnected filaments
• Filaments form the cytoskeleton with microfilaments (actin), microtubules, intermediate filaments, and motor proteins
• The cytoskeleton plays a role in cell shape and movement

Microfilaments

• Microfilaments are small protein filaments that are 4 to 7 nm in diameter
• Microfilaments are scattered within the cytoplasmic matrix or organized into networks and parallel arrays
• Microfilaments are composed of actin protein
• Microfilaments are involved in cell motion and shape changes

Intermediate Filaments

• Intermediate filaments are heterogeneous elements of the cytoskeleton, about 10 nm in diameter
• Keratin and vimentin are classes of intermediate filaments
• The role of intermediate filaments in a cell is unclear
• Intermediate filaments play a structural role
• Intermediate filaments form nuclear lamina
• Intermediate filaments link cells together to form tissues

Microtubules

• Microtubules are thin cylinders that are approximately 25 nm in diameter, comprised of α- and β-tubulin
• Microtubules help maintain cell shape
• Microtubules are involved with microfilaments in cell movements
• Microtubules participate in intracellular transport processes

Secretory Endocytic Pathway

• The secretory endocytic pathway is a complex of membranous organelles and vesicles
• It moves materials from outside the cell, from inside to outside, and within the cell
• The secretory endocytic pathway include the endoplasmic reticulum (ER), Golgi apparatus, and lysosomes

Endoplasmic Reticulum (ER)

• ER is an irregular network of branching and fusing membranous tubules and flattened sacs (cisternae)
• Rough ER has ribosomes attached
• Rough ER synthesize secreted proteins by ER-associated ribosomes
• Smooth ER lacks ribosomes
• Smooth ER synthesize lipids using ER-associated enzymes

Functions of ER

• Transports proteins, lipids, and other materials within the cell
• Major site of cell membrane synthesis

Golgi Apparatus

• The Golgi apparatus is a membranous organelle with stacked cisternae
• The Golgi apparatus has cis and trans faces
• Dictyosomes are stacks of cisternae
• The Golgi apparatus modifies, packages, and secretes materials

Lysosomes

• Lysosomes are membrane-bound vesicles in most eukaryotes
• Lysosomes are involved in intracellular digestion
• Lysosomes contain hydrolases that hydrolyze molecules which enzymes function best under slightly acidic conditions
• Lysosomes maintain an acidic environment by pumping protons into their interior

Secretory Pathway

• The secretory pathway moves materials within the cell, to the plasma membrane, or to the cell exterior
• Proteins destined for the cell membrane, endosomes, lysosomes, or secretion are synthesized by ribosomes on rough ER (RER)
• Proteins are targeted to the RER lumen and released in small budding vesicles from RER
• Released in small vesicles to the cis face of the Golgi apparatus then to the trans face of the Golgi apparatus
• Modification of proteins occurs in the Golgi apparatus which targets protein for final destination
• Transport vesicles are released from the trans face of the Golgi
• Vesicles deliver their contents to endosomes and lysosomes
• Two types of vesicles deliver proteins to the cell membrane
• Constitutive delivery to the membrane
• Secretory vesicles in multicellular eukaryotes store proteins until given a signal to release
• There is also a quality assurance mechanism which unfolds or misfolds proteins. They get secreted into the cytosol, targeted for destruction by ubiquitin polypeptides
• Proteasomes destroy targeted proteins

Endocytic Pathway

• Endocytosis is used by all eukaryotic cells
• Endocytosis brings materials into cells
• Solutes or particles are taken up and enclosed in vesicles pinched from the plasma membrane
• Materials are delivered to lysosomes and destroyed

Types of Endocytosis

• Phagocytosis uses cell surface protrusions to surround and engulf particles
• Clathrin-dependent endocytosis uses clathrin protein-coated pits with external receptors that bind macromolecules
• Caveolae-dependent endocytosis plays a role in signal transduction and transports small and macromolecules

Endocytosis Mechanics

• Clathrin-coated vesicles and some caveolin-coated vesicles deliver contents to endosomes, which have hydrolytic enzymes
• Early endosomes develop into late endosomes that fuse with lysosomes
• Caveosomes fuse with early endosomes

Autophagy

• Autophagy delivers materials to be digested by a route that does not involve endocytosis
• Macroautophagy digests and recycles cytoplasmic components
• A double membrane surrounds a cell component, forming an autophagosome
• An autophagosome fuses with a lysosome When digestion is complete
• Digestion occurs without the release of lysosome enzymes into cytoplasmic matrix
• As contents are digested, products leave the lysosome and can be used as nutrients
• The resulting lysosome turns into what is called a residual body which can release contents to cell exterior by lysosome secretion

Organelles Involved in Genetic Control of the Cell

• Nucleus
• Ribosomes

Nucleus

• A membrane-bound spherical structure that houses genetic material
• Contains a complex of DNA, histones, and other proteins called chromatin
• Five types of histones make up nucleosomes: H1, H2A, H2B, H3, and H4
• Chromatin condenses into chromosomes during division
• A nuclear envelope is a double membrane structure delimiting the nucleus
• The nuclear envelope is continuous with the ER
• There are nuclear pores in the nuclear envelope
• Associated proteins make up the nuclear pore complex
• Nuclear pores enable the transport of molecules into or out of the nucleus

Nucleolus

• Eukaryotic cells contain one or more nucleoli per nucleus
• The nucleolus is an organelle not enclosed by a membrane
• The nucleolus directs the synthesis and processing of rRNA
• rRNA assembles into partial ribosomal subunits within the nucleolus
• Ribosomes mature in the cytoplasm

Eukaryotic Ribosomes

• Larger than 70S bacterial and archaeal ribosomes
• 80S in size, comprised of 60S and 40S subunits
• Attached to ER or free in the cytoplasmic matrix
• The 60S subunit binds to the ER
• Proteins made on ribosomes of Rough ER (RER) are often secreted or inserted into ER membranes as integral membrane proteins
• Free ribosomes synthesize nonsecretory and nonmembrane proteins
• Some proteins are inserted into organelles

Organelles Involved in Energy Conservation

• Mitochondria
• Hydrogenosomes
• Chloroplasts
• Mitochondria, hydrogenosomes, and chloroplasts evolved from bacterial cells that invaded or were ingested by early ancestors of eukaryotic cells
• Mitochondria and chloroplasts are very similar to extant bacteria and cyanobacteria

Mitochondria

• Often called the "power houses of the cell", found in most eukaryotic cells
• Site of tricarboxylic acid cycle activity
• Site where ATP is generated by electron transport and oxidative phosphorylation
• About the same size as bacterial cells
• Reproduce by binary fission (like bacteria)

Mitochondrial Structure

• The outer membrane contains porins similar to the outer membrane of Gram-negative bacteria
• The inner membrane is highly folded to form cristae
• The location of enzymes and electron carriers for electron transport and oxidative phosphorylation
• Matrix enclosed by inner membrane
• Contains ribosomes that are the same size as bacterial
• Contains mitochondrial DNA, possibly circular like bacterial DNA
• Contains enzymes of the tricarboxylic acid cycle and enzymes involved in catabolism of fatty acids

Hydrogenosomes

• Small energy conservation organelles are found in some anaerobic protists
• They descended from a common mitochondrial ancestor
• The have double membrane but no cristae and lack DNA
• ATP is generated by fermentation, rather than respiration
• CO2, H2, and acetate are products of fermentations

Chloroplasts

• Type of plastid and are pigment-containing organelles observed in plants and algae
• They are the site of photosynthetic reactions
• Surrounded by a double membrane
• Stroma is a matrix within the inner membrane
• Stroma contains DNA, ribosomes, lipid droplets, starch granules, and thylakoids
• Thylakoids - flattened, membrane-delimited sacs
• grana (s., granum) are stacks of thylakoids
• The thylakoids are the site of light reactions, trapping light energy to generate ATP, NADPH, and oxygen
• Stroma is the site of dark reactions of photosynthesis (formation of carbohydrates from water and carbon dioxide)
• Algal chloroplasts contain a pyrenoid, which participates in polysaccharide synthesis

External Cell Coverings

• Cilia
• Flagella

Cilia and Flagella

• Flagella (s., flagellum) are 100-200 μm long
• They move in an undulating fashion
• Cilia (s., cilium) are 5-20 μm long
• The beat with two phases, working like oars
• Membrane-bound cylinders that are approximately 2 μm in diameter
• Axoneme is a set of microtubules in a 9+2 arrangement
• Basal body is at the base of flagellum or cilium and directs synthesis of flagella and cilia