Prokaryotes vs. Eukaryotes Unit 2: Cells Part II: Prokaryotes vs. Eukaryotes The differences between these organisms go well beyond the presence or lack of a nucleus This is the first major division of living things on earth a very fundamental difference indeed We can no longer think of prokaryotes as primitive and eukaryotes advanced Prokaryotes vs. Eukaryotes Prokaryotes: Diversity No nucleus Naked DNA in a DNA loop and plasmids Small ribosomes (70s; 50s/30s subunits) Cell walls made of peptidoglycans Flagella not made of microtubules No EMS (endomembrane system) Double membrane bound nucleus DNA organized into chromosomes Large ribosomes (80s; 60s/40s subunits) Cell walls made of cellulose (plants) or chitin (fungi or protist) Flagella made of microtubules EMS present The first forms of life were likely very similar to modern bacteria Rapidly evolving, but surprisingly nearly unchanged over billions of years Prokaryotes can be found in literally every environment and in every available niche on the planet Prokaryotes split into to major groups: Archaebacteria - extremophiles Eubacteria true bacteria Prokaryote Structure Transmission Electron Microscope (TEM) image of a prokaryote (above) Prokaryotes: Structure DNA Loop: a long single fiber in the cytoplasm which contains almost all of the genetic material (the rest is in plasmids); genes are usually kept small and devoid of introns (extra non-coding bits of DNA) highly efficient Ribosomes: freely floating in cytoplasm (unbound); site for protein synthesis Antibiotics like tetracycline bind to the prokaryotic ribosome and interfere with the bacteria s ability to produce proteins 1
Prokaryotes: Structure Cell Wall: provide the cell with shape and structure, and some minimal protection against a hostile environment; most prokaryotes have them Capsule: jelly-like coating that surrounds the cell wall; only some prokaryotes have them; 4 functions of a capsule: Prevents cell from drying out Helps cells stick together or on other surfaces (tissues of other organisms) Helps prokaryotes slide on surfaces Keeps some bacteria from being destroyed by the host organism Prokaryotes: Structure Flagella: solid crystal proteins that stick through the holes in the cell membrane and spin like propellers for locomotion (very different structure from eukaryotic flagella) Pilli: short bristle-like appendages which have 2 functions: Attach bacteria to surfaces Assist in the transfer of DNA from one bacterium to another Prokaryotes: Shape Eubacteria typically come in one of 4 shapes: Coccus (pl. cocci): spere shaped Advantage: less distortion in a dried out organism Bacillus (pl. bacilli): rod shaped Advantage: high surface area Spirillum (pl. spirilla): spiral/helical shaped Advantage: highly motile (corkscrew motion) Spirochete(s): spiral shaped cells with flagella inside the cell membrane Prokaryotes: Movement Chemotaxis: movement of an organism toward or away from a chemical Positive chemotaxis: chemicals that attract organisms toward them are called attractants Negative chemotaxis: chemicals that repel organisms are called repellants Runs and twiddles Prokaryotes: Survival When environmental conditions are unfavorable, bacteria become inactive. Some species form endospores (thick wall surrounding genetic material Endospores go dormant until conditions are favorable Endospores can survive very harsh environmental conditions Boil water 2x 2
Prokaryotes: Reproduction Conjugation Asexual Reproduction Binary Fission: single loop of DNA is copied, both attach to cell membrane; the cell divides by pinching off between the two loops. Sexual Reproduction (sort of) Conjugation: a bridge is formed between cell pili; F plasmid (F=fertility, ~ 25 genes) injected with F pilus; new plasmid is recombined into bacterial DNA Prokaryotes: Reproduction Transformation: a living bacterium absorbs the genetic material of a dead cell or naked genetic material in the environment Transduction: transfer of DNA from a host to another cell by means of a virus Prokaryotes: Metabolics Heterotrophs: must eat to acquire food Photoheterotrophs: can use light to produce ATP, but must get organic carbon from another source Chemoheterotrophs Saprobes: decomposers that absorb nutrients from dead organic material Parasites: absorb nutrients from the body fluids of living hosts Phagotrophs: ingest food and digest it enzymatically within cells or multiple cellular bodies Prokaryotes: Metabolics Autotrophs: can produce their own food Photosynthetic autotrophs (phototrophs): organisms that harness light energy to drive the synthesis of organic compounds from CO 2 Chemosynthetic autotrophs (chemotrophs): organisms that use energy from specific inorganic substances to produce organic molecules from CO 2 and provide life processes Chemoautotrophs: organisms that need only CO 2 as the carbon source; they obtain energy by oxidizing inorganic substances like hydrogen sulfide, ammonia, ferrous or other ions Prokaryotes: Oxygen Prokaryotic oxygen requirements can be used to classify prokaryotes: Obligate aerobes: use oxygen for cellular respiration and cannot survive without it Facultative anaerobes: will use oxygen if present, but can grow by fermentation in an environment void of oxygen Obligate anaerobes: cannot use oxygen and are killed by it 3
Prokaryotes: Archebacteria Archebacteria lack peptidoglycan in their cell walls Archebacteria have a unique lipid composition in their cell membranes Archebacteria have a different rrna structure than eubacteria and eukaryotes Most Archebacteria live in extreme environments Prokaryotes: Archebacteria Examples (subgroups): Methanogens: use elemental hydrogen (H2) to reduce CO2 into methane (obligate anaerobes) Extreme Halophiles: live in high salinity environments Thermoacidophiles: require environments that are hot and acidic Eukaryotes: Diversity Protists: single celled, mostly heterotrophic eukaryotic organisms ie amoeba, euglena, diatoms, etc Fungi: mostly multicellular, heterotrophic, sessile eukaryotic organisms ie mushrooms, molds, rusts (the living kind) Eukaryotes: Diversity Plants: multicellular, autotrophic (photosynthetic), sessile eukaryotic organisms ie trees, grasses, bushes, shrubberies Animals: multicellular, heterotrophic, mostly motile eukaryotic organisms ie sponges, mollusks, fish, insects, reptiles, amphibians, birds, mammals Nucleus Contains primary DNA in the form of chromatin which can be packaged into chromosomes for cellular reproduction Bound by a double membrane (nuclear envelope) with nuclear pores for the exchange of RNA Nucleolus Dense, irregularly shaped body in the nucleus Makes and stores RNA Forms new ribosomes 4
Mitochondrion (pl. mitochondria) Generate ATP (adenosine triphosphate a high energy molecule for cellular energy) Double membrane; inner membrane = cristae, where much of cellular respiration takes place The area inside the cristae is called the matrix Contain their own DNA Why? Plastids Leucoplasts found in roots and tubers Chromoplasts contain accessory pigments Chloroplasts contain chlorophyll pigments, found in leaves and stems and are the primary photosynthetic organelle Ribosomes Non membrane-bound Site for protein synthesis (very numerous) Translate mrna code into proteins Made of RNA and proteins 3 Types 70s - found in prokaryotes 70s (o) associated w/ eukaryotes ER 80s found in cytoplasm of eukaryotes Endoplasmic Reticulum Provides internal framework, support Provides transportation and temporary storage for organic compounds Provides surface area for the synthesis of organic compounds Rough contains ribosomes, site of protein and glycoprotein synthesis (usually for secretion) Smooth no ribosomes, synthesize, secrete, and/or store carbohydrates, steroids, hormones, lipids, or other non-protein products 5
Golgi (complex, apparatus, bodies) Flattened membranous sacs stacked together Sacs are called cisterna Interiors are called the lumen Cis face = forming face (input) Trans face = maturing face (output) Functions: breaks down glycoproteins, concentrates materials into vesicles, forms the cell wall, and produces lysosomes Lysosomes Vesicle w/ highly reactive enzymes which can break down proteins, nucleic acids, and lipids Contain 2 or more hydrolases (enzymes) Proteases Nucleases Lipases Acidic environment (ph 5) where enzymes work best Suicide Bags = programmed cell death Peroxisomes Contain oxidative enzymes which transfer H from various substances to oxygen Purines, fats, alcohol, poisons, hydrogen peroxide can all be broken down by peroxisomes Vacuole Membrane bound body with little or no internal structure Vacuoles hold substances (varies from one cell to another) Water, food, waste, pigments, enzymes Formed by the pinching of the cell membrane Very large in plant cells (central vacuole), smaller in animal cells Cytoskeleton Used to hold and change shape Used for internal organization Used for movement of molecules and/or movement of the cell Used in mitosis for the division of chromosomes Made of smaller organelles Microtubules Actin Fibrils Intermediate Fibrils 6
Cell Wall Maintains cell shape, protection, prevents excessive uptake of water Made of polysaccharide cellulose embedded in a matrix of other polysaccharides and protein Walls of different cells glued together by middle lamella Strengthens with age: secondary walls Cell Wall Cell Membrane (or Plasma Membrane) Semi-permeable membrane surrounding all cells Made of phospholipids, proteins, cholesterol, carbohydrates, glycoproteins, and glycolipids Cell Membrane Fluid-Mosaic Model Must be fluid to work properly Cholesterol controls fluidity based on temperature A mosaic of proteins is embedded and dispersed in the lipid bilayer Integral proteins inserted into the membrane Peripheral proteins not embedded, attached to membrane surface 7
Eukaryotes: Function Movement of substances across the cell membrane Bulk Flow Diffusion Osmosis Facilitated Diffusion Active Transport Vesicle Mediated Transport Cell-Cell Junction Eukaryotes: Function Bulk Flow molecules move all together in the same direction due to force from hydrostatic pressure Diffusion (no energy) The movement of molecules from high concentration to low concentration with no energy requirement (small molecules only) Eukaryotes: Function Osmosis (no energy) Special case of diffusion: movement of water across the membrane from high water potential to low water potential Facilitated Diffusion (no energy) Polar molecules cannot get through by diffusion, so cells use integral membrane proteins to transport them in/out Transport proteins are highly selective Uniport, symport, and antiport proteins Eukaryotes: Function Active Transport (energy) When a substance is moved across the membrane against it s concentration gradient Requires energy and membrane proteins 8
Eukaryotes: Function Vesicle-Mediated Transport Vesicles/vacuoles can fuse with the cell membrane Exocytosis: expulsion of contents outside the cell Endocytosis: bringing in outside molecules Phagocytosis (cell eating) Pinocytosis (cell drinking) Receptor-mediated endocytosis Eukaryotes: Function Cell-Cell Junction Cells organized into tissues must communicate with each other Chemical signals (exocytosis from one, endocytosis into the next) Other junctions Desmosome Tight junction Gap junction plasmodesma 9