Prokaryotes vs. Eukaryotes
Prokaryotes
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Derivation of name:
- Pro: before
- Karyo: nucleus
- Prokaryote: an organism that existed before the nucleus evolved
- Includes all bacteria.
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Characteristics:
- No nucleus
- No organelles (“little organs”) of any kind
- Primary genetic material: one large piece of circular DNA
- Secondary genetic material: many additional very small pieces of circular DNA called plasmids
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Have cell walls
- Prokaryotic cell walls are different from those of plants!
- The cell wall is in addition to a cell membrane, which all cells have.
Eukaryotes
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Derivation of name:
- Eu: good, true
- Karyo: nucleus
- Eukaryote: an organism with a true nucleus
- All organisms other than bacteria
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Characteristics:
- Have a nucleus.
- Have organelles.
- Primary genetic material: linear DNA (not circular)
- No secondary genetic material
- 1000 × as much DNA as prokaryotes
Size
- Eukaryotes are 10 to 100 × bigger in size than prokaryotes
- Compare the eukaryotic cells (labelled “cells”) in the image with the much smaller bacteria cells (labelled “bacteria”).
Evolutionary Relationship between Prokaryotes and Eukaryotes
- The three domains reflect evolutionary relationships.
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Prokaryotes evolved first:
- Domain Bacteria
- Domain Archaea
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Eukaryotes followed:
Prokaryotes and Their Outer “Container”
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Cell membrane (plasma membrane)
- Exists as it does in all living things.
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Cell wall
- Unique to prokaryotes (in this specific, peptidoglycan form)
- Made from peptidoglycan: a mixture of polysaccharides (sugars) and polypeptides (proteins)
- Can differ among bacteria: Gram negative and Gram positive.
- Is attacked by antibiotics like penicillin.
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Capsule
- An additional sticky coat
- Adds protection.
- Helps bacteria stick to surfaces.
Prokaryote Movement
- Some prokaryotes can move, using flagella.
- Half of all prokaryotic species have this capability.
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In deciding which way to go, they show taxis.
- Movement towards or away from a stimulus
- Can be “positive” or “negative.”
- Chemotaxis: movement towards or away from a chemical stimuli
- Phototaxis: movement towards or away from a light stimuli
- Magnetotaxis: movement towards or away from a magnetic stimuli
Cellular and Genomic Organization of Prokaryotes
- What they don't have:
- No extensive internal membrane structures (e.g. organelles)
- Thus, they don't have internal “compartments” like eukaryotes do.
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How is DNA organized?
- Found in a region called the nucleoid, which is not a separate compartment like the nucleus is in eukaryotes.
- The main piece of DNA is circular and located in the nucleoid region.
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Additional smaller circular pieces of DNA called plasmids exist independently of the main DNA.
- Carry only a few genes each.
- Often have genes for antibiotic resistance or other special functions.
- Can be transferred between bacteria during “sexual” encounters.
Prokaryote Reproduction
- They reproduce asexually, by binary fission.
- One bacterium becomes two bacteria.
- They produce two identical copies in the process; they are clones.
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They never have “sex,” exactly, but instead mix up their genetic material in three ways:
- Transformation: bacteria simply absorb, across their cell membrane and cell wall, DNA that is floating around in the environment.
- Conjugation: bacteria directly transfer DNA from one bacterium to another through structures called pilli.
- Transduction: DNA is transferred from one bacterium to another by a virus.
Nutrition Basics
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All organisms need two things to live:
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Energy
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Carbon
- Provides the chemical structure of all living things.
- Carbon is like the “bricks” in a brick building: without it you would have no structure!
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So what is nutrition?
- It is how organisms obtain these two resources.
- There are four different ways for organisms to get energy and carbon…
Four Modes of Nutrition
Pick one from the left, one from the right
Obtaining Energy |
Obtaining Carbon |
Phototrophs
- Photo: light
- Troph: feeder
- Get energy from light
|
Autotrophs
- Auto: self
- Troph: feeder
- Get carbon from CO2
|
Chemotrophs
- Chem: chemical
- Troph: feeder
- Get energy from chemicals
|
Heterotrophs
- Hetero: other
- Troph: feeder
- Get carbon from a source other than CO2
|
Autotrophs (self-feeders): don't need other organisms for carbon
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Photoautotrophs
- Use light as an energy source.
- Use CO2 as a carbon source.
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Examples:
- All photosynthetic prokaryotes (e.g. cyanobacteria, the most famous)
- Plants (eukaryotes)
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Chemoautotrophs
- Use inorganic chemicals as an energy source (H2S, Fe2+).
- Use CO2 as a carbon source.
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Example:
- Some archaea living in hydrothermal vents do this since they have no light deep down in the ocean.
Heterotrophs (other-feeders): need other organisms for carbon
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Photoheterotrophs
- Use light as an energy source.
- Use organic molecules as a carbon source.
- These are rare.
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Chemoheterotrophs
- Use organic molecules as an energy source (e.g. sugar).
- Use organic molecules as a carbon source (e.g. proteins, sugars, fats, nucleic acids, etc.)
- The most common form of nutrition on Earth: used in most prokaryotes, protists, fungi, and animals.
Summary of Nutritional Modes
Mode of Nutrition |
Energy Source |
Carbon Source |
Types of Organisms |
Autotroph
|
Photoautotroph |
Light |
CO2 |
Photosynthetic prokaryotes, including cyanobacteria; plants; certain protists |
Chemoautotroph |
Inorganic chemicals |
CO2 |
Certain prokaryotes (e.g. Sulfolobus) |
Heterotroph
|
Photoheterotroph |
Light |
Organic compounds |
Certain prokaryotes |
Chemoheterotroph |
Organic compounds |
Organic compounds |
Most prokaryotes and protists; fungi; animals; some plants |
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The two most common modes of nutrition:
- Photoautotrophs: use sunlight for energy, CO2 in the atmosphere as a carbon source.
- Chemoheterotrophs: “eat” other organisms for energy and carbon.
Notes on Prokaryotic Chemoheterotrophs
- The most common nutritional mode in prokaryotes.
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The chemoheterotrophic lifestyle among prokaryotes includes two categories:
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Saprobes (decomposers)
- Get their nutrition (energy and carbon) from dead organisms.
- Prokaryotes are the greatest biodegraders on the planet! If something is not biodegradable then bacteria generally can't break it down.
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Parasites
- Get their (energy and carbon) from living organisms.
Prokaryotes and the Nitrogen Problem
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The problem:
- All living organisms need nitrogen, since it is found in proteins and nucleic acids.
- The majority of the nitrogen on the planet is in the atmosphere (N2) and is unusable by living organisms.
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The solution:
- Some prokaryotes can convert unusable N2 into a usable form like NH3.
- This is called nitrogen fixation.
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An example: the symbiotic relationship between prokaryotes and the group of plants called legumes (peas, beans, alfalfa, etc.).
- Bacteria live in nodules on the legume roots and fix nitrogen, thus providing a benefit to the plant.
- The plant, in turn provides sugar and other organic nutrients to the bacteria.
- This is a specific kind of symbiotic relationship called mutualism.
Oxygen: One Bacteria's “Food” is another Bacteria's “Poison”
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For some organisms, oxygen is required for life: they are obligate aerobes (“obliged to use oxygen”)
- Obligate is from “oblige”—to be bound or compelled.
- Aerobe: air/oxygen
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For other organisms, oxygen will poison them: they are obligate anaerobes (“obliged to not use oxygen”)
- An: no, not
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Example from the domain Archaea: methanogens
- Convert CO2 to CH4 (methane).
- Found in the gastrointestinal tract of animals.
- Found in swamps, marshes, and sewer treatment facilities.
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For yet other organisms, either condition is fine: they are facultative anaerobe (“not required to not use oxygen”)
- Facultative: not required or compulsory; optional
Oxygen Revolution
- No oxygen existed on planet Earth after its creation (4.5 billion years ago).
- Life emerged around 3.8 billion years ago in an anaerobic environment.
- Therefore, the original prokaryotes were obligate anaerobes!
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Cyanobacteria evolve between 3.4 and 2.5 billion years ago.
- Able to photosynthesize
- Produce oxygen as a byproduct of photosynthesis.
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Oxygen buildup oxidizes iron in the oceans forming iron oxide (i.e. “rust”).
- Evidence: banded iron formations appear in marine sediments (ancient “rust”).
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Results of the oxygen revolution:
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Once all the iron in the oceans and on land was oxidized…
- Oxygen concentration increased in the oceans.
- Oxygen concentration increased in the atmosphere.
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As a result, obligate anaerobic bacteria become marginalized:
- They have nowhere to run, and nowhere to hide.
- Well, not exactly; they simply have to search for places without oxygen.
- Obligate aerobic bacteria evolve.
- Cyanobacteria is responsible for the creation of oxygen-rich atmosphere, although other organisms now help contribute.