AP Biology 2014
Prokaryotes
From an evolutionary viewpoint bacteria (unicellular organisms) are extremely successful.
 They evolved from a protobiont and have been around since the beginning of life on earth.
 They can thrive in any environment and are incredibly diverse, and exist in massive numbers.
It is their structural, functional, and genetic adaptations that have allowed prokaryotes to be so
successful.
A. Structural Adaptations in Prokaryotes
1. Come in 3 main shapes: Spheres (cocci), rod-shaped (bacillus), spiral shaped (spirochaetes).
 Different shapes (phenotypes) allows for selection.
2. Possess a tough cell wall.
 It provides protection, and keeps the cell’s shape.
 It prevents bursting which allows them to inhabit hypotonic environments.
3. A Capsule
Some bacteria have a sticky polysaccharide capsule that covers the cell wall.
 This allows them to stick or anchor to a surface such as the intestinal lining of animals;
consequently they don’t get washed away as food moves through the intestines.
4. Fimbrae or Pilli
These are hair-like projections that allow bacteria to attach to a host or to other bacterial cells to form a
colony.
 Like a capsule, these adaptations are significant because they prevent bacteria from being
moved from their environment.
 They also allow bacteria in close proximity to exchange plasmids (transformation)
5. Flagella
Some bacteria have flagella for propulsion. These bacteria show “taxis” which is purposeful movement
towards a stimulus such as light, oxygen, or food.
B. Internal Organization of Prokaryotes
Prokaryotes do not exhibit cellular compartmentalization; that is they do not have membrane-bound
organelles.
However, many bacterial species still carry out complex functions like aerobic respiration. The
enzymes of glycolysis, the Krebs cycle and electron transport chains are organized along the plasma
membrane.
 Some aerobic bacteria have infoldings of their plasma membrane like the cristae of
mitochondria.
 Some photosynthetic bacteria also have infoldings of their plasma membrane that resemble the
grana of chloroplasts.
Some bacteria have a basic organization to their metabolic machinery which is important because it
allows for more efficient respiration and photosynthesis.
C. Genomic Organization
The prokaryotic genome is a single, circular chromosome (DNA molecule) that is not associated with
histone proteins, and there is no nuclear envelope.
 Consequently, bacterial DNA is easy and fast to replicate, transcribe, and translate.
 This means fast reproduction and bacteria can make proteins and enzymes very quickly in order
to adjust to changing environmental conditions, like a new food source.
 Ribosomes are smaller than eukaryotic ribosomes.
Plasmids
These are small circular pieces of DNA that operate independently of the main bacterial chromosome.
They are a very significant adaptation because they allow for the rapid transfer of useful genes from
one bacterium to another.
 Plasmids often carry the genes required for the production of pili (bacterial sex) and antibiotic
resistance, both of which increase fitness.
The small size and organizational structure of the prokaryotic genome has contributed greatly to their
success.
 They reproduce very quickly (binary fission occurs once every 1-3 hours).
 This allows for a high mutation rate which creates new genes for selection and adaptation to
new environments.
 They are awesome at adapting to change, good for them, bad for us. See MRSA.
D. Modes of Nutrition
As a group they show great diversity in their modes of nutrition and method of attaining energy. As a
group bacteria can live anywhere because the have the ability to get energy and carbon in a number of
different ways.
1. Photoautotrophs
These use light as an energy source and CO2 as a source of carbon (like plants and certain algae).
 Cyanobacteria, also called blue-green algae are critical to the biosphere.
2. Chemoautotrophs
Use inorganic chemicals like hydrogen sulfide and methane as an energy source and CO2 as a source
of carbon.
3. Photoheterotrophs
Use light as a source of energy and organic compounds for a carbon source.
4. Chemoheterotrophs
Energy source and carbon source are organic compounds such as sugars, and proteins.
E. Metabolism in Prokaryotes
Prokaryotic species include:
 Obligate aerobes: these require oxygen to survive.
 Facultative anaerobes: can survive with or without oxygen.
 Obligate anaerobes: these are poisoned by oxygen.
These are adaptations that allow the exploitation of a variety of environments thus increasing their
fitness as a group.
Nitrogen Fixing Bacteria
An extremely important group of organisms.
 They have the ability to take gaseous nitrogen in the air and “fix” it (via metabolism) into
inorganic nitrates (NO2, NO3). They are the only organisms on earth to do this!
 These nitrates can then be incorporated into other organisms and used to build organic
molecules that contain nitrogen (amino acids, nucleotides etc).
 Nitrogen fixing bacteria live in symbiotic relationships with the root systems of plants. They
get food from the plant and provide usable nitrogen to the plant and hence every heterotroph,
including ourselves, on earth.
Prokaryotic Phylogeny
Molecular systematics (DNA, RNA homologies) have identified new major clades of prokarotes.
Ex. Clamydias and spirochaetes share a common ancestor, interestingly they both cause STD’s.
Role of Prokaryotes in the Biosphere
 Bacteria are crucial in that they decompose dead organisms (detritus) and in doing so recycle
chemical elements between the biotic and abiotic compartments of the ecosystem.
 Nitrogen fixing bacteria add usable nitrogen to the biotic component.
 Many bacteria live in symbiotic relationships with other organisms (bacteria in your gut)

Other bacterial species are parasites or human pathogens.
 Bacteria cause about half of all human disease
 They have been weaponized (anthrax)
Usefulness
 In biotechnology (restriction enzymes, and cloning genes)
 They are agents of bioremediation: used to breakdown oil spills and pollutants.
 Used to make large scale amounts of vitamins and hormones (insulin, growth hormone).