Prokaryotes 20-2

Federoff

Classifying Prokaryotes– The smallest and most

abundant microorganisms on Earth are prokaryotes—unicellular organisms that lack a nucleus.

– Prokaryotes have DNA, like all other cells, but their DNA is not found in a membrane-bound nuclear envelope as it is in eukaryotes. Prokaryote DNA is located in the cytoplasm.

Classifying Prokaryotes– Recently, biologists have divided

prokaryotes into two very distinct groups: Bacteria and Archaea.

– These groups are very different from each other; therefore, biologists now consider each group of prokaryotes as a separate domain. Eukaryotes are the third domain.

Bacteria – The larger of the two

domains of prokaryotes is the Bacteria.

– Bacteria include a wide range of organisms with lifestyles so different that biologists do not agree exactly how many phyla are needed to classify this group.

Bacteria – Bacteria live

almost everywhere—in fresh water, in salt water, on land, and on and within the bodies of humans and other eukaryotes.

Bacteria – Bacteria are

usually surrounded by a cell wall made from peptidoglycan that protects the cell from injury and determines its shape.

Bacteria – In addition,

some prokaryotes have flagella that they use for movement, or pili, which in E. coli serve mainly to anchor the bacterium to a surface or to other bacteria.

Archaea – Under a microscope, archaea look very similar to bacteria.

Both are equally small, lack nuclei, and have cell walls, but there are important differences.

– The walls of archaea lack peptidoglycan, and their membranes contain different lipids.

– The DNA sequences of key archaea genes are more like those of eukaryotes than those of bacteria.

– Based on these observations, scientists have concluded that archaea and eukaryotes are related more closely to each other than to bacteria.

Archaea – Many archaea live in extremely harsh

environments.

Structure and FunctionHow do prokaryotes vary in their structure and function?

– Prokaryotes vary in their size and shape, in the way they move, and in the

– way they obtain and release energy.

Size, Shape, and Movement

– Rod-shaped prokaryotes are called bacilli.

– Spherical prokaryotes are called cocci.

– Spiral and corkscrew-shaped prokaryotes are called spirilla.

Size, Shape, and Movement – Prokaryotes can also be

distinguished by whether they move and how they move.

– Some prokaryotes do not move at all. Others are propelled by flagella. Some glide slowly along a layer of slimelike material they secrete.

Nutrition and Metabolism

– Energy is released from these fuel molecules during cellular respiration, fermentation, or both.

Nutrition and Metabolism: Energy Capture

Nutrition and Metabolism: Energy Release

Growth, Reproduction, and Recombination

– When a prokaryote has grown so that it has nearly doubled in size, it replicates its DNA and divides in half, producing two identical cells. This type of reproduction is known as binary fission.

Growth, Reproduction, and Recombination

– Because binary fission does not involve the exchange or recombination of genetic information, it is an asexual form of reproduction.

– When conditions are favorable, prokaryotes can grow and divide at astonishing rates—some as often as once every 20 minutes!

Growth, Reproduction, and Recombination

– When growth conditions become unfavorable, many prokaryotic cells form an endospore—a thick internal wall that encloses the DNA and a portion of the cytoplasm.

– Endospores can remain dormant for months or even years.

Mutation – Mutations are one of the main ways

prokaryotes evolve.

– Mutations are random changes in DNA that occur in all organisms.

– In prokaryotes, mutations are inherited by daughter cells produced by binary fission.

Conjugation – Many prokaryotes exchange genetic

information by a process called conjugation.

– During conjugation, a hollow bridge forms between two bacterial cells, and genetic material, usually in the form of a plasmid, moves from one cell to the other.

Conjugation

– This transfer of genetic information increases genetic diversity in populations of prokaryotes.

Decomposers – By decomposing dead organisms,

prokaryotes, supply raw materials and thus help to maintain equilibrium in the environment.

– Bacterial decomposers are also essential to industrial sewage treatment, helping to produce purified water and chemicals that can be used as fertilizers.

Producers – Photosynthetic prokaryotes are among

the most important producers on the planet.

– Food chains everywhere are dependent upon prokaryotes as producers of food and biomass.

Nitrogen Fixers – All organisms need nitrogen to make proteins and other

molecules.

– Nitrogen gas (N2) makes up 80 percent of Earth’s atmosphere, but only a few kinds of organisms—all of them prokaryotes—can convert N2 into useful forms.

– The process of nitrogen fixation converts nitrogen gas into ammonia (NH3). Ammonia can then be converted to nitrates that plants use, or attached to amino acids that all organisms use.

– Nitrogen-fixing bacteria and archaea provide 90 percent of the nitrogen used by other organisms.

Nitrogen Fixers – The Rhizobium bacteria often live

symbiotically within nodules attached to roots of legumes, such as clover, where they convert atmospheric nitrogen into a form that is useable by plants.

Human Uses of Prokaryotes – Prokaryotes, especially bacteria, are used in the production

of a wide variety of foods and other commercial products.

– Yogurt is produced by the bacterium Lactobacillus.

– Some bacteria can digest petroleum and remove human-made waste products and poisons from water.

– Other bacteria are used to synthesize drugs and chemicals through the techniques of genetic engineering.

– Bacteria and archaea adapted to extreme environments may be a rich source of heat-stable enzymes that can be used in medicine, food production, and industrial chemistry.