introduction to prokaryotes

prokaryote classification and evolution

Prokaryotes are currently placed in two domains. A domain is the highest taxon in the classification of living things. Its even higher than the kingdom.

classification of prokaryotes

The prokaryote domains are the Bacteria Domain and Archaea Domain, shown in Figure 8.2. All other living things are eukaryotes and placed in the domain Eukarya. (Unlike prokaryotes, eukaryotes have a nucleus in their cells.)

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evolution of prokaryotes

Prokaryotes were the first living things to evolve on Earth, probably around 3.8 billion years ago. They were the only living things until the first eukaryotic cells evolved about 2 billion years ago. Prokaryotes are still the most numerous organisms on Earth. Its not certain how the three domains of life are related. Archaea were once thought to be offshoots of Bacteria that were adapted to extreme environments. For their part, Bacteria were considered to be ancestors of Eukarya. Scientists now know that Archaea share several traits with Eukarya that Bacteria do not share. How can this be explained? One hypothesis is that the first Eukarya formed when an archaean cell fused with a bacterial cell. By fusing, the two prokaryotic cells became the nucleus and cytoplasm of a new eukaryotic cell. If this hypothesis is correct, both prokaryotic domains are ancestors of Eukarya.

prokaryote structure

All prokaryotes consist of just one cell. They share a number of other traits as well. Watch this entertaining video from the Amoeba Sisters to see how prokaryotes differ in structure from eukaryotes: MEDIA Click image to the left or use the URL below. URL:

size and shape of prokaryotes

Most prokaryotic cells are much smaller than eukaryotic cells. Prokaryotic cells are typically only 0.2-2.0 microm- eter in diameter. Eukaryotic cells are about 50 times as big. Prokaryotic cells have a variety of different cell shapes. Figure 8.3 shows three of the most common shapes: spirals (helices), spheres, and rods. Bacteria may be classified by their shape.

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flagella

Most prokaryotes have one or more long, thin "whips" called flagella (flagellum, plural). You can see flagella in Figure 8.4. Flagella help prokaryotes move toward food or away from toxins. Each flagellum spins around a fixed base. This causes the cell to roll and tumble.

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metabolism in prokaryotes

Prokaryotes may have just about any type of metabolism. They may get energy from light or from chemical compounds. They may get carbon from carbon dioxide or from other living things. Most prokaryotes get both energy and carbon from other living things. Many of them are decomposers. They break down wastes and remains of dead organisms. In this way, they help to recycle carbon and nitrogen through ecosystems. Some prokaryotes use energy in sunlight to make food from carbon dioxide. They do this by the process of photosynthesis. They are important producers in aquatic ecosystems. Look at the green streaks on the lake in Figure 8.8. They consist of billions of photosynthetic bacteria called cyanobacteria.

prokaryote metabolism and habitats

Like all living things, prokaryotes need energy and carbon. They meet these needs in a variety of ways and in a range of habitats.

prokaryote reproduction

Prokaryotes reproduce asexually. This can happen by binary fission or budding. In binary fission, a cell splits in two. First, the large circular chromosome is copied. Then the cell divides to form two new daughter cells. Each has a copy of the parent cells chromosome. In budding, a new cell grows from a bud on the parent cell. It only breaks off to form a new cell when it is fully formed.

habitats of prokaryotes

Prokaryotes live in a wide range of habitats. For example, they may live in habitats with or without oxygen. Prokaryotes that need oxygen are described as aerobic. They use oxygen for cellular respiration. Examples include the prokaryotes that live on your skin. Prokaryotes that dont need oxygen or are poisoned by it are described as anaerobic. They use fermentation or other anaerobic processes rather than cellular respiration. Examples include many of the prokaryotes that live inside your body. Like most other living things, prokaryotes have a temperature range that they "like" best. Thermophiles are prokaryotes that prefer a temperature above 45 C (113 F). They might be found in a compost pile. Mesophiles are prokaryotes that prefer a temperature of about 37 C (98 C). They might be found inside the body of an animal such as you. Psychrophiles are prokaryotes that prefer a temperature below 20 C (68 F). They might be found deep in the ocean.

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cell structures

Several other prokaryotic cell structures are also shown in Figure 8.5. They include: cytoplasm. Like all other cells, prokaryotic cells are filled with cytoplasm. It includes watery cytosol and other structures. ribosomes. This is the site where proteins are made. cytoskeleton. This is a network of fibers and tubules that crisscrosses the cytoplasm. The cytoskeleton helps the cell keep its shape. pili. These are hair-like projections from the surface of the cell. They help the cell hold on to surfaces or do other jobs for the cell.

outer layers of a prokaryote cell

The cells of prokaryotes have two or three outer layers. Like all other living cells, prokaryotes have a cell membrane. It controls what enters and leaves the cell. Its also the site of many metabolic reactions. For example, cellular respiration takes place in the cell membrane. Most prokaryotes also have a cell wall. It lies just outside the cell membrane. It makes the cell stronger and more rigid. Many prokaryotes have another layer, called a capsule, outside the cell wall. The capsule protects the cell from chemicals and drying out. It also allows the cell to stick to surfaces and to other cells. You can see a model of a prokaryotic cell in Figure 8.5. Find the cell membrane, cell wall, and capsule in the figure.

biofilms

Some prokaryotes form structures consisting of many individual cells, like the cells in Figure 8.7. This is called a biofilm. A biofilm is a colony of prokaryotes that is stuck to a surface. The surface might be a rock or a hosts tissues. The sticky plaque that collects on your teeth between brushings is a biofilm. It consists of millions of prokaryotic cells.

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prokaryotic dna

All prokaryotic cells contain DNA, as you can see in Figure 8.6. Most of the DNA is in the form of a single large loop. This DNA coils up in the cytoplasm to form a structure called a nucleoid. There is no membrane surrounding it. Most prokaryotes also have one or more small loops of DNA. They are called plasmids.

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genetic transfer

For natural selection to take place, organisms must vary in their traits. Asexual reproduction results in offspring that are all the same. They are also identical to the parent cell. So how can prokaryotes increase genetic variation? They can exchange plasmids. This is called genetic transfer. It may happen by direct contact between cells. Or a "bridge" may form between cells. Genetic transfer mixes the genes of different cells. It creates new combinations of alleles.

instructional diagrams

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questions

Genetic transfer refers to the way that prokaryotes

a. reproduce

b. pass traits to daughter cells

-->  c. increase genetic variation

d. undergo binary fission

___Most prokaryotic cells are much smaller than eukaryotic cells.

-->  a. true

b. false

___Prokaryotes were the first living things to evolve.

-->  a. true

b. false

Which statement about prokaryotes is false?

a. All prokaryotes consist of a single cell

b. You need a microscope to see prokaryotic cells

c. Prokaryotes evolved more than 3 billion years ago

-->  d. Most prokaryotic cells are larger than eukaryotic cells

A prokaryotic cell

a. always has a cell membrane

b. usually has a cell wall

c. may have a capsule

-->  d. all of the above

___Some prokaryotes consist of more than one cell.

a. true

-->  b. false

___Oxygen is toxic to all known prokaryotes.

a. true

-->  b. false

Which structures are lacking in prokaryotic cells?

a. pili

b. ribosomes

-->  c. mitochondria

d. cytoplasm and cytoskeleton

___Prokaryotic cells have hair-like projections called pili.

-->  a. true

b. false

Most prokaryotes have one or more small loops of DNA called

a. chromosomes

b. spirals

c. helices

-->  d. plasmids

___The nucleoid of a prokaryotic cell is surrounded by a membrane.

a. true

-->  b. false

___Most prokaryotes get energy and carbon from other living things.

-->  a. true

b. false

The sticky plaque that collects on your teeth between brushings consists of prokaryotes.

-->  a. true

b. false

Unlike other living things, prokaryotes do not need carbon.

a. true

-->  b. false

Thermophiles are prokaryotes that prefer a temperature below 20 C.

a. true

-->  b. false

Prokaryotes are the most numerous living things on Earth.

-->  a. true

b. false

Archaea share several traits with Eukarya that Bacteria do not share.

-->  a. true

b. false

____layer outside the cell wall of a prokaryote

a. domain

b. cyanobacterium

-->  c. capsule

d. psychrophile

e. nucleoid

f. flagellum

g. mesophile

____type of prokaryote that prefers a temperature of about 37 C

a. domain

b. cyanobacterium

c. capsule

d. psychrophile

e. nucleoid

f. flagellum

-->  g. mesophile

____large coil of DNA in the cytoplasm of a prokaryotic cell

a. domain

b. cyanobacterium

c. capsule

d. psychrophile

-->  e. nucleoid

f. flagellum

g. mesophile

____broadest taxon in modern classifications of living things

-->  a. domain

b. cyanobacterium

c. capsule

d. psychrophile

e. nucleoid

f. flagellum

g. mesophile

____type of prokaryote that prefers a temperature below 20 C

a. domain

b. cyanobacterium

c. capsule

-->  d. psychrophile

e. nucleoid

f. flagellum

g. mesophile

____long, thin whip on a prokaryotic cell

a. domain

b. cyanobacterium

c. capsule

d. psychrophile

e. nucleoid

-->  f. flagellum

g. mesophile

____type of prokaryote that makes food by photosynthesis

a. domain

-->  b. cyanobacterium

c. capsule

d. psychrophile

e. nucleoid

f. flagellum

g. mesophile

Which of the following statements about prokaryotes is false?

a. All prokaryotes lack membrane-bound organelles.

b. Prokaryotes can be seen only with the help of a microscope.

-->  c. Prokaryotes are the least numerous organisms on Earth.

d. Some prokaryotes cause human diseases.

Prokaryote domains include

a. Prokary

b. Eukarya.

-->  c. Archaea.

d. two of the above

Shapes of prokaryotic cells include

a. spirals.

b. spheres.

c. rods.

-->  d. all of the above

A prokaryote uses its whip for

a. feeling its environment.

-->  b. moving.

c. capturing prey.

d. sensing light.

The cell membrane of a prokaryotic cell

a. helps the cell hold onto surfaces.

b. makes the cell stronger and more rigid.

-->  c. provides a site for cellular respiration.

d. two of the above

New combinations of alleles are created in prokaryotes by the process of

a. sexual reproduction.

b. binary fission.

-->  c. genetic transfer.

d. asexual reproduction.

What is the function of the capsule of a prokaryotic cell?

a. protecting the cell from chemicals

b. preventing the cell from drying out

c. controlling what enters and leaves the cell

-->  d. two of the above

diagram questions

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