Lesson 3.2 Prokaryotic Cells

3.2.4 Specific cell processes and cell structures

Now that you are comfortable with the structures of prokaryotic cells, we will apply what you have learned to four specific cell functions: respiration, photosynthesis, protein synthesis, and cell reproduction.

Respiration

Respiration in cells is also referred to as cellular respiration. For us, the term “respiration” means “to breathe” and is most often associated with the process of bringing air into the lungs, and releasing air out of the lungs. For cells, however, the term respiration refers to the biochemical process of creating energy for the cell. Energy within cells is associated with the molecules ATP. Energy is essential to all life processes. Without it, a cell dies.

STOP AND PREDICT:

1. Where do you think respiration takes place in a bacterium?
2. What structures in a bacterial cell may be needed? Explain your choices.

For all living cells, this process can occur with oxygen (aerobically) or without oxygen (anaerobically). In most cases, the amount of energy created will be greater if oxygen is present, than if oxygen is not present. For example, one molecule of the sugar, glucose, used in the process of aerobic cellular respiration in a prokaryote will produce 38 ATP molecules. If the same glucose molecule is used in the process of anaerobic cellular respiration in a prokaryote, 2 ATP molecules can be produced. Prokaryotic organisms that cannot survive in the presence of oxygen (obligate anaerobes) will produce fewer ATP molecules for every molecule of glucose used.

Cellular respiration occurs in the cytoplasm and the plasma membrane of the prokaryotic cell. Most of the enzymes and intermediates of the process are found in the cytoplasm, while the electron transport chain is found specifically in a folded area of the plasma membrane referred to as the mesosome . As the name implies, electrons are moved along this chain which is composed of several protein complexes. Ultimately ATP is produced, which is then used by the cell for a variety of processes.

ACTIVITY CHECKPOINT:

On your bacterial cell diagram sheet, identify the structures involved with respiration by placing a capital “R” next to the structure’s name.

Photosynthesis

While all prokaryotic cells must perform cellular respiration, not all prokaryotes will perform photosynthesis. Photosynthesis is the biochemical process that uses carbon dioxide, water, and sunlight to create nutrient rich products and energy in the form of ATP.

STOP AND THINK!

1. Where do you think photosynthesis takes place in a bacterial cell?
2. What structures in a bacterial cell may be needed? Explain your reasoning.

Prokaryotes do not possess chloroplasts for the process of photosynthesis. A variety of other structures are used, dependent upon which group of photosynthetic prokaryotes you look at. For example, in the Cyanobacteria, a set of thylakoid membranes, containing chlorophyll, is found throughout the cytoplasm. These membranes are where the process of photosynthesis takes place. Green bacteria, on the other hand, possess chlorosomes, which are located on the cytoplasmic side of the plasma membrane. These circular structures contain a form of chlorophyll called bacteriochlorophyll and collect the light energy needed for the process of photosynthesis. Purple bacteria will have their light collecting bacteriochlorophyll located within the plasma membrane itself, rather than in individual chlorosomes. It should be noted that chlorophyll and bacteriochlorophyll are chemically different from one another.

Link to the following site for information on photosynthetic prokaryotes:

Photosynthesis: Comparing Prokaryotes and Eukaryotes

(this site is Flash-based and will not run natively in iOS)

Alternative links for information on photosynthetic prokaryotes:

Photoautotrophs and Photoheterotrophs

Cyanobacteria

Anoxygenic Photosynthetic Bacteria

ACTIVITY CHECKPOINT:

On your bacterial cell activity sheet from Lesson 3.2, identify the structures involved with photosynthesis by placing a capital “P” next to the structure’s name.

Protein synthesis

There are four major groups of macromolecules that are essential to the life of a cell. They are: carbohydrates (sugars), lipids (fats), proteins, and nucleic acids. All cells have the capacity to create proteins through the process of protein synthesis.

STOP AND THINK!

1. Why would a bacterial cell need to create proteins?
2. Where do you think protein synthesis takes place in the bacterial cell?
3. Which specific structures in a bacterial cell may be needed for protein synthesis?

In prokaryotes, protein synthesis requires DNA, RNA, enzymes and amino acids. The DNA is free floating in the cytoplasm and stores all the information needed to create cell proteins.

 RNA is in three forms:

a.        Ribosomal: composes the ribosomes; found in the cytoplasm of the cell; used to assemble the protein

b.        Messenger: created to take the protein code from the DNA to the ribosome; provides the instructions to put the protein together

c.         Transfer: found in the cytoplasm of the cell; used to bring the building blocks of the protein, known as amino acids, to the ribosome

The amino acids are created through a number of biochemical pathways from compounds that the cell utilizes for nutrition. The resulting amino acids are found in the cytoplasm of the cell. The proteins produced by a cell from its amino acids are used for maintaining/repairing cell structures, catalyzing chemical reactions, cell division and a variety of other uses.

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 Figure 3.2.3

Figure 3.2.3 shows the various components required for the creation of proteins in organisms. The messenger RNA contains information from the DNA for a specific protein. At the ribosome, the amino acids which are the individual building blocks of the protein, are assembled in correct order based on the code in messenger RNA. The amino acids are brought to the ribosome by individual TRNA (transfer RNA) molecules. The amino acids are chemically bonded to one another to create the growing peptide chain seen above.

ACTIVITY CHECKPOINT:

On your bacterial cell activity sheet from Lesson 3.2, identify the structures involved with protein synthesis by placing a capital “PS” next to the structure’s name.

Cell reproduction

STOP AND THINK!

1. Why do you think a bacterial cell reproduces?
2. How do you think a bacterial cell reproduces?
3. Draw a diagram to illustrate your idea. Label the cell structures you think are needed for bacterial cell reproduction.

The process of cell reproduction, or binary fission, results in the creation of two genetically identical cells. In prokaryotes, specifically bacteria, it requires DNA, proteins, and the plasma membrane.

The parent cell will almost double in size and will replicate its DNA. Each molecule of DNA will then be attached to opposite ends of the cell in the plasma membrane. A specific protein, referred to as FtsZ, aids in the creation of a division apparatus in the middle of the cell. The plasma membrane will then pinch down, dividing the cytoplasm in half. A new cell wall is synthesized and two genetically identical cells are produced. While it sounds fairly simple, the actual processes and their timing are under strict control.

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Figure 3.2.4

Figure 3.2.4 illustrates the process of binary fission in bacteria.

For an animated view of the process of binary fission,

View this Binary Fission video clip 

Alternative source for the process of binary fission:

Binary Fission

ACTIVITY CHECKPOINT:

On your bacterial cell activity sheet from Lesson 3.2, identify the structures involved with binary fission by placing a capital “BF” next to the structure’s name.