THE CELL THEORY AND PROKARYOTES

YOU MUST KNOW:

  • The differences and similarities between prokaryotic and eukaryotic cells
  • Key ways in which prokaryotes differ from eukaryotes in respect to genome, membrane-bound organelles, size and reproduction.
  • Mechanisms that contribute to genetic diversity in prokaryotes, including transformation, conjugation, transduction, and mutation.
  • The various metabolic mechanisms that bacteria may have.

NOTES:

I. The Cell Theory:

  • All living things are made up of cells.
  • Cells are the basic units of structure and function.
  • Cells come from already existing cells during cellular reproduction.

II. The Compound Light Microscope

We do not spend time on the light microscope here, but you are expected to know the parts of the compound light microscope.

  • Two important parameters of the compound light microscopes are magnification and resolution. Magnification is the ratio of an object's size to its real size (calculated by multiplying the magnifying power of the objective lens by the magnifying power of the eyepiece). Resolution is a measure of the clarity of the image. Because the resolution of a light microscope is limited by the wavelength of light, the effective magnification of compound light microscopes is up to about 1,000 times the actual size of the specimen.
  • The third important characteristic of light microscopes is contrast -- which accentuates differences in parts of a sample. Contrast can be enhanced by staining or some other methods.

III. The Electron Microscope

  • Electron microscopes use a beam of electrons that goes through the specimen or bounces off the surface of it.
  • This type of microscope revels the ultrastructure of cells.
  • There are two types of electron microscopes commonly used in cell biology:
  • Transmission electron microscope (TEM)


  • Scanning electron microscope (SEM)

IV. Cell Fractionation

  • Cell fractionation -- takes cells apart and separates the major organelles and other subcellular structures from one another.
  • The instrument that is used for this is a microcentrifuge. Microcentrifuges spin up to 130,000 revolutions per minute (rpm) and apply forces on particles that are more than 1 million times the force of gravity.

Figure 6.5

  • This method enables researchers to prepare specific cell components, separate them from the rest of the cell for further studying.

V. Prokaryotic Cell Structure:

  • Prokaryotic cells are found in organisms of domains Bacteria and Archea.
  • These cells are only about 1-10 in size. These cells do not contain a nucleus and membrane-bound organelles.
  • There are a few structures that are common among prokaryotic and eukaryotic cells:
  • cell membrane
  • cytosol
  • chromosomes (circular, no histones, concentrated on the nucleoid region)
  • ribosomes

You must be able to draw, label the prokaryotic cell and describe the function of each structure.

VI. Structural and Functional Adaptations of Prokaryotes

  • Prokaryotes are found almost everywhere on this planet.
  • They have been the first organisms to inhabit the Earth and adapted to live in any possible environment. This also resulted in a great diversity of these organisms.
  • Most prokaryotes are unicellular, but some may aggregate to form colonies temporarily or permanently.
  • The three most common shapes of their cells are cocci(ball shaped), bacilli (elongated shape), spiral shape.

A. Cell-Surface Structures

  • The cell wall -- helps to maintain the cell's shape, provides physical protection, prevents the cell from bursting in hypotonic environment. In hypertonic environment the cell membrane moves away from the cell wall (plasmolyze) because of the loss of water. This is the reason why salting meat can be used to preserve it.
  • Bacterial cell walls are made up of peptidoglycan (modified sugar polymers with short polypeptides).
  • Archea have polysaccharides and proteins in their cell wall but they lack peptidoglycan.
  • Gram staining -- staining technique that is used to classify bacteria. Gram positive bacteria has simpler cell wall with a thick layer of peptidoglycan that can trap the Gram stain and stains purple with it. Gram negative bacteria have less peptidoglycan in its cell wall but has an additional outer membrane cover that does not allow the stain to enter the cell. Because of that the stain easily washes off with alcohol and only a faint red color appears.
  • Gram staining is important to determine what type of antibiotics would work better for a patient with a bacterial infection.
  • Many antibiotics also break down the original cell wall structure and destroy the bacteria.
  • Capsule -- sticky layer of polysaccharide or protein outside of the cell wall. The capsule serves as a protector of the cell from dehydration and the immune system and also can attach the bacteria to various surfaces.
  • Fibriae (pilli) -- protein appendages that allow bacteria to stick on surfaces
  • Sex pilli-- longer, fewer pilli that pull cells together before they exchange their DNA

B. Motility

  • Some bacteria are able to move very fast compared to their size.
  • Their motion is helped by flagella. Some bacteria have only a few, others have many flagella. The structure of their flagellum is very different from the eukaryotic flagella. It is shorter, not covered by cell membrane extension and made up of different molecules.
  • Some prokaryotes exhibit taxis when they move toward or away from certain environmental stimulus. (ex. positive and negative chemotaxis)

C. Bacterial Genome (18.3)

  • The main component of the bacterial genome is one double-stranded circular chromosome (DNA molecule) that is associated with a small amount of proteins but is not wrapped aroundhistones. The DNA is tightly packed into the chromosome.
  • Nucleoidregion is the part of the cell where the DNA is located.
  • In addition to the chromosome, the bacterium also has plasmids (small, circular-shaped DNA molecules that carry only a few genes).
  • Bacteria divide by binary fission which is a type of asexual reproduction that takes place very rapidly if conditions are appropriate. Conditions that slow bacterial growth down:
  • Not enough food available
  • Not enough room available
  • Metabolic toxic wastes increase in the environment
  • Competition with other species of bacteria
  • Presence of organisms that eat bacteria
  • Environmental toxins in the environment
  • Mutation can cause the offspring to be different from the parents. These mutations are individually rare, but are substantial in a very quickly reproducing population. These mutations make bacteria better equipped to live in a wide range of environments.
  • Other source of variation in bacteria comes from genetic recombination – combining DNA from two separate sources.

Experiment – to prove that bacteria can acquire genes from their environment:

D. Mechanisms of Gene Transfer and Genetic Recombination in Bacteria

  • Three processes that bring bacterial DNA from different individuals together are transformation, transduction and conjugation
  • Transformation: Altering the bacterial cell’s genotype or phenotype by taking in naked, foreign DNA from the surrounding environment:

Experiment:

  • Transduction – Bacteriophages (viruses that infect bacteria) carry bacterial genes from one host cell to another. This can happen, when the completed bacteriophage that is released from a bacterium takes some of the bacterial genes with it. This defective virus may be able to get into another bacterium but is not able to reproduce. The bacterium survives and carries a new gene from the virus may replace the homologous gene in its own chromosome by crossing over. This new gene will create a recombinant DNA.

  • Conjugation – The direct transfer of genetic material between two bacterial cells that are temporarily joined together by forming a cytoplasmic bridge called a mating bridge. The transfer is one-way. The donor cell carries a plasmid or a segment of its chromosome, called the F factor. The F+ cell (cell with F factor) uses sex pilli to attach to another cell that does not have the F factor (F- cell) and transfers the genes of the F factor into the recipient cell. The recipient cell becomes F+ and the donor cell also remains F+ because it replicated its F factor genes and only the copy was transferred.
  • In some cases, the F factor is built into the donor’s chromosome (also called Hfr cell – high frequency recombinant cell). These Hfr cells can transfer some of its genes with the F factor during conjugation into the F- cell. Usually the conjugation is interrupted before an entire copy of the chromosome is transferred. During crossing over the recipient can incorporate large segments of the received DNA into its own DNA.

  • Certain plasmids can also carry genes that result in antibiotic resistance (R plasmids). If these plasmids are passed on in a population, resistance to certain antibiotics can increase.

E. Transposition of Genetic Elements:

  • Transposable elements (also called “jumping genes”) are segments of the DNA molecule within a cell’s genome that can move from one site in a cell’s DNA to another site. These elements can move from one part of a chromosome to another part, from a chromosome to a plasmid from a plasmid to a plasmid etc.
  • These elements can move by two methods:
  • “cut-and-paste” mechanism – they are cut out of the original DNA segment by enzymes and reinserted into another part of a DNA molecule
  • “copy-and-paste” mechanism – the transposable element replicates at its original site and gets reinserted to a new DNA segment.
  • The simplest transposable elements called insertion sequences exist only in bacteria. They contain only a single gene. These segments can cause mutations by inserting themselves into a functioning gene and destroying the original gene sequence.
  • More complex transposable elements are called transposons. These contain more than just a gene that is necessary for the transposition. They can drag other genes, such as antibiotic resistance genes along. These may help bacteria to adapt to new environments. Transposable elements are also found in eukaryotic genomes.

VII. Nutritional and Metabolic Adaptations

  • Every type of nutrition that is observed in eukaryotes also present in prokaryotes.
  • The four major modes of nutrition in prokaryotes are:
  • Photoautotrophs– photosynthetic organisms that capture the energy of sunlight and use it to produce organic molecules by using CO2 as a carbon source. Ex. Cyanobacteria
  • Chemoautotrophs– needs only CO2 as a carbon source, but oxidizes inorganic substances as a source of energy. Ex. Sulfolobus
  • Photoheterotrophs– use light for energy but must obtain their carbon from organic materials. Ex. Rhodobacter
  • Chemoheterotrophs– consume organic materials as sources of carbon and energy. Ex. Clostridium
  • Prokaryotes also vary with respect to oxygen:
  • Obligate aerobes – use O2 for cellular respiration, cannot grow without oxygen
  • Facultative anaerobes – use cellular respiration if O2 is present, but use fermentation if O2 is not present
  • Obligate anaerobes – poisoned by O2, use only fermentation.
  • Nitrogen metabolism – PLEASE REVIEW THE NITROGEN CYCLE

VIII. Archaea

  • A newer domain of prokaryotes, with some eukaryotic characteristics. For example they have several types of RNA polymerases like eukaryotes, they do not have a peptidoglycan cell wall, have histones to wrap their DNA, their growth is not inhibited by antibiotics, some of their genes have introns.
  • They are adjusted to grow in extreme environments:
  • Extreme halophiles – live in extreme salty environments
  • Extreme thermophiles – they live in very hot environments, their enzymes are more tolerant of high temperatures.

Be able to list and describe some benefits and harms of prokaryotes.

CHAPTER 6: A TOUR OF THE CELL

FOR THIS CHAPTER, YOU WILL USE THE NOTES IN YOUR REVIEW BOOK AND WORK ON THE REVIEW CHART AND REVIEW QUESTIONS BELOW:

  1. Label the parts:
  1. Fill in the missing information:

Organelle / Structure / Illustration and function / Found in Plant Animal Both / Found in Eukaryote Prokaryote Both
Cell Wall
Centrioles
Chromosomes
Cilia or Flagella
Endoplasmis Reticulum
Golgi Complex
Lysosomes
Mitochondria
Nucleus
Peroxisomes
Plasma Membrane
Ribosomes
Vacuoles
  1. Watch the following and explain how cilia and flagella move:
  1. Compare and contrast plant cells and animal cells
  1. Compare and contrast chloroplasts and mitochondria
  1. Describe the endosymbiotic theory.