Exam 1- Essay Questions

In 1884, Hans Christian Gram described a method of staining bacterial cells while not staining the surrounding animal tissues. However, he thought that the staining method he developed was faulty because all bacteria did not stain. In a letter to the editor of the journal in which Gram published his findings, write your response to Gram's concern.

Some of the bacteria (such as pneumucoccus) retained the stain (Gram +) while others did not (Gram -). This discovery is of great use in identification and classification of bacteria. It is also useful in deciding the treatment of bacterial disease, since penicillin is active only against Gram + bacteria; the cell walls of gram - bacteria will not take up either pencil in or gram stain.

Provide evidence to substantiate the hypothesis that eukaryotic cells evolved from prokaryotic cells.

One theory is the Endosymbiont Hypothesis which is: Two bacteria (both prokaryote) were in the presence of each other. One was either engulfed or invaded the cell. In both cases, the prokaryote had its DNA put into the DNA of the bacteria and then new new generations had the new organelle as well. Five supporting evidences for the Endosymbiont Hypothesis: MITOCHONDRIA and CHLOROPLASTS · have two membranes · have their own DNA in the form of a circular chromosome · have their own ribosomes · have ribosomes are more similar to bacterial than eukaryotic ribosomes. · reproduce themselves when the cell divides (Nuclear DNA does not have recipe for mitochondria or chloroplasts)

What is the importance of a phospholipid bilayer to living cells? Where is it in prokaryotic cells?

Stops unwanted substances passing the cell membrane (Keeps cell contents in, along with cytoplasmic pressure/tension and also keeps harmful or unnessarcary substances out). Due to the polarity of a phospolipid, only small un-polar molecules can pass through it. Hydrophilic(+ charge) and hydrophobic. (- charge) In a prokaryotic cell, it is located at the plasma cell membrane which is INSIDE of the cell wall.

In 1877, Robert Koch thought preparing permanently stained slides would be valuable. Why was his assessment correct?

A method was needed to separate these different bacteria. he used permanently staining methods to begin the process of distinguishing microbes and identifying pathogens. These same techniques are essential in studying all microorganism, documenting infection, as well as useful for teaching and research. Staining differentiates gram + and gam - without staining a bunch of cells can be any type of bacteria.

Compare and contrast gram-positive and gram-negative cell walls with regard to (a) sensitivity to antimicrobial agents, (b) resistance to phagocytosis, (c) chemical composition, and (d) decolorization by alcohol.

Both gram-positive and gram-negative bacteria have a cell wall made up of peptidoglycan and a phospholipid bilayer with membrane-spanning proteins. However, gram-negative bacteria have a unique outer membrane, a thinner layer of peptidoglycan, and a periplasmic space between the cell wall and the membrane. In the outer membrane, gram-negative bacteria have lipopolysaccharides (LPS), porin channels, and murein lipoprotein all of which gram-positive bacteria lack. As opposed to gram-positive cells, gram-negative cells are resistant to lysozyme (phagocytosis) and penicillin attack. The gram-negative outer membrane which contains LPS, an endotoxin, blocks antibiotics, dyes, and detergents protecting the sensitive inner membrane and cell wall. LPS is significant in membrane transport of gram-negative bacteria. LPS, which includes O-antigen, a core polysaccharide and a Lipid A, coats the cell surface and works to exclude large hydrophobic compounds such as bile salts and antibiotics from invading the cell. O-antigen are long hydrophilic carbohydrate chains (up to 50 sugars long) that extend out from the outer membrane while Lipid A (and fatty acids) anchors the LPS to the outer membrane. A staining technique used to classify bacteria in which a bacterial specimen is first stained with crystal violet, then treated with an iodine solution, decolorized with alcohol, and counterstained with safranine. Gram-positive bacteria retain the violet stain; gram-negative bacteria do not. That means gram-negative bacteria is decolorized with alcohol.