Lecture 6: Prokaryotic cell organization

Question 1

Describe the major functions of the plasma membrane

Answer 1

It serves as a selectively permeable barrier to mediate nutrient and waste transport. It is also the location of many metabolic processes and detects environmental cues for chemotaxis. It also contains the cytoplasm.

Question 2

Describe the function of the cell wall.

Answer 2

Gives bacteria shape and protection from lysis in solution.

Question 3

Describe the function of the capsule.

Answer 3

It is important for adherence to surfaces and resistance to phagocytosis and bacteriophage infection.

Question 4

What are the fimbriae and pilli and what is their function?

Answer 4

They are small protrusions coming out of the cell that aid in attachment to surfaces and bacterial mating.

Question 5

What are flagella? What is their function?

Answer 5

They are long structures that are attached to a small motor in the membrane. By spinning, they allow the cell to swim in the environment.

Question 6

Describe the composition of the bacterial cytoplasm.

Answer 6

It is very thick and has a high concentration of macromolecules. It contains circular pieces of DNA called plasmids and circular chromosomes. It also has inclusion bodies, which can consist of important nutrients for the cell, and ribosomes.

Question 7

Give one example of a specialized inclusion body in bacteria and its function.

Answer 7

Gas vacuole - uses gas for buoyancy in aquatic environments. This is important for cells that need to access to oxygen or light.

Question 8

What is the periplasmic space? Where is it found?

Answer 8

Periplasmic space is the space between the inner membrane and outer membrane in gram-negatives and between the inner membrane and peptidoglycan in gram-positives.

Question 9

What is the function of periplasmic space?

Answer 9

Contains hydrolytic enzymes and binding proteins for nutrient processing and uptake

Question 10

Describe the main layers of a gram-negative bacterial cell envelope. Explain where the capsule fits in.

Answer 10

From inside to outside: plasma membrane, thin peptidoglycan cell wall, outer membrane. The capsule is not part of the envelope itself, but is an additional layer that is outside the outer membrane.

Question 11

Describe the main layers of a gram-positive bacterial envelope. Explain where the capsule fits into it.

Answer 11

There is an inner cytoplasmic membrane followed by a very thick peptidoglycan cell wall (there is no outer membrane). The capsule is not part of the envelope itself, but is instead a separate outer layer.

Question 12

Describe the structure of a bacterial envelope for mycoplasma.

Answer 12

It consists of just a cytoplasmic membrane that interacts directly with the environment.

Question 13

Explain the importance of the cytoplasmic membrane bilayer being fluid.

Answer 13

The cell needs to be able to grow in size and divide without the bilayer rupturing.

Question 14

Proteins make up […] of the dry weight of the plasma membrane. The two main types are […] and […]

Answer 14

50%, transmembrane proteins, peripheral membrane proteins

Question 15

What is the difference between a saturated and an unsaturated fatty acid tail? Explain.

Answer 15

Saturated fatty acids have no double bond and thus do have have a kink, while unsaturated fatty acids have at least one double bond and have a kink in their structure.

Question 16

Describe the structure of the typical phospholipid in the plasma membrane.

Answer 16

They consist of a hydrophilic head and hydrophobic fatty acid tails, one saturated and one unsaturated.

Question 17

Is there cholesterol in the bacterial cell membrane? If so, describe its purpose.

Answer 17

No, there is no cholesterol in the membrane.

Question 18

What is the difference between the peptidoglycan cell wall of gram-negatives and gram-positives?

Answer 18

In gram-negatives, it is thin, while in gram-positives it is very thick.

Question 19

Name two functions of the peptidoglycan cell wall.

Answer 19

It gives the cell its shape and rigidity and allows the cell to withstand osmotic pressure.

Question 20

Explain why the ability of the peptidoglycan to withstand osmotic pressure is important.

Answer 20

Bacterial cells are always in a hypotonic state because the cytoplasm is very concentrated in molecules, so water always wants to flow into the cell and eventually cause it to burst. The peptidoglycan prevents this because it is so rigid.

Question 21

Explain how penicillin takes advantages of the role of the peptidoglycan layer.

Answer 21

Penicillin inhibits the generation of the peptidoglycan cell wall, so the bacterial cells will grow without more cell wall. This will cause the cell to eventually lose its shape as the wall weakens and lyse, thus killing the bacteria.

Question 22

Describe the structure of the peptidoglycan cell wall.

Answer 22

It is composed of long chains of alternating sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). Connecting these long chains are peptide chains that connect to a pentaglycine interbridge, which serve as cross links. They attach to NAM.

Question 23

What is the peptide interbridge made of?

Answer 23

It is a chain of 5 Gly amino acids in gram-positive. In gram negatives, there is no pentaglycine interbridge, and instead it is just a direct link.

Question 24

How do the peptidoglycan cross links differ between gram-positives and gram-negatives?

Answer 24

In gram-negatives, the peptide interbridge connects D-Ala to DAP, while in gram-positives, it connects D-Ala to L-Lys.

Question 25

In a given peptidoglycan cross link, explain the difference between the two amino acid chains and the reason for this difference.

Answer 25

There is one less D-alanine on the left peptide chain. This is because in the formation of the peptide interbridge, one of the D-alanines was cleaved off, leaving only one behind and two remaining on the other side. The energy from this cleavage allows for the completion of the peptide interbridge.

Question 26

Describe the membrane structure of a mycoplasma.

Answer 26

From inside to outside: inner membrane, peptidoglycan, arabinogalactan, outer membrane