2 Microbial Physiology Module 2: Cytoplasmic Membrane and Cytopl

A) Mechanical support is the correct answer for "not a function," and the other three are well-established, genuine functions.

A) Mechanical support — correctly excluded. The cytoplasmic membrane is a fluid, fragile phospholipid bilayer with no inherent structural rigidity; it cannot withstand internal osmotic (turgor) pressure on its own. That job belongs to the rigid peptidoglycan cell wall, which sits outside the membrane specifically to provide mechanical strength and shape. A cell whose wall is removed (e.g., by lysozyme, forming a protoplast) will lyse in ordinary media because the membrane alone provides no mechanical support.

B) Permeability barrier — a genuine function. The membrane is the cell's selectively permeable barrier, blocking free diffusion of most polar and charged solutes while allowing regulated passage of nutrients and waste via embedded transport proteins.

C) Energy conservation — a genuine function. It houses the electron transport chain components and ATP synthase, generating and harnessing the proton motive force for oxidative phosphorylation (or photophosphorylation in phototrophs).

D) Protein anchor — a genuine function. Numerous proteins are embedded in or anchored to the membrane, including transport carriers, the electron transport chain, cell-wall/lipid biosynthetic enzymes, flagellar motor components, and chemotaxis sensory proteins.

Reference:

Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock biology of microorganisms. Pearson Higher Education.

Rhodopseudomonas palustris is a purple non-sulfur bacterium that, under low-oxygen or anaerobic conditions, develops an intracytoplasmic membrane (ICM) by invaginating its cytoplasmic membrane. This ICM houses the photosynthetic apparatus — reaction centers (RC) and light-harvesting complexes (LH1/LH2) — analogous to thylakoid membranes in chloroplasts and cyanobacteria. The well-established purpose of this structure is to dramatically expand the membrane surface area available for capturing light and carrying out the associated electron transport and photophosphorylation reactions, since these processes are intrinsically membrane-bound.

Direct evidence from related purple non-sulfur bacteria (e.g., Rhodobacter sphaeroides) shows this quantitatively: cells can develop networks of ICM vesicles totaling tens of millions of square nanometers of specialized membrane area, compared to only a few million under high-light (low-ICM) conditions, directly confirming that ICM proliferation is a surface-area expansion strategy for energy transduction. A review on bacterial ICMs states explicitly that prokaryotes "increase their membrane surface area for energy transduction processes" by forming these structures.

Reference:

Jensen, C. M. O. a. G. J. (n.d.). 4.3 Intracytoplasmic membrane | Atlas of bacterial and archaeal cell structure. https://www.cellstructureatlas.org/4-3-intracytoplasmic-membrane.html

Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock biology of microorganisms. Pearson Higher Education.

Bactoprenol (undecaprenyl phosphate) is a C55 isoprenoid lipid carrier that shuttles peptidoglycan precursors (and other cell-envelope building blocks like LPS O-antigen and teichoic acid subunits) across the cytoplasmic membrane during cell wall synthesis — it plays no role in fatty acid biosynthesis . Fatty acid synthesis instead relies on acyl carrier protein (ACP) as its carrier molecule, not bactoprenol. This makes (c) a genuinely false statement and a good distractor because it borrows a real, plausible-sounding membrane-synthesis term but misapplies it to the wrong pathway.

Reference:

Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock biology of microorganisms. Pearson Higher Education.

White, D., Drummond, J., Drummond, J. T., & Fuqua, C. (2012). The physiology and biochemistry of prokaryotes. Oxford University Press, USA.

Membrane lipid distribution is well-established as asymmetric, not symmetric, across both leaflets of bacterial membranes. The Gram-negative outer membrane confines LPS to the outer leaflet and phospholipids to the inner leaflet, and even the inner/cytoplasmic membrane shows measurable phospholipid leaflet asymmetry (e.g., unequal distribution of phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin between leaflets). No standard reference describes cell membrane lipids as symmetric.

Reference:

Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock biology of microorganisms. Pearson Higher Education.

In bacterial (and eukaryotic) phospholipids, fatty acid tails are attached to the glycerol-3-phosphate backbone via ester bonds, not ether bonds. Ether linkages to glycerol-1-phosphate are the defining, distinguishing feature of archaeal membrane lipids, explicitly contrasted against bacterial ester-linked lipids in the literature. Since the stem specifies "bacterial cell membrane," this statement is incorrect as written and should be flagged as false.

Reference:

Madigan, M. T., Bender, K. S., Buckley, D. H., Sattley, W. M., & Stahl, D. A. (2018). Brock biology of microorganisms. Pearson Higher Education.