Final answer:
Nutrient absorption in human cells is primarily mediated by the cell membrane, with active transport often required for water-soluble nutrients. For bacteria, structures like peptidoglycan are more about cell integrity and can influence transport mechanisms but are not direct mediators of nutrient absorption. The correct option is d.
Step-by-step explanation:
Nutrient absorption is primarily mediated by the cell membrane. In the human body, nutrients are absorbed through the mucosa of the intestinal villi, in processes that often require active transport fueled by ATP.
Lipid-soluble nutrients can diffuse directly through the phospholipid bilayer of the plasma membrane, while water-soluble nutrients typically require assistance from transport proteins embedded in the membrane due to the hydrophobic nature of the lipid bilayer.
Once inside the cells, nutrients are processed and either used locally or transported to other parts of the body.
Bacterial cells use mechanisms that can include cell walls composed of peptidoglycan and transport proteins that convey substances across the membrane through active transport, which could also be coupled with active efflux pumps, especially in gram-negative bacteria.
However, the cell wall and peptidoglycan are not directly involved in nutrient absorption for human cells. Rather, they are structural components of bacterial cells that can impact the transport of molecules in and out of these cells. The correct option is d.