Question

It is proposed that the periplasmic space of gram-negative bacteria is iso-osmotic relative to the cytosol. If true, this poses a challenge for these cells when having to deal with turgor pressure. For example, it is generally believed that the rigid periplasmic peptidoglycan layer of these cells could resist the force of turgor pressure if it were exerted outward across the inner plasma membrane; thus, preventing plasma membrane lysis. However, if the periplasm is iso-osmotic with the cytosol, as proposed, the outer membrane must be under stress from the outward force of turgor pressure. Given that it is not thought that the outer membrane is built to withstand high turgor pressure, propose a plausible mechanism for how these cells resist lysis of the outer membrane in the face of high, outward turgor pressure force.

Answer 1

In case of these gram-negative bacteria, the outer membrane (made of lipid) is firmly bound to the cell wall (made of peptidoglycan). This structure is responsible for maintaining cell integrity against the turgor pressure faced from the outside. Since this outer layer is more permeable to substances moving across the membrane, the next layer - the inner membrane - also adds support to withstand turgor pressure. It will not let an osmolyte (such as sucrose) diffuse through it, thereby maintaining the osmotic pressure in the periplasmic space.

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