Question

Cell Biology Short Answer Question: Phospholipid biosynthesis at the interface between the ER and the cytosol presents a number of challenges that must be solved by the cell. Explain how each of the following is handled: 1) The substrates for phospholipid biosynthesis are all water soluble, and yet the end products are not. 2) The immediate site of incorporation of all newly synthesized phospholipids is the cytosolic leaflet of the ER membrane, and yet the phospholipids must be incorporated into both leaflets. 3) Many membrane systems in the cell--e.g., the plasma membrane--are unable to synthesize their own phospholipids, and yet these membranes must expand if the cell is to grow and divide.

Answer 1

1.) The substrates for phospholipid biosynthesis are all water soluble because they are synthesized at the interface between the ER and the cytosol, an area where the production of hydrophobic molecules (like the end products) would not be favorable because of the hydrophilic environment at the interface.

Thus, water soluble, hydrophilic substrates for phospholipid biosynthesis are synthesized in the cytosol. From there, the enzymes from the ER connect these substrates to end up creating the hydrophobic, phospholipid end products.

2.) This challenge is handled by flippases. That is, newly synthesized phospholipids in the cytosolic leaflet of the ER membrane can be flipped and then incorporated to the exoplasmic leaflet with the help of flippases.

3.) Even though membrane systems like the plasma membrane are unable to synthesize their own phospholipids, they can still expand by incorporating phospholipids that have been synthesized elsewhere.

More specifically, vesicles can transport phospholipids to the membrane system, some phospholipids can be transferred from other membranes, and lipid transfer proteins can also help disperse phospholipids.