How cells make membrane lipids

Cells grow and divide, so they constantly need new membrane. In eukaryotic cells, most membrane parts, lipids and proteins alike, are made at the endoplasmic reticulum (ER), the network of membrane tubes and sacs described in Chapter 4.

The building blocks

A phospholipid is assembled from four kinds of parts:

• two fatty acids, which become its two long tails
• one glycerol, a small molecule that serves as the backbone the other parts attach to
• one phosphate
• one polar head group, such as choline

Glycerol ties the parts together. The two fatty acids attach to it side by side, and the phosphate, linked to the head group, attaches next to them, forming the polar head.

Cells take in these parts from food or make them with enzymes in the cytosol. Because some of the parts come from your diet, what you eat can change your membranes. A diet with more unsaturated fats, such as omega-3 fatty acids, can raise the share of those fatty acids in cell membranes throughout the body. The membranes become more fluid, which in turn can change how their proteins function.

Assembly on the smooth ER

Building most lipids is a joint effort between the cytosol and the endomembrane system (the network of internal membranes that includes the ER and the Golgi apparatus). Phospholipids are assembled on the cytosolic leaflet of the smooth ER membrane, the half of the membrane that faces the cytosol:

Choline is only one option; many other head groups can be attached in the last step.

Because all of this happens on one side of the ER membrane, new phospholipids pile up in the cytosolic leaflet. Flip-flop doesn’t happen on its own, so flippases in the ER membrane move about half of the new phospholipids to the other leaflet, the one facing the ER lumen (the space enclosed by the ER). As a result, both leaflets end up with similar amounts of lipid.

Shipping lipids to other membranes

New lipids don’t stay in the ER. They reach the cell’s other membranes by three routes, which differ in where they can deliver:

Mitochondria and chloroplasts also make some lipids of their own, and lipid exchange proteins deliver those lipids to other membranes in the cell.

Check yourselfA phospholipid has just been made in the ER membrane. How does it reach the leaflet facing the ER lumen, and how could it later reach a mitochondrion?

A flippase moves it across to the lumen-facing leaflet. To reach a mitochondrion, a lipid exchange protein would extract it from the ER membrane, carry it through the cytosol, and insert it into the mitochondrial membrane.

Check yourselfHow could a diet rich in omega-3 fatty acids change the membranes of your cells?

Omega-3 fatty acids are unsaturated, and fatty acids from food are used to build membrane phospholipids. Your membranes could end up with more kinked, unsaturated tails, making them more fluid, which can change how membrane proteins function.