Question

Are phospholipids still in the cell membrane?

What keeps cells moving in cold weather?

How does cholesterol control the movement of the cell membrane in cold and warmth?

What kind of proteins are in the cell membrane?

What can go through cell membranes?

what does cholesterol do in the cell membrane?

What does saturation do to the phospholipids

Answer 1

Phospholipids are the major components of the biological membranes. They are amphipathic molecules as they contain a polar head and a non-polar tail.

At cold temperatures, a cell increases the proportion of unsaturated fatty acids and shorter chain fatty acids in the membrane to maintain fluidity.

Biological membranes are mainly composed of lipids and proteins (protein molecules float in the sea of lipids) and provide selective permeability for various molecules into the cell. They are sheet-like, non-covalent assemblies. The lipid molecules of the membrane prevent the free flow of polar molecules whereas proteins serve as receptors for signaling cascades and components of transport systems. Membrane structure and composition is dynamic i.e., it changes according to the environmental conditions. They are asymmetric and fluid in nature i.e., all the lipids and several proteins can easily diffuse rapidly in the plane of the membrane.

The fluidity of the membrane depends on its chemical composition and temperature. Lipids with longer fatty acid chains have more interactions between the hydrophobic fatty acid tails, stabilizing the crystal-like state and making the membrane less fluid. Saturated fatty acids also strengthen membranes and reduce fluidity.

The shift to the higher temperature would increase the fluidity of the membrane by reducing packing of the hydrophobic chains. To prevent this, new phospholipids would be synthesized having longer chains and less number of double bonds. The longer chains would increase the amount of Vander Waals interactions and absence of double bonds reduces the kink in structure, would enhance packing of the fatty acid tails of the phospholipids.

Incorporation of specific lipids, such as sphingomyelin is known reduce the membrane fluidity. So, its synthesis may go up. Cholesterol acts as a positive as well as a negative regulator of membrane fluidity. At high temperatures, it stabilizes the membrane structure, whereas at low temperatures it intercalates between the phospholipids and prevents them from clustering together.

Membrane proteins contain hydrophobic membrane spanning regions.

Only hydrophobic small molecules can pass through biological membranes.

Saturated fats can pack together well. Introducing a double bond (unsaturation) produces a kink in the structure which prevents close packing.