Question

True or false?

True False  In cell membranes, cholesterol decreases membrane fluidity below the transition temperature, but increases membrane fluidity above the transition temperature.
True False  Escherichia coli cells grown at 40°C would be expected to have a lower percentage of unsaturated fatty acids in their membranes than cells grown at 20°C.
True False  If a sphingolipid contains sialic acid (a.k.a. neuraminic acid) in its oligosaccharide headgroup, the headgroup will have a new negative charge of minus one.
True False  Groups lining the pores of transport proteins form weak non-covalent interactions with solute molecules that replace the water molecules that normally hydrate the solute.
True False  In active transport, the transported species moves across the membrane against a concentration gradient.
True False  Myristoyl, palmitoyl, and farnesyl groups anchor otherwise soluble proteins to the extracytoplasmic face of the plasma membrane.
True False  Triacylglycerols have three ester-linked phosphoryl groups that can in turn be ester-linked to serine, choline, ethanolamine, or inositol.
True False  In an amphipathic alpha helix, non-polar amino acid side chains face one side of the helix, and polar uncharged side chains face the other side.

Answer 1

1. Cholesterol increases membrane fluidity by preventing membrane phospholipids from packing together. At high temperature, the cholesterol packs the membrane lipids together to increase fluidity of membrane. Cholesterol is present in animal cells and imparts rigidity to the cell membrane. Membrane are fluids as the phospholipids and proteins that move freely fin the bilayer.

At high temperature, the phospholipid undergoes a phase transition from a solid state to a more fluid-like state at a characteristic melting point temperature. At low temperatures, cholesterol increases membrane fluidity as it prevents fatty acid hydrocarbon chains from packing. Hence, it prevents inhibits the transition from liquid to solid by decreasing the membrane freezing point. At high temperatures cholesterol decreases membrane fluidity. It immobilizes methylene groups in the fatty acid tails of the lipids. Here, cholesterol increases the melting point.

Right option is false.

2. When E.coli is grown at 20⁰C, the fatty acids decrease in length and become unsaturated. Unsaturation in fatty acids will increase the double bonds in the fatty acids, leading to kinks. This allows the fatty acids to become tightly packed, reducing membrane fluidity. The number of saturated fatty acid increase at 40⁰C, to maintain membrane integrity.

Right option is true.

3. Sialic acids are negatively charged α-keto acids with a nine-carbon backbone. They are present in gangliosides. Sialic acid has a COO- group at 1^st position. This negative charge corresponds to net negative charge of -1. Sphingolipids normally have no net charge.

Right option is true.

4. In passive transport of facilitated diffusion, no covalent bonds are formed or broken between the solute and the carrier proteins. Hence, no energy is required. The solute forms weak non-covalent interactions between water molecules associated with it and the carrier protein that can be easily broken when the solute reaches the other side of the membrane. This mechanism provides a hydrophilic trans-membrane passageway for the solute.

Right option is true.

5. In active transport, the ions or solutes are move across a concentration gradients from low concentration to high concentration. For such reverse transport, ATP is required. For example, Na+k+ ATPase pump transports 3 Na ions out of the cell and transport two potassium ions into the cell. This pump operates in a symport with carrier proteins that transport glucose against its concentration gradient.

Right option is true.

6. Proteins with Myristoyl, palmitoyl, and farnesyl groups anchor are present on cytoplasmic side of the plasma membrane. They are involved in cell signaling. The glycolipid anchors the proteins in the extracytoplasmic face of the membranes.

Right option is False.

7. Triacylglycerols have three fatty acyl groups in ester linkage with glycerol. Their carboxyl groups involved in these linkages cannot ionize. Hence they are very hydrophobic. Serine, choline, ethanolamine, or inositol are present in head groups of phospholipids.

Right option if False.

8. Amphipathic a-helices are present on surfaces of water-soluble globular protein. Their hydrophobic helices are on the inside. An amphipathic (or amphiphilic) helix is an a-helix with both hydrophobic and hydrophilic amino acid residues create two faces on opposite sides of the helix. One of the face is hydrophobic while the other is hydrophillic. The nonpolar side chains in the a-helix near the surface will be on one side of the helix. The polar side chain will be on other side, projecting in the water phase.

Option is True.