The Chemistry of Water, Phospholipids, and Membranes

Flashcards covering the chemistry of water, phospholipids, membrane structure, properties like fluidity and permeability, protein types, and membrane asymmetry.

What makes water a polar molecule?

Water has a positive charged dipole (H) and some negative charged dipoles (O), making it a polar molecule.

How do water molecules interact with charged molecules to make them soluble?

Water forms hydration shells around charged molecules, making those molecules soluble in water.

Define amphipathic.

An amphipathic molecule has regions that are hydrophobic and regions that are hydrophilic.

What is the difference between hydrophilic and hydrophobic regions of a molecule?

A hydrophilic region interacts favorably with water (charged or polar molecules), while a hydrophobic region interacts poorly with water (non-polar molecules).

Why do phospholipids spontaneously form a lipid bilayer in water?

Phospholipids self-organize into a lipid bilayer because it is the most energetically favorable configuration for molecules with two fatty acid tails in water.

List the four key properties of the plasma membrane.

The plasma membrane is continuous, fluid (Fluid Mosaic Model), semi-permeable, and non-homogenous (asymmetrical and heterogeneous).

What two main factors influence membrane fluidity?

Membrane fluidity depends on membrane composition (phospholipid species, cholesterol) and temperature.

How does cholesterol affect membrane fluidity and permeability?

Cholesterol inserts into the hydrophobic core, making nearby phospholipids more rigid and less mobile, which reduces membrane fluidity and permeability to small solutes.

What is the relationship between membrane fluidity and membrane permeability?

The more fluid the membrane is, the more permeable it is, as things can more easily 'slip through the cracks'.

What two characteristics of a molecule determine its ability to cross the plasma membrane?

Small molecules cross more readily than large molecules, and uncharged or non-polar molecules cross more easily than charged molecules.

What does it mean for the plasma membrane to be asymmetrical and heterogeneous?

Asymmetrical means the extracellular and intracellular leaflets differ in composition, while heterogeneous means different regions of each leaflet contain microdomains enriched for distinct components.

Where are most phospholipids initially synthesized, and how do some reach the luminal leaflet?

Most phospholipids are initially synthesized on the cytosolic leaflet of the ER, and some are then 'flopped' to the luminal leaflet by flippase.

What are some differences in composition between the exoplasmic (facing out) and cytosolic (facing in) leaflets of the plasma membrane?

The exoplasmic leaflet is enriched in sphingomyelin and phosphatidylcholine, has more saturated tails, higher cholesterol, greater mechanical stiffness, and lower permeability, with protein glycosylation present. The cytosolic leaflet is enriched in phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol, has more unsaturated tails, lower cholesterol, lower mechanical stiffness, and higher permeability, with no protein glycosylation.

Distinguish between the functions of flippase, floppase, and scramblase in maintaining lipid asymmetry.

Flippase is ATP-dependent inward transport (extracellular to cytosolic), floppase is ATP-dependent outward transport (cytosolic to extracellular), and scramblase is energy-independent bi-directional transport, disrupting asymmetry.

What happens to phosphatidylserine (PS) distribution during early stages of cell damage, and what causes this?

Phosphatidylserine asymmetry is lost, and PS becomes expressed on the extracellular leaflet because caspase 3 activates scramblase and inactivates flippases.

What are sphingolipids, and why are they enriched in the extracellular leaflet and lipid rafts?

Sphingolipids have a sphingosine backbone, are saturated, synthesized in the Golgi, have a higher affinity for cholesterol, and are enriched in the extracellular leaflet and lipid rafts because of their dense packing and cholesterol affinity.

What are the three main types of membrane proteins based on their association with the lipid bilayer?

The three main types are integral membrane proteins (fully incorporated), peripheral proteins (temporarily attached), and anchored proteins (covalently attached lipid inserted into core, e.g., GPI-anchored).

Explain the 'Positive In' rule for transmembrane protein orientation.

Proteins will organize so that hydrophilic regions with more of a net positive charge face the cytosol.

What is the glycocalyx, and what are its main functions?

The glycocalyx is a thick layer of glycosylated proteins (glycoproteins) that coats the exterior of the cell. Its functions include providing a physical barrier, aiding cell adhesion, and concentrating circulating growth factors for signaling.

How are membrane proteins stabilized to reduce their lateral diffusion?

Membrane proteins are stabilized by tight junctions, spectrin-actin (cytoskeletal) anchoring, and the formation of lipid rafts.

What are lipid rafts, and what types of proteins are enriched within them?

Lipid rafts are microdomains of proteins and phospholipids held together by high affinity of cholesterol and sphingomyelin. GPI-anchored proteins are predominantly found in lipid rafts.