Membrane Structure and Function

Flashcards reviewing plasma membrane structure, lipid bilayer fluidity and asymmetry, membrane proteins, alpha-helical transmembrane transport, membrane domains, and the glycocalyx.

What are the three primary roles of the plasma membrane?

The roles of the plasma membrane are Signal Reception, Transport, and Flexibility.

What does the term amphipathic mean?

A molecule that possesses both hydrophilic and hydrophobic properties.

What are the structural components of the hydrophilic head and hydrophobic tail of a phospholipid?

The hydrophilic head consists of a phosphate group and a small polar functional group (such as choline, ethanolamine, or serine). The hydrophobic tail consists of hydrocarbon chains that may be saturated or unsaturated.

What is the most common phospholipid in cell membranes, and what causes the kink in its tail?

Phosphatidylcholine is the most common phospholipid; a double bond in its hydrocarbon tail creates the kink.

Why do phospholipids naturally assemble into self-sealing bilayer containers that enclose compartments?

Hydrophilic heads are attracted to water while hydrophobic tails gather together. Any tear creates a free edge exposed to water, which is energetically unfavorable; thus, the bilayer bends and rearranges to eliminate free edges and form closed, sealed compartments.

What is the definition of membrane fluidity?

How molecules within the bilayer move and change places with one another, allowing the bilayer to behave as a two-dimensional fluid.

What are the four ways lipid molecules move within the lipid bilayer, and which movement requires protein assistance?

Lipid molecules move by lateral diffusion, rotation, and flexion of their tails. Moving by flip-flopping from one layer to the other is energetically unfavorable and requires the assistance of a protein.

How do hydrocarbon tail length and saturation affect membrane fluidity?

Short hydrocarbon tails increase fluidity by reducing tail interactions. Double bonds (unsaturated tails) also increase fluidity because their kinks prevent tight packing.

How does cholesterol affect the fluidity and properties of animal cell membranes?

Cholesterol fills the spaces left by the kinks between phospholipids, which makes the bilayer less flexible and less permeable, thereby decreasing fluidity.

What are four reasons why membrane fluidity is essential for cells?

It enables membrane proteins to diffuse rapidly and interact for cell signaling, permits newly synthesized lipids and proteins to diffuse to other cell regions, ensures even distribution of molecules between daughter cells during division, and allows membranes to fuse and mix molecules under certain conditions.

Where are new phospholipids synthesized in the cell, and in which layer are they initially placed?

New phospholipids are manufactured by enzymes on the cytosolic surface of the endoplasmic reticulum (ER) and are deposited exclusively into the cytosolic half of the bilayer.

What is the function of scramblase in the endoplasmic reticulum?

Scramblase moves randomly selected phospholipids from one half of the bilayer to the other to evenly distribute newly made phospholipids between both halves.

How is membrane asymmetry established in the Golgi apparatus?

Phospholipid transporters called flippases use ATP hydrolysis to move specific phospholipids from one side of the bilayer to the other, creating different phospholipid compositions on the two halves.

How is membrane topology preserved when membrane compartments bud and fuse?

The cytosolic monolayer always faces the cytosol, while the non-cytosolic monolayer facing the inside of an organelle remains facing the inside during budding and becomes the outer surface of the cell upon fusion with the plasma membrane.

What are the four key functional classes of membrane proteins?

Transporters and Channels, Anchors, Receptors, and Enzymes.

How do integral and peripheral membrane proteins differ in how they bind to membranes and how they can be experimentally removed?

Integral membrane proteins directly associate with the bilayer (transmembrane, cytosolic monolayer-associated, or lipid-linked) and can only be removed by disrupting the bilayer with detergents. Peripheral membrane proteins are bound indirectly via interactions with other membrane proteins and can be experimentally dissociated by gentle procedures that leave the bilayer intact.

How does an alpha helix enable a transmembrane protein to traverse the hydrophobic interior of the lipid bilayer?

Hydrophobic amino acid side chains face outward toward the lipid hydrocarbon tails, while polar atoms of the polypeptide backbone form hydrogen bonds with one another on the inside of the helix, shielding the polar backbone from the hydrophobic lipid core.

How do multi-pass transmembrane proteins arrange their alpha helices to form a transport channel?

The alpha helices pack side-by-side in a ring with nonpolar amino acids facing outward toward the lipid bilayer and polar amino acids facing inward to create a hydrophilic pore for small, water-soluble molecules.

What is a membrane domain, and what three mechanisms do cells use to confine proteins to specific domains?

A membrane domain is a specific region where a cell confines particular proteins. Proteins are confined by tethering to structures outside the cell, tethering to immobile structures inside the cell, or using barriers like tight junctions.

How are membrane transport proteins distributed between the apical and basolateral surfaces of intestinal cells?

The apical surface contains transport proteins that take up nutrients from the gut lining, while the basolateral surface contains proteins involved in exporting solutes to the blood.

What three carbohydrate-containing components form the glycocalyx?

Glycoproteins (proteins with linked oligosaccharides), proteoglycans (proteins with one or more long polysaccharide chains attached to a small protein core), and glycolipids (lipids with outward-facing sugars).

What are the key functions of the cell surface glycocalyx?

It protects the cell surface, attracts water molecules to give the cell a slimy coat that assists motile cells in squeezing through spaces and prevents blood cells from sticking to vessel walls, and aids in cell-cell recognition and adhesion.