Membrane Structure and Fluidity - BIO 2550

A set of practice flashcards covering membrane structure, lipid organization, fluidity, and related cellular mechanisms based on the lecture notes.

What is the role of organelle membranes in relation to the cytoplasm?

They separate organelle contents from the cytoplasm.

List the main functions of the plasma membrane mentioned in the notes.

Cell communication, import/export of molecules, and cell growth/movement.

Which proteins in the plasma membrane enable cells to receive environmental information?

Receptor proteins.

How many membranes enclose the nucleus and the mitochondrion?

Two membranes.

What general structural feature do all membranes share?

A lipid bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward.

In an aqueous environment, which structure is more energetically favorable for phospholipids: micelles or bilayers?

Bilayers, because the hydrophobic tails are shielded from water.

Where are the hydrophobic tails located in a phospholipid bilayer?

In the interior of the bilayer, away from water.

Do lipids with one tail or two tails tend to form micelles or bilayers?

Lipids with one tail tend to form micelles; lipids with two tails tend to form bilayers.

What primarily determines the fluidity of the plasma membrane?

The hydrophobic tails of the phospholipids.

Name two major factors that influence lipid bilayer fluidity.

Lipid composition (types of lipids) and temperature.

How does unsaturation (double bonds) in fatty acid tails affect membrane fluidity?

More double bonds create kinks, reducing packing and increasing fluidity.

How does tail length affect membrane fluidity?

Shorter tails generally increase fluidity; longer tails decrease fluidity due to more interactions.

What is the effect of cholesterol on membrane fluidity at high temperatures?

Higher cholesterol reduces fluidity, stabilizing the membrane.

What is the effect of cholesterol on membrane fluidity at low temperatures?

Higher cholesterol helps maintain fluidity by preventing tight packing.

Do most bacteria membranes contain cholesterol?

No; most bacteria do not have cholesterol (with few exceptions like Helicobacter).

Where does membrane assembly begin, and how are membranes distributed to other organelles?

Membrane assembly begins at the endoplasmic reticulum (ER); vesicles bud off to Golgi and other membranes.

What is the role of flippases in membranes?

Flippases transfer specific phospholipids to the cytosolic monolayer to maintain lipid asymmetry, mainly in the Golgi and plasma membrane.

What is the role of scramblases?

Scramblases disrupt lipid asymmetry by moving phospholipids between leaflets; can be involved during apoptosis.

If a Golgi-specific flippase is nonfunctional, what would you expect regarding lipid distribution in the Golgi membrane?

Lipids would become randomly distributed between the two leaflets (loss of asymmetry) in the Golgi membranes.

Can phospholipids spontaneously move within a single leaflet (lateral diffusion)?

Yes, phospholipids can spontaneously diffuse laterally within a single leaflet.

Do phospholipids readily flip between leaflets (flip-flop) without enzymes?

No; flipping between leaflets usually requires enzymes (flippases or scramblases).

Which organelles are enclosed by double membranes?

Nucleus and mitochondrion.

What is lipid asymmetry between membrane leaflets called?

Lipid asymmetry.