Structural Lipids and Biological Membranes

Question 1

What is the central structural feature of biological membranes, and what type of molecules form it?

Answer 1

Biological membranes are primarily composed of a double bilayer of amphipathic lipids, which have both polar (hydrophilic) and nonpolar (hydrophobic) regions.

Question 2

What are the three general classes of membrane lipids?

Answer 2

The three general classes are phospholipids (polar head group with phosphate), glycolipids (carbohydrate head group), and sterols (rigid, four-ring structure).

Question 3

What are the main components of a glycerophospholipid?

Answer 3

Glycerophospholipids consist of two fatty acids attached to glycerol-3-phosphate, forming phosphatidic acid as a parent compound, with a polar or charged head group attached via phosphodiester linkage to the glycerol backbone.

Question 4

How does the variation in fatty acid composition affect glycerophospholipids?

Answer 4

The fatty acids attached can vary in saturation, providing flexibility and optimal fluidity within cell membranes by incorporating both saturated and unsaturated fatty acids.

Question 5

How are glycerophospholipids named?

Answer 5

Glycerophospholipids are named as derivatives of the parent compound phosphatidic acid, with names reflecting the head group, such as phosphatidylserine or cardiolipin.

Question 6

What are sphingolipids, and how are they structured?

Answer 6

Sphingolipids are membrane lipids derived from sphingosine. They consist of a long hydrocarbon chain, an amino group on C2, and a hydroxyl group on C1. A fatty acid attaches to C2 through an amide linkage, forming the parent compound ceramide.

Question 7

What roles do sphingolipids play in the cell membrane?

Answer 7

Sphingolipids, typically found on the extracellular side, participate in immune functions and cell recognition, often playing roles in cell-to-cell communication.

Question 8

What are the three subclasses of sphingolipids, and how do they differ in head groups?

Answer 8

The subclasses are:

Sphingomyelins - contain phosphocholine or phosphoethanolamine.
Glycosphingolipids - have carbohydrate head groups.
Gangliosides - contain complex oligosaccharides as head groups.

Question 9

How do glycosphingolipids function in cell recognition, particularly regarding blood types?

Answer 9

Glycosphingolipids on cell surfaces contribute to cell recognition. Variations in glycosphingolipid structures on erythrocytes determine blood types, acting as antigens.

Question 10

What is the general structure of sterols?

Answer 10

Sterols have a rigid structure with four fused rings, three six-membered and one five-membered, with an -OH group on the third carbon.

Question 11

What is the primary sterol synthesized by animals, and why is it important?

Answer 11

Cholesterol is the main sterol in animals, essential for cell membrane structure and serving as a precursor for steroid hormones and other biomolecules.

Question 12

What is the structural difference between a sterol and a steroid?

Answer 12

Sterols contain an -OH group at the third position, while steroids may lack this group and are modified sterols involved in signaling functions, such as testosterone.

Question 13

Where is phosphatidylinositol primarily located in the cell membrane?

Answer 13

Phosphatidylinositol is located on the cytoplasmic side of the cell membrane.

Question 14

What does phospholipase C (PLC) do to phosphatidylinositol-4,5-bisphosphate (PIP2)?

Answer 14

Phospholipase C cleaves the PIP2 headgroup to generate the second messengers IP3 (inositol trisphosphate) and DAG (diacylglycerol).

Question 15

Describe the 7 steps in phosphatidylinositol signaling after an agonist binds to a cell membrane receptor.

Answer 15

An agonist binds to a receptor (e.g., G-protein coupled receptor).
The receptor activates an associated G protein.
The activated G protein stimulates PLC.
PLC cleaves PIP2 into IP3 and DAG.
IP3 activates IP-gated Ca2+ channels, leading to Ca2+ release.
DAG and Ca2+ activate PKC (protein kinase C).
PKC phosphorylates various cellular targets, mediating effects.

Question 16

What are eicosanoids, and how are they produced?

Answer 16

Eicosanoids are autocrine and paracrine signaling molecules synthesized from arachidonate and other polyunsaturated fatty acids, including omega-3 and omega-6 fatty acids.

Question 17

What are some biological functions of eicosanoids?

Answer 17

Eicosanoids play roles in reproduction, inflammation, fever, pain, blood clotting, and blood pressure regulation.

Question 18

How do NSAIDs (nonsteroidal anti-inflammatory drugs) interact with eicosanoids?

Answer 18

NSAIDs inhibit enzymes involved in eicosanoid synthesis, helping to reduce pain, fever, and inflammation.

Question 19

How are steroid hormones synthesized?

Answer 19

Steroid hormones are oxidized derivatives of sterols, synthesized from cholesterol.

Question 20

Describe the pathway of steroid hormone signaling within target cells.

Answer 20

Steroid hormones diffuse across the cell membrane, bind nuclear receptors in the target cell, and the receptor-hormone complex regulates gene expression by interacting with coregulatory proteins.

Question 21

Why do steroid hormones require a carrier protein in the bloodstream?

Answer 21

As hydrophobic molecules, steroid hormones need carrier proteins to move through the aqueous environment of the bloodstream.

Question 22

What role do nuclear receptors play in steroid hormone signaling?

Answer 22

Nuclear receptors bind steroid hormones and interact with coregulatory proteins, facilitating changes in gene expression by regulating transcription.

Question 23

What is a micelle, and which molecules typically form them?

Answer 23

A micelle is a sphere of 10s-100s of amphipathic molecules, formed by molecules like free fatty acids, lysophospholipids, and detergents.

Question 24

How does a bilayer structure form in biological membranes?

Answer 24

A bilayer forms from two lipid monolayers in a 2D sheet driven by the hydrophobic effect, with self-repairing edges to remain energetically favorable.

Question 25

What is a vesicle (liposome), and how is it different from a bilayer?

Answer 25

A vesicle is a spherical bilayer structure that eliminates hydrophobic edges in flat bilayers, creating an enclosed aqueous compartment.

Question 26

Why do two fatty acids favor bilayer and vesicle formation over micelle formation?

Answer 26

Two fatty acids provide an optimal surface area-to-volume ratio, favoring bilayer and vesicle formation over micelle formation.

Question 27

Describe the thickness of biological membranes and its contributing factors.

Answer 27

Biological membranes are 5-8 nm thick, with the lipid bilayer accounting for part of this thickness and proteins protruding from either side adding to it.

Question 28

What is the “fluid mosaic model” of biological membranes?

Answer 28

The fluid mosaic model describes the membrane as a bilayer of phospholipids in which proteins are embedded, creating a flexible, semi-permeable, and self-repairing structure.

Question 29

What creates structural and functional asymmetry in the lipid bilayer?

Answer 29

Asymmetry arises from the different distributions and functions of lipids and proteins on each side of the bilayer.

Question 30

What is the liquid-ordered (Lo) state in lipid bilayers, and what conditions favor it?

Answer 30

The liquid-ordered (Lo) state is a near-solid, highly ordered state, favored by low temperatures and long, saturated fatty acid chains.

Question 31

How does the liquid-disordered (Ld) state differ from the Lo state in lipid bilayers?

Answer 31

The Ld state is more fluid, with hydrocarbon chains in constant motion, favored by high temperatures and short, unsaturated fatty acids.

Question 32

Why are certain regions within the membrane more ordered than others?

Answer 32

Membrane regions vary in order due to different ratios of saturated and unsaturated fatty acids, which affects the fluidity and organization.

Question 33

How do lipids move within the bilayer, and what technique demonstrates this?

Answer 33

Lipids diffuse laterally within the bilayer, demonstrated by Fluorescence Recovery After Photobleaching (FRAP), where bleached areas recover as unbleached lipids move in.

Question 34

What is the significance of the asymmetric distribution of membrane lipids?

Answer 34

The asymmetric distribution supports membrane trafficking and specialized functions by creating distinct lipid compositions on each leaflet.

Question 35

Why is the lipid composition of cellular membranes unique among species, tissues, and organelles?

Answer 35

Lipid composition is tailored to specific cellular needs and functions, though the exact functional significance is still largely unknown.

Question 36

What role does membrane trafficking play in lipid composition?

Answer 36

Membrane trafficking adjusts lipid composition and distribution, contributing to functional asymmetry across different sides of the bilayer.

Question 37

What are membrane rafts, and which lipids are they rich in?

Answer 37

Membrane rafts are lipid microdomains in the outer leaflet of the membrane, rich in long, saturated fatty acids and cholesterol, creating ordered regions.

Question 38

What is the functional role of membrane rafts in cellular processes?

Answer 38

Membrane rafts facilitate the clustering of specific proteins and receptors, supporting interactions and signaling processes within the lipid membrane.