Chapter 6

A comprehensive set of practice flashcards covering lipid structure, membrane organization, diffusion and osmosis, membrane permeability, and the roles of membrane proteins.

What property explains why lipids are insoluble in water?

Lipids have a high proportion of nonpolar C–C and C–H bonds, making them hydrophobic and insoluble in water.

What is an isoprenoid and why is it important?

Isoprenoids are branched hydrocarbon chains built from isoprene units; they serve as pigments, scents, vitamins, and precursors to sex hormones, and are building blocks for other lipids.

Describe the basic structure of a fatty acid.

A fatty acid is an unbranched hydrocarbon chain attached to a polar carboxyl group (–COOH); it typically contains 14–20 carbon atoms and forms the nonpolar ‘tail’ in lipids.

What is the key structural difference between saturated and unsaturated hydrocarbons?

Saturated hydrocarbons have only C–C single bonds; unsaturated hydrocarbons contain one or more C=C double bonds, which can introduce kinks in the chain (cis configuration).

How does bond saturation affect lipid membrane fluidity?

Unsaturated (kinked) tails create spaces and fewer van der Waals interactions, increasing fluidity and permeability; saturated tails pack tightly, reducing fluidity and permeability.

What does hydrogenation do to fats, and what is a potential health concern?

Hydrogenation adds hydrogen to C=C bonds, producing more saturated fats (higher melting points). It can also create trans fats, which have different health effects.

What is a triglyceride and what is its primary biological role?

A triglyceride (triacylglycerol) has three fatty acids linked to glycerol and serves mainly as energy storage; fats are not polymers.

What is a phospholipid and what are its key components?

A phospholipid consists of glycerol linked to a phosphate group and two hydrocarbon tails; the phosphate head is polar (hydrophilic), while the tails are nonpolar (hydrophobic).

Define cholesterol and its significance in membranes.

Cholesterol is a steroid with a four-ring structure; it has a polar hydroxyl group and a nonpolar isoprenoid tail. It is an important membrane component that modulates fluidity and packing of phospholipids.

Name the three main lipid types found in cells discussed in this section.

Fats (triacylglycerols), steroids, and phospholipids.

Why do fats store more energy per gram than carbohydrates?

Fatty acids have a higher ratio of high-energy C–C/C–H bonds to low-energy C–O bonds, yielding more energy per gram when oxidized.

How are fats formed and why are they not considered polymers?

Fats form via dehydration (condensation) between glycerol and fatty acids to make ester linkages; fatty acids are not covalently linked into long chains, so fats are not polymers.

What is a phospholipid’s amphipathic nature and why is it vital for membranes?

Phospholipids have both a hydrophilic (polar) head and a hydrophobic (nonpolar) tail, enabling spontaneous formation of bilayers that form cell membranes.

What structures form spontaneously when phospholipids are in water, and how do they differ?

Micelles form from single-tailed lipids with outward-facing heads; bilayers form from bulky, two-tailed phospholipids, creating membranes.

What is a liposome and how is it used in experiments?

A liposome is a spherical vesicle bounded by a lipid bilayer enclosing an aqueous interior; it serves as a three-dimensional model of a membrane.

What is a planar bilayer and its experimental purpose?

A planar bilayer is a lipid bilayer formed across a hole in a wall separating two solutions; used to study permeability and ion transport under controlled conditions.

What is meant by selective permeability of lipid bilayers?

Some substances cross membranes readily while others cross slowly or not at all, depending on size, polarity, charge, and the presence of transport proteins.

How does cholesterol content affect membrane permeability to glycerol, according to Figure 6.11?

Adding cholesterol decreases membrane permeability; membranes with 0% cholesterol are more permeable than those with 20% or 50% cholesterol, and permeability generally increases with temperature.

How do tail length and saturation status affect membrane permeability?

Shorter and more unsaturated tails create more spaces and fewer van der Waals forces, increasing permeability; long, saturated tails create a denser interior and lower permeability.

What general trend is observed when comparing membranes with many short, kinked tails to those with long, straight tails?

Membranes with many short, kinked tails are more permeable; those with long, straight tails are less permeable.

What is the role of micelles versus phospholipid bilayers in membranes?

Micelles are single-tailed lipids forming spheres; phospholipid bilayers are two-layer sheets that form the foundational structure of membranes.

What are four broad classes of membrane proteins that affect permeability?

Channels, carriers, pumps, and enzymes/regulatory proteins that alter membrane properties and transport.

What is an aquaporin and its function?

A membrane channel that specifically facilitates rapid water transport across the membrane, faster than diffusion through the bilayer due to selective pore residues.

What is the difference between integral and peripheral membrane proteins?

Integral proteins span the lipid bilayer (transmembrane); peripheral proteins associate with the membrane surface and may be on the cytoplasmic or extracellular side.

What is the fluid-mosaic model and who proposed it?

The fluid-mosaic model describes membranes as a dynamic, fluid phospholipid bilayer with proteins interspersed like a mosaic; proposed by Singer and Nicolson in 1972.

What technique provided key evidence for membrane proteins being integral and the fluid-mosaic model?

Freeze-fracture electron microscopy revealed pits and mounds on the fractured membrane, indicating proteins span the bilayer and supporting the fluid-mosaic model.

How do ion channels differ from carrier proteins in transport mechanisms?

Ion channels form pores allowing ions or small molecules to diffuse passively; carriers bind a solute and undergo conformational changes to shuttle it across, often via facilitated diffusion.

What is CFTR and how was its function demonstrated?

CFTR is a chloride channel protein; planar bilayer experiments show electric current when CFTR is present, indicating it conducts Cl− ions.

How does GLUT-1 transport glucose across membranes?

GLUT-1 is a carrier protein that binds glucose on one side, undergoes a conformational change, and releases glucose on the other side, moving down its concentration gradient by diffusion.

What is the sodium–potassium pump (Na+/K+-ATPase) and why is it important?

A pump that uses ATP to move Na+ out of the cell and K+ into the cell, establishing an electrochemical gradient essential for cellular function and driving secondary transport.

Explain secondary active transport with an example.

Energy from ATP-driven pumps creates an electrochemical gradient that powers the transport of another solute against its gradient, such as Na+ driving glucose uptake via a Na+/glucose cotransporter.

What is a protocell and how could fatty-acid membranes have differed from phospholipid membranes in early life?

A protocell is a simple vesicle-like structure with a primitive membrane; fatty-acid membranes could be more permeable to ions and nucleotides, potentially allowing RNA synthesis inside protocells.

What is TTX and how does it affect nerve signaling?

Tetrodotoxin binds to voltage-gated Na+ channels and blocks their ability to conduct Na+, disrupting nerve signaling and muscle function.

How does TTX influence the electrical gradient across the membrane?

TTX binding blocks Na+ movement through voltage-gated channels, preventing the generation or propagation of action potentials and altering ion flow.

What is meant by the term electrochemical gradient?

A gradient that combines a concentration gradient and an electrical gradient, driving the diffusion of ions across membranes.

What is the difference between hypertonic, hypotonic, and isotonic solutions?

Hypertonic: higher solute concentration outside than inside; water leaves the cell. Hypotonic: lower solute concentration outside; water enters. Isotonic: equal solute concentrations on both sides; no net water movement.

Why are fatty acids more permeable in protocell membranes than phospholipid membranes?

Fatty acid membranes are generally more permeable to ions and nucleotides, enabling diffusion of these solutes into protocells, which could support early metabolic processes.

What is the role of membrane proteins in establishing a defined intracellular environment?

Membrane proteins regulate transport (channels, carriers, pumps) to selectively move solutes and maintain internal conditions distinct from the external environment.