Lipids

To which class of biological macromolecules do triglycerides belong?

Lipids

What are the primary solubility and polarity characteristics of lipids?

Lipids are predominantly nonpolar and hydrophobic.

What does it mean for a lipid molecule to be amphipathic?

It possesses both polar (hydrophilic) and nonpolar (hydrophobic) regions.

Which two molecular subunits combine to form a triglyceride?

One glycerol molecule and three fatty acid molecules.

What covalent linkage connects fatty acids to glycerol in a triglyceride?

Ester linkage (−C−O−C=bond)

What type of chemical reaction forms the ester linkages between glycerol and fatty acids?

Dehydration synthesis (condensation reaction), which releases water (H2​O).

What structural feature defines a saturated fatty acid?

It contains no carbon-carbon double bonds (C=C), maximizing the number of attached hydrogen atoms.

What structural feature defines an unsaturated fatty acid?

It contains one or more carbon-carbon double bonds (C=C) in its hydrocarbon chain.

How does a double bond in a cis-unsaturated fatty acid alter its physical shape?

It introduces a bend or kink in the hydrocarbon chain.

How does increasing the proportion of cis-unsaturated fatty acids affect membrane fluidity?

It increases membrane fluidity by preventing fatty acid tails from packing tightly together.

Why do organisms adapted to cold climates typically possess higher proportions of unsaturated fatty acids in their cell membranes?

The kinks in unsaturated fatty acids prevent lipid packing, maintaining membrane fluidity and resisting freezing at low temperatures.

What dual role does cholesterol play in regulating membrane fluidity at different temperatures?

At warm temperatures, cholesterol reduces phospholipid movement (reducing fluidity); at cool temperatures, it prevents tight packing (resisting freezing).

What is the fluid mosaic model of the plasma membrane?

A model depicting the membrane as a dynamic phospholipid bilayer containing a mosaic of laterally mobile proteins and lipids.

How do phospholipids move within a single leaflet of the plasma membrane?

They move laterally rapidly within their own leaflet.

What chemical characteristic allows transmembrane proteins to reside stably across the lipid bilayer?

Amphipathic nature: Nonpolar amino acids interact with hydrophobic lipid tails, while polar/charged amino acids interface with aqueous surroundings.

Which secondary structure commonly spans the hydrophobic core of transmembrane proteins?

α-helices

Why do transmembrane proteins frequently feature an odd number of α-helices anchoring them?

An odd number of passes places the N-terminus and C-terminus on opposite sides of the plasma membrane.

List six major general functions performed by transmembrane proteins.

1. Solute transport

2. Enzymatic catalysis

3. Signal communication / receptors

4. Cell recognition

5. Intercellular adhesion

6. ECM / Cytoskeleton attachment

How do transmembrane proteins function as signal receptors?

They bind extracellular signaling molecules (ligands) and undergo a conformational change that transmits a signal inside the cell.

How do glycoproteins attached to transmembrane proteins aid in cell recognition?

Extracellular carbohydrate chains serve as specific molecular tags recognized by other cells.

What is simple diffusion?

The passive movement of solute particles down a concentration gradient until dynamic equilibrium is reached.

What is the net directional movement of solute particles during simple diffusion?

From high solute concentration ([solute]high​) to low solute concentration ([solute]low​).

What serves as the source of potential energy driving simple diffusion?

The concentration gradient (or electrochemical gradient) itself.

What types of solutes can cross a plasma membrane via simple diffusion?

Membrane-permeable (penetrating) solutes, such as small nonpolar molecules.

Does simple diffusion require cellular energy input (ATP) or specific protein channels?

No; simple diffusion requires neither ATP energy nor transport proteins.

What defines a semipermeable (selectively permeable) membrane?

A membrane that permits the passage of solvent (water) and certain small molecules while blocking nonpenetrating solutes.

What is osmosis?

The net diffusion of water across a semipermeable membrane toward a higher solute concentration.

Why does water cross a semipermeable membrane during osmosis when solute concentrations differ?

Because nonpenetrating solutes cannot cross the membrane to achieve equilibrium, water moves to equalize concentrations instead.

In terms of free water concentration, which way does water move during osmosis?

From high free water concentration ([H2​O]high​) to low free water concentration ([H2​O]low​).

In terms of solute concentration, which way does water move during osmosis?

From a hyposmotic solution (low [solute]) to a hyperosmotic solution (high [solute]).

What is the key difference between osmolarity and tonicity?

Osmolarity measures total concentration of ALL solutes; tonicity depends exclusively on NONPENETRATING solutes and dictates cell volume change.

How does an isotonic solution affect net water movement and cell volume?

Water enters and leaves at equal rates; the cell maintains a constant shape and volume.

How does a hypertonic solution affect net water movement and cell volume in animal cells?

Water moves OUT of the cell, causing the cell to shrink and crenate.

How does a hypotonic solution affect net water movement and cell volume in animal cells?

Water moves INTO the cell, causing it to swell and potentially lyse (burst).

What happens to a plant cell placed in a hypotonic solution?

Water enters the vacuole, creating turgor pressure against the cell wall, making the plant cell turgid (healthy state).

What happens to a plant cell placed in a hypertonic solution?

Water leaves the cell, causing the plasma membrane to pull away from the cell wall (plasmolysis).

How do freshwater protists like Paramecium prevent cell lysis in hypotonic pond water?

They use contractile vacuoles to continuously collect and pump out excess water.

What is facilitated diffusion?

Passive transport of membrane-impermeable solutes across a lipid bilayer down their concentration gradient using transport proteins.

What are the two major classes of transport proteins involved in facilitated diffusion?

Channel proteins (hydrophilic pores) and Carrier proteins (undergo conformational shape changes).

How do simple diffusion and facilitated diffusion differ?

Facilitated diffusion requires transmembrane transport proteins, whereas simple diffusion occurs directly through the lipid bilayer.

What is active transport?

The movement of a solute across a membrane against its concentration gradient, requiring cellular energy expenditure (usually ATP).

How does active transport differ fundamentally from passive transport?

Active transport moves solutes UP (against) their gradient and requires external energy; passive transport moves solutes DOWN their gradient without energy.

What is secondary active transport?

Transport driven indirectly by an electrochemical gradient created previously by primary active transport, rather than direct ATP hydrolysis.

In secondary active transport, what is the functional difference between symport and antiport carriers?

Symporters transport two solutes across the membrane in the SAME direction; antiporters transport two solutes in OPPOSITE directions.