Chapter 10: Lipids, Membranes, and Cellular Transport

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10.1 The Molecular Structure and Behavior of Lipids

Major Functions and Properties of Lipids

Lipids are a diverse group of biomolecules with essential roles in cellular structure and function. Their unique chemical properties distinguish them from other biological macromolecules.

Major functions of lipids include energy storage, membrane structure, and signaling.
Unlike carbohydrates, amino acids, or nucleotides, lipids have limited solubility in aqueous media due to their hydrophobic nature.
Most lipids are amphipathic: they contain both hydrophobic (nonpolar) and hydrophilic (polar) regions.
Example: A phospholipid molecule with a polar head group and nonpolar tail.

Fatty Acids

Fatty acids are the fundamental building blocks of many lipid classes and play a central role in lipid structure and metabolism.

Fatty acids are major constituents of lipids.
Structure: A hydrophilic carboxylate group is attached to one end of a hydrocarbon chain (typically 12–24 carbons).
Unsaturated fatty acids contain one or more cis C=C double bonds; saturated fatty acids contain none.
Fluidity of fatty acids decreases as chain length increases and the number of cis double bonds decreases.
Representative structures: Stearate ion (saturated), Oleate ion (unsaturated).

Fats (Triacylglycerides)

Fats are a primary storage form of energy in many organisms and are composed of fatty acids esterified to glycerol.

In fats (triacylglycerides), glycerol is esterified with three fatty acids.
Fats are used for metabolic energy storage because their carbon atoms are highly reduced.
Fats also provide thermal insulation and can act as a source for energy or heat production.
Example: Tristearin is a simple triacylglycerol, with three stearic acid residues esterified to glycerol.

10.2 The Lipid Constituents of Biological Membranes

Lipids, Micelles, and Bilayers

Membrane lipids self-assemble into structures that form the basis of biological membranes.

Lipids are major constituents of all biological membranes.
Fatty acids tend to form spherical micelles, while lipids with one hydrophilic head group and two hydrophobic tails form bilayers—the structure seen in biological membranes.
Major classes of membrane-forming lipids: glycerophospholipids, glycoglycerolipids, sphingolipids, and glycosphingolipids.

Glycerophospholipids

Glycerophospholipids are the predominant class of membrane lipids in most cells.

They are phospholipids with a glycerol backbone and phosphate-containing head groups.
Common head groups include choline, ethanolamine, serine, and inositol.

Name of Glycerophospholipid	Head Group
Phosphatidylcholine (PC)	Choline
Phosphatidylethanolamine (PE)	Ethanolamine
Phosphatidylserine (PS)	Serine
Phosphatidylinositol (PI)	Inositol

Glycoglycerolipids

Glycoglycerolipids are membrane lipids with carbohydrate groups attached to the glycerol backbone.

They have a carbohydrate linked to their head group.
Common in plant and bacterial membranes.

Sphingolipids

Sphingolipids are a structurally distinct class of membrane lipids based on the amino alcohol sphingosine.

A fatty acid is linked to the amino alcohol sphingosine via an amide bond, forming a ceramide.
Sphingomyelin is a common sphingolipid with a phosphocholine head group.

Glycosphingolipids

Glycosphingolipids are sphingolipids with carbohydrate (glycan) groups attached to their head groups.

They play roles in cell recognition and signaling.

Cholesterol

Cholesterol is a unique membrane lipid that modulates membrane fluidity and serves as a precursor for steroid hormones.

It is based on a tetracyclic hydrocarbon structure and is only weakly amphipathic due to its single hydroxyl group.
Cholesterol disrupts regular fatty acid chain packing in membranes, affecting membrane fluidity.
It is the precursor to all steroids.

10.3 The Structure and Properties of Membranes and Membrane Proteins

Membrane Structure — The Fluid Mosaic Model

The fluid mosaic model describes the dynamic and heterogeneous nature of biological membranes.

Biological membranes consist of lipid bilayers in which membrane proteins are embedded.
Membranes behave as oriented two-dimensional liquids, allowing lateral diffusion of lipids and proteins.
Defined structures or domains, such as protein complexes or lipid rafts, exist within the membrane.

Membrane Proteins

Membrane proteins are essential for a variety of cellular processes, including transport, signaling, and enzymatic activity.

Integral membrane proteins span the lipid bilayer, often as α-helices or β-barrels.
Peripheral membrane proteins associate with the membrane surface.

Membrane Rafts

Membrane rafts are specialized microdomains within the lipid bilayer.

They are dynamic structures rich in cholesterol, sphingolipids, and glycosylphosphatidylinositol.
Rafts play roles in cell signaling and sorting of proteins into organelles.

10.4 Transport across Membranes

Membrane Transport Processes

Transport across biological membranes is essential for maintaining cellular homeostasis and involves several mechanisms.

Nonmediated transport: Simple diffusion across the membrane, more rapid for hydrophobic solutes, slow for polar/charged solutes.
Facilitated transport: Diffusion of certain solutes is accelerated by specific pores, carriers, or permeases.
Active transport: Couples a thermodynamically favorable process (usually ATP hydrolysis) to achieve transport against a concentration gradient.

Major Mediators of Facilitated Transport

Facilitated transport is mediated by protein pores, carrier molecules, and permeases, each with specificity for certain solutes.

Cotransport: Symport versus Antiport

Cotransport systems move two solutes simultaneously across a membrane.

Symport: Transports two solutes in the same direction.
Antiport: Transports two solutes in opposite directions.

Water Channels: Aquaporins

Aquaporins are specialized water channels that facilitate rapid water movement across membranes.

Increase water transport in tissues such as erythrocytes, salivary glands, and kidneys.
Maintain osmotic balance and prevent cell rupture during osmotic stress.

Ion Channels

Ion channels are membrane proteins that allow selective passage of ions such as K+, Na+, and Cl- across membranes.

They are crucial for nerve impulse transmission and muscle contraction.

10.5 Ion Pumps: Direct Coupling of ATP Hydrolysis to Transport

Active Transport

Active transport mechanisms move ions or molecules against their concentration gradients, typically using energy from ATP hydrolysis.

Example: The Na+-K+ ATPase pump.
Stoichiometry of the Na+-K+ ATPase reaction:

Structural Models of the Na+-K+ ATPase

The Na+-K+ ATPase is a membrane protein complex that maintains the electrochemical gradients of sodium and potassium ions across the plasma membrane.

Crystal structures reveal conformational changes during the transport cycle.

Functional Cycle of the Na+-K+ ATPase

The pump alternates between conformations to transport Na+ out of and K+ into the cell, powered by ATP hydrolysis.

ABC Transporters

ATP-binding cassette (ABC) transporters are a large family of active transporters that move a variety of substrates across membranes.

Bind small molecules at the cytoplasmic side and translocate them to the extracellular side using ATP hydrolysis.
Examples include multidrug resistance proteins and the cystic fibrosis transmembrane regulator (CFTR).

10.7 Cotransport Systems

The Sodium-Glucose Cotransport System

Cotransport systems couple the movement of one substance to the favorable movement of another, often using ion gradients as the energy source.

In the sodium-glucose cotransport system, the sodium gradient drives the uptake of glucose against its concentration gradient.