Chapter 6

Lipids: Structure and Function

• Lipids are carbon-containing compounds characterized by insolubility in water (water-insoluble due to nonpolar nature).

  • Insolubility arises from a high proportion of nonpolar C–C and C–H bonds relative to polar functional groups.

  • Lipids dissolve in organic, nonpolar solvents (e.g., benzene).

• Isoprene and isoprenoids

  • Isoprenes are five-carbon units; isoprene chains link end-to-end into isoprenoids (branched hydrocarbon chains).

  • Isoprenoids serve diverse roles: pigments, scents, vitamins, precursors of sex hormones, and building blocks for more complex lipids.

• Fatty acids

  • Simple lipids composed of a hydrocarbon chain attached to a polar carboxyl group (–COOR, in esters; –COOH in free fatty acids).

  • Typical fatty acids contain 14–20 carbon atoms; long nonpolar hydrocarbon tails predominate.

  • Bond saturation matters: double bonds create unsaturation; single bonds yield saturated chains.

  • Double bonds cause the chain to adopt a planar region (C=C) rather than full 3D tetrahedral rotation, reducing packing along the tail.

• Saturation and hydrocarbon structure

  • Saturated hydrocarbons have no C=C bonds; unsaturated hydrocarbons contain C=C bonds.

  • Hydrogenation converts unsaturated to saturated lipids by adding H to double bonds, locking carbons in place (removing cis kink propensity).

  • Saturation and chain length influence packing, physical state, and melting temperature.

• Isoprenoids vs fatty acids (structure and function)

  • Isoprenoids: branched hydrocarbon chains; versatile functions.

  • Fatty acids: unbranched hydrocarbon chains; key building blocks for fats and many lipids.

Lipids: Types Found in Cells and Their Roles

• Steroids

  • Defined by a bulky, four-ring structure; cholesterol is a prominent example.

  • Cholesterol has a polar hydroxyl group and a nonpolar isoprenoid tail; both polar and nonpolar features.

  • Roles include hormones (e.g., estrogens, testosterone) and membrane components.

• Fats (triacylglycerols)

  • Nonpolar molecules formed by three fatty acids linked to glycerol (a three-carbon molecule).

  • Structure: glycerol + three fatty acids; called triglycerides.

  • Primary role in energy storage due to high-energy C–C and C–H bonds.

  • If fatty acids are polyunsaturated, triglycerides tend to be liquid at room temperature.

  • When glycerol-linked fatty acids are not attached to other molecules, they are free fatty acids.

• Phospholipids

  • Consist of glycerol linked to a phosphate group and two hydrocarbon tails (isoprenoids or fatty acids).

  • The phosphate head is bonded to a small organic molecule that is charged or polar (e.g., choline).

  • Domain differences: phospholipids with fatty acid tails are common in Bacteria and Eukarya; phospholipids with isoprenoid tails are found in Archaea.

  • In all domains, phospholipids are crucial components of the plasma membrane.

  • Archaeal phospholipids’ branched isoprenoid tails confer greater membrane stability in extreme environments.

• Amphipathic nature of lipids

  • Lipids have both hydrophobic (nonpolar tails) and hydrophilic (polar heads) regions; this amphipathic character drives their membrane behavior.

  • Cholesterol is also amphipathic due to its hydrophilic hydroxyl group and hydrophobic rings.

  • Amphipathic lipids spontaneously form hollow vesicles (liposomes) in water, which model membranes.

How Lipid Structure Affects Membrane Properties

• Lipid bilayers and solutions

  • Amphipathic lipids in water form either micelles or lipid bilayers depending on tail structure.

  • Micelles: spherical aggregates formed by lipids with single hydrocarbon chains; heads face water, tails point inward.

  • Lipid bilayers: two leaflets with hydrophilic heads facing water and hydrophobic tails facing inward.

• Physical properties of bilayers

  • Tail saturation and length influence packing and van der Waals interactions; straight, long, saturated tails pack tightly, leading to less permeability; kinked, unsaturated tails reduce packing density and increase permeability.

  • Temperature and cholesterol content modulate membrane fluidity and permeability:

  • Higher unsaturation and shorter tails increase permeability by creating gaps and reducing interactions among tails.

  • Cholesterol reduces permeability by increasing tail packing density (bulky rings fit among tails).

• Visual examples (types of lipids and properties)

  • Butter: primarily saturated lipids; high melting point; solid at room temperature.

  • Waxes: extremely long, saturated chains; form stiff solids.

  • Oils: dominated by polyunsaturated chains; liquid at room temperature.

  • Hydrogenation: conversion of polyunsaturated to saturated lipids by adding hydrogen; reduces cis bonds and turns oils into more solid fats.

• Energy storage comparison

  • Fats store more energy per gram than carbohydrates due to higher ratio of C–C and C–H bonds to C=O bonds.

  • This energy difference helps explain fats’ role in long-term energy storage.

Three Major Lipid Types and Membrane Relevance

• Steroids

  • Four-ring steroid structure; cholesterol is an example.

  • Cholesterol’s polar head (hydroxyl) and nonpolar tail contribute to membrane properties.

• Fats (triacylglycerols)

  • Glycerol backbone with three fatty acids; high-energy storage; not polymers.

• Phospholipids

  • Glycerol + phosphate + two hydrocarbon tails; amphipathic; form membranes.

  • In Archaea, isoprenoid tails; in Bacteria/Eukarya, fatty acid tails; tail type affects membrane stability.

Membrane Interactions with Water

• Amphipathic lipids do not dissolve in water; hydrophilic heads interact with water, hydrophobic tails avoid water.

• Water-membrane interactions and the formation of membranes are driven by hydrophobic effects and tail interactions.

• Cholesterol and cholesterol-containing membranes influence packing and stability.

• The amphipathic character underpins the plasma membrane’s barrier function.

Membrane Models and Experimental Systems

• Lipid bilayers as membranes

  • Micelles form from lipids with single tails; bilayers form from phospholipids with two tails.

  • Bilayers are the foundation of cellular membranes.

• Liposomes and planar bilayers as models

  • Liposomes: spherical vesicles bound by lipid bilayers enclosing aqueous solution.

  • Planar bilayers: lipid bilayers formed across a hole in a barrier, separating two aqueous compartments.

• Experimental advantages of planar bilayers

  • Researchers can add defined solutes to one side and measure whether and how quickly they cross the membrane.

  • They can vary lipid composition and assess the impact of proteins or other molecules on permeability.

• Spontaneous formation and entropy considerations

  • Micelles and bilayers form spontaneously in water due to entropic considerations related to water’s structuring around nonpolar tails.

  • The breakdown of water cages around nonpolar tails during aggregation increases overall system entropy, facilitating spontaneous assembly.

Permeability and Response to Lipid Composition

• Permeability trend across lipid bilayers

  • Small, nonpolar molecules cross rapidly.

  • Polar but uncharged molecules (e.g., water) cross more slowly.

  • Larger polar molecules and especially charged solutes cross very slowly without membrane proteins.

• Factors influencing permeability

  • Tail length: longer tails increase packing density and reduce permeability.

  • Saturation: unsaturated tails (with cis double bonds) introduce kinks, increase gaps, and raise permeability.

  • Cholesterol: presence generally lowers permeability by increasing tail packing density.

• Experimental data (typical findings)

  • Membranes with mostly short, unsaturated tails are more permeable to glycerol than membranes with long, saturated tails.

  • Cholesterol reduces glycerol permeability; higher cholesterol results in greater reduction across tested temperatures.

• Temperature effects

  • Membrane fluidity rises with temperature; permeability generally increases as membranes become more fluid.

  • Low temperature can render membranes nearly impermeable to some solutes in the absence of cholesterol.

• Practical takeaway

  • Lipid composition tunes membrane permeability and fluidity, enabling cells to adapt transport properties to environmental conditions.

Planar Bilayers, Liposomes, and Permeability Experiments

• Experimental designs to test permeability

  • Construct liposomes with different lipid compositions: 0% cholesterol, 20% cholesterol, 50% cholesterol.

  • Measure glycerol movement across membranes at various temperatures.

• Expected outcomes (based on hypotheses)

  • Hypothesis: Adding cholesterol reduces permeability to glycerol.

  • Null hypothesis: Cholesterol has no effect on glycerol permeability.

• Interpreting results

  • Data typically show higher permeability in membranes with no cholesterol, with progressively lower permeability as cholesterol content increases.

  • Temperature rise generally increases permeability for all membranes.

• Why cholesterol reduces permeability

  • Cholesterol aligns with phospholipid tails, tightening packing and reducing gap formation in the hydrophobic interior.

Phospholipid Structure and Membrane Organization

• Phospholipid bilayer organization

  • Phospholipids form bilayers with hydrophilic heads facing the aqueous surroundings and hydrophobic tails tucked inside.

  • The bilayer forms the fundamental barrier of the plasma membrane.

• Micelles vs bilayers (brief recap)

  • Micelles are favored by lipids with single hydrocarbon chains.

  • Bilayers are favored by phospholipids with two hydrocarbon tails (bulkier nonpolar region).

• Amphipathic molecules in water

  • Amphipathic lipids tend to assemble into bilayers or micelles because the hydrophobic tails avoid water while hydrophilic heads contact water.

• Artifactual membranes

  • Liposomes and planar bilayers serve as experimental systems to model and manipulate membranes for permeability studies.

Diffusion, Osmosis, and Membrane Transport

• Diffusion across lipid bilayers

  • Spontaneous, random motion allows solutes to move down their concentration gradients when the membrane permits.

  • Passive transport occurs without external energy input.

  • Entropy increase explains diffusion: random motion gradually disperses solutes, increasing overall disorder.

• Osmosis: diffusion of water

  • Osmosis occurs when solutions are separated by a selectively permeable membrane that allows water to pass but restricts some solutes.

  • Water movement is from regions of low solute concentration to high solute concentration, driven by the gradient.

  • Water movement alters vesicle volume; hypertonic outside causes water to exit and vesicle to shrink; hypotonic outside causes water to enter and vesicle to swell or burst; isotonic means no net water movement.

• Quantitative and qualitative aspects

  • Water diffusion can be fast (e.g., aquaporins dramatically accelerate water flow) but still is a diffusion process driven by gradient and temperature.

The Role of Proteins in Membrane Transport

• Integral vs peripheral proteins

  • Integral (transmembrane) proteins span the lipid bilayer; regions facing interior and exterior differ.

  • Peripheral proteins associate with membrane surfaces; some are restricted to the inner or outer surface.

• Channel proteins

  • Form pores that enable selective diffusion of ions and small polar molecules along electrochemical gradients.

  • Some channels are gated (e.g., voltage-gated channels open/close in response to voltage changes).

  • Aquaporins are highly selective water channels; their pore residues act as a filter to exclude solutes others than water.

  • CFTR (cystic fibrosis transmembrane conductance regulator) functions as a chloride channel; its activity can be demonstrated by planar bilayer experiments showing current flow when CFTR is present.

• Carrier proteins (transporters)

  • Bind a solute on one side of the membrane, then undergo conformational change to release on the other side (e.g., GLUT-1 for glucose).

  • Movement is driven by diffusion along the solute’s gradient but requires carrier conformational changes to move the solute.

• Pumps and active transport

  • Pumps use energy (typically ATP hydrolysis) to move ions or molecules in one direction, often against gradients.

  • The Na+/K+-ATPase (sodium–potassium pump) is a classic example:

  • Step 1: three Na+ binding sites with high affinity on the cytoplasmic side.

  • Step 2: Na+ binds from inside; pump autophosphorylates via ATP; phosphate transfer causes conformational change.

  • Step 3: Na+ released to outside; external conformation has high affinity for K+.

  • Step 4: two K+ bind from outside; phosphate is released; pump returns to original conformation with low affinity for K+.

  • Step 5–8: cycle repeats.

  • Stoichiometry: $3 \,\mathrm{Na}^+\text{ exported} : 2 \,\mathrm{K}^+\text{ imported}$

  • Result: establishes an electrochemical gradient across the membrane, storing energy akin to a battery.

• Secondary active transport (cotransport)

  • ATP use is indirect: the Na+/K+-ATPase creates an electrochemical gradient used to power transport of other solutes against their gradients (e.g., Na+-glucose cotransporter).

  • Example: glucose uptake in gut cells is driven by the Na+ gradient; Na+ moves down its gradient as glucose moves up its gradient.

• Summary model of transport

  • Passive diffusion: simple diffusion or facilitated diffusion (channels/carriers) with no energy input.

  • Active transport: pumps requiring ATP energy; primary active transport moves substances against gradients.

  • Secondary active transport uses gradients created by pumps to drive other solutes against their gradients.

Membranes in Evolution: Protocells and Early Life

• Protocells and simple membranes

  • Early life likely used membranes built from fatty acids or simple amphipathic lipids; fatty acids can form bilayers and water-filled vesicles.

  • These membranes could allow passive transport of nucleotides and diffusion of small molecules, supporting early RNA replication inside vesicles.

• Proteins and membrane evolution

  • Proteins can be amphipathic; nonpolar residues in the membrane interior flanked by polar residues near lipid heads facilitate integration.

  • The presence of integral and peripheral proteins would later allow selective permeability and active transport, enabling advanced cellular function.

• Transition to modern membranes

  • The fluid-mosaic model (Singer & Nicolson, 1972) posits membranes as a dynamic mosaic of phospholipids and proteins with proteins spanning the bilayer.

  • Freeze-fracture EM revealed pits and mounds in the interior of membranes supporting the fluid-mosaic model over the older sandwich model.

Diffusion and Osmosis: Applications to Cell Physiology

• Why diffusion/osmosis matter in cells

  • They enable rapid exchange of small molecules and water across membranes, shaping cell volume and internal environment.

  • Osmosis affects cell volume and turgor; hypertonic/hypotonic conditions drive water movement, influencing cell stability.

• Tonicity terms and examples

  • Hypertonic: solution outside is higher solute concentration than inside; water exits, cell shrinks.

  • Hypotonic: outside solution has lower solute concentration; water enters, cell swells or bursts.

  • Isotonic: solute concentrations are equal; no net water movement.

• Implications for early life and protocells

  • Early versus modern membranes: protocells may have relied on fatty acid membranes allowing easier ion passage, enabling uptake of monomers for replication.

  • Modern phospholipid membranes utilize selective permeability, mediated by proteins, to control solute movement.

Tetrodotoxin Case: Voltage-Gated Sodium Channels and Toxins

• Tetrodotoxin (TTX)

  • A toxin that binds to voltage-gated Na+ channels, blocking Na+ movement and disrupting nerve signaling.

  • Found in various animals (e.g., puffer fish); can contaminate water if skin toxins are present.

• Mechanism of action

  • TTX binds to sodium channel pores, preventing Na+ passage and thus blocking nerve impulses.

  • This can interrupt signaling between brain and muscles, potentially causing respiratory or cardiac failure.

• Sodium channels and electrical gradients

  • Voltage-gated Na+ channels respond to the electrical gradient across the membrane; resting membrane potential is typically negative on the inside (~-90 mV in some cells).

  • Activation involves conformational changes in the channel that permit Na+ flow when voltage conditions favor opening.

• Experimental context and question prompts

  • Studies used artificial lipid bilayers to analyze how voltage changes affect channel activity and toxin interaction.

  • Students are asked to model how TTX could alter sodium transport and to formulate predictions about channel behavior under toxin exposure.

Quantitative Highlights and Key Equations (Membrane Transport Context)

• Electrochemical gradients

  • The electrochemical gradient is the combined influence of concentration and electrical gradients on ion movement. A common quantitative expression is: $\Delta \mu = RT \ln\left(\frac{[S]<em>2}{[S]</em>1}\right) + zF\Delta \psi$ where $R$ is the gas constant, $T$ the temperature, $[S]$ concentrations on either side, $z$ the ion’s charge, $F$ Faraday’s constant, and $\Delta \psi$ the membrane potential.

• Na+/K+-ATPase stoichiometry and energy use

  • 3 Na+ are exported for every 2 K+ imported, powered by ATP hydrolysis: $3 \, Na^+ \text{ exported} : 2 \, K^+ \text{ imported}$

  • This pump creates and maintains an electrochemical gradient used for other transport processes (e.g., secondary active transport).

• Resting membrane potential (typical reference)

  • Resting potential is negative inside; common example values around $V_{rest} \approx -90\ \text{mV}$, depending on cell type.

• Permeability trends ( qualitative, not a single equation)

  • Small nonpolar molecules diffuse rapidly across bilayers.

  • Water (polar but uncharged) diffuses slowly unless aquaporins are present; ions require transport proteins to cross.

  • Fatty acid saturation and tail length, plus cholesterol content, modulate bilayer permeability through changes in tail packing and density.

Quick Recap: Core Concepts to Recall

• Lipids share insolubility in water due to nonpolar hydrocarbon-rich structures; amphipathic lipids form membranes with hydrophilic heads and hydrophobic tails.

• Lipid diversity (fats, phospholipids, steroids) underpins energy storage, membrane structure, signaling, and more.

• Membranes form micelles or bilayers depending on lipid type; bilayers form the barrier essential for cellular life.

• Membrane permeability is highly selective; small nonpolar molecules cross readily, ions and large polar solutes require proteins.

• Proteins introduce specificity and control: channels (diffusion), carriers (conformational transport), and pumps (active transport).

• The fluid-mosaic model explains membranes as dynamic, protein-containing, phospholipid bilayers; integral and peripheral proteins contribute diverse functions.

• Osmosis and diffusion drive passive solute and water movement; tonicity governs changes in cell volume.

• Protocells and early membranes may have relied on fatty acids; later evolution introduced protein-mediated transport and complex regulation.

• Toxins like tetrodotoxin reveal the functional architecture of ion channels and the importance of membrane potential in nerve signaling.

Practice and Concept Checks

• Explain how tail saturation and length influence membrane permeability and fluidity.

• Compare and contrast a steroid, a fat, and a phospholipid in terms of structure and function.

• Are free fatty acids amphipathic? Are fats amphipathic? Explain.

• Describe how planar bilayers and liposomes enable studies of membrane permeability.

• Distinguish between channel proteins and carrier proteins; give an example of each.

• Outline the Na+/K+-ATPase cycle and the energy requirements for pumping.

• Explain how glucose uptake is achieved via GLUT-1 and how secondary active transport uses ion gradients.

• Summarize how osmosis affects vesicle volume under hypertonic, hypotonic, and isotonic conditions.

• Describe how tetrodotoxin impacts voltage-gated Na+ channels and the physiological consequences.

$ext{Key equations and constants to memorize:}$

• $3 \ Na^+ \text{ exported} : 2 \ K^+ \text{ imported}$ (Na+/K+-ATPase stoichiometry)

• $\Delta \mu = RT \ln\left(\frac{[S]<em>2}{[S]</em>1}\right) + zF\Delta \psi$ (electrochemical gradient)

• $V_{rest} \approx -90\ \text{mV}$ (typical resting potential reference)

• Fat energy density claim: fats store about twice as much chemical energy per gram as carbohydrates (qualitative statement)

• Fatty acid chain length and saturation relationships; unsaturation introduces kinks, affecting packing and melting point

• Cholesterol's effect: reduces membrane permeability by increasing tail packing

Title

Comprehensive Notes: Lipids, Membranes, and the First Cells