Week 3 Lecture 5

Lipid Structures and Chemical Properties

• Intermolecular Forces in Fatty Acids:

  • Hydrocarbon chains in fatty acids consist of non-polar covalent bonds between carbon and hydrogen atoms.

  • Continuous motion of electrons around atomic nuclei creates short-lived, transient dipole regions with slight negative charges, which are attracted to slight positive charge regions in adjacent atoms.

  • These weak, non-covalent attractive forces formed between temporary partial charges on hydrophobic molecules are defined as van der Waals forces.

  • Van der Waals forces are weaker than hydrogen bonds, but their cumulative strength increases proportionally with the length of the fatty acid hydrocarbon tail.

  • Longer hydrocarbon tails provide larger surface areas for contact, yielding stronger van der Waals interactions and higher structural stability.

• Saturated vs. Unsaturated Fatty Acids:

  • Saturated Fatty Acids:

  • Hydrocarbon chains lack double bonds, allowing fatty acid chains to adopt straight, linear conformations.

  • Straight structures favor tight packing side-by-side, maximizing van der Waals interactions.

  • Require higher thermal energy to disrupt interactions, resulting in higher melting points.

  • Animal fats contain high proportions of saturated fatty acids and are solid at room temperature.

  • Unsaturated Fatty Acids:

  • Hydrocarbon chains contain one or more double bonds (typically in the cis configuration), which introduce permanent kinks and bends into the chain structure.

  • Kinks prevent tight packing of adjacent lipid tails, reducing the number of van der Waals interactions.

  • Require less thermal energy to disrupt, resulting in significantly lower melting points.

  • Plant and fish fats contain high proportions of unsaturated fatty acids and remain liquid at room temperature.

• Melting Point Analysis:

  • In experimental tests measuring the melting points of fat samples of equal fatty acid length:

  • Sample 1 exhibits the highest melting point ($45^\text{o}\text{C}$), indicating it contains the highest proportion of saturated fatty acids with fully straight chains and maximal van der Waals interactions.

  • Sample 2 exhibits an intermediate melting point ($37^\text{o}\text{C}$).

  • Sample 3 exhibits the lowest melting point ($15^\text{o}\text{C}$), indicating the highest proportion of double bonds and unsaturated fatty acids.

• Steroids and Cholesterol:

  • Cholesterol belongs to the steroid class of lipids, characterized by a complex structure containing a carbon skeleton of four fused rings.

  • Structure consists of a polar hydroxyl head group ($-\text{OH}$), a rigid four-ring steroid nucleus, and a nonpolar hydrocarbon tail.

  • Serves as a fundamental structural component in animal cell plasma membranes, modulating membrane rigidity and fluidity.

  • Functions as an essential biochemical precursor in metabolic pathways synthesizing steroid hormones, including estrogen and progesterone.

• Phospholipid Architecture:

  • Phospholipids are the primary structural lipids constituting all cellular membranes.

  • Molecular structure includes:

  • A polar/hydrophilic head group consisting of a charged molecule (such as choline) attached to a phosphate group ($\text{PO}_4^{3-}$) and linked to a glycerol backbone.

  • Two nonpolar/hydrophobic fatty acid tails attached to the glycerol backbone.

  • Presence of a cis double bond in one fatty acid tail creates a distinct structural kink.

Cell Theory and Evolutionary Domains of Life

• Foundational Principles of Cell Theory:

  • All living organisms are composed of one or more cells.

  • The cell is the fundamental structural, functional, and organizational unit of life.

  • All cells arise from pre-existing cells through cell division.

• Three Domains of Life:

  • The tree of life categorizes all living organisms into three major domains: Bacteria, Archaea, and Eukarya.

  • Most of life's biological diversity and deep evolutionary lineage is microbial.

  • Phylogenetically, Eukaryotes are hypothesized to have descended directly from within the domain Archaea.

• Prokaryotes vs. Eukaryotes:

  • Prokaryotes:

  • Defined structurally as organisms belonging to the domains Bacteria and Archaea.

  • Exist strictly as single-celled organisms.

  • Lack internal membrane-bound organelles or a distinct membrane-enclosed nucleus.

  • Eukaryotes:

  • Belong to the domain Eukarya, which encompasses all plants, animals, fungi, and protists.

  • Include both unicellular and complex multicellular organisms.

  • Characterized by complex internal membrane-bound structures (such as the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and chloroplasts).

Membrane Organization, Assemblies, and Dynamics

• Amphipathic Properties and Membrane Bilayer Formation:

  • Phospholipids are amphipathic molecules containing both hydrophilic (water-attracting) polar regions and hydrophobic (water-repelling) nonpolar regions.

  • In aqueous environments, amphipathic phospholipids self-assemble spontaneously into distinct structures to shield hydrophobic tails from water while exposing hydrophilic heads to the aqueous surroundings:

  • Micelles: Spherical structures formed by lipids possessing large, bulky hydrophilic head groups and a single hydrophobic fatty acid tail. Tails pack tightly into the inner core, excluding water completely.

  • Bilayers: Double-layered sheets formed by phospholipids possessing small hydrophilic head groups and two hydrophobic fatty acid tails.

  • Liposomes: Spherical, closed double-layered phospholipid structures containing an enclosed inner aqueous compartment.

• Evolutionary Significance of Liposomes:

  • Liposomes form, break, and reform spontaneously in fluid environments.

  • Additional lipid molecules integrate into existing liposomes, allowing them to grow in size.

  • Macromolecules such as nucleic acids can become trapped inside forming liposomes.

  • Liposomes serve as functional models for primitive proto-cells (protobionts); natural selection favored early leaky membrane assemblies that provided survival advantages and preserved internal chemical conditions.

• Membrane Fluidity and Dynamic Movement:

  • Biological membranes behave as dynamic two-dimensional fluids where molecules diffuse laterally within their respective leaflets.

  • Lateral Diffusion: Phospholipids and embedded proteins rapidly move side-to-side within a membrane layer, occurring at a rate of approximately $10^7\,\text{times per second}$.

  • Transverse Movement ("Flip-Flopping"): Movement of a phospholipid from one membrane leaflet across the hydrophobic core to the opposing leaflet is energetically unfavorable and extremely rare, occurring approximately $\text{once per month}$ without enzymatic assistance (such as flippases).

• Factors Regulating Membrane Fluidity:

  • Fatty Acid Tail Length: Shorter hydrocarbon chains reduce available surface area for van der Waals interactions, increasing membrane fluidity. Longer chains increase interactions and decrease fluidity.

  • Degree of Unsaturation: Unsaturated fatty acid tails introduce kinks that prevent tight lipid packing, increasing membrane fluidity at lower temperatures. Saturated fatty acid tails pack densely, increasing membrane viscosity.

  • Environmental Temperature:

  • Low temperatures decrease thermal kinetic energy, causing lipid packing and potential membrane solidification, impairing cellular function.

  • High temperatures increase kinetic motion, excessively loosening packing and reducing structural stability.

  • Organisms living in cold environments (such as pond unicellular organisms during winter) synthesize higher proportions of unsaturated fatty acids in their plasma membranes to prevent solidification and preserve membrane function.

  • Cholesterol as a Fluidity Buffer:

  • Cholesterol packs tightly between phospholipid tails due to its amphipathic structure.

  • At moderate to high temperatures, cholesterol's rigid steroid ring structure restricts excessive lateral movement of phospholipid tails, reducing membrane fluidity and preventing instability.

  • At low temperatures, cholesterol intercalates between fatty acid chains, preventing them from packing closely together and freezing, thereby maintaining membrane fluidity.

Structural and Functional Classification of Membrane Proteins

• Fluid Mosaic Model:

  • Formulated by S. J. Singer and G. L. Nicolson in 1972.

  • Describes the cell membrane as a dynamic, two-dimensional fluid matrix composed of a phospholipid bilayer in which lipids and proteins freely diffuse laterally.

• Classes of Membrane Proteins:

  • Integral Membrane Proteins:

  • Permanently embedded within the lipid bilayer.

  • Must possess amphipathic structures: hydrophobic regions composed of nonpolar amino acid residues that interact directly with the interior fatty acid tails, and hydrophilic regions that extend into the aqueous cytoplasm or extracellular environment.

  • Transmembrane Proteins: A specific subcategory of integral proteins that span the entire thickness of the lipid bilayer, protruding on both sides.

  • Peripheral Membrane Proteins:

  • Temporarily associated with either the extracellular or intracellular cytoplasmic face of the membrane.

  • Interact via non-covalent interactions (ionic or hydrogen bonds) with polar head groups of lipids or exposed domains of integral membrane proteins.

  • Can be dissociated from membranes easily without disrupting the lipid bilayer core.

• Key Functions of Membrane Proteins:

  • Transporters: Channels or carrier proteins that facilitate the movement of specific ions, polar molecules, or nutrients across the hydrophobic membrane core.

  • Receptors: Transmembrane proteins that specifically bind extracellular signal molecules (ligands) and undergo conformational changes to transmit signals inside the cell.

  • Enzymes: Catalyze specific biochemical reactions at the inner or outer surface of the cellular membrane.

  • Anchors: Structural proteins that physically attach to internal cytoskeletal filaments or external extracellular matrix components, maintaining cell shape and tissue integrity.

Transport Mechanisms and Homeostasis

• Homeostasis and Selective Permeability:

  • Homeostasis: Maintenance of a constant, narrow physiological range of internal cellular conditions necessary for life.

  • Plasma membranes are selectively permeable, controlling which substances cross the membrane boundary.

  • Passively Permeable Substances: Small nonpolar molecules ($\text{O}_2$, $\text{CO}_2$) and small uncharged hydrophobic molecules (fatty acids) readily diffuse directly across the hydrophobic lipid core. Water molecules pass slowly via simple diffusion due to small size.

  • Impermeable / Restrictive Substances: Macromolecules (carbohydrates, proteins, nucleic acids) and charged ions ($\text{Na}^+$, $\text{K}^+$, $\text{Ca}^{2+}$, $\text{H}^+$) cannot pass passively across the hydrophobic interior and strictly require specific transport proteins.

• Passive Transport Mechanisms:

  • Simple Diffusion:

  • Net movement of solute particles down their concentration gradient (from an area of higher concentration to lower concentration).

  • Driven entirely by intrinsic thermal kinetic energy; requires no cellular energy expenditure.

  • Net movement continues until dynamic equilibrium is reached, after which random particle movement continues equally in both directions.

• Osmosis:

  • Special class of diffusion involving net movement of water (solvent) across a selectively permeable membrane.

  • Water moves from regions of lower solute concentration (higher free water concentration) to regions of higher solute concentration (lower free water concentration).

• Facilitated Diffusion:

  • Passive movement of polar or charged substances down their concentration gradient through specific transmembrane transport proteins.

  • Channel Proteins: Transmembrane channels containing hydrophilic pores through which specific ions or water molecules rapidly pass down their concentration gradient.

    • Aquaporins: Dedicated membrane channels that allow bulk, high-speed flow of water molecules across cellular membranes.

  • Carrier Proteins: Transmembrane proteins that bind specific target solutes, undergo conformational changes, and deliver solutes across the membrane.

• Active Transport Mechanisms:

  • Movement of solutes against their established concentration gradient (from low concentration to high concentration).

  • Requires metabolic energy, typically supplied by adenosine triphosphate (ATP) hydrolysis.

  • Primary Active Transport:

  • Uses energy directly from ATP hydrolysis to pump ions across a membrane against their concentration gradient.

  • Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+\text{-ATPase}$): An essential antiporter active transport protein found in animal cell membranes.

  • Operating Cycle Mechanism:

    1. Three cytoplasmic sodium ions ($3\,\text{Na}^+$) bind to high-affinity intracellular sites on the pump protein.

    2. Phosphorylation of the pump by ATP ($\text{ATP} \rightarrow \text{ADP} + \text{P}_i$) triggers a major conformational change, releasing $3\,\text{Na}^+$ ions into the extracellular space against their concentration gradient.

    3. Two extracellular potassium ions ($2\,\text{K}^+$) bind to high-affinity extracellular sites on the phosphorylated pump.

    4. Dephosphorylation ($\text{P}_i$ release) restores the protein's original conformation, transporting $2\,\text{K}^+$ ions into the cytoplasm against their concentration gradient.

• Secondary Active Transport:

  • Electrochemical gradients created by primary active pumps (such as $\text{Na}^+$ or proton concentration gradients) store potential energy.

  • Drive the active transport of other molecules against their concentration gradient via specialized cotransporters:

    • Symporters: Transport two different substances in the same direction across the membrane simultaneously.

    • Antiporters: Transport two different substances in opposite directions across the membrane simultaneously.

  • Cell Size, Shape, and Osmotic Regulation:

• Tonicity Effects on Cells:

  • Hypertonic Environment: External solute concentration is higher than cytoplasmic solute concentration. Net water flows out of the cell via osmosis, causing the cell to shrink/crenate.

  • Isotonic Environment: External solute concentration equals internal solute concentration. No net flux of water occurs, preserving cell volume and normal shape.

  • Hypotonic Environment: External solute concentration is lower than cytoplasmic solute concentration. Net water flows rapidly into the cell, causing swelling and potential cell lysis (bursting).

• Mechanisms for Maintaining Cell Shape and Integrity:

  • Active Ion Pumping: Animal blood cells constantly employ active transport pumps (such as $\text{Na}^+/\text{K}^+\text{-ATPase}$) to adjust intracellular ion concentrations, ensuring the internal environment remains isotonic with the surrounding extracellular fluid.

  • Contractile Vacuoles: Freshwater single-celled protists (such as Paramecium) living in hypotonic surroundings continuously gain water. Specialized organelles called contractile vacuoles accumulate excess internal water and rhythmically contract to pump water out of the cell, preventing osmotic lysis.

* **Cell Wall Support**: Plants, fungi, algae, and bacteria possess a rigid extracellular **cell wall** surrounding the plasma membrane. Incoming osmotic water creates internal hydrostatic pressure (**turgor pressure**) pushing against the wall, maintaining structural cell shape without bursting. Plant cells also possess a large central **vacuole** that absorbs water and maintains turgor pressure.