Water, Carbon, Proteins, Lipids, and Cell Structure: Foundations of Life

Chapter 2 – Water and Carbon: The Chemical Basis of Life

Atoms, Ions, and Molecules: The Building Blocks of Chemical Evolution

All living organisms are composed primarily of a few key elements, and their interactions form the basis of biological molecules and life processes.

• CHONP: Carbon, Hydrogen, Oxygen, Nitrogen, and Phosphorus are the main elements in biological systems.

• Atomic Structure: Atoms consist of protons, neutrons, and electrons. Electrons occupy orbitals arranged in electron shells.

• Valence Shell: The outermost electron shell; the number of unpaired electrons (valence) determines bonding capacity.

• Covalent Bonds: Atoms share electron pairs. Polarity arises from differences in electronegativity; C, N, and H are considered similar in this context.

• Ionic Bonds: Formed when electrons are transferred from one atom to another, resulting in charged ions.

Properties of Water and the Early Oceans

Water's unique properties make it essential for life and chemical evolution.

• Polarity and Hydrogen Bonding: Water is polar, allowing it to form hydrogen bonds with polar solutes and ions (hydrophilic substances).

• Hydrogen Bonds: Occur between hydrogens attached to O, N, or F atoms.

• Hydrophobic Effect: Nonpolar substances are hydrophobic; water forms a 'cage' around them, minimizing disruption of its hydrogen bonding network.

• Van der Waals Interactions: Weak attractions due to transient partial charges from electron movement.

• Other Properties:

  • Cohesion: Water molecules stick together.

  • Adhesion: Water molecules stick to other surfaces.

  • Surface Tension: Water's surface resists external force.

  • Density: Water is denser as a liquid than as a solid (ice floats).

  • High Specific Heat: Water absorbs a lot of energy before changing temperature.

  • Amphoteric: Can act as both an acid and a base.

Chemical Reactions, Energy, and Chemical Evolution

Chemical evolution describes how simple molecules gave rise to complex molecules in early Earth conditions.

• Chemical Evolution: Simple molecules in the early atmosphere or deep-sea vents reacted to form more complex molecules.

• Systems: Closed (no matter exchange) vs. open (matter and energy exchange).

• Chemical Equilibrium: Forward and reverse reactions occur at the same rate.

• Law of Conservation of Matter: Matter is neither created nor destroyed.

• Exothermic/Endothermic Reactions: Exothermic releases energy; endothermic absorbs energy.

• Chemical Energy: A form of potential energy stored in bonds; stronger bonds (more shared electrons or more polar) have lower potential energy.

• Temperature and Kinetic Energy: Higher temperature increases molecular motion and reaction rates.

• Spontaneity: If products have lower potential energy than reactants, the reaction is spontaneous (negative ΔG).

Gibbs Free Energy Equation:

$\Delta G = \Delta H - T \Delta S$

• ΔG: Change in free energy

• ΔH: Change in enthalpy (total energy)

• T: Temperature (Kelvin)

• ΔS: Change in entropy (disorder)

Investigating Chemical Evolution

• Oparin–Haldane Hypothesis: Proposed that life began through a gradual chemical evolution of carbon-based molecules in a "primordial soup." Now considered a formal scientific theory.

• Miller-Urey Experiment: Simulated early Earth conditions and produced organic molecules from inorganic precursors using electrical sparks.

Life Is Carbon Based

Carbon's versatility allows for the formation of a vast array of organic molecules.

• Organic Compounds: Contain carbon atoms bonded to other elements.

• Carbon's Versatility: Can form four covalent bonds, creating complex skeletons.

• Functional Groups: Determine chemical behavior of organic molecules.

  • Amino group (–NH2): Acts as a base.

  • Carboxyl group (–COOH): Acts as an acid.

  • Carbonyl group (–C=O): Site for linkage.

  • Hydroxyl group (–OH): Weak acid.

  • Phosphate group (–PO4): Negatively charged.

  • Sulfhydryl group (–SH): Forms disulfide bonds.

• Polymerization: Monomers join to form polymers via dehydration reactions (removal of water); hydrolysis breaks polymers (adds water, increases entropy).

Chapter 3 – Protein Structure and Function

Amino Acids and Their Polymerization

Proteins are polymers of amino acids, which have a central role in structure and function of cells.

• Amino Acid Structure: Central (alpha) carbon, hydrogen atom, amino group, carboxyl group, and variable R group.

• Ionization in Water: Amino acids ionize, gaining charges that help solubility and reactivity.

• Peptide Bond Formation: Amino acids polymerize via dehydration, forming a C–N bond (peptide bond) and releasing water.

• Peptide Bond Properties: Rigid due to partial double-bond character; other bonds in the backbone are flexible.

• Oligopeptide: Fewer than 50 amino acids; Polypeptide: More than 50 amino acids.

What Do Proteins Look Like?

Protein structure determines function, and is organized into four levels.

• Primary Structure: Unique sequence of amino acids.

• Secondary Structure: Local folding due to hydrogen bonding between backbone groups (carbonyl oxygen and amino hydrogen); forms alpha helices and beta-pleated sheets.

• R Group Influence: Some amino acids (e.g., proline) disrupt certain secondary structures.

• Tertiary Structure: Overall 3D shape due to interactions between R groups (hydrogen bonds, hydrophobic interactions, van der Waals forces, disulfide bridges, ionic bonds).

• Quaternary Structure: Association of multiple polypeptide subunits.

• Macromolecular Machines: Complexes of multiple proteins that perform specific functions.

Folding and Function

Protein folding is crucial for function and is often spontaneous, but can be assisted by other proteins.

• Spontaneous Folding: Driven by hydrophobic interactions and increased entropy.

• Denaturation: Loss of structure leads to loss of function.

• Molecular Chaperones: Proteins (e.g., heat-shock proteins) that assist in folding and prevent aggregation of nonpolar regions.

• Regulation: Some proteins remain disordered until activated or localized.

Protein Functions Are as Diverse as Protein Structures

• Catalysis: Enzymes speed up chemical reactions.

• Structure: Provide support (e.g., collagen).

• Movement: Motor proteins (e.g., myosin, kinesin).

• Signaling: Hormones and receptors.

• Transport: Move substances across membranes (e.g., hemoglobin, channel proteins).

• Defense: Antibodies and immune proteins.

• Enzyme Mechanism: Substrates bind to the active site; enzyme undergoes induced fit to facilitate reaction.

Chapter 8 – Energy and Enzymes: An Introduction to Metabolism

What Happens to Energy in Chemical Reactions?

Energy transformations are central to metabolism and life processes.

• Potential and Kinetic Energy: Chemical energy is potential energy stored in bonds; long, weak bonds have high potential energy.

• Enthalpy (H): Total energy (potential + kinetic) of a molecule.

• Gibbs Free Energy (G): Energy available to do work; determines spontaneity.

• Gibbs Free Energy Equation:

  $\Delta G = \Delta H - T \Delta S$

• Exergonic: ΔG < 0 (spontaneous); Endergonic: ΔG > 0 (nonspontaneous).

• Reaction Rate: Increases with higher concentration or temperature.

Nonspontaneous Reactions May Be Driven Using Chemical Energy

• Energetic Coupling: Exergonic reactions (e.g., ATP hydrolysis) drive endergonic reactions.

• Redox Reactions: Transfer of electrons; reduction (gain of electrons/H), oxidation (loss of electrons/H).

• Electron Carriers: Molecules like FADH2 transport electrons in metabolic pathways.

• ATP: High potential energy due to repulsion of three phosphate groups; hydrolysis releases energy.

• Phosphorylation: Transfer of phosphate group to a molecule, increasing its potential energy.

How Enzymes Work

• Activation Energy: Energy required to reach the transition state; enzymes lower this barrier.

• Transition State: High-energy, unstable intermediate.

• Enzyme Mechanism:

  1. Substrate binds to active site (via weak interactions).

  2. Transition state is stabilized (R groups help binding).

  3. Products are released; enzyme is unchanged.

• Enzyme Kinetics: Michaelis-Menten model; Vmax (maximum rate), Km (substrate concentration at half Vmax).

• Cofactors: Inorganic ions; Coenzymes: Organic molecules; Prosthetic Groups: Tightly bound non-protein components.

• Enzymes do not alter ΔG of a reaction.

What Factors Affect Enzyme Function?

• Temperature: Affects kinetic energy and enzyme structure.

• pH: Influences enzyme structure and charge.

• Inhibitors:

  • Competitive: Bind active site, block substrate.

  • Noncompetitive (Allosteric): Bind elsewhere, change enzyme shape.

• Covalent Regulation: Phosphorylation or peptide bond cleavage can activate/inactivate enzymes.

Enzymes Can Work Together in Metabolic Pathways

• Metabolic Pathways: Series of enzyme-catalyzed reactions; intermediates are products of one step and substrates for the next.

• Catabolic Pathways: Break down molecules, release energy.

• Anabolic Pathways: Build molecules, require energy.

• Feedback Inhibition: End product inhibits an earlier step, regulating pathway activity.

• Retroevolution: New enzymes evolve when original substrates are depleted.

• Patchwork Evolution: Enzymes evolve new functions from existing ones.

• Bioremediation: Use of metabolic pathways to degrade pollutants.

Chapter 6 – Lipids, Membranes, and the First Cells

Lipid Structure and Function

Lipids are hydrophobic molecules essential for energy storage, membrane structure, and signaling.

• Lipids: Carbon-containing, insoluble in water.

• Fatty Acids: Hydrocarbon chains with a polar carboxyl group.

• Saturation:

  • Saturated: No double bonds; straight chains, solid at room temperature (due to van der Waals interactions).

  • Unsaturated: One or more double bonds; kinks prevent tight packing, liquid at room temperature.

• Steroids: Four-ring structure; includes hormones and cholesterol (with isoprenoid tail).

• Fats (Triglycerides): Three fatty acids linked to glycerol via ester linkages; primary role is energy storage.

• Phospholipids: Glycerol backbone, two hydrocarbon chains, phosphate group; amphipathic, form membranes.

Phospholipid Bilayers

• Micelles and Bilayers: In water, phospholipids form micelles (spheres) or bilayers (sheets).

• Liposomes: Artificial vesicles formed from phospholipids in aqueous solution.

• Selective Permeability: Small, nonpolar molecules cross membranes quickly; large or charged molecules cross slowly.

• Membrane Fluidity:

  • Unsaturated phospholipids increase fluidity and permeability.

  • Cholesterol decreases permeability by increasing packing density.

  • Lower temperatures reduce fluidity; more unsaturated lipids can compensate.

• Lateral Motion: Phospholipids move within the bilayer.

How Substances Move across Lipid Bilayers: Diffusion and Osmosis

• Diffusion: Spontaneous movement of molecules from high to low concentration (along the concentration gradient); passive transport.

• Osmosis: Diffusion of water across a selectively permeable membrane; occurs when solute cannot cross but water can.

• Tonicity:

  • Hypertonic: Higher solute concentration outside; cell loses water.

  • Hypotonic: Lower solute concentration outside; cell gains water.

  • Isotonic: Equal solute concentration; no net water movement.

• Protocells: Early cell-like structures with fatty acid bilayers that could harbor nucleic acids.

Proteins Alter Membrane Structure and Function

• Fluid-Mosaic Model: Membranes are a mosaic of lipids and proteins; proteins can be amphipathic.

• Integral/Transmembrane Proteins: Span the membrane.

• Peripheral Proteins: Attached to membrane surface.

• Channel Proteins: Facilitate diffusion of ions/small polar molecules; selective (e.g., aquaporin for water).

• Gated Channels: Open/close in response to signals.

• Facilitated Diffusion: Passive transport assisted by proteins.

• Carrier Proteins: Bind and transport specific molecules (e.g., GLUT-1 for glucose).

• Pumps: Active transport using ATP (e.g., sodium-potassium pump).

• Secondary Active Transport (Cotransport): Uses gradient established by ATP pump to move other solutes against their gradient.

• Example: Cystic fibrosis is caused by a faulty Cl- channel, disrupting osmosis.

Chapter 7: Inside the Cell

Eukaryotic Cell Structures and Their Functions

Eukaryotic cells are compartmentalized, allowing for specialized functions and increased efficiency.

• Large Size: Eukaryotes are larger than prokaryotes but maintain a high surface area:volume ratio via compartmentalization.

• Cytosol: Fluid portion of the cytoplasm; relatively small volume.

• Advantages of Compartmentalization: Separates incompatible reactions, increases efficiency.

• Organelles:

  • Nucleus: Contains DNA.

  • Ribosomes: Protein synthesis.

  • Endoplasmic Reticulum (Smooth/Rough): Protein and lipid synthesis.

  • Golgi Apparatus: Modifies, sorts, and ships proteins.

  • Lysosomes: Digestion and recycling.

  • Vacuoles: Storage (plants/fungi).

  • Peroxisomes: Oxidation reactions.

  • Mitochondria: ATP production.

  • Chloroplasts: Photosynthesis (plants/algae).

  • Cytoskeleton: Structural support, movement.

  • Plasma Membrane: Selective barrier.

  • Cell Wall/Extracellular Matrix: Support and protection.

  • Glyoxysomes: Specialized peroxisomes in plants for processing photosynthesis products.

• Endosymbiosis Theory: Mitochondria and chloroplasts originated from engulfed prokaryotes.

Putting the Parts into a Whole

• Structure-Function Relationship: Cell structures are specialized for their functions.

• Differential Centrifugation: Technique to separate cell components; imaging is now more common for studying cell structure.

Cell Systems I: Nuclear Transport

• Nuclear Envelope: Double membrane surrounding the nucleus; contains nuclear pore complexes.

• Nuclear Pore Complex: Regulates entry/exit of molecules (e.g., rRNA, mRNA exit; nucleoside triphosphates, proteins enter).

• Nuclear Localization Signal (NLS): Sequence that allows proteins to enter the nucleus.

• Chaperone Proteins: Recognize targeting signals and direct proteins to correct locations.

Cell Systems II: The Endomembrane System Manufactures, Ships, and Recycles Cargo

• Protein Targeting: Most organelle proteins are imported after synthesis by free ribosomes.

• Pulse-Chase Experiment: Tracks protein movement; shows proteins start in ribosomes, then move to ER, Golgi, and final destinations.

• Signal Sequence: Directs ribosome to ER; binds signal recognition particle (SRP), pauses translation, then resumes at ER membrane.

• Vesicular Transport: Proteins move from ER to Golgi via vesicles; further modifications and sorting occur.

• Targeting Signals: Additional signals (e.g., phosphate on sugar) direct proteins to specific organelles.

• Protein Recycling: Lysosomes digest proteins via phagocytosis, receptor-mediated endocytosis, or autophagy.

Cell Systems III: The Dynamic Cytoskeleton

• Actin Filaments (Microfilaments): Polymerized globular actin; functions in cell shape, movement, cytokinesis, and organelle transport. Works with myosin for movement (e.g., cell crawling, cytoplasmic streaming).

• Intermediate Filaments: Composed of keratins, lamins; provide structural support, maintain cell shape, and support the nuclear envelope.

• Microtubules: Polymers of alpha and beta tubulin dimers; originate from microtubule organizing centers (MTOC), usually the centrosome. Functions include cell shape, movement (flagella/cilia), chromosome movement, and intracellular transport (motor proteins like kinesin "walk" along microtubules).

• Flagella/Cilia: 9+2 arrangement of microtubules (axoneme); basal body anchors the structure.