Comprehensive Review Notes: Cell Communication, Transport, Metabolism, and Biomolecules

Membrane Transport Mechanisms

• Passive Transport: Solute movement down a concentration gradient requiring no energy input.

  • Simple Diffusion: Nonpolar, hydrophobic molecules (e.g., steroid hormones like testosterone and estradiol, $\text{O}_2$, $\text{CO}_2$) easily diffuse directly across the phospholipid bilayer.

  • Facilitated Diffusion: Polar or charged molecules (e.g., glucose, ions) cross down their concentration gradient with the assistance of channel or carrier proteins.

• Active Transport: Movement of solutes against a concentration gradient requiring energy input (often $\text{ATP}$ hydrolysis) and specific carrier proteins (pumps).

  • Primary Active Transport: Directly uses $\text{ATP}$. Example: $\text{Na}^+/\text{K}^+$ pump (electrogenic pump) exports $3\text{ Na}^+$ ions and imports $2\text{ K}^+$ ions per cycle.

  • Secondary Active Transport (Cotransport): Uses an established ion gradient (such as $\text{Na}^+$) to transport a second solute (such as glucose) against its concentration gradient.

• Transepithelial Transport (Small Intestine Example):

  • Apical Domain: Densely covered with microvilli to increase surface area; contains $\text{Na}^+/\text{glucose}$ symporters that actively import glucose into epithelial cells.

  • Basolateral Domain: Contains passive glucose carrier proteins for facilitated diffusion into extracellular fluid, alongside $\text{Na}^+/\text{K}^+$ pumps to maintain low cytosolic $\text{Na}^+$ levels.

  • Tight Junctions: Join adjacent cells, preventing paracellular leakage and maintaining distinct apical vs. basolateral membrane protein distributions.

Cell Signalling Principles & Pathways

• Information Flow: Transmitted when a cell receives an external signal and responds through biochemical alterations that modify its function.

• Core Signalling Stages:

  1. Reception: A ligand binds a specific receptor protein.

  • Intracellular Receptors: Hydrophobic ligands pass through the plasma membrane to bind receptors in the cytoplasm or nucleus (e.g., testosterone acting as a transcription factor).

  • Surface Receptors: Polar/hydrophilic ligands bind membrane-bound receptors (e.g., G-protein-coupled receptors [GPCRs], ligand-gated ion channels, receptor tyrosine kinases).

  1. Transduction: Multi-step cascade converting the signal to a usable cell form using transducers, including protein kinases, G-proteins, and second messengers ($\text{cAMP}$, $\text{IP}_3$, $\text{DAG}$, $\text{Ca}^{2+}$).

  2. Response: Activation of cellular processes such as altered gene expression, enzyme modification, or changes in membrane transport.

• Sweet Taste Transduction Pathway:

  • A sweet tastant (e.g., sucrose, glucose, aspartame, stevioside) binds the GPCR heterodimer ($\text{T1R2}$ + $\text{T1R3}$) on apical microvilli.

  • Activated G-protein stimulates phospholipase C ($\text{PLC}\beta 2$), which breaks membrane lipid $\text{PIP}_2$ into $\text{IP}_3$ and $\text{DAG}$.

  • $\text{IP}_3$ diffuses through cytoplasm and opens ligand-gated $\text{IP}_3\text{R3}$ $\text{Ca}^{2+}$ channels on the endoplasmic reticulum.

  • Released $\text{Ca}^{2+}$ opens $\text{TrpM5}$ cation channels, causing $\text{Na}^+$ influx and membrane depolarization.

  • Depolarization and $\text{Ca}^{2+}$ open $\text{Panx1}$ hemichannels, releasing $\text{ATP}$.

  • Released $\text{ATP}$ acts via autocrine signaling (stimulating further $\text{ATP}$ release) and paracrine signaling (exciting nearby sensory neurons that transmit signal to the brain).

Energy, Metabolism, and Respiration

• Thermodynamic Concepts:

  • Bond breaking requires energy input; bond formation releases energy. Net reaction energy changes ($\Delta G$) reflect differences between total broken and formed bond energies.

  • Catabolic Pathways: Exergonic ($\Delta G < 0$), spontaneous pathways breaking down complex molecules to release free energy.

  • Anabolic Pathways: Endergonic ($\Delta G > 0$), non-spontaneous pathways building complex molecules requiring energy input.

• Cellular Respiration Stages:

  • Glycolysis (Cytosol): Converts glucose into $2\text{ pyruvate}$, yielding $2\text{ ATP}$ (substrate-level phosphorylation) and $2\text{ NADH}$.

  • Pyruvate Oxidation (Mitochondrial Matrix): Converts pyruvate into $\text{Acetyl CoA}$, producing $2\text{ NADH}$ and $2\text{ CO}_2$ per glucose.

  • Citric Acid Cycle (Mitochondrial Matrix): Oxidizes acetyl groups, yielding $2\text{ ATP}$, $6\text{ NADH}$, $2\text{ FADH}_2$, and $4\text{ CO}_2$ per glucose.

  • Oxidative Phosphorylation (Inner Mitochondrial Membrane): Electron transport chain creates a proton gradient driving chemiosmosis via $\text{ATP}$ synthase, producing $\approx 26\text{--}28\text{ ATP}$.

  • Maximum Yield: $\approx 30\text{--}32\text{ ATP}$ per glucose molecule.

• Evolutionary Connection: Prokaryotes maintain proton gradients and run electron transport/chemiosmosis across their plasma membrane. Eukaryotic mitochondria inherited this mechanism through endosymbiosis.

Large Biological Molecules & Evolution

• Biomolecule Structural Functionality:

  • Carbohydrates: Monosaccharide polymers linked by glycosidic bonds. Structural isomers determine digestibility (e.g., starch $\alpha$-linkages digested by amylase vs. cellulose $\beta$-linkages digested by cellulase).

  • Lipids: Non-polymer hydrophobic molecules.

  • Saturated fats: Contain no double bonds, pack closely, and are solid at room temperature.

  • Unsaturated fats: Contain cis double bonds that introduce kinks, preventing close packing, remaining liquid at room temperature.

  • Phospholipids: Amphipathic molecules (hydrophilic head, hydrophobic tails) that self-assemble into membrane bilayers.

  • Nucleic Acids: Nucleotide polymers (DNA/RNA) storing and expressing genetic information via complementary base pairing ($\text{A--T}$, $\text{C--G}$).

  • Proteins: Amino acid polymers defined by primary ($1^ ext{o}$), secondary ($2^ ext{o}$), tertiary ($3^ ext{o}$), and quaternary ($4^ ext{o}$) structures.

• Molecular Phylogeny Example: Alignment of $\beta$-globin amino acid sequences demonstrates evolutionary relationships: human $\beta$-globin differs by 2 amino acids from gibbons and 8 amino acids from rhesus monkeys, supporting a closer relationship between humans and gibbons.