Biomolecules, Cell Structure, and Membrane Dynamics Vocabulary

The Chemical Basis of Life: Atoms, Molecules, and Water

• Subatomic Particles and Atomic Structure:

  • Protons ($p^+$): Positively charged subatomic particles located within the atomic nucleus. The number of protons defines an element's atomic number.

  • Neutrons ($n^0$): Electrically neutral subatomic particles located within the atomic nucleus. Together with protons, they determine the atomic mass.

  • Electrons ($e^-$): Negatively charged subatomic particles that orbit the nucleus in specific energy shells and orbitals.

  • Atomic Number: Equal to the number of protons in an atom's nucleus ($\text{Atomic Number} = p^+$).

  • Atomic Mass: Equal to the sum of protons and neutrons in an atom's nucleus ($\text{Atomic Mass} = p^+ + n^0$).

  • Neutral Atom Balance: In a neutral, uncharged atom, the number of negatively charged electrons strictly equals the number of positively charged protons ($e^- = p^+$).

  • Isotopes: Atoms of the same element that possess identical numbers of protons but differing numbers of neutrons, resulting in different atomic masses. For example, carbon-12 ($^{12}\text{C}$) possesses $6$ protons and $6$ neutrons, whereas radioactive carbon-14 ($^{14}\text{C}$) possesses $6$ protons and $8$ neutrons.

  • Electron Orbitals and Shells: Electrons occupy three-dimensional region spaces around the nucleus known as orbitals. Each orbital can hold a maximum of $2$ electrons. Orbitals are organized into concentric electron shells of increasing energy levels. The innermost shell holds a maximum of $2$ electrons ($1$ orbital), while outer shells generally hold up to $8$ electrons ($4$ orbitals).

  • Valence Shell and Valence Electrons: The outermost electron shell of an atom is its valence shell, and the electrons inhabiting this shell are valence electrons. Unfilled valence shells create chemical reactivity, driving atoms to gain, lose, or share electrons to achieve stability.

• Chemical Bonds:

  • Unfilled Valence Driving Force: Atoms strive to achieve full valence shells (octet rule for most biological elements). Unfilled shells drive chemical bond formation.

  • Nonpolar Covalent Bonds: Strong chemical bonds formed when two atoms share valence electrons equally due to identical or nearly equal electronegativity.

  • Polar Covalent Bonds: Chemical bonds formed when two atoms with significantly different electronegativities share electrons unequally, creating partial positive (\raisebox{1pt}{\text{ \tiny \textdelta+}}) and partial negative (\raisebox{1pt}{\text{ \tiny \textdelta-}}) charges across the molecule.

  • Ionic Bonds: Chemical interactions formed when an atom with high electronegativity completely strips one or more valence electrons from an atom with low electronegativity, generating ions of opposite charges that attract each other.

  • Cations and Anions:

  • Cation: A positively charged ion formed when an atom loses one or more electrons.

  • Anion: A negatively charged ion formed when an atom gains one or more electrons.

  • Bonding Capacity and Bond Types: The number of unpaired valence electrons determines how many covalent bonds an atom can form:

  • Single Bond: Sharing of $1$ pair of electrons ($2$ total electrons).

  • Double Bond: Sharing of $2$ pairs of electrons ($4$ total electrons).

  • Triple Bond: Sharing of $3$ pairs of electrons ($6$ total electrons).

  • Molecular Formulas: Symbolic representations showing the exact atomic composition and ratio of elements in a molecule (e.g., $\text{H}_2\text{O}$, $\text{CO}_2$, $\text{C}_6\text{H}_{12}\text{O}_6$).

• Organic Functional Groups:

  • Amino Group ($-\text{NH}_2$): Acts as a base; tends to attract a proton ($\text{H}^+$) to form $-\text{NH}_3^+$ in aqueous solution.

  • Carbonyl Group ($-\text{C}=\text{O}$): Polar group found in aldehydes (at terminal carbon) and ketones (within internal carbon chains).

  • Carboxyl Group ($-\text{COOH}$): Acts as an acid; tends to donate a proton ($\text{H}^+$) to form $-\text{COO}^-$ in aqueous solution.

  • Hydroxyl Group ($-\text{OH}$): Highly polar group that renders molecules soluble in water.

  • Phosphate Group ($-\text{PO}_4^{2-}$): Carries negative charges; stores and releases chemical energy and alters protein conformation when transferred.

  • Sulfhydryl Group ($-\text{SH}$): Forms covalent disulfide bridges ($-\text{S}-\text{S}-$) that stabilize protein tertiary and quaternary structures.

• Water Properties and Solutions:

  • Efficient Biological Solvent: Water provides an ideal fluid medium for cellular chemistry due to its bent shape and extreme polarity.

  • Water Polarity: Oxygen is significantly more electronegative than hydrogen, pulling shared electrons toward itself. This generates a partial negative charge (\raisebox{1pt}{\text{ \tiny \textdelta-}}) near oxygen and partial positive charges (\raisebox{1pt}{\text{ \tiny \textdelta+}}) near hydrogens.

  • Hydrogen Bonding: The electrical attraction between the partial positive charge (\raisebox{1pt}{\text{ \tiny \textdelta+}}) of a hydrogen atom in one water molecule and the partial negative charge (\raisebox{1pt}{\text{ \tiny \textdelta-}}) of an oxygen atom in an adjacent water molecule.

  • Hydrophilic vs. Hydrophobic:

  • Hydrophilic: Ions and polar substances that interact favorably with water molecules through electrical attractions and dissolve readily.

  • Hydrophobic: Uncharged, nonpolar molecules that do not interact with water and coalesce together in aqueous environments.

  • Density of Ice vs. Liquid Water: Water expands upon freezing because hydrogen bonds lock molecules into a rigid, open crystal lattice. Ice is less dense than liquid water and floats, insulating aquatic life underneath.

  • High Specific Heat: Water requires a large amount of heat energy absorbed or lost to alter $1\text{ g}$ of water by $1\text{ }^\text{o}\text{C}$, allowing organisms and environments to buffer temperature fluctuations.

  • High Heat of Vaporization: Water requires significant energy to convert from liquid to gas state, enabling evaporative cooling mechanisms.

  • Cohesion and Surface Tension: Cohesion is the attraction between like water molecules via hydrogen bonding, creating high surface tension.

  • Adhesion: The attraction between water molecules and other polar or charged surfaces, facilitating capillary action.

  • Acid-Base Chemistry and pH:

  • Water self-ionizes weakly into hydrogen ions ($\text{H}^+$) and hydroxide ions ($\text{OH}^-$).

  • Acid: A substance that increases the concentration of $\text{H}^+$ in solution.

  • Base: A substance that decreases the concentration of $\text{H}^+$ in solution.

  • pH Scale: Defined as $\text{pH} = -\text{log}_{10}([\text{H}^+])$. A low pH ($< 7$) indicates an acidic solution with elevated $[\text{H}^+]$; a high pH ($> 7$) indicates a basic/alkaline solution with low $[\text{H}^+]$. Neutral pH is $7.0$.

• Levels of Biological Organization:

  • Hierarchical Sequence: Atoms $\rightarrow$ Molecules $\rightarrow$ Cells $\rightarrow$ Tissues $\rightarrow$ Organs $\rightarrow$ Organ Systems $\rightarrow$ Organisms $\rightarrow$ Populations $\rightarrow$ Communities $\rightarrow$ Ecosystems $\rightarrow$ Biosphere.

  • Cell as Fundamental Unit: The cell is the smallest structural and functional unit capable of carrying out all processes of life independently.

  • Non-Living Status of Viruses: Viruses lack independent metabolic machinery, cannot perform cellular respiration, and cannot reproduce without hijacking a host organism's cellular system; hence, they are generally not classified as living organisms.

  • Emergent Properties: Novel characteristics that arise at higher levels of organization that were not present in the individual component parts at lower levels (e.g., consciousness emerging from networks of neurons, or cardiac pumping action emerging from heart muscle cells).

  • Multi-Level Study Approach: Biologists analyze biological processes across multiple levels because lower levels reveal physical and chemical mechanisms, whereas higher levels provide ecological context, functional interactions, and physiological consequences.

Biomolecules

• Carbon and Covalent Bonding Rules:

  • Valence Electrons: Carbon possesses $4$ valence electrons in its outer shell.

  • Bonding Capacity: Carbon forms up to $4$ covalent bonds to achieve a stable octet.

  • Bond Types: Carbon can form single bonds, double bonds, and triple bonds, as well as straight chains, branched structures, and closed ring skeletons.

  • Frequent Bonding Partners: In biological molecules, carbon most frequently bonds with Hydrogen, Oxygen, Nitrogen, and other Carbon atoms.

  • Free Valence Bonding Rules:

  • Hydrogen ($\text{H}$): $1$ valence electron $\rightarrow$ forms strictly $1$ covalent bond.

  • Oxygen ($\text{O}$): $6$ valence electrons ($2$ unpaired) $\rightarrow$ forms strictly $2$ covalent bonds.

  • Nitrogen ($\text{N}$): $5$ valence electrons ($3$ unpaired) $\rightarrow$ forms strictly $3$ covalent bonds.

  • Carbon ($\text{C}$): $4$ valence electrons ($4$ unpaired) $\rightarrow$ forms strictly $4$ covalent bonds.

• Hydrocarbons:

  • Definition: Organic molecules consisting exclusively of carbon and hydrogen atoms.

  • Structural Variations: Hydrocarbons vary in total chain length, presence and location of double or triple bonds, presence of branching, and ring formation.

  • Energy Content: $\text{C}-\text{H}$ and $\text{C}-\text{C}$ nonpolar covalent bonds contain high potential energy due to equal electron sharing. Sugars and fats contain abundant $\text{C}-\text{H}$ and $\text{C}-\text{C}$ bonds, giving them high energy storage capacity per gram.

• Monomers and Polymers:

  • Monomer: A small, repeating molecular subunit that serves as the building block for larger molecules.

  • Polymer: A long molecule consisting of many similar or identical monomers linked together by covalent bonds.

  • Dehydration Reaction (Condensation): A chemical process where two monomers are covalently bonded together with the simultaneous loss of a water molecule ($\text{H}_2\text{O}$). One monomer contributes a hydroxyl group ($-\text{OH}$) and the other contributes a hydrogen ($-\text{H}$).

  • Hydrolysis Reaction: A chemical process that breaks covalent bonds between monomers within a polymer through the addition of a water molecule ($\text{H}_2\text{O}$). Hydrolysis is catalyzed by specialized enzymes to speed up reaction rates.

• Carbohydrates:

  • Classification Types: Monosaccharides (single sugars), Disaccharides (two sugars bonded), Oligosaccharides (short chains), and Polysaccharides (long chains).

  • Common biological sugar: Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) is the primary energy monomer in cellular biology.

  • Suffix Convention: Sugars routinely end with the suffix -ose (e.g., glucose, fructose, galactose, ribose, sucrose, maltose, lactose).

  • Monosaccharide Structure and Ring Formation: In aqueous solutions, five-carbon (pentoses) and six-carbon (hexoses) sugars spontaneously fold from open linear chains into ring structures through a reaction between an internal hydroxyl group and the carbonyl carbon.

  • Disaccharide Formation: Two monosaccharides joined by a dehydration reaction form a glycosidic linkage, an ether-like covalent bond ($\text{R}-\text{O}-\text{R}$).

  • \text{\textalpha}-Glucose vs. \text{\textbeta}-Glucose Orientations:

  • \text{\textalpha}-Glucose: The hydroxyl group ($-\text{OH}$) attached to carbon-1 points down relative to the plane of the ring.

  • \text{\textbeta}-Glucose: The hydroxyl group ($-\text{OH}$) attached to carbon-1 points up relative to the plane of the ring.

  • Significance: Linkage geometry alters molecular shape and digestibility. Enzymes that cleave \text{\textalpha}-linkages cannot hydrolyze \text{\textbeta}-linkages.

  • Polysaccharide Profiles:

  • Starch: Plant storage polysaccharide composed entirely of \text{\textalpha}-glucose monomers. Formed in plant plastids/chloroplasts. Occurs as unbranched helical amylose or branched amylopectin.

  • Glycogen: Animal storage polysaccharide composed of \text{\textalpha}-glucose monomers. Produced and stored primarily in liver and muscle cells. Highly branched helical structure.

  • Cellulose: Structural polysaccharide found in plant cell walls, composed of linear, unbranched chains of \text{\textbeta}-glucose monomers. Hydrogen bonds between parallel chains bundle them into rigid microfibrils and fibers.

  • Chitin: Structural polysaccharide found in fungal cell walls and arthropod exoskeletons, consisting of \text{\textbeta}-glucose monomers with a nitrogen-containing acetylglucosamine side chain.

  • Enzyme Specificity for Linkages: Digestive enzymes like amylase readily break down \text{\textalpha}-glucose linkages found in amylose and glycogen. Breakdown of \text{\textbeta}-glucose linkages in cellulose requires the enzyme cellulase, which humans and most animals lack.

• Lipids:

  • Cellular Locations: Plasma membrane, nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomal membranes, peroxisomal membranes, mitochondrial membranes, chloroplast membranes, transport vesicles, and cytosolic lipid droplets.

  • Biological Functions: Long-term energy storage, membrane structural components, signaling molecules (hormones), thermal insulation, physical cushioning, and protective water-repellent coatings.

  • General Reactivity: Lipids contain extensive nonpolar hydrocarbon chains, making them insoluble in water (hydrophobic) and soluble in organic nonpolar solvents.

  • Fatty Acid Structure: Composed of a terminal carboxyl group ($-\text{COOH}$) attached to a long hydrocarbon chain.

  • Saturated Fatty Acid: Contains only single covalent bonds between carbon atoms ($\text{C}-\text{C}$), maximizing attached hydrogen atoms. Hydrocarbon tails are straight, enabling tight molecular packing; solid at room temperature (e.g., butter, lard).

  • Unsaturated Fatty Acid: Contains one or more double bonds between carbon atoms ($\text{C}=\text{C}$), introducing structural kinks. Kinks prevent close molecular packing; liquid at room temperature (e.g., vegetable oils).

  • Triglycerides (Triacylglycerols): Composed of $1$ glycerol molecule bonded to $3$ fatty acid chains via ester linkages formed through dehydration reactions.

  • Steroids:

  • Carbon Backbone: Lipid structure characterized by a carbon skeleton consisting of $4$ fused rings ($3$ six-membered rings and $1$ five-membered ring).

  • Diversity: Vary by the chemical functional groups or side chains attached to the four-ring core.

  • Examples: Cholesterol (plasma membrane fluidity regulator and precursor for steroid hormones), Estrogen, Testosterone, and Cortisol.

  • Membrane Role: Cholesterol wedges between phospholipid tails in animal cell membranes, acting as a temperature buffer that prevents excessive fluidity at high temperatures and prevents membrane freezing at low temperatures.

  • Phospholipids:

  • Structure: Composed of $1$ glycerol molecule linked to $2$ hydrophobic fatty acid tails and $1$ hydrophilic phosphate group bound to a polar head group.

  • Amphipathic Nature: Possesses both a hydrophilic (polar) head region and a hydrophobic (nonpolar) tail region.

  • Self-Assembly: In aqueous solutions, phospholipids spontaneously form spherical micelles or continuous lipid bilayers, sequestering hydrophobic tails away from water while exposing hydrophilic heads to the environment.

  • Selective Permeability of Lipid Bilayers:

  • Small, Nonpolar Molecules ($\text{O}_2$, $\text{CO}_2$, $\text{N}_2$): Cross the hydrophobic core rapidly via simple diffusion.

  • Small, Uncharged Polar Molecules ($\text{H}_2\text{O}$, glycerol): Pass across slowly.

  • Large, Uncharged Polar Molecules (Glucose, Sucrose): Blocked almost completely without specialized transport proteins.

  • Ions ($\text{Na}^+$, $\text{K}^+$, $\text{Ca}^{2+}$, $\text{Cl}^-$): Unable to penetrate the nonpolar core due to their charge and hydration shells.

  • Unsaturated Tail Effect: Membranes rich in unsaturated fatty acid tails with kinks resist tight packing, maintaining higher membrane fluidity and permeability than membranes dominated by saturated tails.

  • Membrane Proteins Classification:

  • Integral (Transmembrane) Proteins: Penetrate into or span completely across the hydrophobic interior of the lipid bilayer.

  • Peripheral Proteins: Loosely bound to either the exterior or interior surface of the membrane, often anchored to integral proteins or membrane lipids.

• Nucleic Acids:

  • Definition and Monomers: Macromolecules that store, transmit, and express hereditary information. Monomers are nucleotides.

  • Nucleotide Structure: Composed of three distinct components:

  1. A $5$-carbon pentose sugar (Ribose in RNA; Deoxyribose in DNA).

  2. A nitrogenous base.

  3. A phosphate group attached to the $5'$ carbon of the sugar.

  • Nitrogenous Base Categories:

  • Purines: Two-ringed nitrogenous bases $\rightarrow$ Adenine (A) and Guanine (G).

  • Pyrimidines: Single-ringed nitrogenous bases $\rightarrow$ Cytosine (C), Thymine (T, DNA only), and Uracil (U, RNA only).

  • Polymerization and Phosphodiester Linkages: Nucleotides are joined into nucleic acid chains via phosphodiester linkages formed between the $3'$ hydroxyl group ($-\text{OH}$) of one sugar and the phosphate group attached to the $5'$ carbon of the next sugar. Synthesis proceeds exclusively in a $5' \rightarrow 3'$ direction because enzymes add new nucleotides only to the free $3'-\text{OH}$ end.

  • DNA vs. RNA Structural Comparison:

  • Pentose Sugar: RNA contains Ribose (possesses a hydroxyl group $-\text{OH}$ on carbon-2'); DNA contains Deoxyribose (possesses a hydrogen atom $-\text{H}$ on carbon-2').

  • Nitrogenous Bases: RNA uses A, U, C, G; DNA uses A, T, C, G.

  • Strand Configuration: RNA is typically single-stranded and folds into dynamic secondary shapes; DNA exists as a double-stranded antiparallel double helix.

  • Structural Elucidation of DNA:

  • Rosalind Franklin and Maurice Wilkins: Used X-ray diffraction crystallography to discover the helical geometry and repeating spatial dimensions of the DNA molecule.

  • James Watson and Francis Crick: Derived the structural double-helix model of DNA featuring antiparallel sugar-phosphate backbones and specific complementary base pairing.

  • Complementary Base Pairing Rules and Hydrogen Bonding:

  • Adenine (A) pairs with Thymine (T) via $2$ hydrogen bonds.

  • Guanine (G) pairs with Cytosine (C) via $3$ hydrogen bonds.

  • Antiparallel Alignment: The two complementary strands run in opposite directional orientations relative to their sugar-phosphate backbones (one strand runs $5' \rightarrow 3'$, the opposing strand runs $3' \rightarrow 5$').

• Proteins:

  • Core Structure of Amino Acids: Every amino acid possesses a central alpha-carbon (\text{C}_\text{\textalpha}) covalently bonded to four functional groups:

  1. An Amino Group ($-\text{NH}_2$).

  2. A Carboxyl Group ($-\text{COOH}$).

  3. A Hydrogen Atom ($-\text{H}$).

  4. A Variable Side Chain / R-Group ($-\text{R}$).

  • Amino Acid Diversity: Exactly $20$ standard amino acids exist in biological systems, categorized by R-group properties:

  1. Nonpolar / Hydrophobic R-groups: Consist of hydrocarbons or nonpolar rings; avoid water.

  2. Polar / Uncharged Hydrophilic R-groups: Contain polar functional groups ($-\text{OH}$, $-\text{SH}$, $-\text{CONH}_2$); form hydrogen bonds with water.

  3. Acidic / Negatively Charged Hydrophilic R-groups: Contain carboxyl functional groups that lose a proton ($-\text{COO}^-$).

  4. Basic / Positively Charged Hydrophilic R-groups: Contain nitrogenous functional groups that accept a proton ($-\text{NH}^+$, $-\text{NH}_2^+$).

  • Peptide Bond Formation: A covalent peptide bond forms via a dehydration reaction between the carboxyl carbon of one amino acid and the amino nitrogen of another.

  • Polypeptide Structural Directionality:

  • N-terminus (Amino-terminus): The starting end of a polypeptide featuring a free, unbonded amino group.

  • C-terminus (Carboxyl-terminus): The terminal end of a polypeptide featuring a free, unbonded carboxyl group.

  • Four Levels of Protein Structure:

  1. Primary Structure: The precise linear sequence of amino acids in a polypeptide chain joined by peptide bonds. Dictated by gene sequence.

  2. Secondary Structure: Local spatial arrangement of the polypeptide backbone stabilized by hydrogen bonds between backbone carbonyl oxygens and amino hydrogens. Common motifs include the \text{\textalpha}-helix (coiled) and \text{\textbeta}-pleated sheet (folded sheet).

  3. Tertiary Structure: The overall three-dimensional folded shape of a single polypeptide chain, established by interactions between R-groups (hydrophobic interactions, van der Waals forces, ionic bonds, hydrogen bonds, and covalent disulfide bridges between cysteine residues).

  4. Quaternary Structure: Structural arrangement formed when two or more distinct polypeptide subunits assemble into a functional protein complex (e.g., tetrameric Hemoglobin or fibrous Collagen).

  • Impact of Single Amino Acid Substitution: Replacing a single amino acid (e.g., substituting hydrophobic valine for hydrophilic glutamic acid in hemoglobin) can disrupt folding, alter tertiary structure, cause protein aggregation, and eliminate normal biological function (as seen in sickle-cell disease).

  • Protein Folding, Chaperones, and Denaturation:

  • Chaperone Proteins (Chaperonins): Specialized multi-protein complexes that shield newly synthesized polypeptides from improper interactions during folding.

  • Denaturation: Unfolding and destruction of a protein's secondary, tertiary, and quaternary structures caused by environmental stressors such as elevated temperatures, extreme pH shifts, or high salt concentrations. Denaturation leads to complete loss of biological function.

  • Misfolding Consequences: Aggregation of misfolded proteins causes destructive cellular plaques and neurodegenerative conditions, including Prion diseases (Creutzfeldt-Jakob disease), Alzheimer's disease, and Parkinson's disease.

Eukaryotic Cell Structure and Function

• Eukaryotic Features and Compartmentalization:

  • Core Characteristic: Defined by the presence of a double-membrane-bound nucleus containing cellular DNA and membrane-bound internal organelles.

  • Functional Purpose of Compartmentalization: Divides the cytoplasm into isolated functional chambers, allowing incompatible chemical reactions to occur simultaneously, concentrating enzymes and reactants, and enhancing metabolic efficiency.

• Eukaryotic Organelles and Structures:

  • Plasma Membrane: Phospholipid bilayer containing embedded integral/peripheral proteins and cholesterol; regulates substance transport into and out of the cell.

  • Nucleus: Double-membrane enclosed organelle featuring nuclear pores that regulate macromolecular entry/exit. Contains chromatin (DNA bound to histone proteins) and the nucleolus (site of ribosomal RNA synthesis and ribosome subunit assembly).

  • Ribosomes: Non-membrane-bound complexes of ribosomal RNA (rRNA) and proteins; serve as the site of translation (protein synthesis). Found free in the cytosol or bound to the Rough Endoplasmic Reticulum.

  • Rough Endoplasmic Reticulum (RER): Network of folded membrane sacs (cisternae) continuous with the nuclear envelope, studded with ribosomes. Synthesizes, folds, and processes proteins destined for membranes, organelle delivery, or secretion.

  • Smooth Endoplasmic Reticulum (SER): Network of tubular membranes lacking ribosomes. Synthesizes lipids (phospholipids, steroids), metabolizes carbohydrates, detoxifies drugs and poisons, and stores intracellular calcium ions ($\text{Ca}^{2+}$).

  • Golgi Apparatus: Stack of flattened, non-continuous membranous sacs (cisternae) possessing directionality ($cis$ face receives vesicles from RER; $trans$ face ships vesicles out). Modifies, sorts, packages, and tags proteins and lipids for transport.

  • Lysosomes: Acidic, membrane-bound sacs containing hydrolytic enzymes. Break down macromolecules, damaged organelles (autophagy), and ingested foreign material (phagocytosis).

  • Vacuoles: Large membrane-bound sacs. Central vacuoles in plant cells store water, maintain turgor pressure, store ions, and hold enzymes.

  • Peroxisomes: Metabolic compartments that remove hydrogen atoms from organic substrates and transfer them to molecular oxygen ($\text{O}_2$), producing hydrogen peroxide ($\text{H}_2\text{O}_2$). Catalase inside converts $\text{H}_2\text{O}_2$ into harmless water ($\text{H}_2\text{O}$) and oxygen.

  • Mitochondria: Double-membrane organelles containing their own circular DNA and ribosomes. Inner membrane forms deep folds (cristae) enclosing the mitochondrial matrix; site of cellular respiration and ATP generation.

  • Chloroplasts: Double-membrane photosynthetic organelles found in plants and algae. Contain fluid stroma and membrane stacks (thylakoids grouped into grana) containing chlorophyll; site of photosynthesis.

  • Cytoskeleton: Dynamic network of protein fibers extending throughout the cytoplasm, consisting of Microfilaments (actin), Intermediate Filaments (keratin/lamins), and Microtubules (tubulin). Maintains cell shape, anchors organelles, enables cell motility, and acts as tracks for intracellular transport.

  • Cell Wall: Rigid extracellular layer surrounding the plasma membrane in plants (composed of cellulose), fungi (chitin), and algae; provides structural support, protection, and prevents osmotic bursting.

• The Endomembrane System and Secretory Pathway:

  • System Components: Nuclear envelope, Rough ER, Smooth ER, Golgi apparatus, Lysosomes, Vacuoles, Transport Vesicles, and Plasma Membrane.

  • Secretory Pathway Sequence:

  1. Protein Synthesis: Ribosomes synthesize proteins into the lumen of the Rough ER.

  2. ER Processing and Packaging: Proteins are folded and glycosylated in the RER, then packaged into transport vesicles.

  3. Vesicle Transport: Vesicles bud from the RER and travel along cytoskeletal tracks to fuse with the $cis$ face of the Golgi apparatus.

  4. Golgi Processing: Proteins move from $cis$ to $trans$ cisternae, undergoing sequential carbohydrate modifications and sorting.

  5. Final Shipping: Vesicles bud from the $trans$ face and travel to fuse with the plasma membrane.

  6. Exocytosis: Secretory contents are discharged into the extracellular space, while vesicle membrane components insert directly into the plasma membrane.

• Bulk Transport Processes:

  • Exocytosis: Internal transport vesicles fuse with the plasma membrane, releasing bulk contents into the extracellular environment.

  • Endocytosis: Plasma membrane invaginates inward to engulf external substances, pinching off to form an intracellular vesicle:

  • Phagocytosis ("Cell Eating"): Cell engulfs large particles or whole organisms into a phagosome, which fuses with a lysosome for degradation.

  • Pinocytosis ("Cell Drinking"): Cell non-specifically invaginates small droplets of fluid into small vesicles.

  • Receptor-Mediated Endocytosis: Specific target ligands bind to specialized transmembrane receptor proteins concentrated in coated pits (clathrin), triggering vesicle invagination.

Plasma Membrane Structure, Functions, and Transport Mechanisms

• Fluid Mosaic Model and Membrane Dynamics:

  • Structural Arrangement: Plasma membrane consists of a fluid amphipathic phospholipid bilayer with embedded, floating integral proteins and peripheral proteins.

  • Protein Mobility: Many membrane proteins diffuse laterally within the lipid plane; others are anchored in place via attachments to the internal cytoskeleton or external extracellular matrix (ECM).

• Six Major Functional Classes of Membrane Proteins:

  1. Transport Proteins:

  • Channel Proteins: Hydrophilic tunnels that facilitate fast, passive diffusion of specific ions or water (e.g., Aquaporins for water).

  • Carrier Proteins: Bind specific solute molecules and undergo conformational shape changes to move solutes across the membrane.

  1. Enzymatic Activity Proteins: Membrane-bound enzymes that catalyze sequential metabolic steps locally at the membrane surface.

  2. Cytoskeleton and ECM Attachment Proteins: Anchors that non-covalently bind internal cytoskeletal fibers or extracellular matrix components to maintain cell shape and fix protein locations.

  3. Cell-to-Cell Recognition Proteins: Glycoproteins carrying short carbohydrate chains that serve as specific molecular identification tags recognized by other cells.

  4. Intercellular Joining Proteins: Membrane proteins that hook together adjacent cells via junctions (e.g., tight junctions, desmosomes).

  5. Signal Transduction Receptors: Surface receptor proteins possessing binding sites for specific chemical ligands (e.g., G Protein-Coupled Receptors) that transmit extracellular signals into intracellular metabolic responses.

• Active Transport and Electrogenic Pumps:

  • Passive Transport: Net movement of substances down their concentration gradient without cellular energy expenditure ($\text{ATP}$).

  • Active Transport: Movement of solute molecules against their concentration gradient, requiring cellular energy expenditure ($\text{ATP}$ hydrolysis).

  • Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+$ ATPase):

  • Action Mechanism: Uses $1$ molecule of $\text{ATP}$ to actively export $3\text{ Na}^+$ ions out of the cell and import $2\text{ K}^+$ ions into the cell.

  • Net Charge Effect: Net loss of $1$ positive charge per cycle creates a negative charge inside the cell relative to the outside.

  • Membrane Potential and Electrogenic Pumps:

  • Voltage across a cell membrane generated by unequal charge distribution across the bilayer.

  • Electrogenic Pump: A primary active transport protein that generates voltage across a membrane while pumping ions (e.g., Sodium-Potassium Pump in animal cells; Proton Pump $\text{H}^+$ ATPase in plants, fungi, and bacteria).

  • Electrochemical Gradient: Combined force driven by concentration differences (chemical force) and voltage differences (electrical force) that directs ion movement.

  • Secondary Active Transport (Cotransport): Transport mechanism where an electrogenic pump actively moves one solute against its gradient to establish a steep concentration gradient. The downhill diffusion of this primary solute then drives the uphill transport of a second solute against its concentration gradient (e.g., Sucrose-$\text{H}^+$ Cotransporter).

Cell Communication and Signal Transduction

• Distance Scales of Chemical Signaling:

  • Local Signaling: Cell releases signal molecules that diffuse locally through extracellular fluid to act on neighboring cells (e.g., neurotransmitters diffusing across synaptic clefts between adjacent neurons).

  • Long-Distance Signaling: Endocrine cells secrete chemical signals (hormones) directly into bodily fluids (bloodstream) to travel throughout the organism to reach distant target cells.

• Three Stages of Signal Transduction:

  1. Reception: The target cell detects an incoming extracellular signal molecule (ligand) when it binds specifically to a receptor protein located at the cell surface or inside the cell.

  2. Transduction: Binding alters receptor conformation, initiating a multi-step signal cascade (domino effect) that converts the signal into a form capable of triggering a specific biological response.

  3. Response: The transduced signal triggers a specific cellular activity (e.g., enzyme activation, cytoskeletal rearrangement, or turning gene transcription on or off).

• Key Cell Surface Receptors:

  • G Protein-Coupled Receptors (GPCRs): Transmembrane receptors that work with the help of a heterotrimeric G protein:

  1. Extracellular ligand binds GPCR, changing its shape.

  2. Activated GPCR binds an inactive G protein, inducing exchange of bound $\text{GDP}$ for high-energy $\text{GTP}$.

  3. Activated G protein detaches and binds an intracellular enzyme, triggering a downstream signal response.

  • Ligand-Gated Ion Channels: Transmembrane ion channels that open or close a gate in response to the binding of a specific chemical ligand, permitting or blocking the passive flow of specific ions ($\text{Na}^+$, $\text{Ca}^{2+}$) across the membrane.

• Phosphorylation and Dephosphorylation Cascades:

  • Protein Kinases: Enzymes that catalyze the transfer of phosphate groups ($-\text{PO}_4^{3-}$) from $\text{ATP}$ onto target proteins (phosphorylation). Phosphorylation introduces negative charges, inducing shape changes that typically activate the target protein.

  • Protein Phosphatases: Enzymes that rapidly remove phosphate groups from target proteins via hydrolysis (dephosphorylation). Dephosphorylation inactivates proteins and resets signaling pathways.

  • Signal Amplification and Control: Sequential phosphorylation cascades amplify initial weak extracellular signals and provide molecular control points to turn signals off.

Cellular Junctions and Extracellular Structural Connections

• Specialized Intercellular Junctions:

  • Tight Junctions: Continuous belts of specialized proteins that press plasma membranes of neighboring animal cells tightly together, forming liquid-impermeable seals that prevent extracellular fluid from leaking across cell sheets (e.g., intestinal epithelium).

  • Desmosomes: Anchoring junctions that act like structural rivets, holding animal cells together in strong sheets. Constructed from cadherin proteins anchored to intermediate filaments (keratin) in the cytoplasm (e.g., muscle and skin tissues).

  • Gap Junctions: Communicating junctions in animal tissues composed of transmembrane protein complexes (connexons) surrounding a pore. Allow direct passage of water, small ions, amino acids, and chemical messengers between neighboring cytoplasm.

  • Plasmodesmata: Membrane-lined channels traversing rigid plant cell walls, creating continuous cytoplasmic connections (symplast) between neighboring plant cells for water, small solutes, proteins, and RNA molecules.

• Integrative Structural Function:

  • The physical integration between the internal cytoskeleton, plasma membrane proteins, intercellular junctions, and the extracellular matrix (ECM) creates mechanical stability, coordinates cell communication, maintains overall tissue integrity, and directs cell migration and gene expression.