Macromolecules, Cell Structure, Bioenergetics, and Enzymes: Study Notes for Cell Biology

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Macromolecules of the Cell

Amino Acids and Proteins

Proteins are essential macromolecules composed of amino acids, which determine their structure and function. Understanding the properties and organization of amino acids and proteins is fundamental in cell biology.

Amino Acids: The building blocks of proteins. There are 20 standard amino acids, each with a unique three-letter abbreviation (e.g., Ala for Alanine, Gly for Glycine).
Classification: Amino acids are classified as acidic, basic, polar, or nonpolar based on the properties of their side chains (R groups).
Peptide Bond: A covalent bond formed between the carboxyl group of one amino acid and the amino group of another, releasing water (a condensation reaction).
Polypeptide vs. Protein: A polypeptide is a linear chain of amino acids; a protein is a functional molecule that may consist of one or more polypeptides folded into a specific conformation.
Conformation: The three-dimensional shape of a protein. The native conformation is the most stable, functional form.
Disulfide Bond: A covalent bond between two cysteine residues, stabilizing protein structure.
Other Bonds: Hydrogen bonds, ionic bonds, van der Waals interactions, and hydrophobic interactions also stabilize protein structure.
R Groups: The side chains (R groups) of amino acids determine the chemical properties and interactions of proteins.

Levels of Protein Structure:

Primary: Linear sequence of amino acids.
Secondary: Local folding into structures like alpha helices and beta sheets (stabilized by hydrogen bonds).
Tertiary: Overall 3D shape of a single polypeptide.
Quaternary: Association of multiple polypeptide chains.

Chaperones: Proteins that assist in the proper folding of other proteins.

Motifs: Short, recurring structural elements (e.g., helix-turn-helix).

Fibrous vs. Globular Proteins: Fibrous proteins are elongated and structural (e.g., collagen); globular proteins are compact and functional (e.g., enzymes).

Protein Domains: Distinct functional and/or structural units within a protein.

Nucleic Acids: DNA and RNA

Nucleic acids store and transmit genetic information. DNA and RNA differ in structure and function.

DNA: Double-stranded, contains deoxyribose, bases: adenine (A), guanine (G), cytosine (C), thymine (T).
RNA: Single-stranded, contains ribose, bases: adenine (A), guanine (G), cytosine (C), uracil (U).
Pyrimidines: Cytosine, thymine, uracil.
Purines: Adenine, guanine.
Base Pairing: A-T (DNA), A-U (RNA), G-C.
Nucleoside: Base + sugar.
Nucleotide: Base + sugar + phosphate.
Phosphodiester Bond: Links nucleotides in a strand.
Antiparallel: The two DNA strands run in opposite directions.
Complementary: Base pairing rules ensure that each strand can serve as a template for the other.

Carbohydrates

Carbohydrates serve as energy sources and structural components in cells.

Monosaccharides: Simple sugars (e.g., glucose). Classified by the number of carbons and the presence of an aldehyde (aldose) or ketone (ketose) group.
Ring Structure: In aqueous solutions, glucose forms a ring. The ring can be in alpha or beta form, differing at the anomeric carbon.
Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., maltose, lactose, sucrose).
Polysaccharides: Long chains of monosaccharides. Storage polysaccharides include glycogen (animals) and starch (plants). Structural polysaccharides include cellulose (plants) and chitin (fungi, arthropods).
Roles: Energy storage and structural support.

Lipids

Lipids are hydrophobic macromolecules with diverse functions, but they are not polymers.

Definition: Macromolecules defined by their insolubility in water.
Functions: Energy storage, membrane structure, signaling.
Main Classes: Fatty acids, triacylglycerols, phospholipids, glycolipids, steroids, terpenes.
Amphipathic: Molecules with both hydrophobic and hydrophilic regions (e.g., phospholipids).
Saturated vs. Unsaturated Fatty Acids: Saturated have no double bonds; unsaturated have one or more double bonds. Unsaturated fatty acids increase membrane fluidity.
Trans Fats: Unsaturated fats with trans double bonds, associated with health risks.
Phospholipids: Major membrane components. Two main classes: phosphoglycerides and sphingolipids.
Cholesterol: Modulates membrane fluidity and serves as a precursor for steroid hormones.
Terpenes: Lipids derived from isoprene units (e.g., carotenoids, vitamins).

Cells and Organelles

Origin of Life and Cell Properties

The study of cell structure and function includes understanding the origin of life and the diversity of cellular forms.

Stanley Miller's Experiment (1953): Demonstrated that organic molecules could form under prebiotic conditions, supporting the idea of abiotic synthesis of life's building blocks.
Models for Organic Compound Formation: Abiotic synthesis in the atmosphere vs. hydrothermal vent synthesis.
RNA World Hypothesis: Suggests that RNA was the first genetic material and catalyst in early life forms.
Liposomes: Artificially formed vesicles that model cell membranes.
Basic Properties of Cells: Genetic program, metabolism, reproduction, response to stimuli, self-regulation, evolution.

Cellular Diversity and Organization

Three Domains of Life: Bacteria, Archaea, Eukarya. Differ in cell structure, genetics, and biochemistry.
Cell Size: Limited by surface area-to-volume ratio and diffusion rates.
Compartmentalization: Allows specialization of cellular functions.
DNA Storage and Transmission: DNA is stored in the nucleus (eukaryotes) or nucleoid (prokaryotes), replicated and transmitted during cell division.

Eukaryotic Cell Structure

Organelles: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, chloroplasts (plants), vacuoles, ribosomes, cytoskeleton.
Endosymbiont Theory: Mitochondria and chloroplasts originated from symbiotic bacteria.
Outside-In vs. Inside-Out Theories: Competing models for the origin of eukaryotic cells.
Third Membrane Problem: Refers to the challenge of explaining the origin of the double-membrane structure of mitochondria and chloroplasts.
Endomembrane System: Includes nuclear envelope, ER, Golgi, lysosomes, vesicles, and plasma membrane.
Secretory Vesicles: Transport proteins and lipids to the cell surface for secretion.
Lysosomes vs. Peroxisomes: Lysosomes digest macromolecules; peroxisomes break down fatty acids and detoxify. Plants have peroxisomes but not lysosomes.
Ribosomes: Complexes of rRNA and protein; sites of protein synthesis. Prokaryotic and eukaryotic ribosomes differ in size and composition.
Cytoskeleton: Microtubules, microfilaments, intermediate filaments. Not all components are present in all domains.
Extracellular Matrix (ECM): Network of proteins and polysaccharides outside animal cells; not found in all domains.
Cell Walls: Composed of cellulose (plants), chitin (fungi), peptidoglycan (bacteria); structure varies by domain.

Viruses, Prions, and Viroids

Viruses: Infectious particles with genetic material (DNA or RNA) in a protein coat; not considered alive.
Prions: Infectious proteins causing neurodegenerative diseases.
Viroids: Infectious RNA molecules affecting plants.

Bioenergetics: The Flow of Energy in the Cell

Cellular Work and Energy

Cells require energy to perform various types of work and maintain order.

Six Types of Work: Synthetic, mechanical, concentration, electrical, heat, and bioluminescent work.
Phototrophs: Obtain energy from light.
Chemotrophs: Obtain energy from chemical compounds. Subtypes include chemoautotrophs and chemoheterotrophs.
Redox Reactions: Involve transfer of electrons. Oxidation is loss of electrons; reduction is gain of electrons. Reducing agent donates electrons; oxidizing agent accepts electrons.
Matter Flow: Matter cycles through the biosphere via biogeochemical cycles.
Open vs. Closed Systems: Biological systems are open, exchanging energy and matter with their surroundings.
Energy Units: Joule (J), calorie (cal), kilocalorie (kcal). 1 cal = 4.184 J.

Thermodynamics in Biology

First Law: Energy cannot be created or destroyed, only transformed.
Second Law: Entropy (disorder) of the universe increases in spontaneous processes.
Enthalpy (H): Total heat content.
Entropy (S): Measure of disorder.
Thermodynamic Spontaneity: A process is spontaneous if it increases the entropy of the universe.
Free Energy (G): Energy available to do work.
Endergonic vs. Exergonic: Endergonic reactions require energy (); exergonic reactions release energy ().
Equilibrium Constant (Keq): Ratio of product to reactant concentrations at equilibrium. Indicates reaction directionality.
Concentration Ratio: If greater than Keq, reaction favors reactants; if less, favors products.
Standard Conditions: 1 M concentration, 1 atm pressure, 25°C (298 K), pH 7.0.
Relationship:
Steady State: Non-equilibrium state maintained by constant input/output of materials; essential for life.

Enzymes: The Catalysts of Life

Enzyme Function and Kinetics

Enzymes are biological catalysts that accelerate chemical reactions by lowering activation energy.

Activation Energy (Ea): Minimum energy required to initiate a reaction.
Metastable State: Reactants in a stable state before activation energy is overcome.
Transition State: High-energy intermediate during a reaction.
Why Not Increase Temperature? Raising temperature can denature proteins and is not compatible with life.
Catalyst Properties: Increase reaction rate, are not consumed, do not alter equilibrium.
Active Site: Region on enzyme where substrate binds and reaction occurs. Specificity is determined by amino acid residues.
Enzyme Classes: Oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.
Enzyme Sensitivity: Activity is affected by temperature and pH.
Induced Fit Model: Substrate binding induces conformational change in enzyme.
Substrate Activation Mechanisms: Bond distortion, proton transfer, electron transfer.
Ribozymes: RNA molecules with catalytic activity (e.g., ribosomal RNA).

Enzyme Kinetics

Substrate Concentration vs. Velocity: Reaction velocity increases with substrate concentration until saturation.
Vmax: Maximum reaction velocity at saturating substrate concentration.
Km (Michaelis Constant): Substrate concentration at which velocity is half of Vmax; indicates enzyme affinity for substrate.
Turnover Number (Kcat): Number of substrate molecules converted per enzyme per second.
Michaelis-Menten Graph: Plots velocity vs. substrate concentration; hyperbolic curve.
Lineweaver-Burk Plot: Double reciprocal plot (1/v vs. 1/[S]); linearizes Michaelis-Menten equation.
Plot Interpretation: Slope = Km/Vmax; y-intercept = 1/Vmax; x-intercept = -1/Km.

Enzyme Inhibition and Regulation

Inhibition Types: Reversible (competitive, noncompetitive, uncompetitive, mixed) and irreversible (covalent modification).
Competitive Inhibition: Inhibitor competes with substrate for active site.
Noncompetitive Inhibition: Inhibitor binds elsewhere, altering enzyme activity.
Uncompetitive Inhibition: Inhibitor binds only to enzyme-substrate complex.
Mixed Inhibition: Inhibitor can bind to enzyme with or without substrate, affecting both Km and Vmax.
Allosteric Regulation: Enzyme activity modulated by binding of effectors at sites other than the active site.
Covalent Modification: Enzyme activity regulated by addition/removal of chemical groups (e.g., phosphorylation).
Cooperativity: Substrate binding at one site affects binding at other sites (common in multimeric enzymes).
Feedback Inhibition: End product of a pathway inhibits an earlier step.
Zymogen: Inactive enzyme precursor activated by cleavage.

Enzyme Inhibition Type	Binding Site	Effect on Km	Effect on Vmax	Reversibility
Competitive	Active site	Increases	No change	Reversible
Noncompetitive	Allosteric site	No change	Decreases	Reversible
Uncompetitive	Enzyme-substrate complex	Decreases	Decreases	Reversible
Irreversible	Active or allosteric site (covalent)	Varies	Decreases	Irreversible

Example: Penicillin is an irreversible inhibitor of bacterial transpeptidase.

Additional info: Some details, such as the exact six types of work or the full list of lipid classes, were inferred from standard cell biology content.