General Biology: Scientific Inquiry, Cellular Principles, and Macromolecules

The Nature of Science and Scientific Inquiry

• Definition of Knowledge: Knowledge is the complete collection of facts and truths that must be known about an entity, phenomenon, or event.
• Definition of Science: Derived from the Latin verb scire / scientia (meaning "to know"), science is a systematic knowledge-production activity involving the observation of natural phenomena and their subsequent testing through empirical experimentation. It stems from humanity's intrinsic curiosity to understand oneself, the world, and the universe.
• Core Prerequisite of Science: Scientific claims must be supported by verifiable, reproducible empirical evidence. Every piece of scientific information is completely open to examination, verification, and critical review by the global scientific community.
• Core Attributes of a Scientist:
  • Seeks to explain phenomena occurring in the natural world.
  • Primarily motivated by benefiting humanity.
  • Maintains a curious and skeptical mindset.
  • Exercises rational, objective, and realistic reasoning without prejudice.
  • Thoroughly examines prior scientific literature and builds upon established findings.
  • Demonstrates perseverance, determination, and patience.
  • Embraces innovations, leverages imaginative thinking, and generates original ideas.
• The Scientific Process and Inquiry Modes: The scientific endeavor combines two complementary pathways of inquiry:
  • Discovery Science (Descriptive Science): Seeks natural causes for natural phenomena by investigating structures and processes that can be observed and measured either directly or indirectly. Observations must be repeatable and verifiable by independent observers. It primarily employs inductive reasoning (tümevarım), deriving broad generalizations from a large body of specific observations.
  • Hypothesis-Based Science: Triggered by observations made in discovery science, compelling curious minds to formulate testable questions and seek mechanistic explanations. It operates primarily through deductive reasoning (tümdengelim), utilizing "If… then…" logic to proceed from general premises to specific, testable predictions.
• Definition of a Hypothesis: A tentative, testable answer or scientific explanation proposed for an observed problem or question.
• Practical Application of the Scientific Method (The Flashlight Scenario):
  • Observation: The flashlight does not work.
  • Question: What is wrong with the flashlight?
  • Hypothesis: The batteries in the flashlight are dead.
  • Prediction: If the batteries are dead, then replacing them with new batteries will cause the flashlight to turn on.
  • Experiment/Test: Install fresh batteries into the flashlight.
  • Evaluation of Results:
  • If the flashlight illuminates, the hypothesis is supported.
  • If the flashlight fails to illuminate, the hypothesis is refuted; an alternative hypothesis is formulated (e.g., the light bulb is burnt out), and the process is repeated through identical methodological steps.

• Hypotheses vs. Scientific Theories: A scientific theory is significantly broader in scope than an individual hypothesis. A theory is supported by an extensive, accumulating body of diverse evidence, explains a comprehensive framework of natural phenomena, and can generate numerous new testable hypotheses.
• The Distinction Between Scientific Theories and Laws:
  • Scientific Laws: Describe or mathematically formulate how an event consistently occurs under specified physical conditions.
  • Scientific Theories: Well-substantiated, overarching explanations that seek to explain why laws and natural phenomena happen. Theories do not graduate into laws upon accumulating evidence; rather, they explain the mechanisms behind laws. Like hypotheses, theories remain falsifiable if contradictory empirical evidence emerges.
• Five Classical Steps of Scientific Methodology:
  1. Identifying and Defining the Problem: Clarifying the question through skepticism and curious observation.
  2. Data Collection via Observation: Gathering qualitative or quantitative facts under identical conditions.
  3. Formulating a Hypothesis: Synthesizing data into a tentative explanation that is empirically testable, verifiable, and predictive.
  4. Making Predictions and Designing Controlled Experiments: Formulating "If… then…" deductions and testing them under controlled parameters.
  5. Transformation into a Theory: When a hypothesis is repeatedly verified by diverse, independent controlled investigations, it forms or integrates into a scientific theory.
• Types of Scientific Observation:
  • Qualitative Observation (Nitel Gözlem): Relies directly on the five senses without measuring instruments; expresses non-numerical, descriptive data.
  • Quantitative Observation (Nicel Gözlem): Relies on standard measurement instruments to collect objective numerical data (e.g., determining the length or daily growth rate of an earthworm with a ruler). Because quantitative observations do not vary from observer to observer, they form the cornerstone of rigorous scientific experiments.
• Principles of Controlled Experimentation:
  • Runs two simultaneous groups: a control group and an experimental group.
  • Every parameter is kept identical between groups except for the single variable under investigation.
  • Independent Variable (Bağımsız Değişken): The specific factor deliberately manipulated or investigated by the experimenter (e.g., light color).
  • Dependent Variable (Bağımlı Değişken): The measured factor that changes in response to manipulations of the independent variable (e.g., the rate of photosynthesis).
  • Example Setup: A control group of plants is kept under normal white light while experimental groups are exposed to red, green, or purple light; all background conditions (temperature, humidity, atmospheric oxygen concentration) are kept uniform.

Fundamental Themes and Organization in Biology

• Etymology and Scope of Biology: Derived from the Greek bios (life) and logos (study/science). Biology investigates living organisms across their lifespans, interactions among organisms and their biotic/abiotic environments, internal anatomical structures, physiological operations, and cellular and molecular metabolic processes.
• Primary Subdisciplines of Biology:
  • Zoology: The scientific study of animals.
  • Botany: The scientific study of plants.
  • Biotechnology: The application and modification of living organisms or their biological systems for technological and industrial purposes.
  • Histology: The microscopic anatomical examination of biological tissues.
  • Anatomy: The structural study of internal organs and body organization in living organisms.
  • Morphology: The study of external form, appearance, and physical structure.
  • Physiology: The study of the biological functions, activities, and operational mechanisms of tissues, organs, and organ systems.
  • Microbiology: The study of microscopic organisms invisible to the naked eye.
  • Molecular Biology: The molecular investigation of life, focusing on the structures, interactions, and functions of nucleic acids ($\text{DNA}$, $\text{RNA}$), enzymes, hormones, and proteins.
  • Ecology: The scientific study of interactions between organisms and their physical and biological environments.
  • Cytology: The study of cell structure, physiology, and pathology.
  • Genetics: The science of heredity, genes, and the transmission of genetic traits across generations.
• Interdisciplinary Biological Sciences: Includes Biophysics, Biochemistry, Biomathematics, Biostatistics, Bioengineering, Biomedical Science, Bioinformatics, Biogeography, Proteomics, Genomics, Transcriptomics, and Metabolomics.

• Universal Properties of Life:
  • Order (Düzen): Highly coordinated, complex microscopic and macroscopic structural organization (e.g., the precise geometric arrangement of florets in a sunflower).
  • Reproduction (Üreme): The capacity of organisms to reproduce their own kind, upholding the biological principle of biogenesis (e.g., a Japanese macaque nurturing its offspring, or asexual budding in yeast).
  • Growth and Development (Büyüme ve Gelişme): Heritable information encoded in $\text{DNA}$ directs species-specific growth and developmental programs (e.g., embryonic stages of a Costa Rican tree frog).
  • Energy Processing (Enerji Kullanımı): Ingestion and conversion of environmental energy to fuel metabolic work (e.g., a bat feeding on nectar from a saguaro cactus to extract chemical energy stored in molecular bonds to power flight).
  • Response to Environmental Stimuli (Çevresel Uyarılara Tepki Verme): Ability to sense and react to external physical or chemical changes (e.g., a Venus flytrap rapidly closing its modified leaves when trigger hairs detect a cricket).
  • Homeostasis: Regulatory physiological mechanisms that stabilize an organism's internal environment within narrow physiological limits despite environmental fluctuations (e.g., blood vessel vasodilation in the large ears of the black-tailed jackrabbit regulating convective heat dissipation).
  • Evolutionary Adaptation (Evrimsel Uyum): Modifications acquired across generations via natural selection that optimize reproductive fitness in a specific environment (e.g., seasonal white camouflage plumage in the white-tailed ptarmigan blending with snow cover).
• The Hierarchy of Biological Organization: Life is arranged in structural tiers where each subsequent level is built directly upon the underlying tier:
  • Subatomic Particle: Fundamental particles ($p^+$, $n^0$, $e^-$).
  • Atom: Smallest unit retaining elemental properties ($\text{H}$, $\text{C}$, $\text{N}$, $\text{O}$).
  • Molecule: Chemical structures formed by covalent bonds (Water, Glucose, $\text{DNA}$).
  • Organelle: Functional cellular sub-compartment (Mitochondria, Chloroplast, Nucleus).
  • Cell: Fundamental structural and functional unit of life (e.g., a neuron).
  • Tissue: Integrated group of similar cells executing a specialized task (e.g., nervous tissue).
  • Organ: Cooperative assembly of multiple distinct tissues performing specific body functions (e.g., brain, plant leaf).
  • Organ System: Cooperative functional network of distinct organs (e.g., nervous system).
  • Multicellular Organism: An integrated, free-living biological individual (e.g., pronghorn antelope).
  • Species: A group of morphologically and genetically similar organisms capable of interbreeding and producing fertile offspring.
  • Population: A localized group of individuals belonging to the same species coexisting in a specific area.
  • Community: An assemblage of interacting populations belonging to different species within a shared environment.
  • Ecosystem: The holistic functional unit composed of a biological community interacting with all abiotic factors (soil, water, atmosphere).
  • Biosphere: The entire planetary zone of Earth inhabited by living organisms, encompassing terrestrial, aquatic, and atmospheric zones.

• Emergent Properties (Belirleyici Özellikler): New functional properties emerge at each ascending level of biological organization that were absent at lower levels, arising from the specific spatial arrangements and interactions of subcomponents. A living organism is greater than the simple mathematical sum of its parts.
• Reductionism (İndirgemecilik): The scientific strategy of breaking down complex, high-level biological systems into their simpler, constituent chemical and cellular parts to make them accessible to analytical study.
• Correlation of Structure and Function: Throughout all biological scales, physical form reflects physiological function:
  • Avian flight requires specialized aerodynamic wings.
  • Avian bones feature a lightweight honeycomb interior that confers substantial structural strength while minimizing weight.
  • Neurons feature elongated cellular extensions (axons and dendrites) suited for the rapid transmission of electrical impulses.
  • The inner mitochondrial membrane has extensive foldings (cristae) that package a massive surface area of enzymatic membranes within a compact organelle volume.

• Open Systems and Energy Flow: Organisms operate as open thermodynamic systems that continuously exchange matter and energy with their surroundings. Photosynthetic producers capture solar energy and synthesize chemical energy (organic sugars), which consumers ingest. Throughout all biological energy transformations (chemical, potential, thermal, and kinetic), energy continually enters as sunlight and dissipates into the environment as metabolic heat ($\text{Isı}$).

• Dynamic Equilibrium and Catalysis: The cellular oxidation of sugars differs from combustion. While a match burns through uncontrolled, explosive release of heat and light, cellular sugar oxidation is a tightly controlled, multi-step enzymatic pathway regulated by feedback mechanisms.

Biological Diversity, Classification, and Evolution

• Global Biodiversity Estimates: Approximately $1.5 \times 10^6$ ($1.5\text{ million}$) species have been formally identified and documented to date. Among these documented taxa:
  • Plant species: $\approx 280{,}000$
  • Vertebrate species: $\approx 50{,}000$
  • Insect species: $> 750{,}000$
  • Total estimated species richness across the biosphere: Ranges between $5 \times 10^6$ and $30 \times 10^6$ ($5\text{ to }30\text{ million}$) species.
• Taxonomy: The branch of biology dedicated to naming, grouping, and classifying organisms based on evolutionary relationships and morphological patterns.
• Historical Classification Approaches:
  • Empirical / Artificial Classification (Aristotle, 384–322 BCE): Based on superficial outward appearance and living habitats. Grouped plants into herbs (otlar), shrubs (çalılar), and trees (ağaçlar); grouped animals into terrestrial (karada yaşayanlar) and aquatic (suda yaşayanlar). Relied on qualitative observations and analogous organs (analog organlar: organs with different embryonic origins but identical functions, such as the wing of an insect and the wing of a bird). Not scientifically valid.
  • Natural / Phylogenetic Classification: Modern scientific classification based on evolutionary ancestry, utilizing:
  • Homologous organs (homolog organlar): Anatomical structures derived from a common ancestral origin that may have diverged to execute different functions (e.g., human arm, bat wing, whale flipper).
  • Comparative $\text{DNA}$ and gene nucleotide sequencing.
  • Comparative amino acid and protein homology.
  • Comparative embryological developmental pathways.
  • Comparative anatomical organ system organization.
  • Comparative metabolic pathways.
• Binomial Nomenclature: Formalized by Carl Linnaeus; establishes a universal two-part Latin name for every species:
  • Genus name (Cins ismi): The first word, capitalized, identifying shared ancestry.
  • Specific epithet (Tanımlayıcı isim): The second word, lowercase, describing the specific species.
  • Rule of Evolutionary Kinship: Relatedness between organisms is established strictly by the shared genus name, not the specific epithet. For example, Morus nigra (black mulberry) is closely related to Morus alba (white mulberry) because both belong to genus Morus. In contrast, Populus nigra (black poplar) shares only a descriptive epithet (nigra = black) with Morus nigra and bears no close taxonomic affinity.
  • Common binomial examples: Domestic cat (Felis catus / Felis domesticus), Lion (Felis leo), Tiger (Felis tigris), Wolf (Canis lupus).
• Hierarchical Taxonomic Categories: Modern taxonomy ranks life into nested categories called taxa (takson). As one ascends from species to kingdom, individual count increases while shared biological characteristics decrease. As one descends from kingdom to species, individual count decreases while shared biological characteristics increase:
  • Domain
  • Kingdom (Alem / Regnum)
  • Phylum (Şube / Filum)
  • Class (Sınıf / Klasis)
  • Order (Takım / Ordo)
  • Family (Aile / Familya)
  • Genus (Cins / Genus)
  • Species (Tür / Species)

• Taxonomic Classification of the Gray Wolf (Canis lupus):
  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Mammalia
  • Order: Carnivora
  • Family: Canidae
  • Genus: Canis
  • Species: Canis lupus
• The Three Domains of Life: Replacing historical five-kingdom classifications based on morphological criteria, molecular and genomic sequencing established three primary evolutionary domains:
  • Domain Bacteria: Highly diverse, widespread unicellular prokaryotic organisms.
  • Domain Archaea: Prokaryotes that often inhabit extreme environments (hypersaline lakes, boiling hydrothermal vents). Molecular evidence demonstrates that Archaea share a closer evolutionary kinship with Eukarya than with Bacteria.
  • Domain Eukarya: Organisms possessing membrane-bound nuclei and complex internal organelles, classically subdivided into:
  • Kingdom Protista: Unicellular and simple colonial eukaryotes.
  • Kingdom Plantae: Multicellular autotrophic eukaryotes performing oxygenic photosynthesis.
  • Kingdom Fungi: Heterotrophic eukaryotes absorbing nutrients via extracellular decomposition.
  • Kingdom Animalia: Multicellular heterotrophic eukaryotes that ingest other organisms.

Chemical Foundations of Life and Water Chemistry

• Subatomic Structure of the Atom:
  • Nucleus: The dense central core containing positively charged protons ($p^+$) and neutral neutrons ($n^0$), which possess identical mass.
  • Electron Cloud: Contains negatively charged electrons ($e^-$) orbiting the nucleus in defined electron shells.
• Electron Shell Capacities and Chemical Bonding:
  • The innermost electron shell accommodates a maximum of $2$ electrons.
  • The second and third electron shells accommodate up to $8$ electrons.
  • Valence electrons in the outermost shell govern chemical bonding behaviors.

• Elemental Composition of Living Matter: Out of $92$ naturally occurring elements, approximately $25$ elements are essential to life:
  • Major Bulk Elements ($\approx 96\%$ of living mass): Carbon ($\text{C}$), Nitrogen ($\text{N}$), Oxygen ($\text{O}$), and Hydrogen ($\text{H}$).
  • Additional Essential Elements: Sulfur ($\text{S}$), Phosphorus ($\text{P}$), Calcium ($\text{Ca}$), Sodium ($\text{Na}$), Chlorine ($\text{Cl}$).
  • Trace Elements (Eser Elementler): Required in minute quantities for survival, such as Iron ($\text{Fe}$), Iodine ($\text{I}$), and Magnesium ($\text{Mg}$).
• Covalent Bonds and Electronegativity:
  • Nonpolar Covalent Bonds: Formed when electrons are shared equally between atoms of similar electronegativity (e.g., $H_2$, $O_2$).
  • Polar Covalent Bonds: Formed when atoms have different electronegativities, causing unequal electron sharing and generating partial charges (e.g., $H_2O$ where oxygen is partially negative and hydrogens are partially positive).
• Water as the Biological Solvent:
  • Cells are composed of $70\text{ to }95\%$ water.
  • Hydrophilic Substances: Ionic species and polar molecules whose partial charges form electrostatic interactions with the positive ($\delta^+$) and negative ($\delta^-$) poles of water, allowing them to dissolve easily (e.g., sugars, amino acids).
  • Hydrophobic Substances: Nonpolar, uncharged molecules that lack affinity for water and cannot form hydrogen bonds, causing them to aggregate away from water molecules.
• The pH Scale and Acid-Base Chemistry:
  • Calculated as $\text{pH} = -\log_{10}[\text{H}^+]$. Neutrality is defined as $[\text{H}^+] = [\text{OH}^-] = 10^{-7}\,\text{mol}\,\text{dm}^{-3}$ ($\text{pH} = 7.0$).
  • Solutions where $[\text{H}^+] > [\text{OH}^-]$ are acidic ($\text{pH} < 7$); solutions where $[\text{H}^+] < [\text{OH}^-]$ are basic ($\text{pH} > 7$).

• pH Values of Common Biological and Environmental Solutions:
  • $1.0\,\text{M}\text{ HCl}$: $\text{pH} = 0$ ($[\text{H}^+] = 10^0\,\text{mol}\,\text{dm}^{-3}$)
  • Stomach acid, Lime juice: $\text{pH} = 1$ ($[\text{H}^+] = 10^{-1}\,\text{mol}\,\text{dm}^{-3}$)
  • Lemon juice: $\text{pH} = 2$ ($[\text{H}^+] = 10^{-2}\,\text{mol}\,\text{dm}^{-3}$)
  • Acid precipitation: $\text{pH} = 2.5 - 5.5$
  • Vinegar, Orange juice, Tomato juice, Cola: $\text{pH} = 3$ ($[\text{H}^+] = 10^{-3}\,\text{mol}\,\text{dm}^{-3}$)
  • Beer: $\text{pH} = 4$ ($[\text{H}^+] = 10^{-4}\,\text{mol}\,\text{dm}^{-3}$)
  • Black coffee, Tea: $\text{pH} = 5$ ($[\text{H}^+] = 10^{-5}\,\text{mol}\,\text{dm}^{-3}$)
  • Normal rainwater: $\text{pH} = 5.6$
  • Human urine: $\text{pH} = 5.7$
  • Pure water, Saliva, Human blood, Tears: $\text{pH} = 7.0$ ($[\text{H}^+] = 10^{-7}\,\text{mol}\,\text{dm}^{-3}$)
  • Seawater: $\text{pH} = 7.8 - 8.3$
  • Baking soda: $\text{pH} = 9$ ($[\text{H}^+] = 10^{-9}\,\text{mol}\,\text{dm}^{-3}$)
  • Household bleach, Phosphate detergents: $\text{pH} = 10$ ($[\text{H}^+] = 10^{-10}\,\text{mol}\,\text{dm}^{-3}$)
  • Phosphate-free, ammonia-free detergents: $\text{pH} = 11$ ($[\text{H}^+] = 10^{-11}\,\text{mol}\,\text{dm}^{-3}$)
  • Washing soda: $\text{pH} = 12$ ($[\text{H}^+] = 10^{-12}\,\text{mol}\,\text{dm}^{-3}$)
  • Oven cleaner: $\text{pH} = 13$ ($[\text{H}^+] = 10^{-13}\,\text{mol}\,\text{dm}^{-3}$)
  • $1.0\,\text{M}\text{ NaOH}$: $\text{pH} = 14$ ($[\text{H}^+] = 10^{-14}\,\text{mol}\,\text{dm}^{-3}$)

Carbon Chemistry, Molecular Diversity, and Functional Groups

• The Centrality of Carbon: Organic chemistry is the study of carbon-based molecules. Carbon has an atomic number of $6$, with $2$ electrons in its inner shell and $4$ valence electrons in its outer shell. To satisfy the octet rule, carbon shares four electrons via four covalent bonds, yielding a tetrahedral geometry when single-bonded.

• Molecular Diversity of Carbon Skeletons: Carbon backbones vary in length, can be unbranched (e.g., butane) or branched (e.g., isobutane), may feature variable double-bond positions (e.g., 2-butene), or form closed rings.
• Hydrocarbons: Organic compounds consisting exclusively of carbon and hydrogen (e.g., petroleum, gasoline, aliphatic fatty acid tails). Hydrocarbons are hydrophobic and store large amounts of energy released during oxidation.
• Classification of Isomers: Isomers are compounds sharing an identical molecular formula but possessing distinct spatial arrangements, structures, and chemical properties:
  • Structural Isomers (Yapısal İzomerler): Differ in the covalent arrangements of their carbon skeletons or double-bond positions (e.g., butane versus isobutane). The number of structural isomers increases exponentially with carbon chain length.
  • Geometric Isomers (Geometrik İzomerler): Differ in the spatial arrangement of atoms across an inflexible double bond ($\text{C}=\text{C}$), which prevents free rotation along the bond axis. In biological systems, the light-induced geometric isomerization of rhodopsin triggers the visual signal transduction cascade.
  • Enantiomers (Enantiyomerler): Pairs of molecules that are non-superimposable mirror images of each other, occurring when an asymmetric (chiral) carbon is bonded to four chemically distinct atoms or functional groups.

• Pharmacological Significance of Enantiomers:
  • Enantiomers can have dramatically different physiological effects because biological receptors selectively bind one optical isomer.
  • Thalidomide: Prescribed in the early 1960s as a racemic mixture of enantiomers. One enantiomer served as an effective sedative against morning sickness, whereas the mirror-image enantiomer caused severe teratogenic birth defects.
  • Dopa Enantiomers: $\text{L-Dopa}$ actively crosses the blood-brain barrier to alleviate symptoms of Parkinson's disease, whereas its mirror image $\text{D-Dopa}$ is biologically inert.
• Major Biologically Significant Functional Groups: Functional groups substitute for hydrogen atoms along the carbon backbone, imparting distinct chemical properties:
  • Hydroxyl Group ($-\text{OH}$ or $\text{HO}-$): Characteristic of alcohols (names typically end in -ol, e.g., ethanol). The electronegative oxygen atom imparts polarity, forming hydrogen bonds with water and facilitating the dissolution of organic compounds like sugars.
  • Carbonyl Group ($\text{C}=\text{O}$): A carbon atom joined to an oxygen atom by a double bond. Classified as an aldehyde if located at the terminus of a carbon skeleton ($\text{R}-\text{CHO}$), or a ketone if positioned internally ($\text{R}-\text{CO}-\text{R}'$).
  • Carboxyl Group ($-\text{COOH}$): Consists of a carbonyl carbon bonded to a hydroxyl group. Functions as an organic acid (carboxylic acid, e.g., formic acid found in ant venom). Tends to donate a proton ($\text{H}^+$) to solution due to resonance stabilization, yielding a negatively charged carboxylate ion ($-\text{COO}^-$).
  • Amino Group ($-\text{NH}_2$): Consists of a nitrogen atom bonded to two hydrogens. Compounds containing it are classified as amines. Functions as a biological base by accepting a proton ($\text{H}^+$) from the surrounding aqueous environment to form $\text{H}_3\text{N}^+$.
  • Sulfhydryl Group ($-\text{SH}$): Consists of a sulfur atom bonded to a hydrogen atom. Organic compounds possessing this group are called thiols. Two sulfhydryl groups can react to form covalent disulfide bridges ($-\text{S}-\text{S}-$), which stabilize tertiary and quaternary protein structures.
  • Phosphate Group ($-\text{OPO}_3^{2-}$): A phosphorus atom covalently bound to four oxygen atoms. Contributes negative charge to molecules (such as $\text{ATP}$ and nucleic acid backbones) and acts as an energy-transfer group.
• Endocrine Impact of Functional Groups (Estradiol and Testosterone): Both sex hormones share an identical steroid core composed of four fused carbon rings. Their divergent biological roles in vertebrate sexual development arise from differences in their functional groups: estradiol possesses a hydroxyl group ($-\text{OH}$) on its aromatic A-ring, whereas testosterone features a carbonyl group ($=\text{O}$) and an added methyl group ($-\text{CH}_3$).

Macromolecules: Carbohydrates and Lipids

• Macromolecular Polymerization Principles:
  • Polymer: A long molecule consisting of many similar or identical building blocks linked by covalent bonds.
  • Monomer: Smaller repeating molecular subunits that serve as the building blocks of polymers.
  • Dehydration Synthesis / Condensation Reaction: The enzymatic mechanism joining two monomers via covalent bonding, wherein one monomer contributes a hydroxyl group ($-\text{OH}$) and the other contributes a hydrogen ($-\text{H}$), releasing a water molecule ($\text{H}_2\text{O}$).

• Hydrolysis Reaction: The reverse breakdown process where covalent bonds between monomers are disassembled by the addition of a water molecule, restoring the original $\text{H}$ and $\text{OH}$ groups (e.g., biological digestion).

• Carbohydrates: Encompass monosaccharide sugars and macromolecular polymers of sugars.
• Monosaccharides: Simple sugars possessing molecular formulas that are multiples of $\text{CH}_2\text{O}$ (e.g., Glucose, $\text{C}_6\text{H}_{12}\text{O}_6$):
  • Classification by Carbonyl Position: Aldose sugars contain a terminal carbonyl group (e.g., Glyceraldehyde, Ribose, Glucose, Galactose); Ketose sugars contain an internal carbonyl group (e.g., Dihydroxyacetone, Ribulose, Fructose).
  • Classification by Backbone Length:
  • Triose ($\text{C}_3\text{H}_6\text{O}_3$): Glyceraldehyde, Dihydroxyacetone.
  • Pentose ($\text{C}_5\text{H}_{10}\text{O}_5$): Ribose, Ribulose.
  • Hexose ($\text{C}_6\text{H}_{12}\text{O}_6$): Glucose, Galactose, Fructose.
  • Ring Formation in Solution: In aqueous environments, five- and six-carbon sugars cyclize into rings. In glucose, carbon 1 covalently bonds to the oxygen atom attached to carbon 5.
  • Anomeric Forms of Glucose: Cyclization yields two interchangeable stereoisomers depending on the position of the hydroxyl group attached to carbon 1: $\alpha\text{-glucose}$ (hydroxyl group points downward below the ring plane) and $\beta\text{-glucose}$ (hydroxyl group points upward above the ring plane).
• Disaccharides: Formed by joining two monosaccharides through a covalent glycosidic linkage via dehydration synthesis:

• Maltose (Malt Sugar): $\text{Glucose} + \text{Glucose}$, joined by an $\alpha\text{-1,4-glycosidic bond}$.
• Sucrose (Table Sugar): $\text{Glucose} + \text{Fructose}$, joined by a $1,2\text{-glycosidic bond}$; the primary carbohydrate transport form used in plant phloem.
• Lactose (Milk Sugar): $\text{Glucose} + \text{Galactose}$, joined by a $\beta\text{-1,4-glycosidic bond}$.
  • Polysaccharides: Polymers composed of hundreds to thousands of monosaccharides joined by glycosidic bonds. Divided functionally into storage and structural polymers.
  • Storage Polysaccharides:
• Starch: Storage polysaccharide of plants, accumulated as granules within plastids (chloroplasts and amyloplasts). Consists entirely of $\alpha\text{-glucose}$ monomers. Exists in two forms: unbranched amylose (linked exclusively by $\alpha\text{-1,4}$ bonds) and branched amylopectin (linked by $\alpha\text{-1,4}$ bonds with occasional $\alpha\text{-1,6}$ branch points). Most animals produce enzymes to hydrolyze starch into free glucose.
• Glycogen: Extensively branched storage polysaccharide in animals, stored primarily in liver and skeletal muscle cells. It is more heavily branched with $\alpha\text{-1,6}$ linkages than amylopectin. In humans, glycogen stores are depleted within roughly 24 hours in the absence of food intake.
  • Structural Polysaccharides:
• Cellulose: The primary structural material of plant cell walls and the most abundant organic macromolecule on Earth. Composed of $\beta\text{-glucose}$ monomers linked by $\beta\text{-1,4-glycosidic bonds}$.

• Because of the $\beta$ linkage geometry, every adjacent glucose monomer is oriented upside-down relative to its neighbors, producing a straight, unbranched molecule. Parallel cellulose molecules form hydrogen bonds between hydroxyl groups on carbons 3 and 6, aggregating into bundles called microfibrils. Each microfibril consists of approximately $80$ cellulose chains.

• Most animals lack cellulase enzymes and cannot digest cellulose. As insoluble fiber, it stimulates mucus secretion by abrading the intestinal lining. Ruminants (cattle) and termites digest cellulose through symbiotic relationships with cellulolytic gut microbes.
• Chitin: A structural polysaccharide found in the exoskeletons of arthropods and the cell walls of fungi. Monomers consist of a glucose derivative with a nitrogen-containing appendage ($N\text{-acetylglucosamine}$). Pure chitin is flexible and leathery, but it hardens into a rigid shell when encrusted with calcium carbonate ($\text{CaCO}_3$).
  • Lipids: Hydrophobic biological molecules composed predominantly of hydrocarbon assemblies. Include fats, phospholipids, and steroids.
  • Fats (Triacylglycerols / Triglycerides): Formed from one molecule of glycerol (a three-carbon alcohol with three $\text{-OH}$ groups) and three fatty acid molecules (hydrocarbon chains with a terminal carboxyl group, such as palmitic acid). Joined by three consecutive dehydration reactions forming ester linkages between the glycerol hydroxyls and fatty acid carboxyls.

• Saturated vs. Unsaturated Fats:
  • Saturated Fatty Acids: Lack double bonds between carbon atoms in the hydrocarbon tail; the chain is saturated with hydrogen (e.g., stearic acid). Molecules pack together tightly and are solid at room temperature (e.g., butter, animal fats).
  • Unsaturated Fatty Acids: Contain one or more double bonds along the hydrocarbon chain (e.g., oleic acid). These double bonds introduce kinks in the carbon backbone that prevent tight molecular packing, keeping them liquid at room temperature (e.g., plant oils).

• Health Implications of Saturated Fats: Diets rich in saturated fats are a risk factor for cardiovascular diseases, notably atherosclerosis, characterized by the accumulation of lipid plaques along arterial walls. These plaques narrow arteries, reduce vessel elasticity, and restrict blood flow.
• Energy Density: A single gram of fat stores more than twice the metabolic energy of a gram of carbohydrate, making fats a compact energy reserve.
  • Phospholipids: Composed of one glycerol molecule linked to two hydrophobic fatty acid chains and one negatively charged hydrophilic phosphate group (often modified with a charged polar group such as choline).

• Amphipathic Behavior: When suspended in water, phospholipids spontaneously self-assemble into spherical micelles or bilayer sheets, orienting hydrophilic phosphate heads outward toward the aqueous solution and shielding hydrophobic fatty acid tails inward away from water. This forms the structural foundation of all biological cell membranes.
  • Steroids: Lipids characterized by a carbon skeleton consisting of four fused rings. Cholesterol is an essential structural component of animal cell membranes and serves as the precursor for the synthesis of all other steroid hormones, including vertebrate sex hormones.

Macromolecules: Proteins and Nucleic Acids

• Proteins: Polymers that account for more than $50\%$ of the dry mass of most cells. Biological functions include structural support, storage, transport, intercellular signaling, cellular motility, immune defense, and enzymatic catalysis.
• Historical Discovery of Amino Acids:
  • The first amino acid, Asparagine, was discovered in 1806.
  • Threonine was discovered in 1938.
  • For decades, the number of standard amino acids was considered fixed at 20.
  • The discovery of Selenocysteine (Sec, encoded by the $\text{UGA}$ codon) in 1986 broke this rule as the 21st amino acid.
  • In 2002, researchers Krzycki and Chan at Ohio State University discovered the genetic encoding of the 22nd amino acid, Pyrrolysine (Pyl, encoded by the $\text{UAG}$ codon), in the archaeon Methanosarcina barkeri.
  • These discoveries demonstrated that stop codons ($\text{UGA}$ and $\text{UAG}$) can be reprogrammed in specialized biological contexts, opening avenues for protein engineering.
• Amino Acid Structure: All amino acids possess an asymmetric alpha ($\alpha$) carbon covalently bound to four 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 ($\text{R}$ group).
  • The chemical properties, charge, and polarity of the $\text{R}$ group determine whether an amino acid is nonpolar (hydrophobic), polar (hydrophilic), or charged (acidic/basic).
• The Genetic Code and Peptide Synthesis: mRNA nucleotide triplets called codons direct amino acid sequences. With four bases ($\text{A}$, $\text{U}$, $\text{G}$, $\text{C}$), there are $4 \times 4 \times 4 = 64$ possible codons encoding 20–22 amino acids, making the genetic code degenerate.
• Peptide Bond Formation: Joined via a dehydration condensation reaction between the carboxyl carbon of one amino acid and the amino nitrogen of the next, producing a covalent peptide bond. The resulting polypeptide chain has structural polarity with an N-terminus (free amino group) and a C-terminus (free carboxyl group).

• The Four Levels of Protein Structure:

• 1. Primary Structure: The unique linear sequence of amino acids in a polypeptide, determined by the underlying gene sequence.
  • Even a single amino acid substitution can disrupt structure and function. For example, in sickle-cell anemia, replacing a single glutamic acid with valine at position 6 in the 146-amino-acid $\beta\text{-globin}$ chain of hemoglobin causes hemoglobin molecules to aggregate into fibers, deforming disc-shaped red blood cells into sickles.

• 2. Secondary Structure: Local folding and coiling along the polypeptide backbone stabilized by repeating hydrogen bonds between carbonyl oxygens and amide hydrogens of the peptide backbone. The two primary forms are the $\alpha\text{-helix}$ (found in fibrous keratin) and the $\beta\text{-pleated sheet}$ (found in silk fibroin and spider webs, providing high tensile strength and elasticity).

• 3. Tertiary Structure: The overall three-dimensional folding of a polypeptide, stabilized by interactions between variable $\text{R}$ groups:
  • Hydrophobic interactions and van der Waals forces (nonpolar side chains cluster in the protein interior away from water).
  • Hydrogen bonds between polar side chains.
  • Ionic bonds between positively and negatively charged side chains.
  • Covalent disulfide bridges ($-\text{S}-\text{S}-$) formed between the sulfhydryl groups of adjacent cysteine residues.

- *Tertiary Protein Classes:*
  - *Fibrous Proteins:* Elongated, rod-like molecules dominated by secondary structures; insoluble in water (e.g., collagen, actin, myosin).
  - *Globular Proteins:* Compact, spherical, water-soluble molecules with a hydrophobic core and a hydrophilic surface (e.g., myoglobin, casein, lysozyme).

• 4. Quaternary Structure: The overall protein architecture resulting from the assembly of two or more polypeptide subunits into a functional complex (e.g., collagen is a fibrous triple helix of three polypeptides; hemoglobin is a globular tetramer of two $\alpha$ and two $\beta$ subunits, each carrying an iron-containing heme group).

• Chaperonins (Molecular Chaperones): Cylindrical protein complexes that assist in folding nascent polypeptides. They do not specify the final native conformation, but instead provide an isolated, protected microenvironment away from the crowded cytoplasm, preventing protein aggregation during the folding process.

• Nucleic Acids and Gene Expression: The linear sequence of amino acids in a polypeptide is encoded by genes composed of nucleic acids. Biological systems utilize two primary types of nucleic acids: Deoxyribonucleic Acid ($\text{DNA}$) and Ribonucleic Acid ($\text{RNA}$).
• The Central Dogma of Molecular Biology: Genetic information flows directionally from $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$. In eukaryotes, mRNA is transcribed from a $\text{DNA}$ template inside the nucleus, exits through nuclear pores into the cytoplasm, and binds to ribosomes, which translate its sequence into a functional polypeptide chain.

• Functional Classes of RNA:
  • Messenger RNA (mRNA): Carries genetic code from the nucleus to ribosomes in the cytoplasm.
  • Transfer RNA (tRNA): Transports specific amino acids from the cytoplasm to the ribosome during translation.
  • Ribosomal RNA (rRNA): Forms the enzymatic and structural core of ribosomal subunits.
• Nucleotide Architecture: Nucleic acids are polymers of monomers called nucleotides. Each nucleotide consists of:
  1. A nitrogenous organic base.
  2. A pentose sugar (deoxyribose in $\text{DNA}$, ribose in $\text{RNA}$).
  3. An inorganic phosphate group ($-\text{OPO}_3^{2-}$).
• Nitrogenous Base Families:
  • Purines: Feature a double-ring structure composed of a six-membered ring fused to a five-membered ring. Includes Adenine ($\text{A}$) and Guanine ($\text{G}$).
  • Pyrimidines: Feature a single six-membered ring of carbon and nitrogen atoms. Includes Cytosine ($\text{C}$), Thymine ($\text{T}$, found exclusively in $\text{DNA}$), and Uracil ($\text{U}$, found exclusively in $\text{RNA}$).

• Polynucleotide Assembly: Nucleotides polymerize via phosphodiester linkages, in which a phosphate group connects the hydroxyl group on the 3' carbon of one pentose to the phosphate on the 5' carbon of the adjacent pentose. This forms a continuous, repeating sugar-phosphate backbone with nitrogenous bases projecting outward.
• The DNA Double Helix: Consists of two antiparallel polynucleotide strands spiraling around a central axis. Complementary nitrogenous bases pair via hydrogen bonding: Adenine ($\text{A}$) pairs with Thymine ($\text{T}$) via two hydrogen bonds; Guanine ($\text{G}$) pairs with Cytosine ($\text{C}$) via three hydrogen bonds. Semiconservative replication allows each parental strand to serve as a template for synthesizing a complementary strand, ensuring faithful transmission of genetic information across generations.

• Molecular Evolution and Paleogenomics: Because $\text{DNA}$ sequences are passed down through generations, evolutionary relatedness can be measured by comparing $\text{DNA}$ and protein sequences between species. More closely related taxa share higher sequence homology than distantly related taxa.