Comprehensive College Biology Study Notes: Scientific Method, Chemistry of Life, Water, Carbon, and Biological Macromolecules

Introduction to Biology and the Scientific Method

• Science: An evidence-based process of inquiry used to investigate the natural world. It relies on a continuous feedback loop of observations, hypotheses, predictions, and testing of hypotheses.

• Scientific Method: A systematic framework formalised by Sir Francis Bacon in the late 1500s. It consists of six key steps:

  • Observation

  • Question

  • Hypothesis

  • Experimentation

  • Data Analysis

  • Conclusion

• Worked Example of the Scientific Method (Poison Ivy and Acne):

  • Question: Does poison ivy cure acne?

  • Background Research: Conduct literature review on the underlying causes of acne and the active toxins in poison ivy.

  • Formulate Hypotheses:

  • Null Hypothesis ($H_0$): Poison ivy does not affect acne.

  • Alternative Hypothesis 1 ($H_1$): Poison ivy cures acne.

  • Alternative Hypothesis 2 ($H_2$): Poison ivy makes acne worse.

  • Experimental Design & Treatments:

  1. Apply poison ivy extract to the face of children/subjects with acne.

  2. Control group: No treatment.

  3. Placebo group: Placebo treatment.

  • Analyze Results & Draw Conclusions: Poison ivy causes facial skin irritation but does not affect acne.

  • Next Steps:

  • If hypothesis is supported: Report results via peer-reviewed journal articles, academic conferences, etc.

  • If hypothesis is rejected: Formulate a new hypothesis or develop hypotheses regarding alternative acne treatments, then re-test with new experiments.

• Definitions of Core Scientific Terms:

  • Hypothesis: A tentative explanation based on data collected from observations and preliminary experiments. A valid hypothesis must be both testable and falsifiable.

  • Theory: A broad, highly supported hypothesis or set of hypotheses that has consistently survived repeated testing, rigorous experimentation, and extensive observation.

  • Law: A formal statement describing an observed physical or natural phenomenon that remains constant under specified conditions.

Scientific Reasoning, Data, and Biological Themes

• Supernatural vs. Science: Science deals strictly with observable and testable natural phenomena. Science cannot evaluate or prove supernatural concepts because beliefs cannot be empirically measured or tested.

• Scientific Reasoning:

  • Deductive Reasoning: Applies general principles, laws, or premises to predict specific biological outcomes.

  • Inductive Reasoning: Uses a large collection of specific individual observations to derive general principles or universal conclusions.

  • Strong Inference: A method of scientific inquiry involving multiple alternative hypotheses, rigorous testing, and systematic elimination through experimentation.

• Data Types:

  • Qualitative Data: Descriptive, narrative, or categorical observations expressed in words rather than numbers (e.g., "It is sunny").

  • Quantitative Data: Numerical measurements obtained by counting or measuring with standardized units (e.g., $90^\circ$ temperature).

• Types of Scientific Studies & Experiments:

  • Manipulative Experiment: The researcher actively changes only one variable at a time while keeping all other variables constant.

  • Controlled Experiment: Compares an experimental group to a control group that does not receive the experimental treatment.

  • Natural Experiment: Observing and collecting data on natural phenomena or events that have already occurred without direct manipulation (e.g., observing seasonal changes).

  • Blind Study: The experimental subjects do not know whether they are receiving the actual treatment or a placebo, preventing bias.

  • Double-Blind Study: Neither the experimental subjects nor the researchers conducting the experiment know which group received the treatment or the placebo. This setup produces significantly more reliable data.

• Experimental Variables:

  • Independent Variable ($x$-axis): The factor being manipulated, changed, or measured as the presumed cause or predictor.

  • Dependent Variable ($y$-axis): The factor being measured or observed that changes in response to, or is affected by, the independent variable.

• Data Visualization & Graph Types:

  • Bar Graph: Used for comparing discrete counts or averages across different groups ($x$ = cause/category, $y$ = effect/count).

  • Line Graph: Used for displaying continuous data over an ordered sequence or time interval ($x$ = continuous independent variable, $y$ = dependent variable).

  • Scatter Plot: Used for plotting individual data points to identify correlations or relationships between fixed pairs of dependent and independent variables ($x$ and $y$).

• Properties and Functions of Life:

  • Biology: Derived from bio (life) and logy (study of)—the scientific study of life.

  • Cellular Basis: All living organisms are composed of one or more cells.

  • Unicellular: Organisms consisting of a single cell (e.g., bacteria).

  • Multicellular: Organisms composed of many specialized cells.

  • Essential Functions of Life: Nutrition, transport/distribution, respiration, excretion, response to environmental stimuli, regulation/homeostasis (e.g., sweating, shivering), and reproduction.

  • Structural Organization: Organisms display hierarchical structural organization built from one or more cells.

  • Metabolism: The total sum of energy transformations occurring within an organism, including catabolism (digestion and molecular breakdown).

• Domains and Taxonomy of Life:

  • Prokaryotes ("before the nucleus"): Single-celled organisms lacking a membrane-bound nucleus or membrane-bound organelles. Includes Domain Bacteria and Domain Archaea.

  • Eukaryotes: Single-celled or multicellular organisms containing a true nucleus and membrane-bound organelles. Belongs to Domain Eukarya (includes Plants, Animals, Fungi, and Protists).

• Five Unifying Themes in Biology:

  • Positive Feedback: A regulatory mechanism that amplifies a response and pushes a biological process forward until completion.

  • Negative Feedback: A regulatory mechanism that counters a stimulus to move the body back to a normal homeostatic set point.

  • Energy Flow: Energy flows through an ecosystem in a single, one-way direction (entering as sunlight and dissipating as heat).

  • Matter Cycling: Ecosystems cycle matter; chemical elements and nutrients are continuously recycled within the ecosystem.

  • Emergent Properties: Complex properties that arise at higher levels of structural organization that individual components cannot perform on their own ("the whole can do something by itself that individual parts cannot").

• Evolutionary Concepts:

  • Charles Darwin & Natural Selection: Organisms possessing advantageous traits are better suited to survive and reproduce in their environment.

  • Conservative Evolution: Evolutionary change over time that modifies existing genetic structures rather than creating completely new ones from scratch.

  • Genetic Drift: Random changes in gene allele frequencies within a population due to chance events or geographic movement.

  • Gene Flow: The transfer of genetic material between populations when individuals migrate and interbreed.

  • Sexual Selection: A mode of natural selection where certain physical or behavioral traits increase an individual\'s ability to attract mates.

Chemical Basis of Life: Matter, Elements, and Atomic Structure

• Biological Hierarchy of Matter: Biology is fundamentally governed by chemical principles and physical laws. The structural hierarchy from smallest to largest is:   $\text{Atom} \rightarrow \text{Molecule} \rightarrow \text{Macromolecule} \rightarrow \text{Organelle} \rightarrow \text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{Organ System} \rightarrow \text{Organism}$

• Energy and Matter Fundamentals:

  • Energy: The capability to do work. Atoms and molecules within biological systems are held together by chemical energy.

  • Matter: Anything that occupies space and possesses mass. All living organisms are composed of matter.

  • Element: A substance that cannot be broken down into simpler substances by ordinary chemical reactions.

  • Compound: A substance consisting of two or more different elements combined in a fixed ratio, exhibiting chemical characteristics distinct from its component elements.

• Chemical Reactions:

  • Defined as the making and breaking of chemical bonds.

  • Reactants: The starting molecules in a chemical reaction.

  • Products: The final molecules resulting from a chemical reaction.

  • Example Synthesis Reaction:     $2H_2 + O_2 \rightarrow 2H_2O$

  • Exothermic Reaction: A chemical reaction that releases net energy and heat ("exiting").

• Elements Essential to Life:

  • Most chemical compounds in living organisms contain $3$ to $4$ primary elements.

  • Sugars: Composed of Carbon ($C$), Hydrogen ($H$), and Oxygen ($O$).

  • Proteins: Account for $20\%$ of human body mass. Composed primarily of Carbon ($C$), Hydrogen ($H$), Oxygen ($O$), Nitrogen ($N$), and small amounts of Sulfur ($S$).

  • Major Bone & Tooth Constituents: Calcium ($Ca$) and Phosphorus ($P$).

  • Trace Elements: Elements required by organisms in extremely minute quantities (typically less than $0.01\%$ of body mass):

  • Include Boron ($B$), Chromium ($Cr$), Cobalt ($Co$), Copper ($Cu$), Fluorine ($F$), Iodine ($I$), Iron ($Fe$), Manganese ($Mn$), Molybdenum ($Mo$), Selenium ($Se$), Silicon ($Si$), Tin ($Sn$), Vanadium ($V$), and Zinc ($Zn$).

  • Iron ($Fe$): Makes up $0.004\%$ of human body weight; vital for energy processing and oxygen transport in hemoglobin.

  • Iodine ($I$): An essential trace element required specifically by vertebrates (including humans) for thyroid hormone production.

• Atomic Structure:

  • Atom: The smallest functional unit of matter that retains the chemical properties of an element.

  • Subatomic Particles:

  • Protons: Positively charged particles ($+$) located in the atomic nucleus.

  • Neutrons: Uncharged/neutral particles ($0$) located in the atomic nucleus.

  • Electrons: Negatively charged particles ($-$) orbiting in shells around the atomic nucleus.

  • Protons and neutrons possess substantially greater mass than electrons. Uncharged atoms carry equal numbers of positive protons and negative electrons.

• Atomic Mass and Atomic Number Calculations:

  • Atomic Number: Equal to the exact number of protons in an atom\'s nucleus.

  • Atomic Mass (Mass Number): Total number of protons plus neutrons in the nucleus.

  • Calculating Neutrons:     $\text{Number of Neutrons} = \text{Mass Number} - \text{Atomic Number}$

  • Neutral Atom: Possesses an equal number of protons and electrons ($\text{Protons} = \text{Electrons}$).

  • Ion: An atom or molecule with an electrical charge due to an unequal number of protons and electrons.

  • Examples:

  • Carbon ($^{12}_{6}C$): Atomic number = $6$ ($6$ protons), Atomic mass = $12.01\,\text{amu}$ ($6$ protons + $6$ neutrons).

  • Sodium ($^{23}_{11}Na$): Atomic number = $11$, Mass number = $23$. Protons = $11$, Electrons = $11$, Neutrons = $23 - 11 = 12$.

• Isotopes and Radioactive Isotopes:

  • Isotopes: Two or more atoms of the same element that have identical atomic numbers (same number of protons) but differ in atomic mass due to different numbers of neutrons (e.g., Carbon isotopes $^{12}_{6}C$, $^{13}_{6}C$, and $^{14}_{6}C$).

  • Radioactive Isotopes: Unstable isotopes that spontaneously decay over time, giving off subatomic particles and energy.

  • Applications: Widely used as diagnostic tools in medicine and biological research. Radioactive tracers are combined with imaging instruments to track metabolic pathways and atom movement through organisms.

Chemical Bonding and Molecular Interactions

• Electron Shells and Valence Structure:

  • Electron Shells: Three-dimensional regions of space around an atomic nucleus where electrons orbit.

  • Shell capacity: The first shell holds a maximum of $2$ electrons; the second outer shell holds up to $8$ electrons. Once a shell is filled, additional electrons fill the next outer shell.

  • Valence Electrons: Electrons located in the outermost electron shell (the valence shell).

  • Chemical Reactivity: An atom with a completely full valence shell is unreactive or inert (e.g., noble gases). An atom with an incomplete valence shell will chemically react with other atoms to share, gain, or lose electrons to complete its valence shell.

• Ionic Bonds:

  • Definition: A chemical bond formed by the complete transfer of one or more valence electrons from one atom to another, resulting in oppositely charged ions with full outer valence shells.

  • Occurs between metals and non-metals.

  • Example (Sodium Chloride, $\text{NaCl}$): A neutral Sodium ($\text{Na}$) atom with $1$ valence electron donates its electron to a Chlorine ($\text{Cl}$) atom with $7$ valence electrons. This produces a positively charged sodium cation ($\text{Na}^+$) and a negatively charged chloride anion ($\text{Cl}^-$).

• Biological Importance of Ionic Compounds:

  • Ionic compounds form salts. When salt bonds dissociate in water into $\text{Na}^+$ and $\text{Cl}^-$ ions, the resulting sodium ions regulate human blood volume, maintain blood pressure, balance physiological pH, transmit nerve impulses, and preserve the three-dimensional structures of functional proteins.

  • Electronegativity and Covalent Bonding:

• Electronegativity: The measure of an atom\'s ability to attract shared electrons to itself within a chemical bond. Higher electronegativity values indicate a stronger pull on electrons.

• Electronegativity Scale for Bond Types:

  • Difference of $0 - 0.5$: Nonpolar Covalent Bond

  • Difference of $0.5 - 1.9$: Polar Covalent Bond

  • Difference of $\ge 2.0$: Ionic Bond

• Nonpolar Covalent Bond: A covalent bond where two atoms share electrons equally because their electronegativity values are relatively equal.

• Polar Covalent Bond: A covalent bond where electrons are shared unequally between two atoms because one atom is significantly more electronegative. The shared electrons spend more time orbiting the cloud of the more electronegative atom, giving that atom a partial negative charge ($\delta^-$) and the less electronegative atom a partial positive charge ($\delta^+$).

• Dipole Moment: A quantitative measurement of charge separation across a polar covalent molecule. Dipole moment is a vector quantity possessing both direction and magnitude. A greater difference in electronegativity yields a larger dipole moment.

• Worked Example ($\text{HCl}$): Chlorine is substantially more electronegative than Hydrogen. The shared electron pair spends more time around the Chlorine atom, creating a partial positive charge ($\delta^+$) on Hydrogen and a partial negative charge ($\delta^-$) on Chlorine.

• Elements of Life Forming Covalent Bonds: Carbon ($C$), Oxygen ($O$), Nitrogen ($N$), and Hydrogen ($H$) form covalent bonds and together account for $96\%$ of all atoms in the human body. Energy-rich sugars like glucose and fructose are held together by covalent bonds.

  • Hydrogen Bonds:

• Definition: An attractive electrostatic force occurring between a hydrogen atom covalently bonded to a highly electronegative atom in one molecule and an electronegative atom in a neighboring molecule.

• Role in DNA: DNA consists of two twisted strands held together along their entire length by millions of hydrogen bonds connecting complementary nitrogenous bases. While the individual sugar-phosphate backbones are held by strong covalent bonds, the weak hydrogen bonds between strands allow DNA to uncoil, separate, and reform easily during replication.

Water and Its Essential Life-Sustaining Properties

• Chemical Structure of Water: Water ($\text{H}_2\text{O}$) is a polar molecule held together internally by polar covalent bonds. Individual water molecules form hydrogen bonds with adjacent water molecules.

• Cohesion and Adhesion:

  • Cohesion: The attractive force between like molecules (water molecules sticking to other water molecules via hydrogen bonds).

  • Transport in plants: Cohesion works in tandem with adhesion to pull water streams upward from roots to leaves.

  • Surface Tension: A measure of how difficult it is to stretch, deform, or break the liquid surface of a solvent. Cohesion provides water with high surface tension, allowing it to support small organisms exerting minimal force over a large relative surface area.

  • Adhesion: The attractive force between unlike molecules (water sticking to non-water surfaces or polar substrates).

  • Substrate Comparisons (Capillary Action):

  • Office Paper: Adhesion forces are stronger than internal cohesion; surface fibers absorb water readily.

  • Wax Paper: Composed of nonpolar molecules that repel water; low adhesion causes internal cohesion to pull water into rounded droplets.

  • Paper Towel: High adhesion and capillary action allow rapid absorption through microscopic porous spaces.

• Temperature Moderation and Specific Heat:

  • Water strongly resists rapid changes in temperature because it requires significant heat energy input to disrupt its extensive network of hydrogen bonds.

  • When heat is applied to water, the thermal energy is initially absorbed to break hydrogen bonds rather than immediately increasing the kinetic motion of the molecules.

  • Physical States of Water:

  • Gas (Steam): High kinetic energy; hydrogen bonds are completely broken and absent.

  • Liquid Water: Short-lived hydrogen bonds that continuously break, reform, and slip past one another.

  • Solid (Ice): Low kinetic energy; stable, locked hydrogen bonds hold molecules in a crystalline lattice furthest apart, making ice less dense than liquid water and causing water to expand when frozen.

  • High Heat of Vaporization: The exact amount of thermal energy required to convert $1\,\text{g}$ of a liquid into a gas.

  • Evaporative Cooling: As water evaporates from a surface, the highest-energy liquid molecules escape as gas, leaving behind cooler molecules. This allows sweating to absorb excessive body heat and maintain homeostatic thermal regulation.

  • Biological Hydration: Human cells contain high concentrations of water, helping transport nutrients and metabolic wastes while maintaining a stable internal temperature ($\approx 72\%$ of the human body is water).

• Water as a Solvent:

  • Water is an effective universal solvent for polar molecules and ionic compounds (hydrophilic / water-loving).

  • Water cannot dissolve uncharged, nonpolar compounds (hydrophobic / water-fearing), such as oils, fats, and butter.

  • Readily mixes with polar biological substances such as sugars, salts, beverages, and cellular juices to maintain electrolyte balance.

  • Dissolution of Table Salt ($\text{NaCl}$): When $\text{NaCl}$ enters water, the positive ends of polar water molecules attract negative chloride ions ($\text{Cl}^-$) and negative ends attract positive sodium ions ($\text{Na}^+$). This dissociates the crystal lattice, forming hydration shells around the individual ions.

Acids, Bases, pH, and Solutions

• Acids:

  • A chemical substance that donates hydrogen ions ($\text{H}^+$) to a solution, increasing the net concentration of $\text{H}^+$ ions.

  • Acids possess a $\text{pH}$ value less than $7$ ($\text{pH} = 0 - 6.99$).

  • Examples:

  • Citric Acid: Found in lemons and oranges.

  • Salicylic Acid: Used in facial cleansers and acne treatments.

  • Sulfuric Acid ($\text{H}_2\text{SO}_4$): Used in car batteries.       $\text{H}_2\text{SO}_4 \rightarrow 2\text{H}^+ + \text{SO}_4^{2-}$

  • Hydrochloric Acid ($\text{HCl}$): Produced in the stomach to aid digestion.       $\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$

• Bases:

  • A chemical compound that accepts hydrogen ions ($\text{H}^+$) or donates hydroxide ions ($\text{OH}^-$) in solution, lowering the relative concentration of free $\text{H}^+$ ions.

  • Bases possess a $\text{pH}$ value greater than $7$ ($\text{pH} = 7.01 - 14$).

  • Functional Group: Hydroxide ion ($\text{OH}^-$).

  • Example:     $\text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^-$     $\text{OH}^- + \text{H}^+ \rightarrow \text{H}_2\text{O}$

  • Common household bases include soap and baking soda, which neutralize acidic substances.

• Salts and Neutralization Reactions:

  • Neutralization: Occurs when an acid and a base react quantitatively to yield water and an ionic salt.

  • Example Neutralization Reaction:     $\text{NaOH} + \text{HCl} \rightarrow \text{Na}^+ + \text{OH}^- + \text{H}^+ + \text{Cl}^- \rightarrow \text{NaCl} + \text{H}_2\text{O}$

• The pH Scale and Measurement:

  • $pH$ Definition: A logarithmic measurement of the relative concentration of free hydrogen ions ($\text{H}^+$) in an aqueous solution.

  • Logarithmic Scale: Every single $1$-unit change on the $\text{pH}$ scale represents a $10$-fold ($10\times$) change in $\text{H}^+$ ion concentration.

  • Example: A $\text{pH}$ shift from $7$ to $8$ or $0$ to $3$ represents a change by a factor of $10^3 = 1000$.

• $\text{pH}$ Scale Reference Values:

  • $\text{pH} = 0$: Maximum concentration of $\text{H}^+$ ions.

  • $\text{pH} = 1$: Battery acid

  • $\text{pH} = 2$: Lemon juice

  • $\text{pH} = 3$: Vinegar, wine, cola

  • $\text{pH} = 4$: Tomato juice, beer, black coffee

  • $\text{pH} = 5$: Rainwater

  • $\text{pH} = 6$: Urine, saliva

  • $\text{pH} = 7$: Pure water, blood, tears (Neutral: Equal proportions of $\text{H}^+$ and $\text{OH}^-$ ions, where $\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-$)

  • $\text{pH} = 8 - 9$: Seawater (more basic due to dissolved salts), liquid inside small intestine

  • $\text{pH} = 10$: Milk of Magnesia

  • $\text{pH} = 11$: Household ammonia

  • $\text{pH} = 13$: Household bleach

  • $\text{pH} = 14$: Oven cleaner

Carbon and the Molecular Diversity of Life

• Carbon as the Molecular Backbone:

  • Living cells are composed of $70\%$ to $95\%$ water, with the remaining mass made up mostly of carbon-based compounds.

  • The term organic originated from the historical misconception that carbon-based molecules were exclusively produced by living organisms.

  • Organic Chemistry: The study of carbon-containing compounds, primarily involving Carbon ($C$), Hydrogen ($H$), Oxygen ($O$), Phosphorus ($P$), Nitrogen ($N$), and Sulfur ($S$).

• Chemical Properties of Carbon:

  • Carbon possesses an atomic number of $6$ with $4$ valence electrons in its outer shell. It forms $4$ covalent bonds to achieve a stable outer shell.

  • Bonding Flexibility: Carbon forms single, double, or triple covalent bonds.

  • Tetrahedral Geometry: When carbon forms four single covalent bonds, electron pair repulsion produces a tetrahedral shape with bond angles of $109.5^\circ$ (e.g., Methane, $\text{CH}_4$).

  • Carbon Double Bonds: Formed when two pairs of electrons are shared between carbon atoms ($\text{C}=\text{C}$). Double bonds create shorter, stronger, and less flexible bonds that prevent rotation, resulting in planar (flat) molecular geometries.

  • Silicon Comparison: Silicon cannot replace carbon as the basis of life because silicon-based compounds are less stable in aqueous conditions.

• Diversity of Carbon Skeletons:

  1. Chain Length: Carbon chains vary in length; longer carbon chains store greater chemical energy (e.g., Ethane $\text{C}_2\text{H}_6$).

  2. Hydrocarbons: Organic molecules consisting exclusively of carbon and hydrogen atoms (e.g., gasoline, diesel fuel). Hydrocarbons are nonpolar and undergo combustion reactions that release large amounts of energy.

  3. Branching: Carbon skeletons may be unbranched or branched, altering physical and chemical properties (e.g., Butane vs. 2-methylpropane).

  4. Double Bond Position: Double bonds can vary in location along the carbon chain (e.g., 1-butene vs. 2-butene).

  5. Cis-Trans (Geometric) Isomers:

  • Isomers that have identical covalent bond arrangements but differ in spatial orientation due to the inflexibility of double bonds.

  • Cis Isomer: Identical functional groups or hydrogen atoms are positioned on the same side of the carbon double bond. Generally healthier (e.g., cis-monounsaturated fats in olive oil).

  • Trans Isomer: Identical functional groups are positioned on opposite sides of the double bond. Harder for enzymes to digest due to altered spatial geometry.

  1. Ring Formation: Short and intermediate carbon chains can bend to form closed rings (e.g., Cyclohexane $\text{C}_6\text{H}_{12}$).

• Functional Groups in Organic Molecules:

  • Definition: Specific chemical groups attached to carbon skeletons that participate directly in chemical reactions. The number, chemical arrangement, and composition of functional groups determine molecular identity and function.

  • Hydroxyl Group ($-\text{OH}$): Polar due to electronegative oxygen; forms hydrogen bonds; hydrophilic. Found in alcohols and sugars.

  • Carbonyl Group ($>\text{C}=\text{O}$): Polar key component of sugars.

  • Carboxyl Group ($-\text{COOH}$): Polar; acts as an acid by donating $\text{H}^+$.

  • Amino Group ($-\text{NH}_2$): Polar; acts as a base by accepting $\text{H}^+$.

  • Sulfhydryl Group ($-\text{SH}$): Nonpolar; cross-links to stabilize protein structure.

  • Methyl Group ($-\text{CH}_3$): Nonpolar hydrocarbon group.

  • Phosphate Group ($-\text{OPO}_3^{2-}$ / $-\text{PO}_4^{2-}$): Polar; highly reactive; transfers energy between organic molecules.

  • Steroid Hormones (Estrogen vs. Testosterone): Both share a common four-ring carbon backbone, but differ in the specific functional groups attached to the rings, leading to distinct physiological responses.

  • Adenosine Triphosphate (ATP): Consists of an adenosine molecule attached to a chain of three polar phosphate groups; serves as the primary energy currency of the cell.

Biological Macromolecules: Carbohydrates and Lipids

• Overview of Macromolecules:

  • Life relies on four primary classes of large biological molecules: Carbohydrates, Lipids, Proteins, and Nucleic Acids.

  • Enzyme Rule: All enzymes are proteins, but not all proteins are enzymes.

  • Polymers: Long molecules consisting of many similar or identical building blocks (monomers) linked together by covalent bonds.

  • Monomers: The repeated individual structural units/building blocks that construct polymers.

• Synthesis and Breakdown of Polymers:

  • Dehydration Synthesis Reaction (Condensation): Synthesizes polymers by covalently joining two monomers (or a monomer to a growing polymer chain), releasing a water molecule ($\text{H}_2\text{O}$) for every bond formed.

  • Hydrolysis Reaction (Cleavage): Breaks covalent bonds between monomers within a polymer through the addition of a water molecule ($\text{H}_2\text{O}$) ("hydro" = water, "lysis" = breaking).

• Carbohydrates:

  • Organic molecules composed of Carbon, Hydrogen, and Oxygen in a strict $1:2:1$ ratio, with the empirical formula $(\text{CH}_2\text{O})_n$.

  • Serve as fuel sources, energy storage, structural support, and raw materials for ATP production. Carbohydrates are the most abundant macromolecules on Earth.

  • Classes of Carbohydrates:

  • Monosaccharides: Single simple sugar units (e.g., Glucose $\text{C}_6\text{H}_{12}\text{O}_6$).

  • Disaccharides: Two monosaccharides joined covalently by a glycosidic linkage formed via dehydration synthesis (e.g., Maltose formed by linking two glucose molecules via a $1,4$-glycosidic linkage).

  • Oligosaccharides: Short chains containing $2$ to $10$ monosaccharide units.

  • Polysaccharides: Complex polymers containing $10$ or more monosaccharide units.

  • Storage Polysaccharides:

  • Starch: Storage polysaccharide in plants consisting entirely of glucose monomers. Includes Amylose (unbranched) and Amylopectin (branched). Soluble in water, providing short-term quick energy.

  • Glycogen: Highly branched storage polysaccharide in animals stored in liver and muscle tissue. Branching allows rapid hydrolysis to release glucose during high energy demand.

  • Structural Polysaccharides:

  • Cellulose: Structural component of plant cell walls; the most abundant organic compound on Earth. Consists of long, unbranched chains of glucose held in rigid parallel fibers.

  • Chitin: Structural polysaccharide used by arthropods (insects, spiders) to build exoskeletons and by fungi in cell walls. Built from modified glucose monomers.

  • $\beta$-Amyloid: A specialized carbohydrate/protein structural complex involved in cellular plant frameworks.

• Lipids:

  • A diverse group of nonpolar biological molecules that do not dissolve in water (hydrophobic).

  • Three Main Categories:

  1. Fats (Triglycerides): Solid or liquid storage lipids found in animals and plants.

  2. Phospholipids: Primary structural components of cellular membranes.

  3. Steroids: Non-fatty acid signaling molecules that stabilize cell membranes.

• Fats (Triglycerides):

  • Synthesized via dehydration synthesis combining $1$ Glycerol molecule (a 3-carbon alcohol) with $3$ Fatty Acid chains (long hydrocarbon chains ending in a carboxyl group).

  • Saturated Fats:

  • No carbon-carbon double bonds ($\text{C}=\text{C}$) in the fatty acid tail.

  • Chains are fully saturated with hydrogen atoms, creating straight chains that pack tightly together.

  • Solid at room temperature (e.g., butter, cheese, animal fats).

  • Unsaturated Fats:

  • Contain $1$ or more cis double bonds ($\text{C}=\text{C}$) in the fatty acid tail.

  • Double bonds create a kink in the hydrocarbon chain, preventing molecules from packing tightly together.

  • Liquid at room temperature (e.g., olive oil, plant oils).

  • Dietary benefit: Helps reduce bad cholesterol (LDL) and supports cardiovascular health.

  • Functions of Fats:

  1. Long-term energy storage (stores more than twice as much energy per gram as carbohydrates).

  2. Cushions vital internal organs in animals.

  3. Thermal insulation against body heat loss. Low body fat reserves increase susceptibility to hypothermia.

• Phospholipids:

  • Composed of $1$ Glycerol bound to $2$ hydrophobic Fatty Acid tails and $1$ hydrophilic Phosphate head group.

  • Form a phospholipid bilayer in aqueous cellular environments, creating a semi-permeable cell membrane that regulates substance movement into and out of cells.

• Steroids:

  • Lipids that lack fatty acid tails; defined by a characteristic fused backbone of four carbon-based rings.

  • Function as signaling molecules (steroid hormones like Testosterone and Estrogen) and structural membrane components (e.g., Cholesterol maintains membrane fluidity and stability).

Biological Macromolecules: Proteins and Nucleic Acids

• Proteins:

  • Account for over $50\%$ of the dry mass of most cells and perform diverse cellular functions:

  • Structural support (e.g., collagen in animal tissues).

  • Transport (e.g., membrane transport channels, hemoglobin).

  • Chemical signaling (e.g., insulin, growth hormones).

  • Immune defense (e.g., antibodies fighting infections).

  • Enzymatic catalysis (making or breaking chemical bonds).

  • Energy processing and storage.

• Amino Acid Structure:

  • Proteins are constructed from a universal set of $20$ standard Amino Acids.

  • Named amino acids include: Glutamine, Isoleucine, Asparagine, Serine, Threonine, Lysine, Arginine, and Phenylalanine (nonpolar).

  • Basic Structure: Every amino acid consists of a central alpha-carbon ($\text{C}_\alpha$) covalently bound to four components:

  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 characteristics of the variable $\text{R}$ group determine the specific properties (polar, nonpolar, acidic, basic) of each amino acid.

• Peptide Bonds and Polypeptides:

  • Amino acids are joined together covalently by peptide bonds formed through dehydration synthesis reactions between the carboxyl group of one amino acid and the amino group of another.

  • A continuous polymer of amino acids linked by peptide bonds is a Polypeptide.

• Four Levels of Protein Structure:

  1. Primary Structure: The unique linear sequence of amino acids in a polypeptide chain, linked by peptide bonds. Dictated by genetic instructions in DNA. A single amino acid substitution can cause misfolding and disease (e.g., Alzheimer\'s disease, sickle cell anemia).

  2. Secondary Structure: Coils and folds in the polypeptide backbone resulting from hydrogen bonds between backbone carbonyl oxygen atoms ($\text{C}=\text{O}$) and amide hydrogen atoms ($\text{N}-\text{H}$).

  • Alpha-helix ($\alpha$-helix): A delicate helical coil.

  • Beta-pleated sheet ($\beta$-pleated sheet): Parallel or antiparallel folded sheets.

  1. Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain, produced by interactions between variable $\text{R}$ group side chains:

  • Hydrophobic interactions (nonpolar $\text{R}$ groups cluster in the interior away from water).

  • Hydrogen bonding between polar $\text{R}$ groups.

  • Ionic bonds between charged $\text{R}$ groups.

  • Disulfide bridges: Strong covalent bonds formed between sulfur-containing side chains (cysteine residues).

  1. Quaternary Structure: The structural shape resulting from the association of two or more independent polypeptide subunits.

  • Example (Hemoglobin): A functional quaternary protein consisting of $4$ subunits ($2$ $\alpha$-subunits and $2$ $\beta$-subunits) bound to Iron ($Fe$) to transport oxygen.

• Nucleic Acids:

  • Functions: Store, transmit, and express hereditary genetic information.

  • Two Main Types:

  • DNA (Deoxyribonucleic acid): Encodes the inherited genetic blueprint.

  • RNA (Ribonucleic acid): Serves as the active gene copy to direct protein synthesis.

  • Monomer Structure: Polymer of Nucleotides. Each nucleotide contains:

  1. A Phosphate group ($-\text{PO}_4^{2-}$) attached to the $5'$ carbon of the sugar.

  2. A 5-carbon Pentose Sugar (Deoxyribose in DNA; Ribose in RNA).

  3. A Nitrogenous Base attached to the $1'$ carbon of the sugar.

• Structural Comparisons Between DNA and RNA:

  • Pentose Sugar Difference: Deoxyribose in DNA lacks an oxygen atom at the $2'$ carbon position; Ribose in RNA possesses a hydroxyl group ($-\text{OH}$) at the $2'$ carbon position.

  • Nitrogenous Bases:

  • Pyrimidines (single 6-membered ring): Cytosine ($C$), Thymine ($T$, DNA only), Uracil ($U$, RNA only).

  • Purines (6-membered ring fused to a 5-membered ring): Adenine ($A$), Guanine ($G$).

• Directionality and Strands:

  • Both DNA and RNA strands possess strict directionality:

  • $5'$ End: Terminates with a Phosphate group.

  • $3'$ End: Terminates with a Hydroxyl group ($-\text{OH}$) on the pentose sugar.

  • DNA Structure: Double-stranded double helix running in an antiparallel orientation (one strand runs $5' \rightarrow 3'$, the other runs $3' \rightarrow 5'$). Covalent phosphodiester bonds form the sugar-phosphate backbone, while complementary hydrogen bonding joins opposing base pairs:

  • Adenine ($A$) pairs with Thymine ($T$).

  • Guanine ($G$) pairs with Cytosine ($C$).

  • RNA Structure: Single-stranded (can form localized secondary helices); contains Uracil ($U$) instead of Thymine ($T$), pairing $A$ with $U$.

• Three Primary Types of RNA:

  1. Messenger RNA (mRNA): The "blueprint / letter carrier" that transfers genetic coding instructions from nuclear DNA to the ribosome.

  2. Transfer RNA (tRNA): The "worker" that carries specific amino acids to the ribosome during translation.

  3. Ribosomal RNA (rRNA): The structural and catalytic component of the ribosome that synthesizes proteins.