Fundamentals of Human Biology - Homeostasis and Chemistry Vocabulary

Fundamentals of Human Biology and Homeostasis

  • Homeostasis Definition: Homeostasis is the ability of the body to maintain a relatively stable internal environment despite changing external conditions.

  • Role of Physiology: Physiology encompasses the specific mechanisms utilized by the body to maintain homeostasis.

  • Consequences of Homeostatic Failure: Disrupted homeostasis leads directly to injury or death.

  • Feedback Loop Classification: Homeostatic mechanisms are categorized into either positive or negative feedback loops.

Feedback Loop Components and Mechanisms

  • Four Key Components of Feedback Loops:

    • Internal Variable: A chemical or physical property of the body that is monitored for homeostasis (e.g., blood temperature).

    • Setpoint: The desired target value or physiological range of an internal variable (e.g., 97.7−98.6 ∘F97.7 - 98.6\,^\circ\text{F}).

    • Receptor(s): Specialized structures that detect changes in the internal environment away from the setpoint (e.g., blood thermoreceptors).

    • Control Center: A structure that analyzes incoming information from receptors and determines the appropriate output response for effectors. Typically located in the brain or spinal cord (e.g., the hypothalamus).

    • Effectors: Structures capable of carrying out a cellular or physiological response (e.g., skeletal muscle contraction leading to shivering).

  • Negative Feedback Loops:

    • Mechanism: A receptor detects a change in an internal variable away from its setpoint and alerts the control center. The control center activates effectors to negate and reverse the change, restoring the variable back to setpoint.

    • Physiological Outcome: Results in slight, temporary fluctuations above or below the setpoint.

    • Directionality: Negative feedback loops act on internal variables regardless of whether they are above or below setpoint; "negative" refers to negating the direction of deviation, not that the variable is below setpoint.

    • Example: Thermoregulation of blood temperature.

  • Positive Feedback Loops:

    • Mechanism: A receptor detects a change in an internal variable away from setpoint. The control center activates effectors to further move the internal variable away from setpoint, amplifying the original stimulus.

    • Physiological Outcome: Amplifies changes in internal variables away from baseline setpoint.

    • Essential Processes: Critical for specific finite physiological processes, such as childbirth and blood clotting.

Case Study: Blood Calcium Homeostasis in Lontra canadensis

  • Physiological Context: Calcium in the blood is critical for muscle function (including cardiac contraction), neuronal function, and blood clotting physiology.

  • Baseline Setpoint: In Lontra canadensis (North American River Otter), typical blood calcium concentrations range from 8.6−10.2 mg/dL8.6 - 10.2\,\text{mg/dL}.

  • Disruption & Detection: When blood calcium drops to 7.2 mg/dL7.2\,\text{mg/dL}, blood chemoreceptors associated with the endothelial cells of the parathyroid gland detect this deviation below setpoint.

  • Control Center Response: The parathyroid gland functions as the control center and secretes parathyroid hormone (PTH).

  • Effector Action: Parathyroid hormone (PTH) acts on bone tissue and the kidneys, stimulating them to conserve and replace blood calcium to return levels to setpoint.

Structural Complexity of Life

  • Ten Levels of Structural Organization (arranged from simplest to most complex):

    1. Sub-atomic particles

    2. Atoms

    3. Molecules

    4. Macromolecules

    5. Organelles

    6. Cells

    7. Tissues

    8. Organs

    9. Organ systems

    10. Organisms

Chemical Principles: Salts, Electrolytes, and Water

  • Salts:

    • Ionic compounds composed of an anion (negatively charged) and a cation (positively charged).

    • Hydrogen (H+H^+) and Hydroxide (OH−OH^-) ions are explicitly excluded from the definition of salts.

    • Dissociate completely into individual ions when dissolved in water.

  • Electrolytes:

    • Free ions resulting from dissociated salts in solution.

    • Essential for forming specific biological solutions required for electrical conduction, salinity control, and osmolarity maintenance.

    • Most abundant electrolytes in the human body:

    • Sodium (Na+Na^+)

    • Calcium (Ca2+Ca^{2+})

    • Magnesium (Mg2+Mg^{2+})

    • Chloride (Cl−Cl^-)

    • Phosphate (PO43−PO_4^{3-})

    • Bicarbonate (HCO3−HCO_3^-)

  • Biological Roles of Water:

    • The single most abundant chemical compound in living organisms.

    • Serves as a primary transport medium, a reactant in metabolic reactions, and a lubricant in serous membranes.

    • Serous Membranes: Thin, double-layered membranes containing serous fluid (composed primarily of water) between layers, allowing internal organs and structures to slide smoothly without friction.

  • Chemical Properties of Water:

    • Electronegativity & Polarity: Nuclei exert an attractive force on orbiting electrons called electronegativity. Differential electronegativity creates partial charges, making water a polar molecule.

    • Cohesion & Adhesion: Tendency of polar compounds to stick to one another or to other charged molecules via ionic interactions between partial positive and negative charges.

    • Solvent Properties: Water molecules form hydrogen bonds to and around polar solutes, surrounding each molecule to achieve dissolution ("like dissolves like").

    • Polar solvents dissolve polar solutes (e.g., table salt, NaClNaCl).

    • Non-polar solvents dissolve non-polar solutes (e.g., dish soap components dissolving non-polar greases).

    • Reactivity: Participates directly in dehydration synthesis and hydrolysis reactions.

Types of Chemical Bonds

  • Hydrogen Bond: A weak attractive force between two partial electrical charges.

  • Covalent Bond: A strong attractive force between two atoms that share valence electrons (co-valence).

  • Ionic Bond: A moderate attractive force between two oppositely charged ions (cation and anion).

Introduction to Organic Macromolecules

  • Classification:

    • Organic Compounds: Contain carbon atoms; unique to living biological systems.

    • Inorganic Compounds: Do not contain carbon atoms.

  • Polymerization Architecture:

    • Monomers: Small biological molecules used as basic building blocks to synthesize larger molecules. Monomers often possess independent physiological functions.

    • Polymers: Large, repetitive chain molecules constructed from linked monomers.

    • Macromolecule Exception: Lipids are the only class of biological macromolecules that are not true polymers.

  • Four Primary Classes of Biological Macromolecules:

    1. Carbohydrates

    2. Lipids

    3. Proteins

    4. Nucleic Acids

  • Synthesis and Breakdown Mechanisms:

    • Dehydration Synthesis: The process by which polymers are built. A molecule of water (H2OH_2O) is removed to create molecular space for a new covalent bond to form between monomers.

    • Hydrolysis: The process of breaking polymers apart into individual monomers. A covalent bond is broken by adding a molecule of water to occupy the newly exposed bonding sites.

    • HONC Bonding Rules (Number of covalent bonds formed per atom):

    • Hydrogen (HH): 1 bond

    • Oxygen (OO): 2 bonds

    • Nitrogen (NN): 3 bonds

    • Carbon (CC): 4 bonds

Carbohydrates

  • General Properties:

    • Hydrophilic in nature.

    • Naming conventions utilize the root -sacchar (e.g., monosaccharide, disaccharide, polysaccharide) or the suffix -ose (e.g., glucose, sucrose, fructose, lactose).

  • Biological Functions:

    • "Fast fuel" for immediate metabolic energy.

    • Short- and long-term energy storage (glycogen in animals, starch in plants).

    • Structural components (ribose in nucleic acids, cell membrane markers, cellulose in plants).

  • Monosaccharides (Carbohydrate Monomers):

    • Four major biological monosaccharides:

    1. Glucose

    2. Fructose

    3. Ribose / Deoxyribose

    4. Galactose

  • Polysaccharides:

    • Long covalent chains of monosaccharide monomers formed via dehydration synthesis.

    • Glycogen: The major storage polysaccharide in humans. Excess absorbed glucose is converted into glycogen or fat.

    • Storage sites and reserves:

    • Liver Glycogen: Serves as a whole-body blood sugar reserve.

    • Muscle Glycogen: Serves strictly as a local reserve for ATP generation within muscle cells.

  • Muscle Glycogen Pathway:

    1. Muscle contraction demands massive quantities of ATP.

    2. Muscles primarily metabolize glucose to produce ATP.

    3. Dietary glucose absorbed from the intestines is imported into muscle cells and converted into glycogen via glycogenesis.

    4. Under metabolic demand, glycogen is broken down into glucose via glycogenolysis to supply glycolysis for ATP production.

Lipids

  • General Characteristics:

    • Most lipids are non-polar and hydrophobic.

    • Non-polymeric structure.

  • Three Primary Types of Lipids:

    1. Triglycerides

    2. Phospholipids

    3. Steroids

  • Triglycerides:

    • Structure: Composed of 1 glycerol backbone bound to 3 fatty acid chains.

    • Energetics: Exceptionally energy-dense molecules.

    • Primary Functions:

    1. Long-term energy storage

    2. Thermal insulation

    3. Physical cushioning and protection of internal organs

    • Distinctive Note: Triglyceride is a chemical molecule, not synonymous with adipose tissue.

  • Fatty Acid Structure & Classification:

    • Structure: Central carbon chain backbone populated by bound hydrogen atoms.

    • Saturated Fatty Acids: All available carbon bonding sites are fully occupied by hydrogen atoms (no double bonds).

    • Unsaturated Fatty Acids: Containing one or more double covalent bonds between carbon atoms, leaving fewer sites bound to hydrogen.

    • Chain Length Classifications: Short-chain fatty acids (SCFA), Medium-chain fatty acids (MCFA), and Long-chain fatty acids (LCFA).

  • Hydrogenation and Trans Fats:

    • Hydrogenation: Industrial processing that synthetically adds hydrogen atoms to unsaturated fatty acid chains.

    • Trans Fats: Partially hydrogenated fats that behave chemically like saturated fats while remaining structurally unsaturated. They cause severe metabolic disruption to blood lipid regulation and are considered structurally unnatural.

  • Phospholipids:

    • Structure: Produced by modifying a triglyceride—1 glycerol backbone + 2 fatty acid chains + 1 polar phosphate group.

    • Properties: Amphiphilic (containing both hydrophobic fatty acid tails and a hydrophilic phosphate head).

    • Membrane Function: Spontaneously forms lipid bilayers in aqueous solutions. Essential constituent of cell membranes (plasma membrane, mitochondrial membrane, lysosomal membrane, endoplasmic reticulum membrane).

  • Steroids:

    • Parent Molecule: Cholesterol is the precursor for all steroid molecules.

    • Functions: Nervous system activity, plasma membrane structural integrity, and organism-level hormonal signaling (steroid hormones).

    • Biosynthesis: Unique to animal cells. Typically, 15%15\% is obtained through dietary intake, while 85%85\% is synthesized endogenously by the liver.

Proteins

  • General Structure:

    • Polymers constructed from amino acid monomers linked together by specialized covalent bonds called peptide bonds.

    • Translated from mRNA templates at the ribosome.

    • Complex three-dimensional folding directly dictates biological function.

  • Amino Acid Architecture:

    • Composed of 20 distinct amino acids, each containing:

    1. Central carbon atom

    2. Hydroxyl group

    3. Carboxylic acid group

    4. Distinct functional R group (side chain that determines specific chemical characteristics)

  • Polypeptide Chain Terminology:

    • Dipeptide: Two amino acids linked by a peptide bond.

    • Tripeptide: Three amino acids linked by peptide bonds.

    • Polypeptide: A long continuous chain of many amino acids (typically on the order of hundreds).

    • Protein: A fully folded, functional polypeptide chain. Proteins remain inactive until proper folding occurs.

  • Four Levels of Protein Structure:

    • Primary Structure: The linear, sequential chain of amino acids (polypeptide).

    • Secondary Structure: Local structural patterns formed by hydrogen bonding along the polypeptide backbone:

    • Alpha (α\alpha) Helix: Coiled spiral structure stabilized by hydrogen bonds.

    • Beta (β\beta) Sheet: Pleated sheet formed by adjacent strands zig-zagging back and forth, stabilized by hydrogen bonds.

    • Tertiary Structure: The overall 3D shape of a single polypeptide, governed by R group interactions. (Example: Myoglobin, an oxygen transport protein in muscle containing 153153 amino acids, molecular weight 17,699 Da17,699\,\text{Da}, composed of 70%70\% α\alpha-helices).

    • Quaternary Structure: Structural arrangement formed by the assembly of two or more polypeptide subunits.

    • Transthyretin: Functional protein formed by four identical transthyretin subunits.

    • Hemoglobin: Functional protein composed of two α\alpha chains and two β\beta chains bound to heme groups.

  • Forces Driving Protein Folding:

    1. Hydrogen Bonds: Interactions between polar R groups.

    2. Ionic Bonds: Electrostatic interactions between positively and negatively charged R groups.

    3. Hydrophobic Interactions: Aggregation of non-polar R groups inward away from aqueous surrounding fluid.

    4. Disulfide Bonds: Strong covalent bonds formed specifically between cysteine residues.

    5. Van Der Waals Interactions: Weak, short-range attractive forces resulting from transient electron density fluctuations.

  • Protein Denaturation:

    • Unfolding or disruption of the 3D tertiary/quaternary structure causes immediate loss of biological function.

    • Denaturing Agents: Disruptions in pH, temperature, or salinity.

  • Functional Roles of Proteins ("Cellular Tools"):

    • Catalysis (Enzymes)

    • Recognition and immune protection

    • Biological transport

    • Intercellular communication

    • Structural support and cellular framework

    • Cell adhesion and movement

  • Characteristics of Enzymes:

    1. Required for most chemical reactions in the human body.

    2. Increase the rate of chemical reactions.

    3. Decrease the activation energy (energetic cost) of reactions.

    4. Bind specific substrate molecules.

    5. Yield specific chemical products.

    6. Are not consumed or permanently altered by the reaction.

    7. Substrates bind at a specific active site on the enzyme.

    8. Display high substrate specificity.

    9. Nomenclature usually ends with the suffix -ase.

Nucleic Acids and Energy Transfer

  • Structure of Nucleic Acids:

    • Polymers composed of nucleotide monomers.

    • Nucleotide components:

    1. Nitrogenous base

    2. Sugar (Deoxyribose or Ribose)

    3. Phosphate group

  • Comparison of DNA and RNA:

    • Deoxyribonucleic Acid (DNA):

    • Biological Role: Acts as the master cellular genetic code ("Cookbook").

    • Chain Length: Approximately 100 million100\,\text{million} to 1 billion1\,\text{billion} nucleotides in length.

    • Strands: Double-stranded helix with complementary base pairing.

    • Sugar: Deoxyribose.

    • Bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T).

    • Ribonucleic Acid (RNA):

    • Biological Role: Carries out genetic instructions from DNA to synthesize proteins ("Shopping list").

    • Chain Length: Typically ranges from 7070 to 10,00010,000 nucleotides in length.

    • Classes: Comprises 3 functional types.

    • Strands: Typically single-stranded.

    • Sugar: Ribose.

    • Bases: Adenine (A), Guanine (G), Cytosine (C), Uracil (U).

  • Adenosine Triphosphate (ATP):

    • Cellular role as the primary "energy currency" of the cell.

    • Structurally a modified nucleotide specialized for immediate energy transfer.

    • Energy Storage: High-energy covalent bonds link the 2nd and 3rd phosphate groups.

    • Energy Release: Hydrolysis breaks the terminal phosphate bond, freeing energy to drive cellular processes.

  • Categories of Cellular Work Driven by ATP:

    • Transport Work: ATP phosphorylates membrane transport proteins, activating them to pump solutes (e.g., ions) across cell membranes against gradients.

    • Mechanical Work: ATP phosphorylates contractile proteins in muscle cells, causing structural changes that allow cell shortening and muscle contraction.

    • Chemical Work: ATP phosphorylates key chemical reactants, donating energy to drive endergonic (energy-absorbing) chemical synthesis reactions (A+B→A-BA + B \rightarrow A\text{-}B).

Practice Questions and Discussion Review

  • Chemical Bonding Mechanisms:

    • Covalent Bond Formation: Formed when two non-metal atoms share valence electrons to achieve structural stability.

    • Hydrogen Bond Formation: Formed when partial positive charges on hydrogen atoms in polar molecules electrostatically attract partial negative charges on adjacent polar molecules.

  • Role of Water in Polymer Dynamics:

    • Dehydration Synthesis: Water is produced and removed as a byproduct to open bonding sites for monomer linkage.

    • Hydrolysis: Water is consumed and split to satisfy chemical bonding sites as polymers are broken into monomers.