Anatomy and Physiology: The Chemical Level of Organization - Chemical Reactions, Water, and pH

The Fundamental Chemical Reactions and Molecular Formation

• Fundamental Chemical Reactions Concept: The atoms and molecules involved in the three fundamental chemical reactions can be conceptualized and imagined as words according to Figure 2.12.

• Monomers and Polymers:

  • Monomers: These are the basic units used for building larger molecules.

  • Polymers: These consist of two or more monomers that are chemically bonded together.

Detailed Mechanisms of Dehydration Synthesis and Hydrolysis

• Dehydration Synthesis:

  • Definition: A reaction where two monomers are covalently bonded together.

  • Process: One monomer gives up a hydroxyl group ($OH$) and the other monomer gives up a hydrogen atom ($H$) at the specific site of bond formation.

  • Byproduct: A molecule of water ($H_2O$) is released as a byproduct during the reaction.

  • Result: The monomers become linked by a covalent bond.

• Hydrolysis:

  • Definition: A reaction where the covalent bond between two monomers is split.

  • Process: The addition of a water molecule ($H_2O$) is required. A hydrogen atom ($H$) is added to one monomer, and a hydroxyl group ($OH$) is added to the other monomer.

  • Result: The monomers are released from their covalent bond.

Enzymatic Regulation of Chemical Reactions

• Enzymes and Activation Energy:

  • Enzymes function to decrease the activation energy required for a specific chemical reaction to take place (Figure 2.13).

  • Reaction Without an Enzyme: The initial energy input needed for a chemical reaction to begin is high.

  • Reaction With an Enzyme: With the assistance of an enzyme, significantly less energy is required for the reaction to commence.

Physical and Chemical Properties of Water ($H_2O$)

• Biological Composition: Water accounts for approximately $\frac{2}{3}$ of total body mass.

• Molecular Structure:

  • Water is a polar covalent molecule.

  • Hydrogen components within the molecule carry a slight positive charge ($\delta+$).

  • The oxygen component carries a slight negative charge ($\delta-$).

• Hydrogen Bonding:

  • Water readily forms hydrogen bonds.

  • A hydrogen bond (often indicated by a dotted line) occurs when the slightly positive ends ($\delta+$) of water molecules align with the slightly negative ends ($\delta-$) of other water molecules.

• Surface Tension:

  • The combined strength of water's hydrogen bonds results in high surface tension.

  • Example: A water strider is able to walk on the surface of a pond due to this high surface tension.

Water Interaction and Solubility

• General Interaction: Water ($H_2O$) can interact with any polar covalent or ionic compounds.

• Dissociation of Ionic Compounds:

  • Example: Sodium Chloride ($NaCl$): When sodium chloride is placed in water, the crystals dissociate into sodium cations ($Na^+$) and chloride anions ($Cl^-$) rather than staying as $NaCl$ molecules.

  • Each ion becomes completely surrounded by water molecules (Figure 2.15).

• Hydrophilic Compounds:

  • Ionic Compounds: These are hydrophilic because ions like sodium and chloride have positive and negative charges that water can "grab."

  • Polar Covalent Compounds: These are hydrophilic. For example, carbon monoxide molecules possess positive and negative charges that allow water to interact with them.

• Hydrophobic Compounds:

  • Nonpolar Covalent Compounds: These are hydrophobic. For example, Methane ($CH_4$) molecules lack positive and negative charges, leaving nothing for water to interact with.

Biological and Chemical Functions of Water

• Lubricant: Acts to reduce friction between surfaces.

• Reactant for Hydrolysis: Water is a necessary component for the chemical breakdown of polymers.

• Polar Solvent: Capable of dissolving a wide variety of polar and ionic substances.

• Cushioning Agent: Protects organs and tissues from physical trauma.

• High Heat Capacity: Water can absorb a significant amount of heat before its temperature changes.

• High Heat of Vaporization: Requires a large amount of energy to convert from liquid to gas.

• Surface Tension: Provides structural integrity to the surface of water bodies and biological interfaces.

Acid-Base Chemistry and the pH Scale

• Acids:

  • In an aqueous solution, an acid dissociates into hydrogen ions ($H^+$) and anions.

  • Strong Acids: Nearly every molecule of a strong acid dissociates, resulting in a high concentration of $H^+$.

  • Example: Hydrochloric acid ($HCl$) releases $H^+$ and increases the total hydrogen ion concentration of a solution.

• Bases:

  • In an aqueous solution, a base dissociates into hydroxyl ions ($OH^-$) and cations.

  • Strong Bases: Nearly every molecule of a strong base dissociates, resulting in a high concentration of $OH^-$.

  • Mechanism: A base (such as Sodium Bicarbonate, $NaHCO_3$) binds free $H^+$ and therefore decreases the overall $H^+$ concentration of the solution.

• The pH Scale:

  • Range: The scale runs from $0$ to $14$.

  • Measurement: pH is a measurement of the concentration of hydrogen ions ($[H^+]$) in a solution.

  • Neutral: A pH of $7$ is considered neutral.

  • Acidic: Solutions with a pH below $7$ (ranging from $0$ to less than $7$) are acidic.

  • Basic (Alkaline): Solutions with a pH above $7$ (ranging from greater than $7$ to $14$) are basic/alkaline.

• Logarithmic Nature of the pH Scale:

  • As the $[H^+]$ increases, the pH value decreases.

  • Each unit change on the pH scale represents a ten-fold (10x) increase or decrease in $H^+$ concentration.

  • Strength Comparisons:

    • Solution A (pH $5$) vs. Solution B (pH $4$): B is $10x$ stronger than A.

    • Solution B (pH $4$) vs. Solution C (pH $2$): C is $100x$ stronger than B ($10 \times 10$).

    • Solution A (pH $5$) vs. Solution C (pH $2$): C is $1,000x$ stronger than A ($10 \times 10 \times 10$).

pH Values of Common Substances

• pH 0: Battery acid, Hydrochloric acid ($HCl$).

• pH 2: Lemon juice, vinegar.

• pH 2.5 - 3.5: Grapefruit juice, soda, tomato juice.

• pH 5: Black coffee.

• pH 6.3 - 6.6: Milk, urine, saliva.

• pH 7: Neutral (Water).

• pH 7.4: Human Blood.

• pH 8: Sea water.

• pH 9.5: Baking soda.

• pH 10.5 - 11.5: Ammonia solution.

• pH 13.5: Bleaches, oven cleaner, lye.

• pH 14: Liquid drain cleaner.

Class Activity Data

• Acid/Base Classification Activity:

  • Solution A (pH $6.2$): Acid.

  • Solution B (pH $7.4$): Base.

  • Solution C (pH $5.0$): Acid.

  • Solution D (pH $2.4$): Acid.

  • Solution E (pH $11.0$): Base.

• Strength Comparison Activity (Which is stronger?):

  • Between A (pH $6.2$) and B (pH $6.8$): Solution A is a stronger acid.

  • Between C (pH $5.0$) and D (pH $2.4$): Solution D is a stronger acid.

  • Between E (pH $11.0$) and F (pH $8.6$): Solution E is a stronger base.

• Magnitude of Strength Activity (How much stronger is the second vs. first?):

  • Solution A (pH $6.0$) to Solution B (pH $4.0$): B is 100x stronger than A.

  • Solution C (pH $5.0$) to Solution D (pH $1.0$): D is 10,000x stronger than C.

  • Solution E (pH $8.6$) to Solution F (pH $11.6$): F is 1,000x stronger than E.