Biology and Water: The Chemistry of Life

Definition of Biology: Biology is defined as the scientific study of life on Earth.
Central Theme: Evolution by natural selection is the overarching theme and the process of change that has produced the diverse life observed on the planet.
Theoretical Utility: Evolution makes sense of all existing knowledge regarding living organisms.
Ancestry: All organisms living on Earth are modified descendants of common ancestors.
Unsolved Mysteries: The origin of life and the nature of the Last Universal Common Ancestor (LUCA) remain two of the greatest unsolved mysteries in biology.
Biological Inquiry: Biologists ask fundamental questions such as: - How does DNA encode the cellular machinery? - How does a single cell develop into a complex organism? - How does the human brain function? - How do organisms interact within their communities?

Scientific Definition: Life is a self-sustaining chemical system capable of Darwinian evolution.
Essential Characteristics of Life: - Order: The presence of highly ordered structures. - Energy Processing: The utilization of chemical energy to power activities and chemical reactions. - Evolutionary Adaptation: The development of traits over generations that suit individuals to their environments for reproductive success. - Regulation: The maintenance of a stable internal environment (homeostasis). - Reproduction: Organisms producing their own kind. - Response to the Environment: Ability to react to environmental stimuli. - Growth and Development: Controlled by inherited DNA.

Multidisciplinary Nature: Biology is a multidisciplinary science; living organisms are subject to the basic laws of physics and chemistry.
Matter: Anything that has mass and takes up space. Matter is composed of elements.
Element: A substance that cannot be broken down into other substances through chemical reactions.
Compound: A substance consisting of two or more elements in a fixed ratio, possessing characteristics different from its constituent elements.
Molecule: Two or more atoms held together by chemical bonds.
Illustrative Example: Sodium ( $Na$ ) is a metal and Chlorine ( $Cl$ ) is a toxic gas; however, when combined in a fixed ratio, they form Sodium Chloride ( $NaCl$ ), common table salt, which is safe for consumption. Salt is described as being "safer than the sum of its parts."

Essential Elements: Approximately 25 out of the roughly 80 stable elements are essential to life.
Major Elements ( $96.3\%$ of human body mass): - Oxygen ( $O$ ): $65.0\%$ - Carbon ( $C$ ): $18.5\%$ - Hydrogen ( $H$ ): $9.5\%$ - Nitrogen ( $N$ ): $3.3\%$
Minor Essential Elements ( $3.7\%$ of human body mass): - Calcium ( $Ca$ ): $1.5\%$ - Phosphorus ( $P$ ): $1.0\%$ - Potassium ( $K$ ): $0.4\%$ - Sulfur ( $S$ ): $0.3\%$ - Sodium ( $Na$ ): $0.2\%$ - Chlorine ( $Cl$ ): $0.2\%$ - Magnesium ( $Mg$ ): $0.1\%$
Trace Elements: Required in minute quantities (less than $0.01\%$ of mass). These 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 ( $Zn$ ).
Deficiency Effects: - Nitrogen deficiency in plants leads to stunted growth. - Iodine deficiency in humans causes goiter.

Global Context: Water's abundance distinguishes Earth as the "Blue Planet."
Biological Role: Water is the primary biological solvent. Cells are roughly $70\text{--}95\%$ water.
Habitability: Water's abundance is the primary reason Earth is habitable.
Scientific News (January 2025): Research on Asteroid Bennu (OSIRIS-REx mission) published in "Nature" (Sekine; McCoy et al. Nature 637, 1072-1077): - Bennu samples contain salts from ancient brine, providing evidence of water history in the early solar system. - Samples are "volatile-rich" with high levels of carbon, nitrogen, and ammonia. - Findings include 14 of the 20 amino acids used in biology, all 5 nucleobases of DNA/RNA, and approximately 10,000 N-bearing species. - Amino acids found were racemic, suggesting that the left-handed chirality bias of terrestrial life may not come from extraterrestrial impact delivery.

Polarity: Water is a polar molecule where oxygen has a partial negative charge ( $\delta -$ ) and hydrogens have partial positive charges ( $\delta +$ ).
Hydrogen Bonds: Opposite charges allow water molecules to form bonds with each other and interact with polar molecules or dissolved ions.
Four Emergent Properties of Water: - 1. Cohesive Behavior: Hydrogen bonds hold water together (cohesion), assisting water transport against gravity in plants. Adhesion is the attraction between water and different substances (e.g., cell walls). - 2. Moderation of Temperature: High specific heat and high heat of vaporization. - 3. Expansion Upon Freezing: Ice is less dense than liquid water. - 4. Versatility as a Solvent: Dissolves many molecules and exhibits acid/base properties through autoionization.

Autoionization: Water molecules dissociate into Hydronium ( $H^+$ ) and Hydroxide ( $OH^-$ ) ions.
Ion Product Constant: In aqueous solutions at $25^\circ C$ , the product of concentration is $[H^+][OH^-] = 10^{-14}$ .
Neutral Solution: $[H^+] = 10^{-7}$ and $[OH^-] = 10^{-7}$ .
pH Definition: $pH = -\log[H^+]$ .
pH Scale Examples: - Battery Acid: pH 0 - Gastric Juice/Lemon Juice: pH 2 - Vinegar, Wine, Cola: pH 3 - Tomato Juice: pH 4 - Urine/Saliva: pH 6 - Human Blood/Tears: pH 7 (Neutral) - Seawater: pH 8 - Small Intestine: pH 9 - Household Ammonia: pH 11 - Bleach: pH 12-13 - Oven Cleaner: pH 14
Energy Units: - Calorie (cal): Heat to raise $1\,g$ of water by $1^\circ C$ . - Kilocalorie (kcal): $1,000\,cal$ . - Joule (J): $1\,J = 0.239\,cal$ or $1\,cal = 4.184\,J$ .

Question: The pH of a lake is measured to be $4.0$ . What is the $[H^+]$ ? - Response: $10^{-4}\,M$ .
Question: What is the $[OH^-]$ of a lake with pH $4.0$ ? - Response: $10^{-10}\,M$ .

Covalent Connections: Life is built from C, H, O, and N. Valence electrons available for bonding: $C=4$ , $N=3$ , $O=2$ , $H=1$ .
Major Classes of Biological Molecules: 1. Carbohydrates: Sugars join to form Polysaccharides. 2. Lipids: Fatty acids form lipids and membranes. 3. Proteins: Made of Amino acids. 4. Nucleic Acids: Made of Nucleotides (DNA/RNA).