The Chemical Context of Life & Water: Foundations for Biology

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Chapter 2: The Chemical Context of Life

Concept: Matter Consists of Chemical Elements in Pure Form and in Combinations Called Compounds

Understanding the nature of matter and its composition is fundamental to biology. Matter is composed of elements, which can combine to form compounds essential for life.

Matter: Anything that takes up space and has mass.
Element: A substance that cannot be broken down to other substances by chemical reactions (e.g., gold, copper, lead, carbon, oxygen).
Compound: A substance consisting of two or more elements combined in a fixed ratio (e.g., water (H2O), salt (NaCl), ammonia (NH3)).
Essential Elements: Elements required in large amounts for life (e.g., C, O, H, N make up 96% of living matter).
Trace Elements: Elements required in minute quantities (e.g., iron, iodine, magnesium).

Concept: An Element's Properties Depend on the Structure of Its Atoms

Atoms are the smallest units of elements, and their structure determines the properties of the element.

Subatomic Particles: Atoms are composed of protons, neutrons, and electrons.
Protons: Positively charged particles found in the nucleus; determine the atomic number and identity of the element.
Neutrons: Neutral particles in the nucleus; number of neutrons determines isotopes.
Electrons: Negatively charged particles found in electron shells around the nucleus; determine chemical reactivity.

Example: Carbon-12 and Carbon-14 are isotopes of carbon. Both have 6 protons, but Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons.

Atomic Number: Number of protons in an atom.
Mass Number: Sum of protons and neutrons in an atom.

Concept: The Formation of Molecules Depends on Chemical Bonding Between Atoms

Chemical bonds are interactions between atoms that result in the formation of molecules. The type of bond affects the properties of the resulting compound.

Chemical Bonds: Interactions between valence electrons of different atoms.
Covalent Bonds: Formed when valence electrons are shared by two atoms.
Nonpolar Covalent Bonds: Electrons are shared equally (e.g., O2, H2, CH4).
Polar Covalent Bonds: Electrons are shared unequally, resulting in partial charges (e.g., H2O).
Electronegativity: The tendency of an atom to attract electrons in a covalent bond. Elements like nitrogen and oxygen are highly electronegative.
Ionic Bonds: Formed when one atom steals an electron from another, resulting in charged ions (cations and anions) that attract each other (e.g., NaCl).
Hydrogen Bonds: Weak bonds between a hydrogen atom covalently bonded to an electronegative atom (like O or N) and another electronegative atom.
Van der Waals Interactions: Weak, transient connections due to asymmetrical electron distribution within a molecule.

Bond Type	How Formed	Relative Strength	Example
Covalent	Sharing of electrons	Strong	H2O, O2
Ionic	Transfer of electrons	Strong (in dry conditions)	NaCl
Hydrogen	Attraction between H and electronegative atom	Weak	Between water molecules
Van der Waals	Transient, asymmetrical electron distribution	Very weak	Gecko feet adhesion

Chapter 3: Water and the Fitness of the Environment

Concept: The Polarity of Water Molecules Results in Hydrogen Bonding

Water's unique properties arise from its molecular structure and the hydrogen bonds formed between molecules.

Structure: Water (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom.
Polarity: Oxygen is more electronegative, so electrons are pulled closer to it, giving the molecule a partial negative charge near the oxygen and a partial positive charge near the hydrogens.
Hydrogen Bonds: The slightly positive hydrogen of one water molecule is attracted to the slightly negative oxygen of another, forming hydrogen bonds.
Each water molecule can form up to four hydrogen bonds at a time.

Concept: Four Emergent Properties of Water Contribute to Earth's Fitness for Life

Hydrogen bonding gives rise to four key properties of water that are essential for life.

Cohesion: Water molecules stick to each other due to hydrogen bonding. This property allows for surface tension and the movement of water in plants.
Adhesion: Water molecules stick to other substances, aiding processes like water transport in plants.
Transpiration: The movement of water up plant stems against gravity, enabled by cohesion and adhesion.
High Specific Heat: Water can absorb or release large amounts of heat with little temperature change, helping to stabilize temperatures in organisms and environments.
High Heat of Vaporization: Water requires a lot of energy to evaporate, which helps organisms cool off via sweating and moderates Earth's climate.
Expansion Upon Freezing: Ice is less dense than liquid water, so it floats, insulating aquatic life in winter.
Versatile Solvent: Water dissolves many substances, facilitating chemical reactions in cells.

Example: Water's cohesion and adhesion allow it to move from roots to leaves in plants, defying gravity through capillary action.

Additional info: The pH scale is used to measure the concentration of hydrogen ions in a solution, which is important for biological systems. Buffers help maintain stable pH in organisms.