Chapter 2: Chemical Context of Life – General Biology Study Notes

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Chemical Context of Life

Introduction

Understanding the chemical basis of life is essential for studying biology. Living organisms are composed of matter, which is made up of elements and atoms. The unique properties of water and the structure of atoms play a critical role in biological processes.

Properties of Water

Cohesion and Adhesion

Water molecules exhibit two important properties: cohesion and adhesion. These properties are vital for many biological functions, such as the transport of water in plants.

Cohesion: Water molecules are attracted to each other due to hydrogen bonding. This attraction allows water to form a continuous column in plant vessels (xylem), enabling the upward movement of water from roots to leaves.
Adhesion: Water molecules are also attracted to other polar substances, such as the walls of xylem tubes. This helps water "stick" to surfaces and move against gravity.
Example: When water evaporates from leaves, cohesion and adhesion work together to pull more water upward to replace the lost molecules.

Matter, Elements, and Atoms

Definitions and Key Concepts

Matter: Anything that has mass and occupies space.
Atom: The smallest unit of matter that retains the properties of an element.
Element: A substance that cannot be broken down into other substances by chemical means. Elements are made of atoms.
Essential Elements: Elements necessary for life. The top four for humans are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N).
Trace Elements: Elements required in minute amounts, such as iron (Fe) and zinc (Zn).

Table: Major and Trace Elements in the Human Body

Element	Approximate % in Body	Role
Oxygen (O)	65%	Component of water and organic molecules
Carbon (C)	18%	Backbone of organic molecules
Hydrogen (H)	10%	Component of water and organic molecules
Nitrogen (N)	3%	Component of proteins and nucleic acids
Trace Elements	<0.01%	Required for enzyme function and other processes

Compounds

Definition and Examples

Compound: A substance consisting of two or more different elements combined in a fixed ratio.
Examples: Water (H2O) and Carbon Dioxide (CO2).

Structure of Atoms

Subatomic Particles

Protons: Positively charged particles found in the nucleus.
Neutrons: Neutral particles found in the nucleus.
Electrons: Negatively charged particles orbiting the nucleus.
Atomic Number: Number of protons in an atom; defines the element.
Atomic Mass: Sum of protons and neutrons in the nucleus.

Table: Subatomic Particles

Particle	Charge	Location
Proton	+1	Nucleus
Neutron	0	Nucleus
Electron	-1	Electron shell/orbit

Isotopes

Definition and Applications

Isotopes: Atoms of the same element with different numbers of neutrons.
Radioactive Isotopes: Unstable isotopes that decay over time, releasing energy. Used in radiometric dating and medical diagnostics.

Electron Shells and Energy Levels

Organization and Chemical Behavior

Electrons are arranged in shells around the nucleus. Each shell has a specific energy level.
The outermost shell is called the valence shell. Atoms are most stable when their valence shell is full (usually 8 electrons, except for hydrogen and helium).
Atoms with incomplete valence shells tend to form chemical bonds to achieve stability.

Chemical Bonds

Types of Bonds

Covalent Bonds: Atoms share electrons. Can be single, double, or triple bonds. Strongest type of bond.
Polar Covalent Bonds: Electrons are shared unequally, resulting in partial charges (e.g., in water molecules).
Nonpolar Covalent Bonds: Electrons are shared equally between atoms of the same element.
Ionic Bonds: Atoms transfer electrons, forming ions (charged atoms). Oppositely charged ions attract each other.
Hydrogen Bonds: Weak attractions between a hydrogen atom in one molecule and an electronegative atom (like oxygen or nitrogen) in another molecule. Important for the properties of water and biological molecules.

Table: Types of Chemical Bonds

Bond Type	Description	Strength
Covalent	Electron sharing	Strong
Ionic	Electron transfer	Moderate
Hydrogen	Attraction between polar molecules	Weak

Electronegativity

Definition and Trends

Electronegativity: The ability of an atom to attract electrons in a covalent bond.
Electronegativity increases across a period and decreases down a group in the periodic table.
Oxygen is highly electronegative, which makes water a polar molecule.

Water: Unique Properties and Biological Importance

Thermal Properties

High Specific Heat: Water absorbs a lot of heat before its temperature rises, helping organisms maintain stable internal temperatures.
High Heat of Vaporization: Water requires significant energy to change from liquid to gas, which is important for cooling mechanisms like sweating.
Density of Ice: Water becomes less dense as it freezes, allowing ice to float and insulate aquatic life.

Solvent Properties

Universal Solvent: Water can dissolve more substances than any other liquid due to its polarity.
Solute: The substance being dissolved.
Solvent: The dissolving agent (water in biological systems).

Acids, Bases, and pH

Definitions and Biological Relevance

Acid: Substance that increases hydrogen ion (H+) concentration in solution. pH < 7.
Base: Substance that reduces hydrogen ion concentration, often by accepting H+ or releasing OH-. pH > 7.
pH Scale: Measures hydrogen ion concentration; logarithmic scale where each unit represents a tenfold difference.
Formula:
Example: A solution with pH 2 has 1000 times more H+ ions than a solution with pH 5.

Buffers

Buffer: Substance that minimizes changes in pH by accepting or donating H+ ions.
Example: Bicarbonate buffer system in blood:

Summary Table: Water Properties and Biological Importance

Property	Description	Biological Importance
Cohesion	Water molecules stick together	Transport in plants
Adhesion	Water molecules stick to other surfaces	Movement in xylem
High Specific Heat	Resists temperature change	Homeostasis
Universal Solvent	Dissolves many substances	Biochemical reactions
Density of Ice	Ice floats on water	Insulates aquatic life

Additional info: Some context and definitions have been expanded for clarity and completeness, including the explanation of chemical bonds, electronegativity, and the pH scale.