The Chemical Basis of Life: Chapter 2 Study Notes

The Chemical Basis of Life

Introduction

This chapter explores the fundamental chemical principles underlying biological systems. Understanding the nature of elements, atoms, molecules, and chemical bonds is essential for grasping how life functions at the molecular level.

Elements and Atoms

Elements

An element is a pure chemical substance consisting of one type of atom, defined by its number of protons. Elements are the building blocks of matter and cannot be broken down by chemical means.

• Definition: A substance composed of only one type of atom.

• Examples: Hydrogen (H), Carbon (C), Oxygen (O).

• Biological Relevance: Living organisms are primarily composed of a few key elements: Oxygen, Carbon, Hydrogen, Nitrogen, Calcium, Phosphorus, Potassium, Sulfur, Sodium.

• Abundance: These elements make up about 95% of the human body by weight.

Atoms

An atom is the smallest unit of an element that retains its chemical properties. Atoms are composed of subatomic particles.

• Nucleus: Central part of the atom containing protons (positively charged) and neutrons (no charge).

• Electron Shells: Surround the nucleus and contain electrons (negatively charged).

• Atomic Structure: Electrons are attracted to the nucleus and occupy specific energy levels or shells.

Atomic Number, Mass Number, and Isotopes

Atomic Number and Symbol

Each element is represented by a unique symbol and atomic number.

• Symbol: One or two-letter code (e.g., H for Hydrogen, C for Carbon).

• Atomic Number: Number of protons in the nucleus; defines the element.

• Example: Carbon always has 6 protons; Oxygen always has 8 protons.

lm Mass Number

The mass number is the sum of protons and neutrons in the nucleus.

• Protons and Neutrons: Each has a mass of approximately 1 atomic mass unit (AMU).

• Electrons: Mass is negligible compared to protons and neutrons ($0.0005489$ AMU).

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Stable Isotopes: Have a constant number of neutrons.

• Radioactive Isotopes: Nuclei are unstable and emit particles to regain stability.

• Example: Carbon has three isotopes: $^{12}_6C$, $^{13}_6C$, $^{14}_6C$ (Carbon-12, Carbon-13, Carbon-14).

• Applications: Radioactive isotopes are used as tracers in biological research and medical imaging.

Periodic Table of Elements

Organization

The periodic table arranges elements by increasing atomic number and groups elements with similar chemical properties.

• Groups: Vertical columns with similar properties.

• Periods: Horizontal rows indicating increasing atomic number.

• Key Elements for Life: Highlighted in the table (e.g., C, H, O, N, P, S).

Electron Shells and Chemical Bonding

Electron Shells

Electrons occupy specific shells around the nucleus, each with a maximum capacity.

• Shell 1: Maximum 2 electrons

• Shell 2: Maximum 8 electrons

• Shell 3: Maximum 8 electrons

• Valence Shell: Outermost shell; determines chemical reactivity.

• Stability: Atoms with full valence shells are chemically stable.

Chemical Bonds

Chemical bonds form when atoms interact to achieve stable electron configurations.

• Ionic Bonds: Formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions that attract each other.

• Covalent Bonds: Formed by the sharing of electrons between atoms.

• Polar Covalent Bonds: Unequal sharing of electrons, leading to partial charges.

• Nonpolar Covalent Bonds: Equal sharing of electrons.

• Hydrogen Bonds: Weak attractions between partial charges in polar molecules, especially important in water.

Molecules and Compounds

Definitions

• Molecule: Two or more atoms bonded together.

• Compound: A molecule containing two or more different elements.

• Chemical Formula: Indicates the types and numbers of atoms in a molecule (e.g., $H_2O$, $C_6H_{12}O_6$).

Chemical Reactions

Chemical reactions involve making and breaking chemical bonds, transforming reactants into products.

• Law of Conservation of Matter: Matter is not created or destroyed, only rearranged.

• Example Reaction: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

Properties of Water

Structure and Polarity

Water is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen atoms.

• Polarity: Oxygen is more electronegative, creating partial negative and positive charges.

• Hydrogen Bonds: Form between water molecules due to polarity.

Unique Properties

• Cohesion: Water molecules stick together due to hydrogen bonding.

• Adhesion: Water molecules cling to other polar surfaces.

• Surface Tension: High due to cohesive forces.

• Temperature Moderation: Water absorbs and releases heat slowly, stabilizing temperatures.

• Evaporative Cooling: As water evaporates, it removes heat, cooling the surface.

• Density of Ice: Ice is less dense than liquid water due to hydrogen bond-induced lattice structure, allowing ice to float.

• Solvent Properties: Water dissolves many substances due to its polarity, forming aqueous solutions.

Acids, Bases, and pH

Definitions

• Acid: Substance that increases the concentration of hydrogen ions ($H^+$) in solution.

• Base: Substance that reduces the concentration of hydrogen ions, often by increasing hydroxide ions ($OH^-$).

• pH Scale: Measures hydrogen ion concentration; ranges from 0 (most acidic) to 14 (most basic), with 7 as neutral.

• Logarithmic Nature: Each pH unit represents a tenfold change in $H^+$ concentration.

Buffer Systems

Buffers help maintain stable pH in biological systems by binding or releasing $H^+$ ions as needed.

• Function: Minimize changes in pH when acids or bases are added.

• Example: Carbonic acid-bicarbonate buffer system in blood.

Summary Table: Key Elements in Biology

Element	Symbol	Biological Role
Oxygen	O	Component of water, cellular respiration
Carbon	C	Backbone of organic molecules
Hydrogen	H	Component of water, organic molecules
Nitrogen	N	Proteins, nucleic acids
Calcium	Ca	Bone structure, signaling
Phosphorus	P	Nucleic acids, energy transfer (ATP)
Potassium	K	Nerve function, fluid balance
Sulfur	S	Proteins (amino acids)
Sodium	Na	Nerve function, fluid balance

Additional info:

• Radioactive isotopes are used in medical diagnostics and research, but can be hazardous due to radiation.

• Water's solvent properties are crucial for biochemical reactions and transport of substances in living organisms.

• Buffers are essential for maintaining homeostasis in biological systems.