Introduction to General Biology: Elements of Life, Chemical Bonds, Water, and the Scientific Method

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Elements Essential to Life

The Four Major Elements

Living organisms are primarily composed of a small number of chemical elements. Four elements make up approximately 96% of the mass of living matter.

Oxygen (O)
Carbon (C)
Hydrogen (H)
Nitrogen (N)

These elements are fundamental to the structure and function of biomolecules such as proteins, nucleic acids, carbohydrates, and lipids.

Atomic Structure: Atomic Number and Mass Number

Atoms are characterized by two key numbers:

Atomic Number: The number of protons in the nucleus of an atom. This defines the element.
Mass Number: The sum of protons and neutrons in the nucleus.

Example: For an element X with atomic number 75 and mass number 78:

Atomic number = 75
Mass number = 78

Chemical Bonds in Biology

Types of Chemical Bonds

Atoms combine to form molecules through chemical bonds. The main types of bonds relevant to biology are:

Ionic Bonds: Formed when electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other.
Covalent Bonds: Formed when two atoms share one or more pairs of electrons.
Polar Covalent Bonds: A type of covalent bond where electrons are shared unequally due to a difference in electronegativity between the atoms.
Nonpolar Covalent Bonds: Electrons are shared equally between atoms with similar electronegativity.

Electronegativity and Bond Polarity

Electronegativity is a measure of an atom's ability to attract shared electrons. The difference in electronegativity between two atoms determines the type of bond:

If the difference is large (>1.7), the bond is usually ionic.
If the difference is moderate (0.4–1.7), the bond is polar covalent.
If the difference is small (<0.4), the bond is nonpolar covalent.

Example: In ammonia (NH3):

Electronegativity of N = 3.04, H = 2.20
Difference = 0.84 (polar covalent bond)
Nitrogen is more electronegative, so the shared electrons are pulled closer to N, making the bond polar.

The Scientific Method in Biology

Steps of the Scientific Method

The scientific method is a systematic approach to understanding the natural world. It involves the following steps:

Make an observation: Notice something interesting or unexplained.
Form a question: Ask a specific, testable question about the observation.
Form hypotheses: Propose possible explanations (hypotheses) that can be tested.
Design an experiment: Plan a way to test the hypothesis under controlled conditions.
Make a prediction and test the hypothesis: Predict what will happen if the hypothesis is correct and perform the experiment.
Get a result that supports or contradicts your hypothesis: Analyze the data to draw conclusions.

Example: Observation: Seeds grow at different rates at different temperatures. Question: Will seeds grow faster at 35°C? Hypothesis: The highest number of seeds will germinate at 35°C. Experiment: Grow seeds at various temperatures and record germination rates.

Properties of Water

Importance of Water in Biology

Water is essential for life and covers about 71% of Earth's surface. It makes up a significant portion of living cells and is involved in many biological processes.

Accounts for about 60% of body weight in humans.
Original cells are thought to have evolved in water ~3.5 billion years ago.

Covalent and Polar Covalent Bonds in Water

In a water molecule (H2O):

Oxygen and hydrogen are joined by polar covalent bonds.
Oxygen is more electronegative than hydrogen, so electrons are pulled closer to oxygen, making it partially negative and hydrogen partially positive.
This polarity makes water molecules attract each other via hydrogen bonds.

Hydrogen Bonds

Hydrogen bonds are weak, temporary bonds that form between the partially positive hydrogen atom of one molecule and the partially negative atom (often oxygen or nitrogen) of another molecule.

Hydrogen bonds are responsible for many of water's unique properties.
They are constantly breaking and reforming in liquid water.

Properties of Water Due to Hydrogen Bonding

High surface tension: Water molecules stick together (cohesion) and to other surfaces (adhesion).
Universal solvent: Water can dissolve many substances, especially those that are ionic or polar.
High specific heat: Water can absorb a lot of heat before its temperature rises, helping to stabilize temperatures in organisms and environments.

Water as a Solvent

A solution is a homogeneous mixture of substances. The solute is the substance dissolved, and the solvent is the dissolving agent.

Example: Dissolving table salt (NaCl) in water:
- NaCl is the solute
- Water is the solvent
- The result is a saltwater solution

Hydrophilic, Hydrophobic, and Amphipathic Molecules

Hydrophilic substances: Have an affinity for water; usually ionic or polar compounds.
Hydrophobic substances: Repel water; usually nonpolar molecules.
Amphipathic molecules: Contain both hydrophilic (polar) and hydrophobic (nonpolar) regions (e.g., fatty acids, phospholipids).

Amphipathic molecules are crucial in forming biological membranes, as their dual nature allows them to form bilayers in aqueous environments.

Acids, Bases, and pH

Dissociation of Water

Water can dissociate into ions:

Hydrogen ion:
Hydroxide ion:

The reaction is:

or more simply:

In pure water, the concentrations of and are equal, making the solution neutral (pH = 7).

Acids and Bases

Acids: Substances that increase the concentration of ions in solution (e.g., HCl dissociates into $H^+$ and ).
Bases: Substances that decrease the concentration of ions, often by increasing (e.g., NaOH dissociates into and $OH^-$).

The pH Scale

The pH scale measures the concentration of ions in solution:

Pure water: M, so pH = 7 (neutral).
Acidic solutions: pH < 7 (higher concentration).
Basic (alkaline) solutions: pH > 7 (lower concentration).

Each unit change in pH represents a tenfold change in concentration.

Buffers

Buffers are substances that minimize changes in pH by binding or releasing ions as needed. They are essential in biological systems to maintain stable internal conditions.

Example: Bicarbonate () acts as a buffer in blood, soaking up excess ions to prevent drastic pH changes.

Summary Table: Types of Chemical Bonds

Bond Type	Description	Example
Ionic	Transfer of electrons; attraction between oppositely charged ions	NaCl (sodium chloride)
Nonpolar Covalent	Equal sharing of electrons	O2 (oxygen gas)
Polar Covalent	Unequal sharing of electrons due to difference in electronegativity	H2O (water), NH3 (ammonia)
Hydrogen Bond	Weak attraction between a hydrogen atom and an electronegative atom (O or N)	Between water molecules

Key Takeaways

Four elements (O, C, H, N) make up most of living matter.
Chemical bonds (ionic, covalent, hydrogen) are fundamental to molecular structure and function.
Water's unique properties are due to its polarity and hydrogen bonding.
The scientific method is the foundation of biological inquiry.
Acids, bases, and buffers are crucial for maintaining pH balance in biological systems.