Chemical Bonds and the Properties of Water in Biology

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Chemical Bonds in Biological Molecules

Covalent Bonds

Covalent bonds are fundamental to the structure of biological molecules, involving the sharing of electrons between atoms to achieve stability.

Covalent Bond: A chemical bond formed when two atoms share one or more pairs of valence electrons.
Single Bond: Involves the sharing of one pair (2) of valence electrons between two atoms (e.g., H–H in hydrogen gas).
Double Bond: Involves the sharing of two pairs (4) of valence electrons (e.g., O=O in oxygen gas).
Example: In a water molecule (H2O), each hydrogen atom shares one electron with oxygen, forming two single covalent bonds.

Bond Notation

Different notations are used to represent covalent bonds in molecules:

Structural Formula: Shows the arrangement of atoms and bonds (e.g., H–H for hydrogen gas).
Electron Distribution Diagram: Illustrates the sharing of electrons between atoms.
Lewis Structure: Uses dots to represent valence electrons (e.g., H:H for a single bond).
Molecular Formula: Gives the number and type of atoms (e.g., H2).

Name and Molecular Formula	Electron Distribution Diagram
Hydrogen (H2)	H–H or H:H
Oxygen (O2)	O=O or O::O
Water (H2O)	H–O–H
Methane (CH4)	H \| H–C–H \| H

Electronegativity and Bond Types

Electronegativity

Electronegativity is a key concept in understanding chemical bonding, especially in biological systems.

Definition: Electronegativity is an atom’s ability to attract and hold onto electrons in a chemical bond.
Atoms in a molecule may attract shared electrons to different degrees.
The more electronegative an atom, the more strongly it pulls shared electrons toward itself.
If one atom is much more electronegative than the other, the bond is likely to be ionic.

Electronegativity Trend in the Periodic Table

Electronegativity increases across a period (left to right) and decreases down a group (top to bottom).

Fluorine (F) is the most electronegative element.
Elements on the right and top of the periodic table are more electronegative.

Bond Polarity

Nonpolar Covalent Bond: Electrons are shared equally between two atoms; no partial charges develop.
Polar Covalent Bond: Electrons are shared unequally, resulting in partial positive (δ+) and partial negative (δ–) charges on the atoms (e.g., H2O).
Ionic Bond: Complete transfer of electrons from one atom to another, resulting in full charges (e.g., Na+ and Cl– in NaCl).

Bond Type	Electronegativity Difference	Electron Sharing	Example
Nonpolar Covalent	< 0.5	Equal	H2, O2
Polar Covalent	0.5 – 2.0	Unequal	H2O
Ionic	> 2.0	Transferred	NaCl

Hydrogen Bonds and the Properties of Water

Hydrogen Bonds

Hydrogen bonds are weak attractions that play a crucial role in the structure and function of biological molecules.

Form when a hydrogen atom covalently bonded to a highly electronegative atom (like oxygen or nitrogen) is attracted to another electronegative atom.
Weaker than covalent or ionic bonds but essential for the properties of water and the structure of DNA and proteins.
Example: Hydrogen bonds between water molecules give water its unique properties.

Emergent Properties of Water

Water’s unique properties make it essential for life on Earth. These properties arise from its molecular structure and hydrogen bonding.

Ability to Moderate Temperature: Water can absorb or release large amounts of heat with only a slight change in its own temperature.
Cohesive Behavior: Water molecules stick together due to hydrogen bonding, resulting in high surface tension and capillary action.
Expansion Upon Freezing: Water is less dense as a solid than as a liquid, so ice floats on water.
Versatility as a Solvent: Water can dissolve a wide variety of substances, making it the “solvent of life.”

Specific Heat Capacity of Water

Water has a high specific heat capacity, which helps stabilize temperatures in organisms and environments.

Definition: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.
For water, the specific heat is 1 cal/(g·°C).
This property allows water to buffer temperature changes, protecting living organisms from rapid temperature fluctuations.

Cohesion and Surface Tension

Cohesion refers to the attraction between water molecules, leading to high surface tension and capillary action.

Surface Tension: The measure of how difficult it is to stretch or break the surface of a liquid.
Capillary Action: The movement of water up narrow tubes against gravity, important for water transport in plants.

Expansion Upon Freezing

Unlike most substances, water expands when it freezes, making ice less dense than liquid water.

This property allows ice to float, insulating aquatic life in winter.
If ice sank, bodies of water would freeze solid from the bottom up, making life impossible in many aquatic environments.

Water as a Solvent

Water’s polarity allows it to dissolve many substances, facilitating chemical reactions in living organisms.

Solution: A homogeneous mixture of two or more substances.
Solvent: The dissolving agent (water in aqueous solutions).
Solute: The substance being dissolved.
Water forms hydration shells around ions and polar molecules, keeping them in solution.

Hydrophilic and Hydrophobic Substances

Hydrophilic: Substances that have an affinity for water (e.g., salts, sugars, proteins).
Hydrophobic: Substances that do not interact with water (e.g., oils, fats).

Acids, Bases, and the pH Scale

Acids and Bases in Aqueous Solutions

Acids and bases are substances that alter the concentration of hydrogen ions (H+) and hydroxide ions (OH–) in a solution.

Acid: Increases the concentration of H+ in a solution.
Base: Increases the concentration of OH– or decreases H+.
Water can dissociate into H+ and OH– ions.

The pH Scale

The pH scale measures the concentration of hydrogen ions in a solution, ranging from 0 (most acidic) to 14 (most basic).

pH = -log [H+]
At 25°C, the product of [H+] and [OH–] is
Neutral solution: [H+] = [OH–] = M, so pH = 7
Acidic solution: [H+] > [OH–], pH < 7
Basic solution: [H+] < [OH–], pH > 7
pH and pOH are related:

pH	Example
1	Battery acid
2	Gastric juice
7	Pure water (neutral)
8	Seawater
13	Oven cleaner

Elements Essential to Life

Major Elements in the Human Body

Living organisms are composed primarily of a few key elements.

Element	Symbol	Percentage of Body Mass
Oxygen	O	65.0%
Carbon	C	18.5%
Hydrogen	H	9.5%
Nitrogen	N	3.3%
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 (less than 0.1%): 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).

Summary

Chemical bonds, especially covalent and hydrogen bonds, are essential for the structure and function of biological molecules.
Water’s unique properties, arising from its polarity and hydrogen bonding, make it indispensable for life.
The pH scale and the balance of acids and bases are critical for maintaining homeostasis in living organisms.
Only a small number of elements make up the majority of living matter, with trace elements also playing vital roles.