The Molecules of Cells: Organic Compounds and Functional Groups

The Molecules of Cells

Introduction: Biological Molecules and Human Health

Biological molecules are essential for the structure and function of living cells. Their roles are evident in processes such as digestion and metabolism, which directly impact human health.

• Lactose is a sugar found in milk and dairy products, which are rich in protein and minerals.

• To digest lactose, the intestine must produce the enzyme lactase.

• Lactose intolerance occurs when individuals lack lactase in the small intestine, leading to symptoms such as nausea, cramps, and bloating.

• Lactase is a protein enzyme produced by a gene on DNA. If the gene is inactive, lactase is not produced.

• Lactose intolerance illustrates the importance of biological molecules in cellular function and health.

Organic Compounds: The Basis of Life's Molecular Diversity

Properties of Carbon

Carbon is the foundational element of organic molecules due to its ability to form four covalent bonds, resulting in a vast diversity of compounds.

• Organic compounds are molecules based on carbon.

• Hydrocarbons are compounds composed only of carbon and hydrogen.

• Carbon atoms can bond in chains of various lengths, forming the carbon skeleton of organic molecules.

Carbon Skeletons: Structure and Variation

Carbon skeletons can be arranged in multiple ways, influencing the properties of organic molecules.

• Chains may be unbranched (e.g., butane) or branched (e.g., isobutane).

• Skeletons may include double bonds (e.g., 1-butene, 2-butene) or be arranged in rings (e.g., cyclohexane, benzene).

• Compounds with the same molecular formula but different structures are called isomers.

Table: Types of Carbon Skeletons

Type	Example
Unbranched chain	Butane
Branched chain	Isobutane
Double bond	1-Butene, 2-Butene
Ring structure	Cyclohexane, Benzene

Double Bonds and Isomers

Double bonds can occur at different positions in the carbon skeleton, resulting in isomers with unique properties.

• Example: 1-butene and 2-butene both have the formula $C_4H_8$ but differ in the location of the double bond.

• Isomers have the same molecular formula but different structures and properties.

Ring Structures

Some hydrocarbons, such as cyclohexane and benzene, have carbon skeletons arranged in rings. These molecules are typically nonpolar due to their C-H bonds.

• Cyclohexane and benzene are examples of ring-structured hydrocarbons.

• Hydrocarbons are generally nonpolar and insoluble in water.

Functional Groups: Determinants of Molecular Properties

Definition and Importance

Functional groups are specific groups of atoms within molecules that are responsible for characteristic chemical reactions. They determine the properties and reactivity of organic compounds.

• The unique properties of an organic compound depend on the size and shape of its carbon skeleton and the functional groups attached.

• Functional groups are usually polar, making compounds hydrophilic and soluble in water.

Table: Functional Groups of Organic Compounds

Functional Group	Structure	Example
Hydroxyl	–OH	Alcohol (Ethanol)
Carbonyl	–C=O	Aldehyde (Formaldehyde), Ketone (Acetone)
Carboxyl	–COOH	Carboxylic acid (Acetic acid)
Amino	–NH2	Amine (Amino acids)
Phosphate	–OPO32–	Organic phosphate (ATP)

Hydroxyl Group

The hydroxyl group consists of a hydrogen atom bonded to an oxygen atom. It is characteristic of alcohols and increases solubility in water.

• Example: Ethanol, Methanol, Propanol

• General formula: $–OH$

Carbonyl Group: Aldehydes and Ketones

The carbonyl group is a carbon atom double-bonded to an oxygen atom. Its position determines whether the compound is an aldehyde or a ketone.

• Aldehyde: Carbonyl group at the end of the carbon skeleton (e.g., formaldehyde, ethanal).

• Ketone: Carbonyl group within the carbon chain (e.g., acetone, acetophenone).

Table: Examples of Aldehydes and Ketones

Aldehydes	Ketones
Formaldehyde	Acetone
Acetaldehyde (Ethanal)	Acetophenone
Propionaldehyde (Propanal)
Benzaldehyde

Carboxyl Group

The carboxyl group consists of a carbon double-bonded to an oxygen and bonded to a hydroxyl group. It acts as an acid by donating a proton (H+).

• Compounds with carboxyl groups are called carboxylic acids.

• Example: Acetic acid ($CH_3COOH$)

Amino Group

The amino group is composed of a nitrogen atom bonded to two hydrogen atoms. It acts as a base by accepting a proton.

• Organic compounds with amino groups are called amines.

• Amino acids contain both carboxyl and amino groups and are the building blocks of proteins.

Phosphate Group

The phosphate group consists of a phosphorus atom bonded to four oxygen atoms. It is usually ionized and involved in energy transfer.

• Compounds with phosphate groups are called organic phosphates.

• ATP (adenosine triphosphate) contains three phosphate groups and is essential for cellular energy.

• Phosphate group formula: $–OPO_3^{2–}$

Structure of ATP

ATP is composed of adenosine (adenine + ribose) and three phosphate groups. It is the primary energy carrier in cells.

• ATP structure includes a chain of three phosphate groups attached to adenosine.

• Energy is released when the terminal phosphate bond is broken.

Effect of Functional Groups: Biological Significance

Functional groups are responsible for the distinct properties and biological activities of molecules, such as hormones.

• Male and female sex hormones differ mainly in their functional groups, leading to different physiological effects.

• Example: Estradiol (female hormone) and testosterone (male hormone) have similar carbon skeletons but different functional groups, resulting in distinct biological functions.

Additional info: Functional groups are central to the chemistry of life, influencing molecular interactions, solubility, and reactivity. Their study is foundational for understanding biochemistry and molecular biology.