General Biology: Organic Molecules, Isomerism, and Functional Groups

Organic Molecules in Biology

Key Elements in Organic Molecules

Organic molecules are the foundation of biological systems, composed primarily of a few key elements.

• Carbon is the central element in organic chemistry, forming the backbone of most biological molecules.

• Other essential elements include hydrogen, oxygen, and nitrogen.

• These elements combine in various ways to form the diverse molecules necessary for life.

Example: Carbohydrates, proteins, lipids, and nucleic acids are all organic molecules built primarily from these elements.

Experimental Approaches in Biology

Biological investigations often rely on experimental approaches to understand the nature of life and its chemical basis.

• Vitalism is the outdated belief that life is governed by a force beyond physical and chemical laws.

• Modern biology presumes that the activities of living organisms can be explained by the same physical and chemical laws that govern non-living matter.

• Simple organic compounds can be synthesized in the laboratory from inorganic precursors, as demonstrated by classic experiments (e.g., Miller-Urey experiment).

Example: The Miller-Urey experiment showed that organic molecules could form under prebiotic conditions, supporting the idea that life's chemistry is not unique to living organisms.

Isomerism in Organic Molecules

Types of Isomers

Isomers are molecules with the same molecular formula but different structures or spatial arrangements.

• Structural isomers: Differ in the covalent arrangement of their atoms.

• Cis-trans isomers: Have the same covalent bonds but differ in spatial arrangements due to inflexible double bonds.

• Enantiomers: Are mirror images of each other and cannot be superimposed; important in pharmaceuticals and biology.

Example: L-dopa and D-dopa are enantiomers with different biological activities.

Cis-Trans Isomerism

Cis-trans isomers occur due to the restricted rotation around double bonds in hydrocarbons.

• Cis isomer: Similar groups are on the same side of the double bond.

• Trans isomer: Similar groups are on opposite sides of the double bond.

• These isomers have different physical and chemical properties.

Example: The difference between cis- and trans-fats in nutrition.

Enantiomers and Biological Activity

Enantiomers are a type of stereoisomer important in biology and medicine.

• They have identical physical properties except for the direction in which they rotate plane-polarized light and their interactions with other chiral molecules.

• Often, only one enantiomer is biologically active.

Example: Thalidomide and L-dopa are drugs where only one enantiomer is effective or safe.

Hydrocarbons and Their Properties

Hydrocarbon Structure

Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen.

• They can be straight-chained, branched, or contain rings.

• Double or triple bonds introduce rigidity and affect the molecule's flexibility and reactivity.

Example: Ethene (C2H4) is a simple hydrocarbon with a double bond.

Effect of Double Bonds

• Double bonds make hydrocarbon chains more constrained in structure, reducing flexibility.

• They also affect the molecule's polarity and hydrogen content.

Functional Groups in Organic Molecules

Common Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and chemical reactivity.

• Hydroxyl group (-OH): Found in alcohols; increases solubility in water.

• Carbonyl group (C=O): Found in aldehydes and ketones; important in sugars.

• Amino group (-NH2): Found in amino acids; acts as a base.

• Sulfhydryl group (-SH): Found in some amino acids; forms disulfide bonds in proteins.

Example: The carbonyl group is present in both glucose (an aldehyde) and fructose (a ketone).

Functional Groups and Protein Structure

• The sulfhydryl group (-SH) helps stabilize protein structure by forming covalent disulfide bonds between cysteine residues.

Functional Groups and Biological Molecules

• The phosphate group is a key component of ATP, the energy currency of the cell.

• Hydroxyl groups increase solubility in water, making molecules more hydrophilic.

ATP and Energy in Cells

ATP Structure and Function

ATP (adenosine triphosphate) is the primary energy carrier in cells.

• ATP contains three phosphate groups, a ribose sugar, and an adenine base.

• Hydrolysis of ATP releases energy by removing a phosphate group, forming ADP (adenosine diphosphate).

Equation:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• ATP has more energy than ADP because of the high-energy phosphate bond.

Table: Types of Isomers

The following table summarizes the main types of isomers discussed:

Type of Isomer	Definition	Example
Structural Isomer	Same molecular formula, different covalent arrangement	Butane vs. isobutane
Cis-Trans Isomer	Same covalent bonds, different spatial arrangement around double bond	Cis-2-butene vs. trans-2-butene
Enantiomer	Mirror images, non-superimposable	L-dopa vs. D-dopa

Table: Common Functional Groups

Functional Group	Structure	Properties	Example
Hydroxyl	-OH	Polar, forms hydrogen bonds	Ethanol
Carbonyl	C=O	Polar, found in sugars	Acetone
Amino	-NH2	Acts as a base	Glycine
Sulfhydryl	-SH	Forms disulfide bonds	Cysteine
Phosphate	-OPO32-	Contributes negative charge, energy transfer	ATP

Summary

• Organic molecules are built from a small set of elements, with carbon as the central atom.

• Isomerism (structural, cis-trans, enantiomers) leads to molecular diversity and biological specificity.

• Functional groups determine the chemical properties and reactivity of organic molecules.

• ATP is the main energy currency in cells, with its energy stored in phosphate bonds.