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Chapter 4: Carbon and the Molecular Diversity of Life – Study Notes

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Carbon and the Molecular Diversity of Life

Overview: Carbon – The Backbone of Life

Carbon is the foundational element in the chemistry of life, forming the backbone of the vast majority of biological molecules. Its unique properties allow for the formation of large, complex, and diverse molecules essential for life.

  • Living organisms are primarily composed of carbon-based compounds.

  • Although cells are 70–95% water, most of the remaining material consists of carbon compounds.

  • Proteins, DNA, carbohydrates, and other molecules that distinguish living matter are all composed of carbon compounds.

Concept 4.1: Organic Chemistry – The Study of Carbon Compounds

Organic chemistry is the branch of chemistry that studies compounds containing carbon. Most organic compounds also contain hydrogen, and often other elements such as oxygen and nitrogen.

  • Examples of organic compounds: carbohydrates, lipids, proteins, nucleic acids.

Concept 4.2: Carbon Atoms Can Form Diverse Molecules by Bonding to Four Other Atoms

Properties of Carbon

Carbon's versatility stems from its atomic structure:

  • Carbon has four valence electrons, allowing it to form up to four covalent bonds with other atoms.

  • This enables carbon to form a wide variety of molecules, including chains, rings, and branched structures.

Valence is the number of covalent bonds an atom can form. For carbon, valence = 4.

Major Elements in Organic Molecules

Element

Valence

Common Bonds

Hydrogen (H)

1

Single

Oxygen (O)

2

Double or two single

Nitrogen (N)

3

Triple or three single

Carbon (C)

4

Single, double, triple, or combinations

Shapes and Diversity of Carbon Compounds

  • Carbon chains can vary in length, branching, and the presence of rings.

  • Covalent bonds can be single, double, or triple, affecting molecular shape and function.

  • Examples: methane (CH4), ethane (C2H6), ethene (C2H4).

Hydrocarbons

  • Hydrocarbons are molecules consisting only of carbon and hydrogen.

  • They are nonpolar and hydrophobic, making them good energy stores (e.g., fats).

Carbon's Bonding Partners

  • Most frequent partners: hydrogen, oxygen, nitrogen.

  • Together, these elements form the "building code" for the architecture of living molecules.

Isomers

Isomers are compounds with the same molecular formula but different structures and properties.

  • Structural isomers: Differ in the covalent arrangement of atoms (e.g., pentane vs. 2-methylbutane).

  • Cis-trans isomers: Differ in spatial arrangement around a double bond (cis: same side; trans: opposite sides).

  • Enantiomers: Mirror images of each other, differing in arrangement around an asymmetric carbon. Only one enantiomer is usually biologically active.

Example: Thalidomide enantiomers—one reduced morning sickness, the other caused birth defects.

Pharmacological Importance of Enantiomers

Drug

Effect

Effective Enantiomer

Ineffective Enantiomer

Ibuprofen

Reduces inflammation and pain

S-Ibuprofen

R-Ibuprofen

Albuterol

Relaxes bronchial muscles

R-Albuterol

S-Albuterol

Concept 4.3: A Few Chemical Groups Are Key to Molecular Function

Chemical groups called functional groups can replace hydrogens attached to carbon skeletons, giving molecules unique properties. The number and arrangement of these groups determine molecular function.

Biologically Important Functional Groups

  • Hydroxyl group (–OH): Alcohols; polar, forms hydrogen bonds (e.g., ethanol).

  • Carbonyl group (>C=O): Ketones (within carbon skeleton) and aldehydes (at end); found in sugars (e.g., acetone, propanal).

  • Carboxyl group (–COOH): Organic acids; acts as an acid (e.g., acetic acid).

  • Amino group (–NH2): Amines; acts as a base (e.g., glycine).

  • Sulfhydryl group (–SH): Thiols; forms "cross-links" in proteins (e.g., cysteine).

  • Phosphate group (–OPO32–): Organic phosphates; involved in energy transfer (e.g., ATP).

  • Methyl group (–CH3): Methylated compounds; affects gene expression and function of sex hormones.

Functional Groups Table

Group

Structure

Compound Name

Example

Hydroxyl

–OH

Alcohol

Ethanol

Carbonyl

>C=O

Ketone/Aldehyde

Acetone/Propanal

Carboxyl

–COOH

Carboxylic acid

Acetic acid

Amino

–NH2

Amine

Glycine

Sulfhydryl

–SH

Thiol

Cysteine

Phosphate

–OPO32–

Organic phosphate

Glycerol phosphate

Methyl

–CH3

Methylated compound

5-Methyl cytosine

ATP: An Important Source of Energy for Cellular Processes

  • Adenosine triphosphate (ATP) is the primary energy-transferring molecule in cells.

  • ATP consists of adenosine attached to three phosphate groups.

  • Hydrolysis of ATP (reaction with water) releases energy for cellular work:

Phospholipids: Key Component of Cell Membranes

  • Phospholipids are the most prevalent molecules in cell membranes.

  • They have hydrophilic phosphate heads and hydrophobic fatty acid tails, forming a bilayer that separates the cell from its environment.

Summary

  • The versatility of carbon enables the diversity of organic molecules essential for life.

  • Variation at the molecular level underlies biological diversity.

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