Study Notes on Chemical Principles in Cell Biology

Chemical Principles in Cell Biology

Chapter two addresses the five chemical principles that are important to cell biology. It highlights the essential nature of certain atoms and molecular structures that contribute to cellular functions. Notably, carbon is a key atom in forming all cellular molecules, and all cells are encompassed by a selectively permeable plasma membrane. Two subsequent chapters will delve deeper into membrane structure and function as well as membrane transport.

Synthesis of Biological Macromolecules

This section touches on the synthesis of biological macromolecules through two primary processes: polymerization and self-assembly. Biological macromolecules comprise small, water-soluble organic molecules that cells either derive from other cells or synthesize from simple inorganic substances and ions such as phosphate ions, carbon dioxide, or ammonium. These inorganic precursors from the environment provide the building blocks for the synthesis of organic molecules essential for forming larger macromolecules like polysaccharides, proteins, lipids, and nucleic acids. Each of these macromolecules plays a crucial role in cellular functions and structures.

Polymerization Process

Definition

Polymerization is the process whereby small organic molecules, referred to as monomers, are covalently bonded together to form larger macromolecules or polymers.

Description of the Process

The polymerization process is characterized by two main steps: activation of monomers and condensation reactions.

  1. Activation of Monomers

    • Typically, an input of energy is required for activation, usually in the form of ATP, which supplies a carrier molecule to the monomers. This activation is crucial for enabling the covalent bonds to form between the monomers.

  2. Dehydration/Condensation Reaction

    • This reaction involves removing a water molecule as a hydroxyl group from one monomer and a hydrogen from another, allowing the monomers to bond and form a covalent linkage. The removal of a water molecule enables the joining of monomers, facilitating the repeated synthesis of larger polymers.

    • These polymerization reactions are catalyzed by specific enzymes that recognize the respective monomers. Directionality is inherent in these macromolecules, which is vital for their functional roles.

Directionality of Macromolecules

  • Proteins have distinct N-terminus and C-terminus, while nucleic acids exhibit a free 5’ end and a free 3’ end.

Hydrolysis Reaction

  • To separate the monomers once they are bonded in a polymer, a hydrolysis reaction can be employed, which adds water back to break apart the covalent bonds connecting the monomers.

Biological Macromolecules

The three macromolecules that are recognized as polymers include nucleic acids, proteins, and polysaccharides, while lipids do not qualify as long polymers.

Monomers of Biological Macromolecules

  • Proteins: Composed of amino acids (20 different types).

  • Nucleic Acids: Composed of nucleotides (four types in DNA/RNA).

  • Polysaccharides: Composed of monosaccharides, which may be identical or similar, bonded through glycosidic linkages.

Classification of Carbohydrates

Carbohydrates consist of carbon, hydrogen, and oxygen in a ratio of approximately 1:2:1, with the empirical formula represented as $CnH{2n}O_n$.

Monosaccharides

  • The simplest carbohydrates are monosaccharides, which vary in carbon content (usually 3-6 carbons). They can be classified as:

    • Aldose Sugar: Contains a carbonyl group at the terminal position.

    • Ketose Sugar: Contains a carbonyl group on an interior carbon.

Ring Form and Structural Variations

Monosaccharides primarily exist in ring forms in aqueous solutions. For instance:

  • Alpha Monosaccharide: Hydroxyl group on carbon 1 projecting down (e.g., alpha glucose).

  • Beta Monosaccharide: Hydroxyl group on carbon 1 projecting up (e.g., beta glucose).

Polysaccharides

Polysaccharides, which are larger carbohydrate structures, are formed through glycosidic linkages of monosaccharides. When two monosaccharides bond, a disaccharide is formed, and oligosaccharides consist of 3-10 monosaccharides. Examples include:

  • Cellulose: A structural polysaccharide within plant cell walls.

  • Starch: An energy storage polysaccharide in plants, formed from alpha glucose through alpha glycosidic linkages.

    • Amylose: A simpler form of starch with helical structure.

    • Amylopectin: A branched starch variant.

  • Glycogen: The energy storage polysaccharide in animals, which also consists of alpha glucose and is more branched than starch.

Synthesis of Glycogen

  • Glycogen is synthesized through steps similar to those described for other polysaccharides. Instead of ATP, uridine triphosphate (UTP) is used for activation. Hydrolysis of UTP releases a pyrophosphate and supplies the energetic monomer for subsequent condensation reactions, mediated by glycogen synthase.

Nucleic Acids

Nucleic acids, including DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are paramount for storing, transmitting, and expressing genetic information. Different types of RNA play diverse roles, such as:

  • Messenger RNA (mRNA): Carries genetic information from DNA.

  • Ribosomal RNA (rRNA): Forms the structure of ribosomes.

  • Transfer RNA (tRNA): Brings amino acids to ribosomes.

  • MicroRNA (miRNA) and Small Interfering RNA (siRNA): Involved in gene regulation.

Structure of Nucleic Acids

Nucleic acids are constructed from nucleotides, each consisting of:

  • A five-carbon sugar (ribose in RNA, deoxyribose in DNA).

  • A nitrogenous base (either pyrimidines or purines).

  • One or more phosphate groups.

  • The presence of ribose enhances RNA reactivity compared to DNA.

Nomenclature of Nucleotides and Nucleosides

Understanding nomenclature is critical:

  • Nucleoside: Comprises a five-carbon sugar bound to a nitrogenous base.

  • Nucleotide: Includes the nucleoside plus one or more phosphate groups.

Phosphodiester Bonds

Nucleotides are polymerized through condensation reactions that form phosphodiester bonds, which also create directionality within nucleic acids (5’ to 3’).

Structure of DNA

Typically exists as a double helix, with:

  • A sugar-phosphate backbone outlining the structure.

  • Complementary base pairing (G-C and A-T) stabilized by hydrogen bonds.

    • Three hydrogen bonds between C and G, two between A and T, contribute to the integrity of the DNA structure.

Structure of RNA

RNA typically exists as a single strand but can form complex structures (hairpin loops) due to intra-strand complementary base pairing. RNA's structure allows flexibility in function compared to the more stable double helix configuration of DNA.