Important Biological Molecules and Cellular Processes

Important Biological Molecules

Atoms
- The four most important elements that make up approximately 99% of the human body are:
- Hydrogen
- Carbon
- Nitrogen
- Oxygen
- Additional elements (sodium, magnesium, phosphorus, sulfur, chloride, potassium, calcium) constitute about 0.9% of the body but are still essential for biological functions.

Organic Compounds: These compounds contain carbon along with hydrogen and often oxygen.
Electron Shells:
- First Shell: Maximum of 2 electrons.
- Second and Subsequents Shells: Maximum of 8 electrons.
- Valence Electrons: Understanding valence electrons is crucial for understanding the types of bonds that can form between atoms.
Atoms generally prefer to have a ‘full’ outer shell, leading them to donate, accept, or share electrons. This property forms the basis of chemical bonding, which is essential for life.
Water in Cells:
- Around 70% of a cell is composed of water.
- Within water molecules, two hydrogen atoms are covalently bonded to one oxygen atom, creating polar characteristics.

pH: Significantly varies in the human body and includes various buffering mechanisms to maintain balance.
Water (H₂O): At a pH of 7, the concentrations of both hydroxide (OH⁻) and hydrogen (H⁺) ions are equal.
Acids and Bases:
- Acids: Substances that release protons (H⁺).
- Bases: Substances that accept protons.
- The dissociation of water can be represented as:
  H_2O
  ightleftharpoons H^+ + OH^-

Cells primarily consist of four types of macromolecules:
- Proteins
- Lipids
- Function as structural and functional components of cells.
- Carbohydrates: Exist in mono-, di-, and polysaccharides, constructed from monomers through condensation reactions.

Anabolic Reactions: Reactions that require energy to construct molecules from smaller units.
Catabolic Reactions: Reactions that release energy through the breakdown of larger molecules into smaller ones.

Activation Energy: The minimum energy required for a chemical reaction to proceed.
- Enzymes reduce the activation energy, which enhances the rate of chemical reactions.
- Enzymes have active sites for specific substrate binding, forming an enzyme-substrate complex.
- Allosteric Binding Sites: Regions on the enzyme that can increase or decrease enzyme activity.

ATP Formation: ATP formation from ADP and inorganic phosphate involves a condensation reaction, which is energetically unfavorable and must be coupled with a highly favorable reaction.

Common Activated Carriers in Metabolism:
- ATP: Carries phosphate groups.
- NADH, NADPH, FADH₂: Carry electrons and protons.
- Acetyl CoA: Carries acetyl groups.
- S-Adenosylmethionine: Carries methyl groups.
- Uridine diphosphate glucose: Carries carboxyl groups.

Glycolysis converts glucose into pyruvate through a series of enzymatic reactions, categorized into:
- Energy Investment Phase: Two ATP are utilized.
- Energy Generation Phase: Results in a net synthesis of 2 ATP and 2 NADH per glucose molecule.
Glycolysis also known as the Embden-Meyerhof pathway.
The pathway involves 10 steps, each catalyzed by specific enzymes:
- Initial steps involve cleavage of six-carbon sugars into two three-carbon sugars, doubling the number of molecules with each step.

The citric acid cycle starts with the reaction of acetyl CoA and oxaloacetate to form citrate (citric acid).
For each complete cycle:
- Produces 3 NADH, 1 GTP, and 1 FADH₂.
- Releases 2 CO₂ as waste products.
Connection to biosynthesis, as glycolysis and the citric acid cycle furnish precursors for amino acids, nucleotides, lipids, sugars, etc.

Amino Acids: Serine, Alanine, Aspartate, and others.
Nucleotides: Purines and pyrimidines.
Lipids: Fatty acids, cholesterol.
Macromolecules: Glycolipids, glycoproteins, etc.
Arrows in metabolic diagrams represent enzyme-catalyzed reactions and the pathways leading to the production of essential biological molecules.