A&P 2.9 Carbohydrates

Definition and Terminology:
- Biological macromolecules, often colloquially referred to as "macros" in nutrition and exercise contexts, are large biological molecules essential for cellular structure and function.
- They are classified into four major categories:
  - Carbohydrates (commonly called carbs or sugars, though the category includes complex structures like fiber).
  - Lipids (commonly referred to as fats, characterized by abundant carbon-carbon and hydrocarbon bonds).
  - Proteins.
  - Nucleic Acids.
Nutritional Packaging Labels:
- Standard nutrition facts packaging explicitly displays three of the four major categories: carbohydrates, lipids/fats, and proteins.
- Nucleic acids are omitted from nutrition labels despite being biological macromolecules because they do not serve as a primary energy source or contribute meaningfully to daily caloric intake.
- Although complex biochemistry allows nucleic acids to be metabolized into energy, the human body will only burn nucleic acids after completely depleting its reserve stores of carbohydrates, lipids, and proteins.

Hierarchy of Carbohydrates:
- Carbohydrates are categorized based on the number of simple sugar units (monomers) linked together.
- The prefix mono- denotes one single carbohydrate unit.
- The prefix di- denotes two carbohydrate units joined together.
- The prefix poly- denotes many (typically defined as three or more) carbohydrate units linked together.
Monosaccharides:
- Monosaccharides represent single, simple sugar molecules.
- Key examples of monosaccharides to master include:
  - Glucose
  - Fructose
  - Galactose

Representations in Chemical Structures:
- In biochemical line-angle drawings (stick-and-line formulas), implicit conventions are used:
  - Unlabeled line intersections and angles represent carbon ( $C$ ) atoms.
  - Associated hydrogen ( $H$ ) atoms attached to those carbons are also implied by default.
Polarity and Solubility:
- Glucose contains numerous polar covalent bonds formed between oxygen ( $O$ ) and hydrogen ( $H$ ) atoms.
- In an oxygen-hydrogen bond, oxygen exerts a higher electronegativity, pulling shared electrons closer and creating a partial negative charge ( $\delta^-$ ) on oxygen and a partial positive charge ( $\delta^+$ ) on hydrogen.
- These partial charges throughout the glucose molecule make it highly attractive to polar water molecules ( $H_2O$ ).
- Overall Net Charge: Despite having localized partial charges due to polar covalent bonds, a complete glucose molecule carries no net electrical charge (it is neutral, not an ion).

Dehydration Synthesis Reactions:
- Simple carbohydrates can be covalently linked together through biochemical synthesis reactions or broken apart via cleavage reactions.
- When two monosaccharides combine, hydroxyl groups ( $-OH$ ) and hydrogen atoms on the respective molecules react:
  - The reaction creates a single oxygen bridge connecting the two sugar units.
  - The remaining atoms—two hydrogen atoms and one oxygen atom—are released as a single molecule of water ( $H_2O$ ).
- This type of reaction is termed a dehydration synthesis reaction because it combines smaller molecules into a larger compound while producing water as a byproduct.
Key Disaccharides:
- Disaccharides are formed by covalently bonding two monosaccharides together:
  - $\text{Glucose} + \text{Fructose} \rightarrow \text{Sucrose}$ (common table sugar/sweetener).
  - $\text{Glucose} + \text{Glucose} \rightarrow \text{Maltose}$ .
  - $\text{Glucose} + \text{Galactose} \rightarrow \text{Lactose}$ .
- Correction of Common Misconception: Students frequently confuse lactose and galactose. Galactose has a longer name but is the monosaccharide, whereas lactose is the larger disaccharide.

Polymerization Concepts:
- A polymer is a long molecular chain composed of repeating structural units (monomers) linked sequentially through chemical bonds ("daisy-chaining").
- Biological macromolecules that routinely form polymers include:
  - Carbohydrates (monomer: monosaccharide)
  - Proteins (monomer: amino acid)
  - Nucleic Acids (monomer: nucleotide)
- In theory, polymers can repeat indefinitely, though cellular size places physical constraints on their total length.
- Lipid Exception: Fatty acids and lipids can form complex molecular structures, but they do not form true, potentially infinite polymeric chains.

Polysaccharide Examples:
- Polysaccharides consist of long chains of repeating monosaccharide units.
- Major biological polysaccharides include glycogen, starch, and cellulose.
Starch:
- Produced exclusively by plant cells for energy storage.
- Composed of two structural forms of glucose chains:
  - Amylose: A linear, unbranched single chain of glucose monosaccharides linked end-to-end.
  - Amylopectin: A branched complex structure consisting of multiple glucose chains extending outwards.
Glycogen:
- Serves as the primary glucose storage polysaccharide in animal and human bodies ("animal starch").
- Enables the body to store excess circulating blood glucose for future energy needs rather than burning it immediately.
- Primary Storage Sites: Human liver and skeletal muscle tissue store limited reserves of glycogen.
- Structural Comparison: Glycogen features a highly branched structure, exhibiting significantly more branching points than plant amylopectin.
- Metabolic Function: When systemic energy levels drop, enzymes break down glycogen polymers to liberate single glucose molecules back into the bloodstream.

Question: Consider the chemical reaction where glucose and glucose act as reactants to produce maltose as the final product ( $\text{Glucose} + \text{Glucose} \rightarrow \text{Maltose}$ ). Which terms accurately describe this chemical process?
- Answer: This reaction is categorized as both an anabolic reaction and a synthesis reaction (specifically, dehydration synthesis).
- Explanation: It is anabolic because it utilizes energy to construct a larger, more complex disaccharide from smaller monosaccharide subunits. It is a synthesis reaction because smaller precursor molecules combine to form a single larger output molecule.