Carbohydrate (Sugars)

Description

Greek: σάκχαρον (sákkharon) – sugar

Carbohydrates are a broad class of organic molecules composed primarily of carbon, hydrogen, and oxygen, typically in a ratio of 1:2:1. They serve as a major source of energy for living organisms, providing quick and accessible fuel through processes like cellular respiration. Carbohydrates also play structural roles, forming key components of cell walls in plants (such as cellulose) and exoskeletons in arthropods (chitin). Additionally, they are involved in cell recognition and signaling through glycoproteins and glycolipids on cell surfaces.

Summary

Carbohydrates in traditional diets come primarily from whole, nutrient-dense sources like fruits, vegetables, and properly prepared grains and tubers. These natural carbohydrates provide vitamins and minerals that support digestion and overall health. While carbohydrates supply quick energy, the foundation emphasizes balanced intake alongside nutrient-rich fats and proteins for optimal metabolic function. Processed sugars and refined carbs are discouraged due to their role in promoting inflammation, insulin resistance, and chronic disease. Traditional carbohydrate sources, prepared properly are useful survival foods to be used in moderation.

Absorption of Carbohydrates

Carbohydrate digestion begins in the mouth, where salivary amylase starts breaking down starch into smaller polysaccharides. Digestion continues in the small intestine, where pancreatic amylase further breaks down polysaccharides into disaccharides and monosaccharides. Enzymes on the surface of intestinal cells (such as maltase, lactase, and sucrase) then break down disaccharides into monosaccharides like glucose, fructose, and galactose. These monosaccharides are absorbed by enterocytes via specific transporters and transported into the bloodstream. Glucose and galactose are absorbed through active transport, while fructose is absorbed by facilitated diffusion. Once in the bloodstream, these sugars are transported to tissues where they are used for immediate energy or stored as glycogen in the liver and muscles.

Functions

  • Energy source: Carbohydrates provide quick energy, primarily as glucose, which is essential for brain function and muscle activity.

  • Energy storage: Excess glucose is converted into glycogen and stored mainly in the liver and skeletal muscles, serving as a short-term energy reserve that can be rapidly mobilized during increased energy demand.

  • Cell recognition and signaling: Carbohydrate moieties attached to proteins and lipids (glycoproteins and glycolipids) on cell surfaces play critical roles in cellular communication, immune system recognition, and pathogen defense.

  • Protein sparing: By supplying accessible energy, carbohydrates reduce the need for the body to break down proteins for fuel, allowing proteins to focus on tissue repair, enzyme production, and other vital functions.

Chemical Structure

Molecular formula: Varies by type

  • Monosaccharides: e.g., Glucose – C₆H₁₂O₆

  • Disaccharides: e.g., Sucrose – C₁₂H₂₂O₁₁

  • Polysaccharides: (C₆H₁₀O₅)ₙ, where n is the number of repeating sugar units

Molecular mass:

  • Monosaccharides: ~180 atomic mass units (amu)

  • Disaccharides: ~342 amu

  • Polysaccharides: Can range from thousands to millions of amu depending on polymer length

Atomic composition:

  • Carbon (C), Hydrogen (H), Oxygen (O)

  • Typically in a ratio of 1:2:1 (C:H:O) for simple sugars

Bond types:

  • C–C and C–H bonds: Single covalent (sigma, σ) bonds

  • C–O bonds: Single covalent bonds connecting hydroxyl groups and glycosidic linkages

  • C=O bonds: Present in the carbonyl group (aldehyde or ketone) in monosaccharides

Functional groups:

  • Hydroxyl groups (–OH): Present on each carbon (except carbonyl carbon)

  • Carbonyl group (C=O): Present as either an aldehyde (aldose) or ketone (ketose)

  • Glycosidic bonds (C–O–C): Link monosaccharide units in disaccharides and polysaccharides

Bond order:

  • C–C, C–O, and C–H: bond order 1 (single bonds)

  • C=O: bond order 2 (double bond)

Bond length (approximate):

  • C–C bond: ~154 pm

  • C–O single bond: ~143 pm

  • C=O double bond: ~120 pm

Electron configuration (typical carbon atom): 1s² 2s² 2p²

Molecular polarity:

  • Monosaccharides and disaccharides: Polar, due to multiple hydroxyl and carbonyl groups

  • Polysaccharides: Generally polar, but less soluble in water depending on structure (e.g., cellulose is insoluble)

Solubility:

  • Simple carbohydrates: Highly water-soluble

  • Complex carbohydrates: Solubility decreases with size and branching; some are insoluble (e.g., cellulose)

Functionality:

  • Rapid energy source (monosaccharides)

  • Energy storage (glycogen)

  • Structural support (e.g., cellulose in plants, not in humans)

  • Cellular recognition and signaling (via glycoproteins and glycolipids

Physiological Functions of Carbohydrates

Rapid Energy Supply

Carbohydrates, primarily in the form of glucose, serve as a quick and efficient energy source for the body, particularly for tissues with high energy demands such as the brain, red blood cells, and skeletal muscles during intense activity. Glucose is rapidly metabolized through glycolysis, the citric acid cycle, and oxidative phosphorylation to produce ATP. Carbohydrates yield approximately 4 kcal/g, making them suitable for immediate energy needs but less efficient than fats for long-term energy storage.

Short-Term Energy Storage

Excess dietary glucose is converted into glycogen and stored primarily in the liver and skeletal muscle. Liver glycogen maintains blood glucose levels between meals, while muscle glycogen serves as a local energy reserve during physical exertion. However, glycogen storage is limited (~100–120 g in liver and ~300–400 g in muscle), making it a short-term energy buffer rather than a long-term reserve.

Non-Essential Nutrient

Carbohydrates are not essential in the human diet. The body can synthesize glucose internally via gluconeogenesis, using substrates such as amino acids (from protein), glycerol (from fats), and lactate. This endogenous glucose production is sufficient to meet the needs of glucose-dependent tissues, particularly during fasting, carbohydrate restriction, or ketogenic diets. Humans can survive indefinitely without dietary carbohydrates when adequate fat and protein are available.

Cellular Recognition and Immune Function

Carbohydrates play a key role in cell-cell recognition, immune response, and signaling through glycoproteins and glycolipids on the cell membrane. These carbohydrate-containing molecules are involved in immune surveillance, pathogen detection, and tissue differentiation, forming a critical part of the glycocalyx that lines cell surfaces.

Protein-Sparing Effect

In the fed state, carbohydrates provide energy that spares proteins from being catabolized for gluconeogenesis. This allows dietary and structural proteins to fulfill their primary roles in tissue maintenance, enzyme function, and immune defense, rather than being used as an energy substrate.

References