Carbohydrate Study Guide

📖 Core Concepts Carbohydrate – a sugar or sugar‑derivative; empirical formula $(CH{2}O){n}$ (C:H:O = 1:2:1). Classification – Monosaccharide: single unit, cannot be hydrolyzed. Disaccharide: two monosaccharides linked by a glycosidic bond (e.g., sucrose). Polysaccharide: long chain of many monosaccharides (starch, glycogen, cellulose, chitin). Structural features – polyhydroxy aldehydes/ketones; may carry N‑acetyl, sulfate, carboxyl, phosphate or deoxy modifications. Energy & storage – glucose is the primary fuel; stored as glycogen (≈300–500 g in muscle + liver). Glycemic Index (GI) – measures how fast a carb raises blood glucose relative to pure glucose (GI = 100). Glycoconjugates – sugars attached to proteins/lipids (glycoproteins, proteoglycans, glycolipids); N‑linked glycans can act as on/off switches for protein activity. Dietary fiber – insoluble cellulose, resistant starch, inulin; not digested by human enzymes but confers health benefits. --- 📌 Must Remember Empirical formula: $(CH{2}O){n}$ (ratio 1 : 2 : 1). GI benchmark: pure glucose = 100. Energy yield: 1 g carbohydrate → ≈4 kcal (≈16 kJ); 1 g fat → ≈9 kcal (≈38 kJ). Glycogen stores: 300–500 g total (muscle + liver). Glycolysis net gain: 2 ATP per glucose (after an initial 2‑ATP investment). Major storage polysaccharides: starch = plants; glycogen = animals. Structural polysaccharides: cellulose (plants), chitin (arthropods/fungi). Fiber benefits: improves digestion, blunts post‑prandial glucose, lowers cholesterol, increases satiety. --- 🔄 Key Processes Glycolysis (anaerobic/ aerobic entry) Step 1‑2: Invest 2 ATP → glucose‑6‑P → fructose‑6‑P. Step 3‑5: Split fructose‑1,6‑bisphosphate → two triose‑phosphates. Step 6‑10: Pay‑off phase – generate 4 ATP and 2 NADH → net +2 ATP. Citric Acid Cycle (Krebs) – Pyruvate → acetyl‑CoA → series of reactions → CO₂ + H₂O + 30 ATP equivalents per glucose (including oxidative phosphorylation). Glycogen Synthesis (storage) – Glucose → UDP‑glucose → glycogen synthase adds α‑1,4‑linked glucose; branching enzyme creates α‑1,6 branches. Glycogenolysis (mobilization) – Glycogen phosphorylase removes glucose‑1‑P units; debranching enzyme handles α‑1,6 points. N‑linked Glycosylation – Step 1: Transfer of pre‑assembled oligosaccharide from dolichol‑linked donor to Asn in the consensus Asn‑X‑Ser/Thr motif. Step 2: Trimming and remodeling in the ER/Golgi to generate mature glycan. --- 🔍 Key Comparisons Monosaccharide vs. Disaccharide vs. Polysaccharide Size: single unit ↔ two units ↔ many units. → Hydrolyzable: mono = no, di = yes, poly = yes (except structural polymers). Available vs. Unavailable Carbohydrates Absorption: small intestine ↔ large intestine (fermentation). Refined vs. Whole‑Food Carbohydrates Digestibility: rapid, high GI ↔ slower, lower GI, more fiber. Starch vs. Glycogen Source: plant storage ↔ animal storage; both α‑1,4/α‑1,6 linked but glycogen is more highly branched. Cellulose vs. Chitin Monomer: glucose ↔ N‑acetylglucosamine; Function: plant structural ↔ arthropod/fungal structural; Digestibility: humans lack cellulases, most lack chitinases. --- ⚠️ Common Misunderstandings “All carbs raise blood sugar the same.” → GI varies; fiber blunts the rise. “Low‑carb diets automatically cause weight loss.” → Long‑term loss depends on total calories, not macronutrient ratio. “Fiber provides calories.” → Most dietary fiber is indigestible; calories come from fermentation by gut microbes (minimal). “Cellulose is an energy source.” → Humans cannot hydrolyze β‑1,4‑glucose linkages; it’s purely structural. “Carbohydrate restriction always improves lipid profile.” → Evidence is mixed; benefits are not guaranteed. --- 🧠 Mental Models / Intuition CH₂O → “Carb‑brick” – think of each carbon as a brick with two hydrogens and one oxygen; stacking bricks gives polymers. Glucose = “Universal Currency” – all carbohydrate fuels are converted to glucose (or pyruvate) before entering energy pathways. Glycogen = “Short‑term bank” – stored in muscle (local “checking account”) and liver (central “savings”). Spend quickly, refill with meals. Fiber = “Traffic jam” – slows the flow of glucose into bloodstream, giving a smoother ride. --- 🚩 Exceptions & Edge Cases Cellulose & Chitin – structurally important but not digestible by human enzymes; require microbial cellulases/chitinases. High‑GI foods can be healthy when paired with protein/fat (overall glycemic load may be modest). Some disaccharides (e.g., lactose) need lactase; deficiency → intolerance – not a “unavailable carbohydrate” in the technical sense. N‑linked vs. O‑linked glycosylation – the outline details N‑linked; O‑linked follows Ser/Thr but may be omitted in some contexts. --- 📍 When to Use Which Assessing blood‑glucose impact: use GI for rate, GL (GI × carb grams/100) for total load. Diet counseling: prioritize whole‑food carbs over refined when aiming for lower GI and higher fiber. Metabolic pathway choice: Anaerobic, high‑intensity → rely on glycolysis (fast ATP, no O₂). Aerobic, prolonged activity → feed pyruvate into the citric acid cycle for maximal ATP yield. Choosing storage vs. immediate use: Post‑exercise: favor glycogen replenishment (high‑glycemic carbs). Weight‑maintenance: choose low‑GI, fiber‑rich carbs to moderate insulin response. --- 👀 Patterns to Recognize “‑ose” suffix → monosaccharide (glucose, fructose, ribose). “‑an” suffix → polysaccharide (starch, glycogen, cellulose). High GI foods → simple sugars, refined grains, low fiber → rapid glucose spike. Glycosylation motifs → Asn‑X‑Ser/Thr for N‑linked; look for “‑N‑” in protein sequences. Energy yield per gram → 4 kcal/g for carbs, 9 kcal/g for fats → quick rule for calorie calculations. --- 🗂️ Exam Traps | Trap | Why it’s tempting | Correct reasoning | |------|-------------------|--------------------| | “All carbohydrates provide 4 kcal/g.” | Remembered from nutrition tables. | True for digestible carbs; fiber’s caloric contribution is negligible (fermentation yields <1 kcal/g). | | “Low‑carb diets increase metabolism.” | Popular diet hype. | No consistent evidence of higher energy expenditure; weight loss = calorie deficit. | | “Cellulose can be used for energy like starch.” | Both are polysaccharides of glucose. | Human enzymes cannot cleave β‑1,4 linkages; cellulose is structural only. | | “GI alone predicts blood‑sugar response.” | GI is a well‑known number. | Glycemic Load matters: GI × carb amount; a large portion of low‑GI food can still raise glucose significantly. | | “Glycogen stores 500 g in muscle only.” | Numbers remembered vaguely. | Total body storage is 300–500 g split between muscle and liver. | | “All disaccharides are glucose + fructose.” | Sucrose is a familiar example. | Disaccharides vary (e.g., lactose = glucose + galactose, maltose = glucose + glucose). | | “N‑linked glycans always activate proteins.” | “On‑off switch” wording. | N‑linked glycans can modulate activity either way (activate or inhibit) depending on context. | ---