Carbohydrates Comprehensive Study Notes

Complex vs. Simple Carbohydrates

• Complex carbohydrates
  • Provide vitamins, minerals and dietary fibre.
  • Major food sources: breads, legumes, rice, pasta, starchy vegetables (e.g., potatoes, corn).
  • Digested and absorbed more slowly ➔ steadier blood-glucose profile.
• Simple carbohydrates
  • Naturally present in fruits, milk and many vegetables.
  • Refined/simple sugars (cake, candy, table sugar) supply energy but lack vitamins, minerals and fibre.
  • Rapidly absorbed ➔ sharp rise in blood glucose.

Dietary Sources & Meal-Planning Notes

• Blood-glucose control: carbohydrates should be eaten at each meal/snack so that administered insulin can be matched to carbohydrate load + physical activity level.
• Typical carbohydrate-rich foods (slide list)
  • Starches: rice, pasta, bread, potatoes, pizza, noodles, scones, muffins, breakfast cereals, hot chips, sandwiches, lentils, baked beans.
  • Dairy: milk, yoghurt (plain, sweet, savoury), dairy alternatives.
  • Fruit: fresh, canned, dried, fruit juice, tinned halves, orange juice.
  • Snacks/Condiments: biscuits, cake, muffins, chips, ketchup, barbecue sauce, cream sauce, spaghetti sauce, café mocha.
  • Beverages: regular soda, smoothies.

Examples of Foods That Contain Carbohydrate (condensed slide table)

• Starches: bread, tortillas, chapati, crackers, pasta, noodles, cereal grains (rice, wheat, bulgur, couscous).
• Dairy: milk (whole/low-fat), yoghurt, soy milk.
• Sweets: candy, cake, cookies, ice-cream, sherbet, pie.
• Starchy vegetables: potatoes, corn, peas.
• Fruits: all fresh, canned, frozen and dried types.
• Drinks: fruit juice, soft drinks, smoothies.
• "Hidden" sources (often forgotten): sauces (spaghetti, BBQ, ketchup), chips, café mocha.

Definition & General Features of Carbohydrates

• Large group of organic compounds containing carbon (C), hydrogen (H), oxygen (O).
• Contain both
  • Carbonyl group C = O (aldehyde or ketone)
  • Hydroxyl groups -OH.
• General empirical formula for a monosaccharide: (CH2O)n.

Biological Roles / Uses of Carbohydrates

• Primary energy source: substrate for glycolysis.
• Energy storage: starch (plants) & glycogen (animals).
• Structural components:
  • Cell membranes: glycoproteins & glycolipids.
  • Plant cell walls: cellulose.

Classification of Carbohydrates

• Monosaccharides – single sugars (monomers).
• Disaccharides – two monosaccharide units.
• Oligosaccharides – 3!\text{–}!14 monosaccharides (mentioned in table).
• Polysaccharides – hundreds/thousands of monosaccharides.

Monosaccharides

General Properties

• Sweet, crystalline, low molecular mass, water-soluble.
• Reducing sugars because free aldehyde/ketone can donate electrons.

Classification by Carbon Number (n in (CH2O)n)

• Trioses (3C) – e.g., glyceraldehyde (aldose), dihydroxyacetone (ketose).
• Tetroses (4C) – e.g., erythrose.
• Pentoses (5C) – e.g., ribose, deoxyribose, ribulose.
• Hexoses (6C) – e.g., glucose, fructose, galactose.

Structural Isomerism: α- and β-Glucose

• Both share formula C6H{12}O_6.
• Differ at C1 (anomeric carbon):
  • α-glucose – \text{OH} group below plane.
  • β-glucose – \text{OH} group above plane.

Carbonyl Type

• Aldoses (aldehyde at C1) – e.g., glucose, ribose.
• Ketoses (ketone at C2) – e.g., fructose, ribulose.

Reducing Sugar Concept

• Aldoses & ketoses reduce metal ions (Cu²⁺ → Cu⁺ in Benedict’s reagent).
• Sucrose is not reducing: both anomeric carbons are tied in its glycosidic bond.

Disaccharides

Formation

• Two monosaccharides undergo condensation (release H_2O) forming a glycosidic bond C–O–C.
• Reaction is reversible ➔ hydrolysis.

Major Disaccharides & Bonds

• Maltose – glucose + glucose, α-1,4 bond (reducing sugar).
• Sucrose – glucose + fructose, α-1,2 bond (non-reducing).
• Lactose – glucose + galactose, β-1,4 bond (reducing).

Physiological Functions (from table)

• Maltose: respiratory substrate in germinating barley.
• Sucrose: main transport sugar in plants (cane sugar/beet).
• Lactose: energy source in mammalian milk.

Polysaccharides

General Characteristics

• Very large, usually tasteless, generally insoluble in water.
• Chains may be straight/coiled, branched/unbranched.

Starch (Plant Storage)

• Composed of α-glucose.
• Two fractions:
  • Amylose
  • Unbranched helix, ≈300 glucose units.
  • Linkage: α-1,4 throughout.
  • Amylopectin
  • Branched, up to \sim1500 units.
  • Backbone: α-1,4; branches every \approx30 residues via α-1,6.

Glycogen (Animal Storage)

• Also α-glucose; stored in liver & muscle.
• Shorter chains, more frequent branching than amylopectin (α-1,4 main, α-1,6 branch).

Cellulose (Plant Structure)

• Composed exclusively of β-glucose.
• Straight chains linked by β-1,4 bonds.
• Parallel chains form microfibrils via extensive hydrogen bonding ➔ high tensile strength of cell walls.

Functional Importance (Consolidated)

• Energy (immediate & stored), structural integrity, cell-cell recognition (glycoproteins/glycolipids), intermediates in respiration/photosynthesis.

Summary Tables (as per slides)

• Monosaccharides
  • Trioses: glyceraldehyde, dihydroxyacetone – intermediates in glycolysis/photosynthesis.
  • Pentoses: ribose (RNA, ATP, NAD), deoxyribose (DNA), ribulose (CO₂ acceptor in Calvin cycle).
  • Hexoses: glucose (major respiratory substrate), fructose (nectar), galactose (component of lactose).
• Disaccharides
  • Maltose, sucrose, lactose – sources & functions listed above.
• Polysaccharides
  • Starch (plant storage), glycogen (animal storage), cellulose (plant cell wall support).
• Oligosaccharides
  • 3–14 residues; attached to membrane proteins/lipids for cell recognition.

Laboratory Food Tests

1. Benedict’s Test – Reducing Sugars

• Procedure
  • Mix 2 mL sample + 2 mL Benedict’s reagent.
  • Heat to boil while shaking.
• Positive results
  • Colour transition: blue → green → yellow → orange → brick-red precipitate.
• Chemistry
  • \text{Cu}^{2+}{(blue)} + e^- \rightarrow \text{Cu}^+{(red\;Cu_2O)}.
  • Semi-quantitative: further colour change ⇒ higher reducing sugar concentration.

2. Test for Non-Reducing Sugar (Sucrose Example)

• Step 1: Confirm negative Benedict’s result (blue remains).
• Step 2: Hydrolyse 2 mL sucrose with 1 mL dilute HCl (boil ≈1 min).
• Step 3: Neutralise with NaHCO_3.
• Step 4: Repeat Benedict’s test ➔ brick-red ppt indicates hydrolysed glucose/fructose.

3. Iodine/Potassium Iodide (I₂/KI) Test – Starch

• Add few drops I₂/KI to 2 mL 1 % starch solution or directly to food slice (e.g., potato).
• Blue-black colour complex forms with helical amylose ⇒ presence of starch.

Ethical / Practical / Real-World Connections

• Nutrition counselling emphasises complex carbs and fibre for metabolic health.
• Low-carb dieting (cartoon reference “sick and tired of this Low Carb diet”) illustrates public perception vs. biochemical need for carbohydrates.
• Proper matching of insulin to carbohydrate intake is vital in diabetes management.

Equations & Key Numbers (collated)

• General monosaccharide: (CH2O)n.
• Glucose: C6H{12}O_6; ring formation involves bond between C1 and O on C5.
• Benedict redox: \text{Cu}^{2+} + e^- \rightarrow \text{Cu}^+.
• Amylopectin branching ≈ every 30 residues; up to 1500 glucose units.
• Amylose helix ≈ 300 glucose units.

Connections to Previous / Broader Topics

• Monosaccharide trioses (glyceraldehyde, dihydroxyacetone) are intermediates in glycolysis & Calvin cycle.
• ATP, NADH, FAD contain pentose sugars – illustrating the centrality of carbohydrates in metabolic energy transfer.
• Cellulose’s β-1,4 link highlights stereochemistry’s role in digestibility (humans cannot hydrolyse β-1,4).