Carbohydrates – AP Biology Notes

Overview
- Carbohydrates are energy molecules essential for quick energy, energy storage, raw materials, and some structural roles.
- General composition: elements C, H, and O with the empirical formula
- ext{CH}2 ext{O} or, for many sugars, ext{(CH}2 ext{O)}_x
- A common specific example: ext{C}6 ext{H}{12} ext{O}_{6}
- Functions: fast energy, energy storage, raw materials, structural materials.
- Monomer: sugars; examples include sugars in general, starches, and cellulose.

Basic Formula and Classification of Carbohydrates

Monomer, oligomer, polymer concepts
- Monosaccharides: simple sugars (one sugar unit)
- Disaccharides: two monosaccharides linked together
- Polysaccharides: long chains (polymers) of monosaccharides
Sugars are named mainly with the suffix -ose.
Carbohydrate categories by number of carbons:
- 3C = triose (e.g., glyceraldehyde)
- 5C = pentose (e.g., ribose)
- 6C = hexose (e.g., glucose)
Common examples and formulas:
- Glyceraldehyde (triose): ext{C}3 ext{H}6 ext{O}_3
- Ribose (pentose): ext{C}5 ext{H}{10} ext{O}_5
- Glucose (hexose): ext{C}6 ext{H}{12} ext{O}_6
Functional groups determine sugar type
- Triose sugars: ext{C}3 ext{H}6 ext{O}_3
- Pentose sugars: ext{C}5 ext{H}{10} ext{O}_5
- Hexose sugars: ext{C}6 ext{H}{12} ext{O}_6
- Classification by carbonyl group:
- Aldose (aldehyde group at one end)
- Ketose (ketone group within the molecule)
- Aldoses vs Ketoses (examples):
- Aldoses: glyceraldehyde, ribose, glucose, galactose
- Ketoses: dihydroxyacetone, ribulose, fructose
- Aldose vs Ketose illustrations show terminal vs internal carbonyl groups in structural drawings
Rings in solution
- 5C and 6C sugars commonly form ring structures in aqueous solution
- Carbons in the sugar rings are numbered as they become part of the ring structure
- In biology, ring formation brings conformational changes that affect reactivity and linkage
- Context: rings are formed in cells and are central to how sugars link to form polysaccharides

Numbering and Carbon Skeletons

Carbons in sugars are numbered to track linkages and reactions
- For general sugars: C1, C2, C3, C4, C5, C6 (for hexoses common in biology)
- In nucleotides, sugar carbons are often denoted with primes (e.g., 1′, 2′, 3′, 4′, 5′) to distinguish from the base carbons
The carbon skeleton contains energy stored in C–C bonds, which is harvested during cellular respiration
- Those carbon bonds and their positions become important when considering energy yield and polymer structure

Simple, Disaccharides, and Polysaccharides

Simple sugars (Monosaccharides): one sugar unit
- Example: glucose
Disaccharides: two monosaccharides linked together
- Example: sucrose (table sugar; glucose + fructose)
- Example: maltose (glucose + glucose)
Polysaccharides: long polymers of sugars
- Examples: starch (plants), glycogen (animals, stored in liver & muscles), cellulose (plants), chitin (arthropods & fungi)
- Functions: energy storage (starch, glycogen) and structural support (cellulose, chitin)

Building Sugars: Dehydration Synthesis

Key process: dehydration synthesis links monosaccharides by removing water (H2O) to form a glycosidic bond
- General pattern: two monosaccharides combine to form a disaccharide with the release of water
- Example 1: glucose + glucose → maltose + water
- Example 2: glucose + fructose → sucrose + water
Chemical representation (examples):
- Glucose monomer units: ext{C}6 ext{H}{12} ext{O}_6
- Disaccharide (maltose) formula: ext{C}{12} ext{H}{22} ext{O}_{11}
- Overall dehydration synthesis equations:
- ext{C}6 ext{H}{12} ext{O}6 + ext{C}6 ext{H}{12} ext{O}6
  ightarrow ext{C}{12} ext{H}{22} ext{O}{11} + ext{H}2 ext{O}
- In solution, ring-opening and glycosidic bonds determine sugar linkage patterns (e.g., α- vs β-linkages in starches and cellulose)
Glycosidic linkages
- Linkages between sugar units are called glycosidic bonds
- Different linkages (e.g., α-1,4; β-1,4; α-1,6) influence digestibility and structural properties

Polysaccharide Diversity and Function

Molecular structure determines function
- Isomers of glucose lead to different polymers (starch vs cellulose)
- Starch: α linkages; some branching (amylose and amylopectin) for energy storage in plants
- Glycogen: highly branched storage polysaccharide in animals (liver and muscles) for rapid energy release
- Cellulose: β linkages; linear chains form strong structural fibers in plants
- Chitin: similar to cellulose, but with acetylated amino groups; structural support in arthropods and fungi
Linear vs Branched Polysaccharides
- Linear: starch contains both amylose (mostly linear) and amylopectin (branched)
- Branched: glycogen is highly branched
- Branching effects on digestion:
- Branching generally allows faster digestion and more rapid energy release due to more accessible ends for enzymatic action
- Linear structures like cellulose are less readily digested by many animals

Digestive Implications and Real-World Relevance

Digesting starch vs cellulose
- Starch: easily digested by human and other animal enzymes (e.g., amylase)
- Cellulose: difficult to digest; many animals lack cellulase; digestion often requires gut bacteria
Herbivores vs carnivores
- Cellulose is the most abundant organic compound on Earth; herbivores have evolved mechanisms to digest cellulose via symbiotic bacteria in their digestive systems
- Most carnivores cannot digest cellulose efficiently and rely on other food sources for energy
Ruminants and gut microbiota
- Ruminant animals (e.g., cows) harbor bacteria in their stomachs that break down cellulose, enabling efficient energy extraction from plant material
- The presence of symbiotic bacteria allows cellulose digestion that the animal itself cannot perform
- Some herbivores engage in coprophagy (re-eating feces) to maximize nutrient extraction from microbial fermentation products
- Examples and notes from the transcript:
- Cow can digest cellulose well; do not need extra sugars beyond cellulose for energy
- Gorilla digestion of cellulose is less efficient; may supplement diet with additional sugars from fruit or other sources
- Visual reminder: “Ruminants” and “Coprophage” terms appear in the material as key concepts describing microbial digestion assistance and nutrient recycling in some species
Ethical, practical, and ecological considerations
- Dietary trends like low-carbohydrate diets raise questions about energy sources and health impacts; carbohydrates are a major energy source in many diets
- Dependence on gut microbes for fiber digestion underlines the importance of microbiomes for nutrient availability in herbivores and the ecological implications of dietary changes

Quick Reference: Key Formulas and Terms

General carbohydrate formula: ext{(CH}2 ext{O)}x
Common sugar formulas:
- Glucose: ext{C}6 ext{H}{12} ext{O}_6
- Ribose: ext{C}5 ext{H}{10} ext{O}_5
- Glyceraldehyde: ext{C}3 ext{H}6 ext{O}_3
Monosaccharides, disaccharides, polysaccharides definitions
Dehydration synthesis example (glucose + glucose → maltose + water):
- ext{C}6 ext{H}{12} ext{O}6 + ext{C}6 ext{H}{12} ext{O}6
  ightarrow ext{C}{12} ext{H}{22} ext{O}{11} + ext{H}2 ext{O}
Major polysaccharides and roles
- Starch (plants) – energy storage
- Glycogen (animals) – energy storage in liver and muscles
- Cellulose (plants) – structural material; indigestible to many animals without bacteria
- Chitin (arthropods & fungi) – structural material

Connections to Foundations and Real-World Context

Link to cellular respiration: C–C bonds in glucose store energy that is harvested in respiration to produce ATP
Structure-function relationship: polymer geometry (branched vs linear, α vs β linkages) determines digestibility and biological role
Ecological and evolutionary relevance: herbivore adaptation to cellulose digestion via gut microbiota; co-evolution of gut flora with host species; influence on diet choices and energy budgets
Practical implications: dietary choices (e.g., low-carb trends) affect energy availability and metabolic pathways; understanding starches, glycogen, and cellulose helps explain food processing and nutrition