Carbohydrates – Comprehensive Study Notes (BIO 1010C: Chapter 3, Module 3)

Carbohydrates: Role and Overview

Carbohydrates (sugars) are macromolecules with roles in energy storage, cell structure, and cell recognition/identity.
Classes:
- Monosaccharides (single-sugar units)
- Disaccharides (two sugars linked by a glycosidic bond)
- Polysaccharides (long polymers of monosaccharides)
General chemical formula: $(CH2O)n$
Meaning of name:
- “carbo” = carbon; “hydrate” = water; n can vary from 3 to over a thousand
Key functional features:
- Contain one carbonyl group (C=O), several hydroxyl groups (O–H), and many C–H bonds
- Polar groups make carbohydrates hydrophilic
- Sugar names typically end in -ose
Roles in biology (summary):
- Energy storage
- Cell structure
- Cell recognition and identity
Conceptual framework: monosaccharides link to form polysaccharides; the major questions include how linkages form, how they vary, and how structure relates to function

The Structure of Monosaccharides

Monosaccharide monomers vary in four main ways: 1) Location of the carbonyl group:
- Aldose: carbonyl group at the end of the chain
- Ketose: carbonyl group in the middle
  2) Number of carbon atoms:
- Trioses (3), Pentoses (5), Hexoses (6), etc.
  3) Spatial arrangement of their atoms (structural isomers): different hydroxyl group orientations create distinct isomers
  4) Linear vs ring forms: sugars commonly form ring structures in watery solutions
General features:
- Common monosaccharides have 3–7 carbon atoms
- End with the suffix -ose; carbonyl group is C=O
- Aldoses have carbonyl at the end; ketoses have carbonyl in the middle
- Typical formulas include triose, pentose, hexose examples
Common examples:
- Glyceraldehyde (aldose, triose)
- Dihydroxyacetone (ketose, triose)
- Ribose (aldose, pentose)
- Glucose (aldose, hexose)
- Galactose (aldose, hexose; epimer of glucose at C-4)
Structural isomers (example):
- Glucose vs. Galactose are structural isomers with the same carbon skeleton but different spatial arrangement around one carbon
Hexose monosaccharides (example):
- Glucose, Galactose, Fructose (fructose is a ketose)
- All have the formula $C6H{12}O_6$
Ring forms: carbohydrates often exist as rings in solution; ring forms include α- and β- anomers of glucose
- α-Glucose: OH on C-1 is below the plane of the ring
- β-Glucose: OH on C-1 is above the plane of the ring

Monosaccharides: Quick Facts and Notation

General properties:
- Monosaccharides usually have 3–7 carbons
- End with -ose
- Contain a carbonyl group C=O (aldehyde or ketone)
- Aldoses: carbonyl at the end of the chain; Ketoses: carbonyl in the middle
- Trioses, Pentoses, Hexoses as categories
Examples (structure shown conceptually):
- Glyceraldehyde (aldose, triose)
- Dihydroxyacetone (ketose, triose)
- Ribose (aldose, pentose)
- Glucose (aldose, hexose)
Structural isomers: different arrangement of hydroxyl groups in space creates unique monosaccharides

How Monosaccharides Link to Form Polysaccharides

Monosaccharides are the monomers that make up polysaccharides
Linkage formation:
- Condensation (dehydration) reaction between two hydroxyl groups forms a covalent glycosidic linkage
- Linkages can be broken by hydrolysis
Polysaccharides are long chains (polymers) of monosaccharides
End result: highly diverse polysaccharide structures with different properties and functions

Glycosidic Linkages: α and β Forms

Two common glycosidic linkages:
- α-1,4-glycosidic linkage (between C-1 and C-4 of adjacent monosaccharides; OH on C-1 is down in the ring orientation)
- β-1,4-glycosidic linkage (between C-1 and C-4 of adjacent monosaccharides; OH on C-1 is up in the ring orientation)
Key geometric difference: C-1 hydroxyl groups are on opposite sides of the plane of the glucose rings
These linkages can form between any two hydroxyl groups on adjacent monomers; the linkage type (α vs β) and position (1,4 vs 1,6) determine overall shape and properties
Visual cue from examples: α-glucose vs β-glucose structures show the orientation difference and how a glucose residue can be "flipped" in β-linkage

Disaccharides: Common Dietary Sugars

Definition: two monosaccharides linked by a glycosidic bond
Examples:
- Maltose = glucose + glucose
- Sucrose = glucose + fructose
- Lactose = glucose + galactose

Polysaccharides: Structure and Function

Polysaccharides are polymers of monosaccharide monomers; may be branched or unbranched; may contain more than one type of monosaccharide
Typical molecular weight: often > 10,000 daltons
Structural types and examples:
- Starch (in plants): energy storage
- Composed of α-glucose monomers
- Forms a helix
- Amylose: unbranched, α-1,4 linkages
- Amylopectin: branched, α-1,4 with α-1,6 branch points ~ every 30 monomers
- Glycogen (in animals): energy storage in liver and muscle
- Highly branched α-glucose polymer
- Branching occurs about every 10 monomers
- Cellulose (plants and some algae): structural polymer in cell walls
- Made of β-glucose monomers
- β-1,4-glycosidic linkages
- Every other glucose is flipped, leading to a linear molecule
- Hydrogen bonds form between parallel strands, contributing to strength
- Chitin (fungi and arthropod exoskeletons)
- Monomer: N-acetylglucosamine (NAG)
- Similar to cellulose: β-1,4 linkages with alternating flip; linear strands with interstrand hydrogen bonds
- Peptidoglycan (bacterial cell walls)
- Structural polymer with alternating monosaccharides linked by β-1,4-glycosidic bonds
- Short amino acid chains form peptide bonds between adjacent strands
Structural takeaway: specific glycosidic linkages and monomer composition determine whether a polysaccharide is energy storage, structural, or other functional category

Starch, Glycogen, Cellulose, Chitin, and Peptidoglycan: Quick Distinctions

Starch: α-glucose; plant storage; amylose (unbranched, α-1,4); amylopectin (branched, α-1,4 and α-1,6)
Glycogen: α-glucose; animal storage; highly branched; more frequent branching than amylopectin
Cellulose: β-glucose; plant cell wall; β-1,4 linkage; linear; strong hydrogen bonding between strands
Chitin: N-acetylglucosamine; fungal walls and arthropod exoskeletons; β-1,4 linkage; nitrogen-containing; similar structural logic to cellulose
Peptidoglycan: bacterial walls; β-1,4 linkage; peptide cross-links between strands; provides rigidity
Summary table (conceptual):
- Starch: energy storage in plants; α-glucose; unbranched (amylose) vs branched (amylopectin)
- Glycogen: energy storage in animals; highly branched α-glucose
- Cellulose: structural support in plant cell walls; β-glucose; β-1,4 linkage; linear, hydrogen-bonded
- Chitin: structural support in fungi and arthropod exoskeletons; NAG; β-1,4 linkage; nitrogen-containing
- Peptidoglycan: bacterial structural support; β-1,4 linkage; peptide cross-links

Carbohydrates as Structural Support and Dietary Fiber

Structural polymers that form fibers or sheets:
- Cellulose, chitin, and peptidoglycan form long strands with interstrand hydrogen bonds
- Some fibers are highly resistant to hydrolysis (not easily broken down by many organisms)
- These fibers can exclude water, contributing to their stability
Dietary fiber: carbohydrate fibers important for digestive health; not all are readily hydrolyzed in the digestive system

Carbohydrates and Cell Identity

Carbohydrates on cell surfaces indicate cell identity and mediate recognition/signaling
Glycoproteins: carbohydrates attached to proteins on the cell surface
Glycolipids: carbohydrates attached to lipids on the cell surface
Key roles:
- Cell–cell recognition: identify cells as self vs. non-self
- Cell–cell signaling: communication between cells via carbohydrate-containing molecules
Visualization: glycoproteins and glycolipids are displayed on the outer surface of the plasma membrane

Carbohydrates and Energy Storage

Primary energy role: store chemical energy captured during photosynthesis in carbohydrates
Photosynthesis (simplified overall equation):
- $CO2 + H2O + ext{sunlight} ightarrow (CH2O)n + O_2$
How energy is used:
- When energy is needed, cells break down glucose to produce ATP
- Overall process (catabolic):
- $(CH2O)n + O2 + ADP + ext{Pi} ightarrow CO2 + H_2O + ATP$
ATP use: energy to drive endergonic reactions and perform cellular work
Energy density concept:
- Carbohydrates store more energy than carbon dioxide but less than fatty acids because of bond types and proportions
- General energy trend (per molecule): fatty acids > carbohydrates > CO₂ due to bond composition (more C–C and C–H bonds in fats)

Key Takeaways: What Do Carbohydrates Do?

Provide energy storage and fuel via glucose and polysaccharides
Serve as precursors to larger molecules (nucleotides, amino acids)
Provide fibrous structural materials (cellulose, chitin, peptidoglycan)
Indicate cell identity and mediate cell–cell interactions (glycoproteins, glycolipids)
Contribute to energy flow and metabolism through ATP production

Connections to Foundational Principles and Real-World Relevance

Structure–function relationships: stereochemistry (α vs β; 1,4 vs 1,6 linkages) directly influence polymer properties (helical vs linear, digestibility, strength)
Energy metabolism: carbohydrates are central to energy capture and ATP production, linking to cellular respiration and catabolic/anabolic pathways
Cell communication: carbohydrate decorations on membranes are essential for tissue organization, immune recognition, and signaling
Diet and health: dietary fiber has physiological benefits and is a practical implication of carbohydrate structure in digestion

Key Terms to Remember

Monosaccharide, disaccharide, polysaccharide
Aldose, ketose
Triose, pentose, hexose
Glycosidic linkage: α-1,4; β-1,4; α-1,6
Ring forms: α-glucose, β-glucose
Amylose, amylopectin, glycogen
Cellulose, chitin, peptidoglycan
Glycoprotein, glycolipid
Photosynthesis equation: $CO2 + H2O + ext{sunlight} ightarrow (CH2O)n + O_2$
ATP synthesis: $(CH2O)n + O2 + ADP + ext{Pi} ightarrow CO2 + H_2O + ATP$