Carbohydrates

Structure, function, and types of carbohydrates in biological systems

🍞 Carbohydrates

Overview

Carbohydrates: Carbon (C), Hydrogen (H), Oxygen (O) in 1:2:1 ratio → (CH₂O)ₙ

Functions:

Energy storage (starch, glycogen)
Structural support (cellulose, chitin)
Cell recognition
Energy currency (glucose)

Types of Carbohydrates

1. Monosaccharides (Simple Sugars)

Glucose (C₆H₁₂O₆): Primary energy source
Fructose: Fruit sugar
Galactose: Component of lactose
Ribose/Deoxyribose: RNA/DNA components

2. Disaccharides

Formed by dehydration synthesis (removes H₂O):

Maltose = Glucose + Glucose
Sucrose = Glucose + Fructose
Lactose = Glucose + Galactose

Broken by hydrolysis (adds H₂O)

3. Polysaccharides

Storage:

Starch (plants): α-glucose polymer, stored in plastids
Glycogen (animals): highly branched α-glucose, liver/muscle

Structural:

Cellulose (plants): β-glucose polymer, cell walls
- Most abundant organic polymer on Earth
- Humans cannot digest (lack cellulase)
Chitin: Modified glucose with N-groups
- Fungal cell walls, arthropod exoskeletons

Key Concepts

α-glucose vs. β-glucose:

α: OH on C1 below ring → forms starch/glycogen (digestible)
β: OH on C1 above ring → forms cellulose (indigestible)

Glycosidic bonds:

Formed by dehydration synthesis
Broken by hydrolysis
Enzyme specificity determines digestibility

📚 Practice Problems

1Problem 1easy

❓ Question:

Compare and contrast starch, glycogen, and cellulose in terms of: (a) monomer composition, (b) type of glycosidic linkages, (c) structure, and (d) biological function.

💡 Show Solution

Comparison of Polysaccharides:

(a) Monomer Composition:

All three are polymers of glucose (C₆H₁₂O₆)

Starch: α-glucose
Glycogen: α-glucose
Cellulose: β-glucose

(b) Glycosidic Linkages:

Starch:

Amylose: α-1,4 glycosidic bonds (unbranched)
Amylopectin: α-1,4 bonds with α-1,6 branches every 24-30 glucose units

Glycogen:

α-1,4 bonds with α-1,6 branches every 8-12 glucose units (highly branched)

Cellulose:

β-1,4 glycosidic bonds (linear, unbranched)

(c) Structure:

Starch: Helical coils (amylose) and branched structure (amylopectin)

Glycogen: Highly branched, compact globular structure

Cellulose: Long, straight, unbranched chains that form hydrogen bonds with adjacent chains → microfibrils

(d) Biological Function:

Starch:

Energy storage in plants
Found in roots, tubers, seeds
Easily digestible by amylase enzymes

Glycogen:

Energy storage in animals
Stored in liver and muscle cells
Highly branched for rapid glucose release

Cellulose:

Structural support in plant cell walls
Provides rigidity and strength
Most humans cannot digest (lack cellulase enzyme)
Dietary fiber in human nutrition

$\boxed{\text{Storage: starch/glycogen (α-glucose); Structural: cellulose (β-glucose)}}$

2Problem 2easy

❓ Question:

Compare and contrast starch, glycogen, and cellulose in terms of: (a) monomer composition, (b) type of glycosidic linkages, (c) structure, and (d) biological function.

💡 Show Solution

Comparison of Polysaccharides:

(a) Monomer Composition:

All three are polymers of glucose (C₆H₁₂O₆)

Starch: α-glucose
Glycogen: α-glucose
Cellulose: β-glucose

(b) Glycosidic Linkages:

Starch:

Amylose: α-1,4 glycosidic bonds (unbranched)
Amylopectin: α-1,4 bonds with α-1,6 branches every 24-30 glucose units

Glycogen:

α-1,4 bonds with α-1,6 branches every 8-12 glucose units (highly branched)

Cellulose:

β-1,4 glycosidic bonds (linear, unbranched)

(c) Structure:

Starch: Helical coils (amylose) and branched structure (amylopectin)

Glycogen: Highly branched, compact globular structure

Cellulose: Long, straight, unbranched chains that form hydrogen bonds with adjacent chains → microfibrils

(d) Biological Function:

Starch:

Energy storage in plants
Found in roots, tubers, seeds
Easily digestible by amylase enzymes

Glycogen:

Energy storage in animals
Stored in liver and muscle cells
Highly branched for rapid glucose release

Cellulose:

Structural support in plant cell walls
Provides rigidity and strength
Most humans cannot digest (lack cellulase enzyme)
Dietary fiber in human nutrition

$\boxed{\text{Storage: starch/glycogen (α-glucose); Structural: cellulose (β-glucose)}}$

3Problem 3easy

❓ Question:

Compare and contrast starch, glycogen, and cellulose in terms of: (a) monomer composition, (b) type of glycosidic linkages, (c) structure, and (d) biological function.

💡 Show Solution

Comparison of Polysaccharides:

(a) Monomer Composition:

All three are polymers of glucose (C₆H₁₂O₆)

Starch: α-glucose
Glycogen: α-glucose
Cellulose: β-glucose

(b) Glycosidic Linkages:

Starch:

Amylose: α-1,4 glycosidic bonds (unbranched)
Amylopectin: α-1,4 bonds with α-1,6 branches every 24-30 glucose units

Glycogen:

α-1,4 bonds with α-1,6 branches every 8-12 glucose units (highly branched)

Cellulose:

β-1,4 glycosidic bonds (linear, unbranched)

(c) Structure:

Starch: Helical coils (amylose) and branched structure (amylopectin)

Glycogen: Highly branched, compact globular structure

Cellulose: Long, straight, unbranched chains that form hydrogen bonds with adjacent chains → microfibrils

(d) Biological Function:

Starch:

Energy storage in plants
Found in roots, tubers, seeds
Easily digestible by amylase enzymes

Glycogen:

Energy storage in animals
Stored in liver and muscle cells
Highly branched for rapid glucose release

Cellulose:

Structural support in plant cell walls
Provides rigidity and strength
Most humans cannot digest (lack cellulase enzyme)
Dietary fiber in human nutrition

$\boxed{\text{Storage: starch/glycogen (α-glucose); Structural: cellulose (β-glucose)}}$

4Problem 4easy

❓ Question:

Compare and contrast starch, glycogen, and cellulose in terms of: (a) monomer composition, (b) type of glycosidic linkages, (c) structure, and (d) biological function.

💡 Show Solution

Comparison of Polysaccharides:

(a) Monomer Composition:

All three are polymers of glucose (C₆H₁₂O₆)

Starch: α-glucose
Glycogen: α-glucose
Cellulose: β-glucose

(b) Glycosidic Linkages:

Starch:

Amylose: α-1,4 glycosidic bonds (unbranched)
Amylopectin: α-1,4 bonds with α-1,6 branches every 24-30 glucose units

Glycogen:

α-1,4 bonds with α-1,6 branches every 8-12 glucose units (highly branched)

Cellulose:

β-1,4 glycosidic bonds (linear, unbranched)

(c) Structure:

Starch: Helical coils (amylose) and branched structure (amylopectin)

Glycogen: Highly branched, compact globular structure

Cellulose: Long, straight, unbranched chains that form hydrogen bonds with adjacent chains → microfibrils

(d) Biological Function:

Starch:

Energy storage in plants
Found in roots, tubers, seeds
Easily digestible by amylase enzymes

Glycogen:

Energy storage in animals
Stored in liver and muscle cells
Highly branched for rapid glucose release

Cellulose:

Structural support in plant cell walls
Provides rigidity and strength
Most humans cannot digest (lack cellulase enzyme)
Dietary fiber in human nutrition

$\boxed{\text{Storage: starch/glycogen (α-glucose); Structural: cellulose (β-glucose)}}$

5Problem 5medium

❓ Question:

Explain the process of dehydration synthesis (condensation reaction) in forming a disaccharide. Use the formation of sucrose from glucose and fructose as an example. Include the molecular formula and type of bond formed.

💡 Show Solution

Dehydration Synthesis (Condensation Reaction):

Formation of sucrose from glucose + fructose

Reactants:

Glucose: C₆H₁₂O₆ (α-glucose, 6-carbon aldose)
Fructose: C₆H₁₂O₆ (β-fructose, 6-carbon ketose)

Process:

Hydroxyl groups (-OH) from each monosaccharide approach
- OH from C1 of glucose
- OH from C2 of fructose
Dehydration occurs:
- One monosaccharide loses H
- Other monosaccharide loses OH
- These combine to form H₂O (water)
Bond formation:
- Oxygen links the two monosaccharides
- Forms glycosidic bond (C-O-C)
- Specifically: α-1,2 glycosidic linkage

Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + \text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{enzyme}} \text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O}$

$\text{Glucose} + \text{Fructose} \rightarrow \text{Sucrose} + \text{Water}$

Key Features:

Anabolic reaction (builds larger molecules)
Endergonic (requires energy input, usually ATP)
Enzyme-catalyzed (sucrase in reverse, or sucrose synthase)
Reversible via hydrolysis

Bond Formed: $\boxed{\text{Glycosidic bond (α-1,2 linkage)}}$

Reverse Reaction (Hydrolysis): Adding water breaks the glycosidic bond, releasing glucose and fructose. This is how we digest disaccharides.

Note: Sucrose is "table sugar" - the primary transport sugar in plants (phloem sap).

6Problem 6medium

❓ Question:

💡 Show Solution

Dehydration Synthesis (Condensation Reaction):

Formation of sucrose from glucose + fructose

Reactants:

Glucose: C₆H₁₂O₆ (α-glucose, 6-carbon aldose)
Fructose: C₆H₁₂O₆ (β-fructose, 6-carbon ketose)

Process:

Hydroxyl groups (-OH) from each monosaccharide approach
- OH from C1 of glucose
- OH from C2 of fructose
Dehydration occurs:
- One monosaccharide loses H
- Other monosaccharide loses OH
- These combine to form H₂O (water)
Bond formation:
- Oxygen links the two monosaccharides
- Forms glycosidic bond (C-O-C)
- Specifically: α-1,2 glycosidic linkage

Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + \text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{enzyme}} \text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O}$

$\text{Glucose} + \text{Fructose} \rightarrow \text{Sucrose} + \text{Water}$

Key Features:

Anabolic reaction (builds larger molecules)
Endergonic (requires energy input, usually ATP)
Enzyme-catalyzed (sucrase in reverse, or sucrose synthase)
Reversible via hydrolysis

Bond Formed: $\boxed{\text{Glycosidic bond (α-1,2 linkage)}}$

Reverse Reaction (Hydrolysis): Adding water breaks the glycosidic bond, releasing glucose and fructose. This is how we digest disaccharides.

Note: Sucrose is "table sugar" - the primary transport sugar in plants (phloem sap).

7Problem 7medium

❓ Question:

💡 Show Solution

Dehydration Synthesis (Condensation Reaction):

Formation of sucrose from glucose + fructose

Reactants:

Glucose: C₆H₁₂O₆ (α-glucose, 6-carbon aldose)
Fructose: C₆H₁₂O₆ (β-fructose, 6-carbon ketose)

Process:

Hydroxyl groups (-OH) from each monosaccharide approach
- OH from C1 of glucose
- OH from C2 of fructose
Dehydration occurs:
- One monosaccharide loses H
- Other monosaccharide loses OH
- These combine to form H₂O (water)
Bond formation:
- Oxygen links the two monosaccharides
- Forms glycosidic bond (C-O-C)
- Specifically: α-1,2 glycosidic linkage

Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + \text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{enzyme}} \text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O}$

$\text{Glucose} + \text{Fructose} \rightarrow \text{Sucrose} + \text{Water}$

Key Features:

Anabolic reaction (builds larger molecules)
Endergonic (requires energy input, usually ATP)
Enzyme-catalyzed (sucrase in reverse, or sucrose synthase)
Reversible via hydrolysis

Bond Formed: $\boxed{\text{Glycosidic bond (α-1,2 linkage)}}$

Reverse Reaction (Hydrolysis): Adding water breaks the glycosidic bond, releasing glucose and fructose. This is how we digest disaccharides.

Note: Sucrose is "table sugar" - the primary transport sugar in plants (phloem sap).

8Problem 8medium

❓ Question:

💡 Show Solution

Dehydration Synthesis (Condensation Reaction):

Formation of sucrose from glucose + fructose

Reactants:

Glucose: C₆H₁₂O₆ (α-glucose, 6-carbon aldose)
Fructose: C₆H₁₂O₆ (β-fructose, 6-carbon ketose)

Process:

Hydroxyl groups (-OH) from each monosaccharide approach
- OH from C1 of glucose
- OH from C2 of fructose
Dehydration occurs:
- One monosaccharide loses H
- Other monosaccharide loses OH
- These combine to form H₂O (water)
Bond formation:
- Oxygen links the two monosaccharides
- Forms glycosidic bond (C-O-C)
- Specifically: α-1,2 glycosidic linkage

Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + \text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{enzyme}} \text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O}$

$\text{Glucose} + \text{Fructose} \rightarrow \text{Sucrose} + \text{Water}$

Key Features:

Anabolic reaction (builds larger molecules)
Endergonic (requires energy input, usually ATP)
Enzyme-catalyzed (sucrase in reverse, or sucrose synthase)
Reversible via hydrolysis

Bond Formed: $\boxed{\text{Glycosidic bond (α-1,2 linkage)}}$

Reverse Reaction (Hydrolysis): Adding water breaks the glycosidic bond, releasing glucose and fructose. This is how we digest disaccharides.

Note: Sucrose is "table sugar" - the primary transport sugar in plants (phloem sap).

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