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Carbohydrates
Structure & Function

Explore the molecular building blocks of life: from simple sugars to complex polysaccharides, and how their structure determines biological function.

AP Biology

What Are Carbohydrates?

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen. They follow the general formula \( \text{C}_n(\text{H}_2\text{O})_n \), which literally means "hydrated carbons."

Energy
Glucose fuels cellular respiration: \( \text{C}_6\text{H}_{12}\text{O}_6 \)
Structure
Cellulose and chitin provide rigid support in cell walls and exoskeletons
Storage
Starch (plants) and glycogen (animals) store energy for later use

Monosaccharides: The Simplest Sugars

Monosaccharides are single sugar units that cannot be hydrolyzed into simpler carbohydrates. They are classified by the number of carbon atoms they contain.

Glucose (6C)
\( \text{O} \)
\( \text{OH} \)
\( \text{OH} \)
\( \text{OH} \)
\( \text{CH}_2\text{OH} \)
 Fructose (6C)
\( \text{O} \)
\( \text{OH} \)
\( \text{OH} \)
\( \text{OH} \)
 Aldose (aldehyde group) 6-membered ring Ketose (ketone group) 5-membered ring

Classifying Sugars by Carbon Number

Monosaccharides are named by their number of carbons. The most biologically important are pentoses (5C) and hexoses (6C).

Type Carbons Formula Examples
Triose 3 \( \text{C}_3\text{H}_6\text{O}_3 \) Glyceraldehyde
Pentose 5 \( \text{C}_5\text{H}_{10}\text{O}_5 \) Ribose, Deoxyribose
Hexose 6 \( \text{C}_6\text{H}_{12}\text{O}_6 \) Glucose, Fructose, Galactose

Dehydration Synthesis: Building Polymers

Monosaccharides link together through dehydration synthesis (also called condensation), forming a glycosidic bond and releasing a water molecule.

Glu
\( \text{OH} \)
 Glu
\( \text{H} \)
\( -\text{H}_2\text{O} \)
 Glu bond Glu +
\( \text{H}_2\text{O} \)
 Dehydration Synthesis (removes water) Hydrolysis (adds water to break)

Common Disaccharides

Disaccharides are formed when two monosaccharides join via a glycosidic bond. Each disaccharide has its own specific combination of monomers.

Sucrose
Glucose + Fructose
Found in: table sugar, sugarcane, fruits
Lactose
Glucose + Galactose
Found in: milk and dairy products
Maltose
Glucose + Glucose
Found in: germinating grains, malt

Polysaccharides: Complex Carbohydrates

Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. Their function depends on the type of monomer, the type of bond, and whether the chain is branched or unbranched.

Starch (Amylopectin) Branched, energy storage (plants) Cellulose Unbranched, structural (plants) Glycogen Highly branched, storage (animals) Chitin Unbranched, structural (arthropods/fungi) N N N N N N = nitrogen-containing group (NAG)

Alpha vs Beta Glycosidic Bonds

The orientation of the glycosidic bond determines whether a polysaccharide is used for energy storage or structural support: this is one of the most important distinctions in carbohydrate biology.

Alpha (\( \alpha \)) Linkage
OH group on Carbon 1 points downward. Produces helical, easily digestible chains. Found in: starch, glycogen.
Beta (\( \beta \)) Linkage
OH group on Carbon 1 points upward. Produces straight, rigid chains that resist digestion. Found in: cellulose, chitin.

Storage vs Structural Polysaccharides

Feature Storage Structural
Function Energy reserve Physical support
Bond type \( \alpha \)-glycosidic \( \beta \)-glycosidic
Chain shape Helical / branched Straight / rigid
Examples Starch, Glycogen Cellulose, Chitin
Organisms Plants (starch), Animals (glycogen) Plants (cellulose), Arthropods (chitin)
Digestible by humans? Yes No (fiber)
An illustrative potato storage cell shows a cellulose wall outside and an amyloplast containing starch granules inside. Cellulose has beta 1-to-4 linkages; starch has alpha 1-to-4 linkages and alpha 1-to-6 branches.
In potato storage cells, cellulose supports the extracellular wall while starch is stored inside amyloplasts. The enlarged structures are schematic.

Polysaccharide geometry and biological function

Cut potato storage tissue with pale granules, alongside a beetle and an enlarged conceptual exoskeleton section.Amylose, amylopectin and glycogen use α-linked glucose chains; amylopectin and glycogen also have α-1,6 branches. Cellulose and chitin use unbranched β-1,4 backbones.

Linkage changes polymer function

Linkage changes polymer function. Amylose, amylopectin and glycogen use α-linked glucose chains; amylopectin and glycogen also have α-1,6 branches. Cellulose and chitin use unbranched β-1,4 backbones.Amylose, amylopectin and glycogen use α-linked glucose chains; amylopectin and glycogen also have α-1,6 branches. Cellulose and chitin use unbranched β-1,4 backbones.

Different enzymes act at different sites

Different enzymes act at different sites. Amylase hydrolyzes internal α-1,4 bonds. Glycogen phosphorylase acts at nonreducing ends to release glucose-1-phosphate; neither enzyme directly cleaves α-1,6 branch bonds.Amylase hydrolyzes internal α-1,4 bonds. Glycogen phosphorylase acts at nonreducing ends to release glucose-1-phosphate; neither enzyme directly cleaves α-1,6 branch bonds.

How to Identify Carbohydrates

Use this step-by-step checklist when encountering a carbohydrate question on the AP exam.

1
Check the formula. Does it follow \( \text{C}_n(\text{H}_2\text{O})_n \) or contain only C, H, and O in roughly a 1:2:1 ratio?
2
Count the monomers. Is it a single sugar (mono), two sugars (di), or many sugars (poly)?
3
Identify the bond type. Alpha (\( \alpha \)) bonds: storage function. Beta (\( \beta \)) bonds: structural function.
4
Determine branching. Branched = quick access to energy (glycogen). Unbranched = long-term structure or compact storage (amylose).
5
Match to organism. Starch = plants, glycogen = animals, cellulose = plant cell walls, chitin = arthropods/fungi.

Example 1: Glucose Structure

Problem: Glucose has the molecular formula \( \text{C}_6\text{H}_{12}\text{O}_6 \). Verify that it fits the general carbohydrate formula \( \text{C}_n(\text{H}_2\text{O})_n \).

1
Identify n: There are 6 carbon atoms, so \( n = 6 \).
2
Apply the formula: \( \text{C}_6(\text{H}_2\text{O})_6 = \text{C}_6\text{H}_{12}\text{O}_6 \).
3
Check the ratio: H:O = 12:6 = 2:1. This matches the 2:1 hydrogen-to-oxygen ratio. ✓

Example 2: Sucrose Hydrolysis

Problem: Write the hydrolysis reaction for sucrose (\( \text{C}_{12}\text{H}_{22}\text{O}_{11} \)). What products form?

Sucrose +
\( \text{H}_2\text{O} \)
 enzyme Glucose + Fructose
\( \text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + \text{C}_6\text{H}_{12}\text{O}_6 \)
Note: glucose and fructose are structural isomers (same formula, different arrangement)

Example 3: Why Can't We Digest Cellulose?

Problem: Both starch and cellulose are polymers of glucose. Why can humans digest starch but not cellulose?

Starch (\( \alpha \)-1,4 bond)
 Glu Glu Glu All monomers same orientation
Cellulose (\( \beta \)-1,4 bond)
 Glu Glu Glu Alternate monomers are flipped
Human amylase can cleave \( \alpha \)-bonds ✓
Human enzymes cannot cleave \( \beta \)-bonds ✕
 Cellulose passes through as dietary fiber

Common Mistakes

✕ Confusing Isomers
Glucose, fructose, and galactose all share \( \text{C}_6\text{H}_{12}\text{O}_6 \) but are structural isomers with different arrangements. They are NOT the same molecule.
✕ Mixing Up Reactions
Dehydration synthesis removes water to build polymers. Hydrolysis adds water to break them. Don't reverse these.
✕ Forgetting Bond Type
Students often forget that the \( \alpha \) vs \( \beta \) bond distinction determines function. Same monomer (glucose) + different bond = completely different polymer.

Summary: Major Carbohydrates

Molecule Type Monomer(s) Function
Glucose Monosaccharide - Cellular fuel
Sucrose Disaccharide Glu + Fru Transport sugar (plants)
Starch Polysaccharide Glucose (\( \alpha \)) Energy storage (plants)
Glycogen Polysaccharide Glucose (\( \alpha \)) Energy storage (animals)
Cellulose Polysaccharide Glucose (\( \beta \)) Structure (plant walls)
Chitin Polysaccharide NAG (\( \beta \)) Structure (arthropods)

Question 1 of 5

Which of the following correctly describes the process by which a disaccharide is formed from two monosaccharides?

Question 2 of 5

Glucose, fructose, and galactose all have the molecular formula \( \text{C}_6\text{H}_{12}\text{O}_6 \). These molecules are best described as:

Question 3 of 5

Starch and cellulose are both polymers of glucose, yet they have very different properties. The primary reason for this difference is:

Question 4 of 5

An animal cell needs to store glucose for rapid energy release. Which polysaccharide would it use, and why?

Question 5 of 5

A student observes that adding the enzyme sucrase to a sucrose solution produces glucose and fructose. This reaction is an example of: