Carbohydrates

🍞 Carbohydrates: The Cell's Quick Energy Source

Carbohydrates are organic molecules built from carbon, hydrogen, and oxygen — usually in the ratio $(CH_2O)_n$. They are the most abundant biomolecules on Earth and serve as both immediate energy and structural support.

Where you'll meet carbohydrates

Big idea

Glucose is the universal cellular fuel — it enters glycolysis and ultimately drives ATP production in respiration.

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Monomers: Monosaccharides

The simplest carbohydrates are monosaccharides ("single sugars"). They are classified by carbon count:

• Triose (3C) — glyceraldehyde
• Pentose (5C) — ribose, deoxyribose
• Hexose (6C) — glucose, fructose, galactose

Building Up

Bond Formation

Two monosaccharides join through a glycosidic linkage formed by dehydration synthesis (loss of $H_2O$).

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Carbohydrates: Structure & Function

Monosaccharides

Simple sugars with the general formula $(CH_2O)_n$:

Disaccharides

Two monosaccharides linked by a glycosidic bond:

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Polysaccharides

Key distinction: Starch and glycogen use alpha (α) linkages (digestible), while cellulose uses beta (β) linkages (indigestible by most animals).

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Why Cells Need Carbohydrates

Carbohydrates serve four core biological roles:

Storage vs Structural

• Storage polysaccharides use α-glycosidic linkages that coil → easy to mobilize when energy is needed.
• Structural polysaccharides use β-glycosidic linkages that form rigid straight fibers → great for support, hard to digest.

This single linkage difference (α vs β) is why we can digest starch but not cellulose.

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Cell-Surface Sugars: Identity Cards

Short carbohydrate chains attached to membrane lipids and proteins (glycolipids and glycoproteins) point outward from the cell surface like name tags.

Roles of cell-surface carbohydrates

• Cell–cell recognition during embryonic development
• Blood type determination (ABO blood groups differ only in their terminal sugar)
• Pathogen attachment — many viruses and bacteria latch on to specific sugar patterns
• Immune system targeting — antibodies often recognize sugar epitopes

Key term

The total carbohydrate coating of a cell is called the glycocalyx.

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🎯 AP Review: Carbohydrates

Must-know synthesis points

1. Monomer/polymer relationship — monosaccharides → glycosidic bond (dehydration synthesis) → di/polysaccharides; polymers split by hydrolysis.
2. α vs β linkage — α coils for storage (starch, glycogen); β straightens for structure (cellulose, chitin).
3. Function follows structure — branching (glycogen, amylopectin) speeds energy release; straight β-fibers (cellulose) supply tensile strength.
4. Cell-surface sugars (glycocalyx) — recognition, signaling, immunity, blood typing.

High-yield FRQ traps

• Don't confuse glycogen (animal storage) with glucagon (a hormone — also raises blood sugar but is a peptide).
• Lactose intolerance is a missing lactase enzyme, not a sugar shortage.
• Glycemic load is a behavioral concept; AP wants the biochemistry.

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