B. Biochemistry · Biochemistry

Carbohydrates: Sugars, Starches and Cellulose

B3.2 B3.3 B3.5 B2.3 B2.1 A1.12About 75 minutes

Learning goals

I can describe the structure of monosaccharides, disaccharides and polysaccharides and explain their functions in cells.
I can distinguish α-glucose from β-glucose and explain why that small difference makes starch digestible and cellulose not.
I can show how glycosidic bonds form by condensation and break by hydrolysis, and calculate formulas and molar masses of sugars.
I can build and draw 3-D models of glucose and of a short polysaccharide.

Words to know

monosaccharide
A single sugar unit, with the general formula (CH₂O)ₙ, usually with n = 3 to 7. Examples: glucose, fructose, galactose, ribose.

disaccharide
Two monosaccharides joined by a glycosidic bond. Examples: maltose, sucrose, lactose.

polysaccharide
A polymer of many monosaccharides. Examples: starch, glycogen, cellulose, chitin.

glycosidic bond (linkage)
The covalent C–O–C bond that joins two sugars, formed by a condensation reaction. It is named by the carbons it joins, such as α(1→4).

aldose / ketose
A sugar with an aldehyde group (glucose, galactose) or a ketone group (fructose).

α- and β-glucose
The two ring forms of glucose. In α-glucose the –OH on carbon 1 points below the ring; in β-glucose it points above.

glycogen
The highly branched storage polysaccharide of animals and fungi, stored mainly in liver and muscle.

cellulose
A structural polysaccharide of β-glucose that forms plant cell walls. Its chains are straight and hydrogen-bond into strong fibres.

Paper, pasta and a potato

A sheet of paper, a bowl of pasta and a baked potato are all made mostly of the same sugar: glucose. You can digest the pasta and the potato, but you could eat paper all day and get almost no energy from it. The difference is one tiny detail: which way an –OH group points on one carbon atom. This lesson shows how that detail changes everything.

Remember from Grade 11

In SBI3U you learned the names: monosaccharides (glucose, fructose, galactose), disaccharides (maltose, sucrose, lactose) and polysaccharides (starch, glycogen, cellulose), and that cellulose is fibre we can't digest. Now you will see why, at the level of structure.

Monosaccharides

Carbohydrates are made of C, H and O, usually in the ratio 1 : 2 : 1, written (CH₂O)ₙ. Every monosaccharide has several hydroxyl groups (which make it very soluble in water) and one carbonyl group.

Trioses (3 C): such as glyceraldehyde, made in photosynthesis and in glycolysis.
Pentoses (5 C): ribose (in RNA and ATP) and deoxyribose (in DNA).
Hexoses (6 C), all C₆H₁₂O₆: glucose and galactose are aldoses (aldehyde group); fructose is a ketose (ketone group). Glucose and galactose differ only in which way the –OH points on carbon 4.

Rings in water

In water, almost all glucose molecules close up into a ring: the oxygen of the –OH on carbon 5 bonds to carbon 1. When the ring closes, the new –OH on carbon 1 can point below the ring (α-glucose) or above it (β-glucose). The two forms switch back and forth in solution, but once glucose is locked into a polymer, the form is fixed.

Haworth projections of α- and β-glucose (H atoms on the ring carbons are left out). The only difference is the circled –OH on carbon 1: down in α, up in β.

Glycosidic bonds: building disaccharides

Two monosaccharides join by condensation: an –OH from one and an –H from an –OH on the other leave as water, and an oxygen bridge (C–O–C) forms. This is a glycosidic bond, named by the carbons it links and the form of carbon 1.

Maltose = α-glucose + glucose, α(1→4) linkage. Formed when starch is digested.
Sucrose (table sugar) = glucose + fructose, linked between carbon 1 of glucose and carbon 2 of fructose. Plants transport sugar this way.
Lactose (milk sugar) = galactose + glucose, β(1→4) linkage. People who stop making the enzyme lactase as adults cannot digest it well.

Each disaccharide is C₁₂H₂₂O₁₁: two hexoses (C₁₂H₂₄O₁₂) minus one H₂O. Hydrolysis reverses the reaction.

Worked example

Example: Formulas and molar masses of sugar chains

Find the molecular formula and molar mass of (a) maltose and (b) maltotriose, a chain of three glucose units. (c) Write a general formula for a chain of n glucose units. Use C = 12.01, H = 1.01, O = 16.00 g/mol.

Glucose is C₆H₁₂O₆: molar mass = 6(12.01) + 12(1.01) + 6(16.00) = 72.06 + 12.12 + 96.00 = 180.18 g/mol. Water is 2(1.01) + 16.00 = 18.02 g/mol.
(a) Maltose: 2 glucose − 1 H₂O = C₁₂H₂₂O₁₁; molar mass = 2(180.18) − 18.02 = 342.34 g/mol.
(b) Maltotriose: 3 glucose units are joined by 2 bonds, so 2 waters are lost: C₁₈H₃₆O₁₈ − 2 H₂O = C₁₈H₃₂O₁₆; molar mass = 3(180.18) − 2(18.02) = 540.54 − 36.04 = 504.50 g/mol.
(c) n units, n − 1 waters lost: C₆ₙH₁₂ₙ₋₂₍ₙ₋₁₎O₆ₙ₋₍ₙ₋₁₎ = C₆ₙH₁₀ₙ₊₂O₅ₙ₊₁. For a very long chain this is close to (C₆H₁₀O₅)ₙ, the formula you often see for starch and cellulose.

(a) C₁₂H₂₂O₁₁, 342.34 g/mol; (b) C₁₈H₃₂O₁₆, 504.50 g/mol; (c) C₆ₙH₁₀ₙ₊₂O₅ₙ₊₁ ≈ (C₆H₁₀O₅)ₙ.

Polysaccharides: storage and structure

Storage polysaccharides are made of α-glucose:

Starch (plants) is a mixture of amylose, unbranched α(1→4) chains that coil into a helix, and amylopectin, α(1→4) chains with α(1→6) branches about every 24 to 30 glucose units.
Glycogen (animals and fungi) is like amylopectin but far more branched, about every 8 to 12 units. More branches mean more chain ends, and enzymes remove glucose from the ends, so glycogen can release glucose very quickly when your liver or muscles need it.
Storing glucose as a polymer is useful because a polymer is insoluble (or nearly), so it doesn't draw water into the cell by osmosis the way the same number of free glucose molecules would.

Structural polysaccharides:

Cellulose is made of β-glucose joined by β(1→4) bonds. Because the –OH on carbon 1 points up, every second glucose is flipped over relative to its neighbours. The result is a straight, flat chain. Chains lie side by side and form many hydrogen bonds between their –OH groups, making strong microfibrils: the stuff of plant cell walls, cotton, wood and paper.
Chitin is like cellulose, but each glucose carries a nitrogen-containing group. It forms the tough exoskeletons of insects and crustaceans and the cell walls of fungi.

Why can't we digest cellulose? Enzymes are shape-specific. Human amylase fits α(1→4) bonds; it cannot bind β(1→4) bonds. Cows, deer and termites can live on plants because microorganisms in their guts make cellulase. For humans, cellulose passes through as insoluble fibre.

Four polysaccharides of glucose (each circle is one glucose unit). The α-linked storage forms coil and branch; cellulose's β-linked chains are straight and hydrogen-bond side by side into strong fibres. Not to scale.

Polysaccharide	Monomer	Linkage	Shape	Function / where
Amylose (starch)	α-glucose	α(1→4)	Unbranched helix	Energy storage in plants
Amylopectin (starch)	α-glucose	α(1→4), α(1→6) branches	Branched	Energy storage in plants
Glycogen	α-glucose	α(1→4), many α(1→6)	Highly branched	Storage in liver and muscle
Cellulose	β-glucose	β(1→4)	Straight, H-bonded fibres	Plant cell walls
Chitin	Modified β-glucose (with N)	β(1→4)	Straight fibres	Exoskeletons, fungal walls

Summary of the polysaccharides.

Other jobs for carbohydrates

Short sugar chains attached to proteins (glycoproteins) and lipids (glycolipids) stick out from the cell surface. They act as identification tags: your ABO blood type is determined by which sugars are at the ends of these chains on your red blood cells. Ribose and deoxyribose are part of RNA, DNA and ATP.

Just for fun

Why doesn't eating a paper plate give you a sugar rush?

Your amylase only reads α-bonds. Cellulose is written entirely in β.

Hands-on activity

Paper glucose: build α and β chains

You need: 2 sheets of paper, a pencil, scissors (careful), tape, and a ruler.

Draw 10 hexagons about 5 cm across and cut them out. These are glucose rings. On each, mark the corners C1 (right) and C4 (left), like the diagram above. Put the ring oxygen at the top-right corner.
On 5 hexagons, draw a short line with OH pointing down at C1 (α-glucose). On the other 5, draw OH pointing up at C1 (β-glucose). Mark an OH at C4 on all of them (pointing down, as in the diagram).
α chain: join the 5 α units C1 to C4, keeping all of them face up. For an α(1→4) bond, the C1 OH and the next unit's C4 OH must both point down: they line up. Tape them. Look at the chain from the side: the bonds all angle the same way, so a long chain would curve into a helix.
β chain: try joining β units C1 to C4 all face up. The C1 OH points up but the next C4 OH points down: they don't meet. Flip every second unit over so the OHs line up, and tape. The chain is now straight and flat.
Draw both chains in your notebook and label each glycosidic bond α(1→4) or β(1→4).

What to look for: to make the β chain you had to flip alternate units; that is exactly what happens in cellulose, and it is why cellulose chains are straight and pack into fibres while starch coils.

Your turn

Name the two monosaccharides in (a) sucrose, (b) lactose, (c) maltose.

How do α-glucose and β-glucose differ?

Why is glycogen more branched than amylopectin useful to an animal?

What type of reaction forms a glycosidic bond, and what is released?

Why do cellulose chains form strong fibres?

Glucose and fructose are both C₆H₁₂O₆. What kind of isomers are they, and what group differs?

More practice: warm-up

What is the general formula of a monosaccharide?

Name the storage polysaccharide in plants and the one in animals.

Which sugar is in RNA?

What bond joins sugars together?

Which polysaccharide forms the exoskeleton of a lobster?

More practice: on your own

A starch molecule has 2000 glucose units. How many water molecules are needed to hydrolyze it completely to glucose?

Find the molar mass of lactose. (Glucose and galactose are both 180.18 g/mol; water is 18.02 g/mol.)

Explain why storing glucose as starch rather than as free glucose helps a potato cell avoid taking in too much water.

Termites eat wood but cannot make cellulase. How do they get energy from it?

Compare amylose and cellulose in three ways.

Challenge

Show that the general formula for a straight chain of n glucose units, C₆ₙH₁₀ₙ₊₂O₅ₙ₊₁, gives the right formula for maltose.

Some people have lactose intolerance. Explain at the molecular level why eating lactose causes problems for them, and why lactose-free milk does not.

Amylose and amylopectin have the same monomer. Suggest why a plant makes both, using what you know about structure.

Check your understanding

Quiz

Which linkage is found in cellulose?

What is the main structural difference between α-glucose and β-glucose?

Which polysaccharide is most highly branched?

Lactose is made of:

The molecular formula of a disaccharide made from two hexoses is:

Why can humans digest starch but not cellulose?

For parents and teachers

Big idea: This lesson covers B3.2 (structure and function of carbohydrates), the role of hydroxyl and carbonyl groups (B3.3), condensation and hydrolysis (B3.5), and 3-D models (B2.3).

How to help: Ask your teen to explain the "paper vs pasta" puzzle from the start of the lesson in under a minute. If they can say "same monomer, different bond orientation, so a different shape, and enzymes are shape-specific," they have the key idea. The paper model activity needs scissors.

Common difficulties:

Mixing up which disaccharide contains which monosaccharides (try a mnemonic: Lactose has gaLactose; Sucrose is table Sugar = glucose + fructose; Maltose = glucose + glucose, from Malt or starch).
Thinking cellulose isn't made of glucose.
Forgetting to subtract one water per bond when working out formulas and molar masses.