Carbohydrates

Essential Question

How do cells use sugars and starches for energy and structure?

Bread, fruit, and rice are rich in carbohydrates, and so are the cell walls of every plant. Carbohydrates are sugars and the chains built from them. In this lesson, you’ll learn how single sugars join into larger molecules, why starch, glycogen, and cellulose do different jobs even though they are all built from glucose, and what happens to the glucose in your blood after a meal.

Monosaccharides

• Monosaccharide: one sugar unit
• Examples: glucose, fructose, galactose
• Glucose, C₆H₁₂O₆, is the main fuel of cells
• C, H, and O are in a ratio of about 1 : 2 : 1

Carbohydrates are made of carbon, hydrogen, and oxygen, usually in a ratio of about 1 carbon to 2 hydrogen to 1 oxygen. The simplest carbohydrates are monosaccharides, or simple sugars. Glucose, fructose, and galactose all have the formula C₆H₁₂O₆, but their atoms are arranged differently, which gives each sugar different properties. Glucose is made by photosynthesis, travels in the blood of animals, and is the main fuel that cells break down to release energy.

Disaccharides

• Disaccharide: two monosaccharides joined
• Joined by dehydration synthesis
• Sucrose = glucose + fructose
• Lactose = glucose + galactose

Two monosaccharides can join by dehydration synthesis to make a disaccharide. Table sugar (sucrose) is made of glucose and fructose. Milk sugar (lactose) is made of glucose and galactose. Notice that the formula of the disaccharide is the sum of the two sugars minus one molecule of water: two C₆H₁₂O₆ molecules contain 12 C, 24 H, and 12 O, and removing H₂O leaves C₁₂H₂₂O₁₁. During digestion, hydrolysis splits disaccharides back into monosaccharides.

Polysaccharides: Storage and Structure

• Polysaccharide: a polymer of many sugar units
• Starch stores energy in plants
• Glycogen stores energy in animals
• Cellulose makes up plant cell walls

A polysaccharide is a polymer of many monosaccharides. Starch, glycogen, and cellulose are all polymers of glucose, but the way the glucose units are bonded and arranged is different. Plants store extra glucose as starch, and animals store it as glycogen, which is stored in the liver and muscles. Cellulose is a structural molecule. Its straight chains line up side by side and are held together by hydrogen bonds, forming strong fibers that make plant cell walls rigid.

Same Monomer, Different Job

• Starch, glycogen, and cellulose all contain only glucose
• Bonds and branching make the shapes different
• Shape decides the job
• Humans cannot digest cellulose

Polysaccharide	Found in	Job	Can humans digest it?
Starch	Plants (potatoes, rice, wheat)	Stores energy	Yes
Glycogen	Animals (liver and muscle)	Stores energy	Made and broken down in the body
Cellulose	Plant cell walls	Gives support	No, it passes through as fiber

Because the bonds in cellulose are different from the bonds in starch, the enzymes in the human digestive system can break down starch but not cellulose. Cellulose passes through as dietary fiber. Some animals, such as cows, have microorganisms in their digestive systems that can digest it. This is an example of a major idea in this unit: the structure of a molecule determines what it can do.

Using Carbohydrates

• Digestion breaks starch into glucose by hydrolysis
• Glucose enters the blood
• Cells use glucose as a fuel
• Extra glucose is stored as glycogen

When you eat starch, enzymes break it into glucose by hydrolysis, and the glucose is absorbed into the blood. The graph shows sample data for blood glucose after a meal rich in carbohydrates. The level rises quickly to a peak at 30 minutes and then falls back toward the starting value as cells take up glucose to use as fuel and extra glucose is stored as glycogen in the liver and muscles. When blood glucose is low, glycogen is broken down to release glucose again.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Counting Atoms in Sucrose

Glucose and fructose each have the formula C₆H₁₂O₆. (a) Find the formula of sucrose, which forms when one glucose and one fructose join by dehydration synthesis. (b) A starch molecule is a chain of 500 glucose units. How many molecules of water were released when it formed? How many are used when it is digested completely to glucose?

Show the solution

1. (a) Add the atoms: C: 6 + 6 = 12. H: 12 + 12 = 24. O: 6 + 6 = 12. That gives C₁₂H₂₄O₁₂.
2. Remove water: dehydration synthesis removes H₂O, which is 2 H and 1 O. H: 24 − 2 = 22. O: 12 − 1 = 11. The formula is C₁₂H₂₂O₁₁.
3. (b) Starch: a chain of 500 units has 500 − 1 = 499 bonds, so 499 molecules of water were released. Complete digestion by hydrolysis uses 499 molecules of water.
4. Answer: (a) C₁₂H₂₂O₁₁. (b) 499 released and 499 used.

Example 2: Glucose After a Meal

Use the graph of blood glucose. (a) What was the highest blood glucose level, and when did it occur? (b) By how many mg per dL did it rise from 0 to 30 minutes? What percent increase is that? (c) Explain why the level fell after 30 minutes.

Show the solution

1. (a) Peak: 140 mg per dL at 30 minutes.
2. (b) Rise: 140 − 90 = 50 mg per dL. As a percent of the starting level: 50 ÷ 90 × 100 = 56%.
3. (c) Explanation: cells take glucose out of the blood and use it as fuel, and the liver and muscles store extra glucose as glycogen. Both processes lower the level in the blood.
4. Claim, evidence, reasoning: after eating, glucose rose by 56% (evidence) because digestion of carbohydrates adds glucose to the blood, and it returned to about 90 mg per dL by 180 minutes (evidence) because glucose was used and stored (reasoning).
5. Answer: (a) 140 mg per dL at 30 minutes. (b) 50 mg per dL, a 56% increase. (c) Cells use glucose and extra is stored as glycogen.

Example 3: Which Polysaccharide?

Name the polysaccharide in each situation, using the diagram. (a) A marathon runner’s muscles break down a stored polysaccharide for quick glucose. (b) A tree trunk resists bending. (c) A potato tuber stores energy for the plant.

Show the solution

1. (a) Glycogen: it is the storage polysaccharide in animals, and its many branches let enzymes release glucose quickly.
2. (b) Cellulose: its straight, hydrogen-bonded chains form strong fibers in plant cell walls.
3. (c) Starch: plants store energy as starch.
4. Answer: (a) Glycogen. (b) Cellulose. (c) Starch.

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Practice

Classify sugars by size, find the formula of a disaccharide, calculate the water released when a polysaccharide forms, read a graph of blood glucose, and explain why starch and cellulose do different jobs. Try it on your own first, then check your work with the answer key.

• Carbohydrates Practice Worksheet (PDF)
• Carbohydrates Practice Answer Key (PDF)

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Lesson Slides

• 5.2 Carbohydrates Slides (PDF)

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Big Ideas

• Monosaccharides are simple sugars, such as glucose
• Disaccharides form by dehydration synthesis
• Starch and glycogen store energy; cellulose gives support
• Shape and bonding decide the job of a polysaccharide
• Glucose is the main fuel of cells

Back to the essential question: cells use carbohydrates in two main ways. Glucose is a fuel that cells break down to release energy, and extra glucose is stored as starch in plants and glycogen in animals. Cellulose uses the same monomer for a different job, as the strong fibers of plant cell walls, because its bonds and shape are different. Next, you’ll look at a class of molecules that stores even more energy per gram: lipids.

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