Title: Starch, glycogen and cellulose

Learning objectives•How are α-glucose monomers arranged to form polymers of starch and glycogen?•How are β-glucose monomers arranged to form the polymer cellulose?•How do the molecular structures of starch, glycogen and cellulose relate to their function

Key words: starch, glycogen, cellulose, condensation reactions

Reminder - Session 3 starts on Friday afternoon

Write down as many things as you can remember about carbohydrates…

… or glucose in particular

Do Now…….!

Organic compounds which comprise of only C, H and O

Carbo-Hydr-ate

What are they?

Types of Carbohydrate

MonosaccharidesDisaccharidesPolysaccharides

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What are polysaccharides?

Polysaccharides are polymers containing many monosaccharides linked by glycosidic bonds. Like disaccharides, polysaccharides are formed by condensation reactions.

The major polysaccharides are starch and cellulose in plants, and glycogen in animals.

Polysaccharides are mainly used as an energy store and as structural components of cells.

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The structure of starch

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Properties and uses of starch

Starch is the major carbohydrate storage molecule in plants.

Starch is produced from glucose made during photosynthesis. It is broken down during respiration to provide energy and is also a source of carbon for producing other molecules.

It is usually stored as intracellular starch grains in organelles called plastids.

Plastids include green chloroplasts (e.g. in leaves) and colourless amyloplasts (e.g. in potatoes).

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Iodine test for starch

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What is cellulose?

Unlike starch, cellulose is very strong, and prevents cells from bursting when they take in excess water.

Cellulose is another polysaccharide and is the main part of plant cell walls. It is the most abundant organic polymer.

Cellulose consists of long chains of beta glucose molecules joined by beta1–4 glycosidic bonds.

The glucose chains form rope-like microfibrils, which are layered to form a network.

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The structure of cellulose

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What is glycogen?

Animals do not store carbohydrate as starch but as glycogen.

Glycogen has a similarstructure to amylopectin, containing many alpha 1–6 glycosidic bonds that produce an even more branched structure.

Glycogen is less dense and more soluble than starch, and is broken down more rapidly. This indicates the higher metabolic requirements of animals compared with plants.

Glycogen is stored as small granules, particularly in muscles and liver.

Use page 156 – 157

Complete the summary questions when you have made notes on starch, cellulose and glycogen

Task 1

Task 2

Complete the summary sheet for starch, glycogen and cellulose.

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Plant Cell Structure

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Starter Questions

1. Which polysaccharide makes up plant cell walls?

2. Which monosaccharide makes up this polysaccharide?

3. Is this polysaccharide branched or unbranched?

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Cellulose

• Main structural sugar in plants

• Structural component of plant cell walls

• Very strong

• Also permeable to numerous substances

• About 33% of plant matter

• Most common organic compound on Earth

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Starch

• Amylose (α helix) – 20% of starch

• Amylopectin (branched starch) – 80%

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Amylopectin

Use the Microscopes and bioviewers to look at the plant cells.

Identify as many of the specific organelles as you can and record as a sketch in your book

Task 3

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Question

1. Compare and contrast the structures of glycogen and cellulose, showing how each molecule’s structure is linked to its function.

[10 marks]

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Mark Scheme

[1] Gycogen is a chain of α-glucose molecules

[2] Cellulose – chain of β-glucose molecules

[3] Glycogen’s chain is compact but very branched, whereas

[4] Cellulose’s chain is very long, straight and unbranched

[5] and these chains in cellulose are bonded to form fibres

[6] Glycogen’s structure makes it a good food store in animals

[7] The branches allow enzymes to access the glycosidic bonds

[8] to break the food store down quickly

[9] Cellulose’s structure makes it a good structure in cell walls

[10] The fibres/ H bonds provide strength

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Polysaccharides: true or false?

Write an account of the structure and function of chloroplastsInclude

• Key structures and their functions• Diagrams

Task 4

Task 5

Complete the Application on pg 160 – 161 on specialised cells and finish off by completing the exam style questions.

Variation in Carbohydrates

Beta glucose Condensation reaction

Alpha glucose

Disaccharides

Polysaccharides: Structure and function

Starch

Glycogen

Cellulose

Homework

• Write an account of the structure and function of chloroplasts

• Include– Key structures and their functions– Diagrams– Endosymbiotic theory of chloroplast evolution and

evidence for it

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Glossary

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What’s the keyword?

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What’s the carbohydrate?

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Multiple-choice quiz