AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Metabolism: conversion of glucose to starch, glycogen and cellulose

    Glucose is a soluble sugar made in photosynthesis and used in respiration.

    Checked against the AQA 8464 specification (opens in a new tab) on .

    • Point 5 of 10
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Glucose is a soluble sugar made in photosynthesis and used in respiration. Excess glucose is converted into storage or structural polymers by condensation reactions, each glucose unit joining with the loss of a water molecule. Plants convert glucose to starch for storage in leaves, roots and seeds, and to cellulose for cell walls. Animals, including humans, convert glucose to glycogen for storage in the liver and muscles. Starch and glycogen are compact, insoluble and easily hydrolysed back to glucose when needed; cellulose is strong and structural. A useful method is to compare the three products by monomer, solubility, function and location, for example starch in a potato tuber, glycogen in liver cells and cellulose in a plant cell wall.

    What earns the marks

    • Glucose is a monosaccharide; starch, glycogen and cellulose are polysaccharides formed by joining many glucose units together.
    • The conversions are condensation reactions: each glucose unit added releases one molecule of water.
    • Starch is the storage polysaccharide in plants, found in leaves, roots, tubers and seeds.
    • Glycogen is the storage polysaccharide in animals and fungi, stored in the liver and muscles.
    • Cellulose is a structural polysaccharide that forms plant cell walls and gives strength.
    • Starch and glycogen are insoluble and compact, so they do not affect cell water potential and are readily hydrolysed to glucose for respiration.
    • Cellulose molecules form straight chains held by hydrogen bonds, giving strength; it is not used as an energy store in humans.
    • All three are polymers of glucose but differ in the arrangement of glucose units and in the bonds between them, so their properties differ.

    Common mistakes and how to put them right

    • Saying glucose is converted to glycogen in plants.

      Put it right: , plants store starch, while animals and fungi store glycogen.

    • Describing the conversion as hydrolysis.

      Put it right: , joining glucose units is condensation, which releases water; hydrolysis uses water to break the polymer down.

    • Stating that cellulose is a storage molecule.

      Put it right: , cellulose is structural, forming plant cell walls, and is not broken down for energy in humans.

    Exam tips

    • Link each product to its function and location, for example starch in a potato tuber, glycogen in liver cells, cellulose in a plant cell wall.
    • Use the word condensation and state that water is released when glucose units join.
    • Compare the three polysaccharides in a table of monomer, solubility, function and where found to cover several marking points concisely.

    Try the lesson

    The start of the Metabolism: conversion of glucose to starch, glycogen and cellulose lesson, made from checked teaching notes for this specification point.

    Have a go first

    In which molecule are glucose units stored in a plant cell?

    Listen

    Glucose is a soluble sugar made in photosynthesis and used in respiration. Excess glucose is converted into storage or structural polymers by condensation reactions, each glucose unit joining with the loss of a water molecule. Plants convert glucose to starch for storage in leaves, roots and seeds, and to cellulose for cell walls. Animals, including humans, convert glucose to glycogen for storage in the liver and muscles. Starch and glycogen are compact, insoluble and easily hydrolysed back to glucose when needed; cellulose is strong and structural. A useful method is to compare the three products by monomer, solubility, function and location, for example starch in a potato tuber, glycogen in liver cells and cellulose in a plant cell wall. A strong answer earns marks for these points. Glucose is a monosaccharide; starch, glycogen and cellulose are polysaccharides formed by joining many glucose units together. The conversions are condensation reactions: each glucose unit added releases one molecule of water. Starch is the storage polysaccharide in plants, found in leaves, roots, tubers and seeds. Glycogen is the storage polysaccharide in animals and fungi, stored in the liver and muscles. A common slip: Saying glucose is converted to glycogen in plants. Instead, , plants store starch, while animals and fungi store glycogen. Exam tip: Link each product to its function and location, for example starch in a potato tuber, glycogen in liver cells, cellulose in a plant cell wall.

    See it

    A top-down flowchart. At the top is a node labelled Glucose (monosaccharide). An arrow leads down to a node describing condensation reactions, in which each glucose unit is joined with the loss of a water molecule. From this node three arrows lead to three product nodes: Starch, Glycogen and Cellulose. From Starch an arrow leads to a node saying storage in plants: leaves, roots, tubers, seeds. From Glycogen an arrow leads to a node saying storage in animals and fungi: liver and muscles. From Cellulose an arrow leads to a node saying structural: plant cell walls, gives strength.
    It shows glucose being converted by condensation reactions into starch, glycogen and cellulose, and where each product is found and what it does.

    Key words

    Say what each word means before you turn the card.

    Condensation

    Try answering aloud or in your head before turning the card.

    Starch

    Try answering aloud or in your head before turning the card.

    Glycogen

    Try answering aloud or in your head before turning the card.

    Cellulose

    Try answering aloud or in your head before turning the card.

    See every definition
    Condensation
    A reaction in which glucose units are joined together, with the loss of a water molecule each time a unit is added.
    Starch
    The storage polysaccharide in plants, found in leaves, roots, tubers and seeds, which is compact, insoluble and easily hydrolysed back to glucose.
    Glycogen
    The storage polysaccharide in animals and fungi, stored in the liver and muscles, which is compact, insoluble and readily hydrolysed to glucose for respiration.
    Cellulose
    A structural polysaccharide that forms plant cell walls and gives strength, with straight chains held by hydrogen bonds, and is not used as an energy store in humans.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. At the top is a node labelled Glucose (monosaccharide). An arrow leads down to a node describing condensation reactions, in which each glucose unit is joined with the loss of a water molecule. From this node three arrows lead to three product nodes: Starch, Glycogen and Cellulose. From Starch an arrow leads to a node saying storage in plants: leaves, roots, tubers, seeds. From Glycogen an arrow leads to a node saying storage in animals and fungi: liver and muscles. From Cellulose an arrow leads to a node saying structural: plant cell walls, gives strength.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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