Saccharides (Edexcel IAL Biology)

Types of Saccharide

Spec point — 1.2

• Carbohydrates are one of the main carbon-based compounds in living organisms

• All molecules in this group contain C,H and O

  • Carbon atoms are key to the structure of organic compounds because

    • Each carbon atom can form covalent bonds; this makes the compounds very stable

      • Covalent bonds are so strong they require a large input of energy to break them

    • Carbon atoms can form covalent bonds with oxygen, nitrogen and sulfur

    • Carbon atoms can bond to form straight chains, branched chains, or rings

• Carbon compounds can form small, single subunits, or monomers, that bond with many repeating subunits to form large molecules, or polymers

  • This is a process called polymerisation

• The three types of carbohydrates are:

  • monosaccharides

    • The monomers of carbohydrates; they can join together to make carbohydrate polymers
    • Monosaccharides are simple carbohydrates
    • Monosaccharides are sugars, e.g. triose (3C) sugars, pentose (5C) sugars and hexose (6C) sugars

  • disaccharides

    • Two monosaccharides can join together via condensation reactions to form disaccharides
    • Examples include lactose and maltose

  • polysaccharides

    • Polysaccharides are carbohydrate polymers; repeated chains of many monosaccharides
    • Starch (including amylose and amylopectin), glycogen, and cellulose are examples of polysaccharides

Exam Hint Although cellulose is an important polysaccharide you do not need to know about it in this topic.

Saccharide Structure & Function

Spec point — 1.2

Monosaccharides

Monosaccharide structure

• Glucose is a well known example of a monosaccharide

  • Glucose is a hexose sugar

  • The six carbons that make up glucose form a ring structure

    • Carbons 1-5 form a ring, while carbon 6 sticks out above the ring

• Glucose comes in two forms; alpha ($α$) and beta($β$)

  • The forms of glucose are almost identical; they differ only in the location of the H and OH groups attached to carbon 1

    • Alpha glucose has the H above carbon 1 and the OH group below

      • Remember = alpha has the H above

    • Beta glucose has the H below carbon 1 and the OH group above

      • Remember = beta has the H below

Alpha glucose (top) has the hydrogen above carbon 1 and the OH group below, while beta glucose (bottom) has the hydrogen below carbon 1 and the OH group above

Monosaccharide function

• The main function of monosaccharides is to store energy within their bonds

  • When the bonds are broken during respiration, energy is released

• The structure of glucose is related to its function as the main energy store for animals and plants

  • It is soluble so can be transported easily
  • It has many covalent bonds which store energy

• Monosaccharides can combine through condensation reactions to form larger carbohydrates

• Some monosaccharides are used to form long, structural fibres, which can function as cellular support in some cell types

Disaccharides

Disaccharide structure

• Common examples of disaccharides include

  • Maltose

    • Contains two molecules of glucose linked by a 1,4 glycosidic bond
    • This means that the glycosidic bond is located between carbon 1 of one monosaccharide and carbon 4 of the other

  • Sucrose

    • Contains a molecule of glucose and a molecule of fructose linked by a 1,2 glycosidic bond
    • This means that the glycosidic bond is located between carbon 1 of one monosaccharide and carbon 2 of the other

  • Lactose

    • Contains a molecule of glucose and a molecule of galactose linked by a 1,4 glycosidic bond

Sucrose is a disaccharide formed from a molecule of glucose (left) and a molecule of fructose (right) joined together by a 1,2 glycosidic bond

Disaccharide function

• The function of disaccharides is to provide the body with a quick-release source of energy

  • Disaccharides are made up of two sugar molecules so they're easily broken down by enzymes in the digestive system into their respective monosaccharides and then absorbed into the bloodstream

• Due to the presence of a large number of hydroxyl groups, disaccharides are easily soluble in water

  • These hydroxyl groups form hydrogen bonds with the water molecules when dissolved in aqueous solutions

• Just like monosaccharides they are sweet in taste

  • Sucrose, also known as table sugar, is an example

Polysaccharides

Starch: structure

• Starch is constructed from two different polysaccharides

  • Amylose

    • Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules

      • A helix is a spiral shape

    • The helix shape enables it to be more compact and thus more can be stored

  • Amylopectin

    • A branched molecule containing 1,4 glycosidic bonds between α-glucose molecules and 1,6 glycosidic bonds
    • The branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added to for storage

• Both forms of starch are insoluble, so they do not dissolve and won't alter the water potential of cells

Amylose is a helix-shaped polysaccharide found in starch, the storage polysaccharide in plants

Amylopectin is a branched polysaccharide found in starch

Starch: function

• Starch is the storage polysaccharide of plants; it is stored as granules in plastids

  • Plastids are membrane-bound organelles that can be found in plant cells

    • They have a specialised function, e.g. amyloplasts store starch grains and chloroplasts carry out photosynthesis

• The amylose in starch has a helical structure which makes it very compact, meaning that much can be stored in a small space

• The amylopectin in starch has branches that provide many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added for storage

Glycogen: structure

• Glycogen contains both 1,4 and 1,6 glycosidic bonds

• Glycogen is more branched than amylopectin

  • The branching provides more terminal glucose molecules which can either be added to or removed by hydrolysis; this allows the quick storage or release of glucose to suit the demands of the cell

    • This is essential in animal cells as animals are very metabolically active

• Glycogen is compact, which means that much can be stored in a small space

  • Liver and muscles cells have a high concentration of glycogen, present as visible granules; this enables a high cellular respiration rate

• Glycogen is insoluble

Glycogen is a highly branched molecule used as a storage polysaccharide in animals and fungi

Glycogen: function

• Glycogen is the storage polysaccharide of animals and fungi

• Glycogen is more branched than amylopectin

  • The branching provides more terminal glucose molecules which can either be added to or removed by hydrolysis; this allows the quick storage or release of glucose to suit the demands of the cell

    • This is essential in animal cells as animals are very metabolically active

• Glycogen is compact which means that much can be stored in a small space

  • Liver and muscles cells have a high concentration of glycogen, present as visible granules; this enables a high cellular respiration rate

Summary of Storage Polysaccharides Table

Types of Carbohydrate Summary Table