Saccharides (Edexcel International AS Biology) : Revision Note

Types of Saccharide

• 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, disaccharides, and polysaccharides

Monosaccharides

• Monosaccharides are the monomers of carbohydrate; they can join together to make carbohydrate polymers

  • Monosaccharides are simple carbohydrates

  • Monosaccharides are sugars

• There are different types of monosaccharide formed from molecules with varying numbers of carbon (C) atoms, for example

  • Triose (3C), e.g. glyceraldehyde

  • Pentose (5C), e.g. ribose

  • Hexose (6C), e.g. glucose

Disaccharides

• Two monosaccharides can join together via condensation reactions to form disaccharides

  • A condensation reaction is one in which two molecules join together via the formation of a new chemical bond, with a molecule of water being released in the process

  • The new chemical bond that forms between two monosaccharides is known as a glycosidic bond

Polysaccharides

• Starch, glycogen, and cellulose are examples of polysaccharides

  • Polysaccharides are carbohydrate polymers; repeated chains of many monosaccharides joined by glycosidic bonds in a condensation reaction

Starch

• Starch is the storage polysaccharide of plants

  • It is stored as granules inside plant cells

  • Plants make glucose during photosynthesis and the molecules of glucose are joined to make the polysaccharide starch

• Starch is constructed from two different polysaccharides

  • Amylose and amylopectin 

Glycogen

• Glycogen is the storage polysaccharide of animals and fungi

  • It is highly branched and not coiled

• 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 

Saccharide Structure & Function

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 () and beta ()

  • 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