Saccharides (Edexcel International AS Biology)

• 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) eg. glyceraldehyde
  • Pentose (5C) eg. ribose
  • Hexose (6C) eg. 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 

Monosaccharides: 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

Monosaccharides: 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 fibers, which can be used as cellular support in some cell types

The glycosidic bond

• To make monosaccharides more suitable for storage they are bonded together to form disaccharides and polysaccharides
  • Polysaccharides are insoluble so have less influence on the process of osmosis
• Disaccharides and polysaccharides are formed when two hydroxyl (OH) groups on different monosaccharides interact to form a strong covalent bond called a glycosidic bond
• The name of the glycosidic bond that forms depends on the location of the OH groups on the monosaccharides concerned, e.g.
  • If the OH groups are located on carbon 1 of one monosaccharide and carbon 4 of the other, a 1,4 glycosidic bond forms
  • If the OH groups are located on carbon 1 of one monosaccharide and carbon 6 of the other, a 1,6 glycosidic bond forms
• Every glycosidic bond results in one water molecule being released, thus glycosidic bonds are formed by a condensation reaction

Glycosidic bonds form through condensation reactions, during which a water molecule is released. When two glucose molecules are joined by a glycosidic bond, the resulting disaccharide is maltose

Glycosidic bonds can link monosaccharides together to form polysaccharides such as amylopectin, a form of starch. Amylopectin contains 1,4 and 1,6 glycosidic bonds

Breaking the glycosidic bond

• The glycosidic bond is broken when water is added in a hydrolysis reaction
  • Hydro = water
  • Lysis = to break
• Examples of hydrolytic reactions include the digestion of food in the alimentary tract and the breakdown of stored carbohydrates in muscle and liver cells for use in cellular respiration

Glycosidic bonds are broken when water is added in a hydrolysis reaction

Disaccharides: 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

Disaccharides: 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: structure

• Polysaccharides may be
  • Branched or unbranched
    • Being branched increases the rate at which a polysaccharide can be broken down
  • Straight or coiled
    • Being straight makes the molecules suitable for constructing cellular structures e.g. cellulose
    • Being coiled makes a molecule more compact and suitable for storage e.g. amylose in starch
• Starch and glycogen are useful as storage polysaccharides because they are
  • Compact; large quantities can be stored
  • Insoluble; they will have no osmotic effect, unlike glucose which would increase the solute concentration of a cell and causing water to move in by osmosis

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

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

Amylopectin is a branched polysaccharide found in starch

Glycogen: structure

• Glycogen is highly branched and not coiled
• It 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 a highly branched molecule used as a storage polysaccharide in animals and fungi

Polysaccharide: function

• Starch and glycogen are storage polysaccharides; they are adapted for this function by being
  • Compact
    • Large quantities can be stored
  • Insoluble
    • They will have no osmotic effect on cells, unlike glucose which can dissolve and raise the solute concentration of cell cytoplasm, causing water to move into cells by osmosis

Starch

• 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

• 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