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Carbohydrate Polymers

  • Carbohydrates are used in many different ways in an organism. Here polymers of Glucose that are involved in storage and structure are explored.

Energy Storage

  • Glucose units contain a lot of bonds that can be broken down to release energy during respiration to create ATP. The breakdown occurs in a series of steps which are driven by shape-specific Enzymes. In plants and animals, only α glucose can be broken down in respiration as only the enzymes which fit its shape are present.
  • α Glucose can form long chains with thousands of subunits called and Amylose molecule. Glucose units are bonded together by Condensation Reactions forming (1→4) Glycosidic Bonds. Amylose molecules tend to form coiled springs due to the way in which the the glucose units bond, making it quite compact. Large molecules such as amylose differ from glucose in that they are not water soluable.
  • Iodine molecules can become trapped within the 'coils' of the Amylose chain, which causes iodine (in Potassium Iodide solution) to change colour from yellow-brown to blue-black.
  • Starch consists of a mixture of Amylose and a branched carbohydrate chain called Amylopectin. The branches are formed when a one end of a chain joins with a glucose in another, forming a (1→4) Glycosidic Bond.
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  • Glycogen is almost identical to starch but differs in that the chains of (1→4) linked glucoses are shorter, giving it a more highly branched structure. This branching allows for the fast breakdown of the molecule during respiration as it means that there are more ends which enzymes can start the proccess of hydrolysis from.
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  • Engergy storage molecules like Starch and Glycogen:
    • are insoluble in water and so do not affect the water potential of cells.
    • store glucose molecules in chains so that they can esily be 'broken off' and used in respiration.

Structural

  • β Glucose chains, like the one above, are called Cellulose molecules, and can contain 10000 glucose units. They are stronger than Amylose and are only found in plants. Cellulose is the most abundant polysaccharide found in nature.
  • Cellulose fibres are arranged in a very specific way and can be described as being like a fractal. Long Cellulose chains bunch together, held by Hydrogen bonds, to form Microfibrils. These Microfibrils are bunched with other Microfibrils, held by more Hydrogen bonds, to form Macrofibrils.
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  • Macrofibrils have a very high mechanical strength, similar to that of steel. In plant cell walls, they criss-cross over each, forming a cross-hatched structure, held by Hydrogen bonds, which is very strong. This also allows water to move though and along the cell wall. The strength of the cell walls prevent the cell form bursting, as it would in an animal cell, when water passes into the cell. The pressure cause by the water makes the cell Turgid, supporting the plant through Turgor Pressure.
  • Microfibrils can have special roles. For example, in Guard Cell Walls, the arrangement of microfibrils allows the Stomata to open and close. Cell walls can also be reinforced with other substances, or made waterproof.
  • Other Carbohydrate Polymers are used by a number of other organisms to provide support, such as Peptidoglycan, which forms the basis of bacterial cell walls, and Chitin, which makes up the exoskeleton of insects.

Comparing Cellulose and Amylose

  • Cellulose and Amylose can be compared in terms of their structure and function.
  • Both Cellulose and Amylose consist of Polypeptide Chains of Glucose bonded together with Glycosidic Bonds. Both molecules are insoluble in water. However, Cellulose is composed of β Glucose and forms long straight chains, in which every other Glucose monomer flipped over and are very strong, whereas Amylose is composed of α Glucose, forms a coiled chain and tends to be found in granules.
  • Because of their structure, Cellulose and Amylose have very different functions. Amylose is used as an energy storage in starch, whereas Cellulose plays a structural role.