AS Bio: Biological Molecules- Carbohydrates

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Monosaccharides

Are basic, single units that make up large carbohydrates.

Example: Glucose, Fructose, Galactose

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Disaccharides

When two monosaccharides are joined together.

Glucose + Glucose = Maltose

Glucose + Fructose = Sucrose

Glucose + Galacose = Lactose

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Glycosidic Bonds

Are formed between monosaccharides when they are joined together. A molecule of water is released as it is a condensation reaction.

Example:  Glucose + Glucose are joined with a glycosidic bond to form Maltose and a molecule of water is released.

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Polysaccharides

When more then two monosccharides are joined together in condensation reaction ( glycosidic bonds).

Example: Starch, Cellulose and Glycogen

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Starch

  • Is the main material of energy storage in plants. Plants store excess glucose as starch, so that it can be made into glucose when it is needed.
  • Insoluble in water and does not affect water potential.
  • Prevents osmosis.
  • Starch is composed of Amylose and Amylopectin
  • Amylose: long unbranched chain of alpha-glucose. It is coiled which make it compact and good for storage.
  • Amylopectin: long branched chain of alpha glucose. The branches allow enzymes to attach and break down the molcules glycosidic bonds, meaning glucose can be released quickly.
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Cellulose

  • Is a component of cell walls in plants due to its strong structural feature of microfibrils.
  • Long, unbranched chains of beta-glucose that form straight chains.
  • Cellulose chains are joined with glycosidic bonds and hydrogen bonds.
  • Insoluble in water
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Glycogen

  • Is the main energy storage material in animals. Animals store excess glucose and glycogen, so that it can be made into glucose when it is needed.
  • Long, alpha-glucose chain with lots of branches, which allows glucose that was stored to be released quickly for movement etc. 
  • Its structure is compact making it god for storage. 
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Isomers

Are molecules with the same molecular formula but have different structures.

Example:

Alpha Glucose- H is on the top right of the simple structure with OH on the bottom right.

Beta Glucose- OH is on the top right of the simple structure with H on the bottom right.

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Benedicts Test For Reducing Sugars

Test for Redcing Sugars:

1) Add Benedicts reagent (which is Blue) to a sample and heat it in a waterbath until it boils.

2) If the test is positive it will form a coloured precipitate (solid particles in the solution/sample).

3) The higher the concentration of the reducing sugar the more the colour will change, this allows you to compare the amount reducing sugar in the sample. Then you can weigh the percipitate to get accurate results.

Colour of Precipitate: Blue -> Green -> Yellow -> Orange -> Red

If the results of the reducing sugar test are negative, non reducing sugar caould be present. To check, the sugar needs to be broken down down into monosaccharides.

1)Get a new sample of test solution, adding dilute hydrocholoric acid and heat it in the waterbath until it boils. Then neutralise it with sodium hydrogencarbonate.

2) Continue Benedicts test as normal and record the results (colour and positive/negative)

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Iodine Test For Starch

[Find out what is left after the digestion of starch.]

1) Add iodine dissolved in potassium iodide solution to the test sample.

2) If there is startch present the sample will be dark blue-black colour. If starch is not present the sample will remain a brown-orange colour.

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