Carbohydrates

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Iodine-Potassium Test - Presence of Starch

  • Iodine soluton reacts with starch
  • Results in a colour change from orange-brown to blue-black
  • Qualitative test - accurate concentration cannot be determined
  • The depth of blue-blackgives an idea of relative concentration
  • Like Benedict's, it is qualitative 

IF STARCH IS PRESENT IN THE SOLUTION BEING TESTED:

  • The solution will change from orange-brown to blue-black

IF STARCH IS NOT PRESENT IN THE SOLUTION BEING TESTED:

  • The solution will remain orange-brown and not change colour
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Disaccharides and Polysaccharides

FORMATION OF DISACCHARIDES

  • Two monosaccharides joined together by a glycosidic bond
  • Happens in a condensation reaction (elimination of water)
  • 1-4 glycosidic bonds are formed because the bond is between carbon no. 1 on one monosaccharide and no. 4 on the other monosaccharide
  • The disaccharide molecule is straight and not twisted, and so the glycosidic bond formed is an alpha-1, 4 glycosidic bond

EXAMPLES OF DISACCHARIDES

  • MALTOSEglucose + glucose - in germinating seeds
  • SUCROSE glucose + fructose - transport in phloem of flowering plants
  • LACTOSEglucose + galactose - found in mammalian milk

POLYSACCHARIDES

  • Large, complex polymers
  • Monosaccharides are their monomers
  • Linked by glycosidic bonds
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Polysaccharides - Starch

WHY IS GLUCOSE CONVERTED TO STARCH?

  • Glucose is soluble in water, and so consequently would draw water into the cell by osmosis
  • Starch is insoluble, can't diffuse out of cell, compact molecule, lots of energy in C-H and C-C bonds

STARCH

  • Main glucose storage in plants. Found in high concentration in seeds and storage organs
  • Made of alpha-glucose molecules bonded in two different ways, forming AMYLOSE and AMYLOPECTIN

AMYLOSE

  • Liner, unbranched molecules
  • Consists of alpha-1, 4 glycosidic bonds
  • Coils into an alpha-helix

AMYLASE

  • Chains of glucose monomers joined with alpha- 1,4 glycosidic bnds
  • Cross-linked with alpha-1, 6 glycosidic bonds. Results in a branched structure
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Polysaccharides - Glycogen

  • Main storage product in animals
  • Similar to amylopectin in that it has both alpha- 1, 4 and alpha - 1, 6 glycosidic bonds
  • The difference between amylopectin and glycogen is that glycogen has shorter alpha- 1,4 linked chains and so as a result are more branched
  • Readily hydrolysed to alpha-glucose, which is soluble and can be transported to where energy is required
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Polysaccharides - Cellulose

STRUCTURE

  • Structural polysaccharide consisting of long, parallel beta-glucose units
  • Glucose monomers are joined by beta - 1, 4 glycosidic bonds
  • The beta-link rotates the adjacent glucose molecules by 180 degrees. This allows the formation of hydrogen bonds between (OH) groups of adjacent parallel chains, contributing to structural stability
  • Cellulose molecules are cross-linked to form microfibrils. Microfibrils are held in bundles called fibres. Each cell wall has several layers of fibres running parallel, at an angle to adjacent layers
  • The laminated structure contributes to the strength of the cell wall

PERMEABILITY 

  • Cellulose fibres are freely permeable as spaces between fibres allow water and its solutes to penetrate through, reaching the cell membrane
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Monosaccharides

  • Monosaccharides are building blocks for larger molecules
  • They have the general formula (CH2O)n
  • The names of monosaccharides is determined by the number of carbons they have (e.g. hexose has six, triose has three)

GLUCOSE

  • Hexose sugar (six carbons)
  • Glucose has two isomers: alpha-glucose and beta-glucose. The only difference between the two is the positioning of an OH group
  • Result in biological differences when they form polymers

FUNCTIONS

  • A source of energy in respiration (C-H and C-C bonds are broken to release energy)
  • Building blocks for larger molecules (i.e. disaccharides and polysaccharides)
  • Intermediates in reactions
  • Constituents of nucleotides (e.g. deoxyribose in DNA and ribose in RNA, ATP and ADP)
  • Intermediates in reactions (trioses are intermediates in reactions of respiration)
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Polysaccharides - Chitin

STRUCTURE

  • Structural polysaccharide
  • Resembles cellulose, but has added amino acids forming a heteropolysaccharide
  • Strong, waterproof and lightweight
  • Monomers are rotated 180 degrees as in cellulose
  • Long parallel chains are crosslinked by hydrogen bonds, forming microfibrils

USES

  • Found in exoskeletons of insects and in fungi cell walls
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Benedict's Test (1)

  • Detects reducing sugars in a solution
  • Equal volumes of Benedict's reagent and the solution being tested are heated to at least 70 degrees celsius 
  • If a reducing sugaris present, then the solution will turn from blue through green, yellow and orange to red
  • This is because the reducing sugars donate an electron to reduce blue copper (II) oxide ions in copper sulphate to red copper (I) oxide

NON-REDUCING SUGARS

  • All monosaccharides and some disaccharides, e.g. maltose, are reducing
  • Non-reducing sugars must first be broken down to constituent monosaccharides. This is done by heating with hydrochloric acid. An alkali must be added as the reagent needs alkaline conditions to work. This is then heated as before
  • If the solution turns red then a non-reducing sugar was present initially

ENZYMES

  • The enzyme sucrase can be used to break down sucrose to its constituent monosaccharides. Enzymes are specific, so sucrase only works with sucrose
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Benedicts's Test (2)

BIOSENSORS

  • Using a biosensor means a value of the concentration of sugar is given
  • Quantative measurement

USES

  • Important in monitoring diabetes
  • Gives an accurate measurement of blood glucose
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