AS/A-level Biology - ​​Sugar

When it comes to AS/A-level Biology, do you remember how to test for starch and reducing sugar?

• Iodine solution 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-black gives an idea of relative concentration.
• E.g. Benedict's Test (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.

• Large, complex polymers.
• Monosaccharides are their monomers.
• Linked by glycosidic bonds.

When it comes to AS/A-level Biology, do you know 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.

• 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.

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

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

• 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.

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

• 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 cross-linked by hydrogen bonds, forming microfibrils.
• Found in exoskeletons of insects and in fungi cell walls.

• Two monosaccharides joined together by a glycosidic bond.
• This 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.
• For example,
  Maltose = glucose + glucose - in germinating seeds
  Sucrose = glucose + fructose - transport in phloem of flowering plants
  Lactose = glucose + galactose - found in mammalian milk

• 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).

• 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.
• 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 (trioses are intermediates in reactions of respiration)
• Constituents of nucleotides (e.g. deoxyribose in DNA and ribose in RNA, ATP and ADP).

• 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 sugar is 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 sugars.
• 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.

Biosensors

• Using a biosensor means a value of the concentration of sugar is given.
• Quantative measurement.
• Important in monitoring diabetes.
• Gives an accurate measurement of blood glucose.

That's the end of the topic!

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