Presence of Starch - Iodine-Potassium Test
- 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.
Polysaccharides - 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.
Polysaccharides - Amylose
- Liner, unbranched molecules.
- Consists of alpha-1, 4 glycosidic bonds.
- Coils into an alpha-helix.
Polysaccharides - Amylopectin
- Chains of glucose monomers joined with alpha- 1,4 glycosidic bonds.
- Cross-linked with alpha-1, 6 glycosidic bonds.
- Results in a branched structure.
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.
Polysaccharides - Cellulose
- 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.
Polysaccharides - Chitin
- 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.
Formation of Disaccharides
- 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).
Monosaccharides - 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.
- 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.
- 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.
- The enzyme sucrase can be used to break down sucrose to its constituent monosaccharides.
- Enzymes are specific, so sucrase only works with sucrose.
- 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!
Drafted by Bonnie (Biology)