C2 — Carbohydrates Flashcards
Glycosidic bond
A glycosidic bond (C-O-C) is defined as the bond formed between the anomeric carbon of one sugar unit and another carbon on the other sugar unit.
Polysaccharides
Polysaccharides are macromolecules with a few hundred to a few hundred thousand monosaccharides joined by glycosidic bonds.
Describe one structural similarity and one structural difference between amylopectin and amylose. [2]
- Both are made of α-glucose monomers ;
- Both contains monomers linked by α-glycosidic bonds ;
- Both exists as helical chains ;
1. Amylopectin is a branched molecule but amylose is unbranched ;
2. Monomers can be linked by α(1,4) or α(1,6) glycosidic bonds in amylopectin but monomers are linked by only α(1,4) glycosidic bonds in amylose ;
Explain how the structure of starch makes it suitable for its function. [3]
- Ref. to amylose being helical, hence compact ; R: α-helix
- Ref. to amylopectin forming branches, hence compact ;
- Ref. to amylopectin forming branches, hence providing large number of free ends for hydrolysis by amylase at any one time ;
- Ref. to (amylose / amylopectin / starch, containing) thousands of glucose monomers that act as a large store of carbon ; I: store of energy
- Ref. to (amylose / amylopectin / starch, containing) thousands of glucose monomers for use as respiratory substrate (upon hydrolysis) ;
- Ref. to starch being large and insoluble, hence is osmotically inactive / does not affect water potential ;
Describe how the alpha (1,4) glycosidic bond may be broken to release two molecules of glucose. [2]
- Ref. to hydrolysis
- enzyme-catalysed / maltase-catalysed, cleavage (of α(1,4) glycosidic bond) ; R: amylase
- via addition of 1 molecule of water ;
Monosaccharides
The hydroxyl group that is bonded to the anomeric carbon (e.g. carbon atom 1 in glucose) can assume 2 configurations: (ABBA)
- alpha-Glucose below the plane of the ring (away)
- beta-Glucose above the plane of the ring (same)
Freely soluble in water, insoluble in non-polar solvents.
General formula: (CH2O)n
Uses:
- Energy sources (respiratory substrate) to produce ATP during cellular respiration
- Building blocks for the synthesis of disaccharides and polysaccharides
- Raw material for synthesis of other organic molecules, such as nucleotides, amino acids and fatty acids
Aldose / Aldo sugar
The monosaccharide which has an aldehyde group.
- are strong reducing agents
- C=O groups usually at the end of the carbon chain
Ketose/ keto sugar
A monosaccharide that contains a ketone group, where C=O groups usually at is usually at the middle of the carbon chain.
Storage polysaccharides — Starch
- Main function: storage
- Monomer: alpha-glucose
- Bonds: alpha(1,6) glycosidic bonds (gives rise to branches) + alpha(1,4) glycosidic bonds (linear, unbranched chains) (higher percentage)
- Composition: consists of both unbranched amylose (10 – 30%) + branched amylopectin (70 – 90%)
- Solubility in water: soluble -> not affect water potential
- Occurrence: stored in plant cells as starch grains either within the chloroplasts, or within the amyloplasts, which are specialised plastids for
starch storage. - Colour in iodine test: blue black
Storage polysaccharides — glycogen (branched)
Main function: storage
Monomer: alpha-glucose
Bonds: α(1,6) glycosidic bonds occur every 8 - 12 glucose units (give rise to branches) + α(1,4) glycosidic bonds (linear, unbranched chain)
- Solubility in water: insoluble, does not affect water potential
- Structure: more extensively branched and compact than amylopectin
- Occurrence: found mainly in the liver and skeletal muscle in the form of cytoplasmic granules
- Colour in iodine test: red violet
Characteristics of storage polysaccharides (starch + glycogen)
- large molecule -> insoluble -> does not affect water potential (WP) within cells and living organisms
- Composed of several hundreds to thousands of glucose monomers -> store large number of carbon in glucose and acts as respiratory substrate
- Glucose units are linked by α(1,4) glycosidic bonds. -> Starch may be easily hydrolysed by enzymes present in plants and most organisms. OR enzyme glycogen phosphorylase in animals
- Amylopectin molecules are highly branched due to the presence of α(1,6) glycosidic bonds. -> Amylopectin is highly compact + A large number of free ends are available for hydrolysis by amylase
- Anomeric carbon is involved in few free anomeric hydroxyl groups -> Starch is an unreactive and chemically-stable glycosidic bond formation, leaving compound.
+ Only for amylose:
- Amylose molecules are helical in shape. -> Amylose is compact.
Structural polysaccharides — cellulose
Monomer: beta-glucose
Bonds: β- (1,4) glycosidic bond
Composition: alternate monomers are inverted, forming a long, unbranched straight chain. Many chains run parallel to each other and their hydroxyl groups (OH) project outwards from each chain.
Solubility in water: Full permeability to water and solutes
Occurrence: cell wall of plant cells
Structure: unbranched
Characteristics:
- large molecule -> insoluble -> make a good structural material
- β-glucose units are linked by β(1,4) glycosidic bonds, which have a different molecular shape from α(1,4) glycosidic bonds found in amylose and amylopectin.
Amylase can only hydrolyse α(1,4) glycosidic bonds found in starch.
Few organisms produce cellulase. Hence, cellulose is stable.
- Alternate inverted β-glucose units linked by β(1,4) glycosidic bonds allow cellulose to form long, unbranched and straight chains.
+
Extensive hydrogen bonds form between parallel chains.
+
Straight parallel chains can be grouped into microfibrils, which
eventually cluster
into macrofibrils.
-> Cellulose provides high tensile strength for structural support.
Components of starch — amylose (unbranched)
Bonds: α(1,4) glycosidic bonds -> linear, unbranched chains
Composition: consists of hundreds to thousands of α-glucose residues joined by α(1,4) glycosidic bonds
Solubility in water: Poorly soluble in water as it is bulky, it does not exert osmotic influence in the cell.
Structure: helical structure that is compact.
6 glucose units per turn in the helix.
Iodine test colour: blue black
Components of starch — amylopectin
Bonds: α(1,6) glycosidic bonds
-> (give rise to branches) + α(1,4) glycosidic bonds -> linear, unbranched chains
Composition: Branch points occur at every 12 - 30 residues and average branch length is between 24 - 30 residues.
Structure: somewhat branched helical chain
Iodine test colour: red violet
(Looks like water down the drain)
