Biological Molecules - Carbohydrates
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Transcript of Biological Molecules - Carbohydrates
Carbohydrates
Carbohydrates are one if the 4 most impt classes of biological molecules that make up the bodies of all living organisms
Carbohydrates, proteins, lipids and nucleic acids account for over 90% of total dry mass of the cell
These four most impt classes of biological molecules are organic (contains Carbon, Hydrogen and Oxygen)
Insoluble carbohydrate polymers serve as structural and protective elements in cell walls and connective tissues of animals
Other carbohydrate polymers lubricate skeletal joints and participate in the recognition and adhesion between cells
More complex carbohydrate polymers covalently attach to proteins or lipids (Glycoproteins / Glycolipids) and involved in receiving signals from external surroundings
The unique feature of carbon atom
Small atom, low in mass
Each carbon is able to form 4 strong and stale covalent bonds with a variety of other atoms like hydrogen, oxygen, nitrogen, phosphorous and sulfur
There is a tetrahedral arrangement when a carbon atom is bond to 4 other atoms
Carbon has the unusual ability to bond with itself, forming carbon-carbon bonds, thus building up large carbon skeletons with ring and/or chain structures
Classes of Carbohydrates
Carbohydrates are hydrates of carbon and hence have a general formula of Cx(H2O)y
They are made up of units containing carbon, hydrogen and oxygen atoms in the ratio of 1:2:1
There are 3 main classes of carbohydrates-Monosaccharides-Disaccharides-Polysaccharides
Monosaccharides
A monosaccharide is a carbohydrate that cannot be hydrolysed to simpler carbohydrates
They are classified according to the number of carbon atoms in the molecule
Number of C atoms Name Examples
3C Triose Glyceraldehyde
4C Tetrose Threose
5C Pentose Ribose, Ribulose
6C Hexose Glucose, Galactose, Fructose
7C Heptose Sedoheptulose
Of these, 5C and 6 C are the most common
Structure of Monosaccharides
General formula of (CH20)n where n>3
Monosaccharides show isomerism
Glucose and Fructose are Structural isomers (a big difference in structure)
Glucose and Galactose are stereoisomers (one portion of the chain is mirrored)
There are 3 types of isomers of monosaccharides
-Aldose and Ketose-Open-chain form and ring form-alpha isomer and beta-isomer
Aldose and Ketose
Aldose has an aldehyde functional group
Ketose has a keto group
Open-chain form and ring form
Linear form is rare due to its instability
Almost all glucose molecules in a solution spontaneously react to form one of the two ring structures, the alpha and beta forms of glucose
Both alpha and beta forms exist in equilibrium, but beta forms more commonly as it is slightly more stable than the alpha form
The ring form of monosaccharides are used to make disaccharides and polysaccharides
In glucose, C1 combines with the oxygen atom on C5 to form a 6-sided structure known as a pyranose ring
In the case of fructose, C2 links to the oxygen atom in C5 rather than C1 to form a 5-sided structure known as a furanose ring
Alpha-isomer and beta-isomer
Glucose can exist in 2 possible ring forms
The -OH on C1 can project below the plane of the ring (with C6 on the top) to form alpha-glucose, or it can be prosecuted
above the plane of the ring (same plane as C6) to form a beta-glucose
A glucose molecule in solution can switch spontaneously from the open-chain form to either of the two ring forms and back again
Alpha and beta glucose have similar properties, however, the relatively small difference in the structure explains why storage polysaccharides like starch and glycogen are made up of alpha-glucose while structural polysaccharides like cellose are made up of beta-glucose
Properties of Monosaccharides
Physical:-Small-Low Mr-sweet-Crystalline in appearance-Readily soluble in water (polar), hence exerts osmotic pressure
Chemical:-All monosaccharides are reducing sugars-Monosaccharides can be oxidized by relatively mild oxidizing agents like Cu2+ present in Benedict’s solution-The carbonyl carbon is oxidized t a carboxyl group. Sugars capable of reducing ferric Cu2+ are called reducing sugars
-All carbohydrates are the source of energy or plants and animals, but monosaccharides can act as respiratory substrates-Monosaccharides are raw materials for synthesis of other carbohydrates like polysaccharides, proteins, lipids, nucleic acids and coenzymes-In plant cells, the osmotic pressure monosaccharides exert accounts for the turgidity of plant cells, elongation of young plant cells and transportation of water from cell to cell and opening and closing of the stomata
Disaccharides
A disaccharide is made by joining two monosaccharides
Examples are maltose, lactose and sucrose
Disaccharides are formed by glycosidic linkages
General formula of disaccharides: C12H22O11
Formation of disaccharides from two monosaccharides is a condensation reaction
Glycosidic bonds are usually formed between C1 and C4, hence the bond name would be ,4 glycosidic bond
This reaction is reversible through the process of hydrolysis
Properties of disaccharides
Physical:-Small-Low Mr-SweetCrystalline in appearance-Polar, thus exert osmotic pressure
Chemical:
-Some disaccharides are reducing (maltose and lactose) while others aren’t (sucrose)-Disaccharides can be hydrolysed into monosaccharides by:
-incubating it with a dilute acid at 100*C-incubating it with an enzyme at room temp
Functions of Disaccharides
Disaccharides have the same functions as monosaccharides (respiratory substrates, raw materials for synthesis)since the latter are produced when disaccharides are hydrolysed
Disaccharides are also osmotically active substances and thus have the same osmotic functions as that of monosaccharides
Disaccharide Composition Functions
Maltose Glucose + Glucose Respiratory substrateProduction of starch digestion by amylase
Lactose Glucose + Galactose Respiratory substrateFound exclusively in milkImpt carbohydrate source for young mammals
Sucrose Glucose + Fructose Respiratory substrateChief form of transported sugars in plants due to high solubility (Sugars largely stored as starch)Storage in some plants (onion)
Polysaccharides
A polysaccharide is a polymer whose constituent monomers (usually hexoses) are monosaccharides
Example, starch, glycogen, cellulose
General formula of (C6H10O5)n where n = number of hexose units linked together in a polysaccharide
A polysaccharide consists of many repeating units of monosaccharides joined together by glycosidic bonds, forming long straight chains (in cellulose), helical chains (in amylose), or branched chains (in amylopectin)
If 1,4 glycosidic bonds and 1,6 glycosidic bonds are formed between 2 alpha isomers, then they are called alpha(14) glycosidic bond/linkage and alpha(16) glycosidic bond/linkage respectively
Properties of Polysaccharides
Physical:-Not sweet-Not crystalline in appearance-Insoluble in water, thus does not exert osmotic pressure
Chemical:-Non-reducing-Can be hydrolysed to disaccharides and monosaccharides by concentrated acids and in the presence of amylase
Functions of Polysaccharides
Based on functions, polysaccharides can be divided into 2 groups, Storage polysaccharides ad Structural polysaccharides
Storage polysaccharides function mainly as food and energy stores, while structural polysaccharides are used to make certain structural materials of cells
Starch: Storage Polysaccharides in Plants
Main storage polysaccharide in plant (absent in animals)
Consist of two components - amylose and amylopectin, both of which are made up of many alpha-glucose units
A suspension of amylose in water gives a blue-black colour with iodine-potassium iodide solution, whereas that of amylopectin gives a red-violet colour (this forms the basis for the test of starch)
Amylose:
It is an unbranched polymer of several hundreds to few thousands of alpha-glucose linked together by alpha(14) glycosidic bonds, which cause the amylose chain to coil helically into more compact shape
For every complete turn of the helix, there are 6 alpha-glucoses units
Helix is held together by hydrogen bonds formed between -OH groups
Since most of the -OH groups of the alpha-glucose units project into the interior of the helix, cross-linking of amylose chains is not possible
Amylose is not polar, unless the temp is high enough to overcome the hydrogen bonds holding the helix together
Amylopectin:
It is a branched polymer of alpha-glucose units whose number is more than twice of that in amylose
The alpha-glucose units in the main chain are linked by alpha(14) glycosidic bonds, while the side chains are attached to the main chain by alpha(16) glycosidic bonds
Branching decreases the ability of the chains to bind together with one another and increases the binding of water molecules to the chains, amylopectin is more soluble than thee unbranched amylose in water
Both components of starch form a complex 3D structure where the amylose helices are entangled in the branches of the amylopectin molecules
Structure of Amylopectin
In plants, it is packed into spherical plastids called starch grains, which occur particularly in chloroplasts of plant cells or in specialized structures such as seeds or potato tubers
2 ways to hydrolyse starch back to glucose:-chemical method: incubate starch with dilute acid @ 100*C-enzymatic method: incubate starch with enzymes at room temp
Glycogen: A highly branched storage polysaccharide in animals
Animal equivalent of starch
Found in liver and skeletal muscles of vertebrae animals and many fungi
Glycogen is the most branched out of the 3, so it is also the most soluble
Cellulose: A structural Polysaccharide in Plants
Most abundant organic molecule on earth (abt 50% of carbon found in plants is in cellulose)
Structural component of ALL cell walls
Polymer of about 10,000 beta-glucose units linked by beta(14) glycosidic bonds, forming unbranched, linear chain rather than a helix chain, as the glycosidic bonds occur alternatively on opposite ends
H-bonds may form between the protruding -OH groups on adjacent cellulose chains, resulting in rigid cross-linking between chains
Cross-link chains associate in groups to form micelles, which are arranged in larger bundles to form micro fibrils, which combine to form macro fibrils, which are of great tensile strength
The tensile strength provides the support and mechanical strength to plant cells and prevent them from bursting when placed in hypotonic solutions
There are large spaces between the cellulose macro fibrils to allow water and solute molecules to pass through the cell walls