structure and properties of biological molecules

7/30/2019 Structure and Properties of Biological Molecules

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Structure and Properties of Biological Molecules

Starch

Starch is a polysaccharide consisting of 2 substances, amylose (linear polysaccharide)

and amylopectin (highly branched polysaccharide). Both has many glucose units joined by

glycosidic bonds. Natural starch contains 10-20% amylose and 80-10% amylopectin.

When starch is placed in heated water, the granules swells and bursts and the semi-crystalline

structure is disrupted causing the amylose to drain out of the granule to create a network that

holds water (increases the mixtures viscosity: this process is called starch gelatinization)

Pure starch is white, tasteless and odorless. The test for starch is by adding iodine solution

which gives a blue/black colour if starch is present.

Starch is soluble in water when heated, it coagulates into a wheat-paste that can be used as a

gluing agent. When it is cooled, the semi-crystalline structure slowly recovers and thickens,

eliminating water.

Amylases hydrolyse starch into constituent sugars. Alpha-amylases hydrolyses starch givingglucose and maltose. Beta-amylase cuts starch into maltose unit. Acids also hydrolyse starch

into simpler carbohydrates.

Starch is never found in animal cells. Instead, glycogen is present.

It has a pryanose structure and belongs to the aldehyde group.

Uses of starch: sugars in processed foods, gluing agent, adhesive in paper-making.

Cellulose

Cellulose is a polysaccharide with the formula (C6H10O5)n and a polymer of -Glucose.

Is a straigh chain polymer:no coiling or branching. It belongs to the Alkyl group.

Cellulose molecules bind strongly to each other. Cellulose fibres have very high tensile

strength and are freely permeable, located in cell walls.

No taste, odorless and hydrophillic, biodegradable (though slow).

Insoluble in most organic solvents and water.

Can be broken down into its glucose units by treating with concentrated acids at high

temperature. Ruminants and termites can digest cellulose with the help ofsymbiotic

microorganisms in their guts.


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Humans can digest cellulose to some extent (is a hydrophilic bulking agent for feces) can

also be called dietary fiber.

Heated water and acetone can also break down cellulose strands.

Mainly obtained through wood pulp and cotton. Main uses of cellulose: producing paperboard

and paper, biofuel (experimenting).

Cellulolysis is a hydrolysis reaction whereby cellulose is broken down into cellodextrins or

completely into glucose units.

Cellulose consists of long parallel chains. In order to maintain its structure, adjacent molecules flip over. Thus

every second molecule of glucose is upside-down

Glycogen

A multibranched polysaccharide a form of energy storage in animals and fungi. In

humans, glycogen are stored primarily in the liver and muscles.

Has a linear(14) glycosidic bond and then branches to (16) glycosidic bondsbetween the first glucose of the new branch and a glucose on the stem chain.

Similar structure to amylopectin but has more extensive branches.

Glycogen is cleaved from the nonreducing ends of the chain by the enzyme glycogen

phosphorylase to produce monomers ofglucose-1-phosphate, which is then converted

to glucose 6-phosphate by phosphoglucomutase. A special debranching enzyme is

needed to remove the alpha(1-6) branches in branched glycogen and reshape the chain into

linear polymer.

Soluble in water and insoluble in alcohol. Also, its solubility ranges with temperature.


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Amylopectin

Polysaccharide, highly branched polymer of glucose. One of the 2 components of starch.

The branched structure renders amylopectin soluble in cold water, whereas amylose forms a

colloidal dispersion in hot water (helps with thickening).

Broken down by amylase. Is hydrolysed quicker than amylose due to the many end points

available for enzymes to attach onto. Has lower density.

Glucose units are linked in a linear way with (14) glycosidic bonds. Branching takes place

with (16) bonds occurring every 24 to 30 glucose units.

Galactose

A monosaccharide, less sweet than glucose a C-4 epimer (like and -glucose) of

glucose. Exists in both open chain and cyclic form.

Galactose is part ofLactose.

Converted into glucose by three principal enzymes called Leloir pathway.


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GALT( galactose-1-phosphate uridylyltransferase) gene gives instructions to make anenzyme called galactose-1-phosphate uridylyltransferase which helps process galactose.

Found in dairy products, sugar beetsIt is also synthesized by the body, where it forms part

ofglycolipids and glycoproteins in several tissues.

Soluble in water (683.0 g/L) and very slightly soluble in alcohol.

Lactose

A disaccharide sugar formed from galactose and glucose (eiher or -pyranose).

Lactose crystals have a characteristic tomahawk shape that can be observed with a light

microscope.

Has a formula ofC12H22O11. Broken down by lactase into its 2 subunits.

Found most notably in milk and added to pills as a filler because of its physical properties eg

compressibility and low price. Can be used to dilute heroin for similar reasons.

Soluble in water (21.6 g/100 mL) and very slightly in ethanol (0.0111 g at 40 C per 100 g).

structure and properties of biological molecules

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