Biological Molecules

Carbohydrates, lipids and proteins

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Carbohydrates

Carbohydrates are made of monosaccharides which are single sugar molecules. Some examples are glucose, ribose, galactose and fructose (normally end in 'ose')

There are two types of glucose - alpha and beta. They are arranged only slightly different with H and OH molecules swapped around. So the H of alpha glucose is at the top whereas the H of the beta glucose is at the bottom. Ways of remembering this is Beta Bottom OR ay up (in a northerner accent)

Alpha molecules tend to be spiraled whereas beta molecules tend to be straight chains.

Monosaccarides join together to form larger carbohydrates by forming glycosidic bonds.

This is achieved by condensation a reaction that involves the loss of a water molecule to join two molecules together. The other type of reaction, hydrolysis, is the splitting of molecules by the addition of a water molecule. You can remeber this as hydro is water and condensation is formed when steam leaves a liquid.

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Benedicts Test

Reducing sugars

These are sugars that can donate electrons to other molecules, so this includes all monosaccharides and some disaccharides.

You add blue benedicts reagent to the solution and heat, if reducing sugars are present the solution should turn brick red due to the formation of a red precipitate.

Non-reducing sugars

You have to break them down into monosaccharides by boiling the solution with dilute hydrochloric acid (HCl) and then neutralising with sodium hydrogen carbonate. After you have done this you can then go on to do the reducing sugars test

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Carbohydrates

Polysaccharides are many sugars joined together.

Examples are:

Starch - The main storage material in plants, made up of two other polysaccharides of alpha-glucose:

Amylose which is a long, unbranded chain of alpha glucose, it has a coiled structure due to the angles of the glycosidic bonds.
This gives it a compact, coiled structure which makes it really good for storage

Amylopectin which is a long branched chain of alpha glucose.
It's side branches make it particularly good for the storage of glucose, the enzymes that break down the molecule can get at the glycosidic bonds easily, to break them and release the glucose.

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Carbohydrates

Glycogen

Similar structure to amylopectin except that it had loads more side branches.

It's compact and found in animal liver and muscle cells.

Lots of branches = stored glucose released quickly = important for energy release.

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Carbohydrates

Cellulose

Long unbranched chains of beta-glucose. The bonds between the sugars are straight therefore the chains are straight.

They are linked by hydrogen bonds to form strong fibres called microfibrils. The strong fibres mean cellulose has high tensile strength and can provide structural support for cells.

Also enzymes cannot break down cellulose.

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Test for Starch

Add iodine dissolved in potassium iodide solution to the test sample.

If starch is present then the test sample should turn blue-black

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Lipids

Most fats and oils are triglycerides which are composed of one molecule of glycerol and three fatty acids attached to it.

The triglyceride's (fatty acids) have long hydrocarbon chains.

These comprise of hydrophobic 'tails' that repel and are insoluble in water.

When they are put in water, fat and oil molecules clump together in globules to reduce the surface area in contact with water.

Triglycerides are formed by condensation reactions by three fatty acid joining to a glycerol molecule, forming an ester bond (o-c=o)

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Phospholipids

Found in cell membranes

In a phospholipid a phosphate group replaces one of the fatty acid molecules.

It is ionised to attract water molecules

so it had both a hydrophilic (water loving) and hydrophobic (water hating) side.

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