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Biology: Molecules Of Life

1. Carbohydrates

2. Lipids

3. Proteins

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1. Carbohydrates

* Includes sugars and polymers of sugars.

* Monosaccharides=single sugars, they are a carbon ring with hydrogen and oxygen.

Glucose is a monosaccharide. When it forms a ring, the hydroxyl group attached to carbon #1 is positioned either below or above. Below is called alpha glucose, above is called beta.

* Disaccharides=double sugars, two monosaccharides linked together with a glycosidic bond from the process "condensation". (e.g. Lactose is a disaccharide formed from glucose molecule joined to a galactose molecule)

* Polysaccharides=more than 3 monosaccharides linked together. Polysaccharides are macromolecules. Some of them serve as storage material. When needed, they are hydrolyzed to provide sugar for cells, using process "hydrolysis".

Starch is a storage polysaccharide, consisting only of glucose molecules, joined by 1-4 linkages (number 1 carbon and number 4 carbon). Plants store starch as granules in cells, these granules are called plastids.

Animals store glycogen, a polymer of glucose.

Amylopectin is branched with 1-6 linkages at branched points, but less branched than glycogen.

Chitin is another important polysaccharide, used in arthropods in their exoskeletons. It's similar to cellulose, but the beta glucose of chitin has nitrogen.

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2. Lipids

Lipids do NOT consist of polymers, but are large biological molecules.

They are grouped together because they are hydrophobic.

Consists mainly hydrocarbons, may have some oxygens causing polar bonds.

* Fats

Are NOT polymers, but are large.

Fatty acids molecules join to glycerol with an ester bond--hydroxyl-carboxyl bond. They are also called triglycerides.

Contains 3 fatty acids linked to a glycerol molecule.

Saturated fatty acid=fatty acids with no double bonds.

Unsaturated fatty acid=fatty acids w/ one or more bonds--resulting in "kinked" tail if contain cis-double bond.

* Phospholipids

Has only 2 fatty acids-glycerol, instead of 3. The remaining joins to a phosphate group, negatively charged.

Variety of phospholipids can be formed by linking small molecules (often polar).

Phospholipids are ambivalent to water--having hydrophilic phosphate head (has affinity to water), and hydrophobic tail.

So they form bilayers, shielding hydrophobic parts from water.

* Steroids

Are lipids with carbon skeleton with 4 fused rings.

They differ in functional groups attached to the ensemble rings.

Cholesterol is crucial for animals, but too much can cause atherosclerosis.

Saturated fats and trans fats are bad for health by affecting cholesterol levels.

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3. Proteins

* Polypeptides

Are made of amino acids.

Amino acids has a center asymmetric carbon (alpha carbon), with four different things attached to it--1 amino group, 1 carboxyl group, 1 R group (which varies, from one single hydrogen atom to enormous carbon skeleton with different functional groups attached. The chemical and physical properties of R group determines the unique characteristics of a particular amino acid.)

Amino acids link together by peptide bonds to form polypeptides.

Amino and carboxyl groups are showed in ionized form--what they exist as at pH7 (cell conditions)--these are called zwitter ions.

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 (c) copyright Susanna Shiu & Queenie Liew, since September 2007

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