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Biochemistry. Carbohydrates. Carbohydrates. Literally, it means “carbon water”, and all carbohydrates have the general formula C n (H 2 O) m —where n and m can be the same or nearly so

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Literally, it means “carbon water”, and all carbohydrates have the general formula Cn(H2O)m—where n and m can be the same or nearly so

Carbohydrates are made by plants, where 75% of the solid plant is carbohydrate. They are involved in both the structure and energy storage for the plant

Though not a major part of animal tissue, they are very important in the structure of the exoskeletons of crustaceans and other arthropods

classes of carbohydrates
Classes of Carbohydrates

Monosaccharides—simple sugars, cannot be hydrolyzed

Disaccharides—composed of two monosaccharides

Polysaccharides—made of many, up to 3000, monosaccharides

Monosaccharides are further classed by how many carbons: 3=triose, 4=tetrose, 5=pentose, 6=hexose

They are also classed by the type of carbonyl group in the molecule: aldehyde=aldose ketone=ketose

Examples: ribose is an aldopentose, fructose is a ketohexose


Properties: colorless crystalline solids highly water soluble with a sweet taste

Important monosaccharides: glucose, fructose, galactose, ribose, deoxyribose

Glucose is the most common hexose, an aldose found in nature, primarily in fruit juices, and is the immediate source of energy within cells

Glucose is a main component of many di- and polysaccharides


Structure: straight chain, not common in nature

Ring forms—two variations, linkage is an acetal between 1st and 5th carbons

Short form:

The #1 carbon makes the difference in the forms

optical isomerism
Optical Isomerism

Biochemical exhibit an additional type of isomerism, the twisting of light due to differences in the molecule

A carbon with four different atoms attached to it will have chirality, that is it cannot be superimposed upon its mirror image, just like a left hand glove cannot go onto your right hand

optical isomerism7
Optical Isomerism

Chirally different molecules affect polarized light by bending it in opposite directions, either to the right (dextrorotatory) or left (levorotatory)

Living organisms will only use one or the other optical isomer due to enzyme matching…example Splenda

other monosaccharides
Other Monosaccharides

Fructose—a ketose, very sweet, found in honey

straight form:

ring form, a hemiketal

Galactose—aldohexose linked with glucose in lactose, very similar to glucose except for the #4 carbon


The most important 5 carbon sugars are ribose and deoxyribose, found in the nucleic acids RNA and DNA respectively

ribose: deoxyribose:



reducing sugars
Reducing Sugars

If a sugar contains a free or potentially free aldehyde or ketone group, it will react with Cu+2 (Benedict’s) or Ag+ (Tollen’s) reagents. Positive results are brick red or black, depending on the reagent

These tests can be used to detect sugar in urine

homework 15a
Homework 15a

p. 389 CYU all

p. 398ff 2, 3, 7, 11, 13


Maltose: malt sugar, a combination of two a-glucose molecules by condensation

Lactose—milk sugar, combination of glucose and galactose by a b-1,4 linkage


Sucrose—table or cane sugar, a combination of glucose and fructose by an a-1,4 linkage. Since both the aldehyde and ketone of the two sugars are involved in the linkage, sucrose is not a reducing sugar


Starch—very long chain/branched chain structure with molar mass between 150,000 and 6 million. It is connected like maltose, a-1,4 .

Starch is the storage form of glucose used by plants, and is one of the primary sources of glucose in our diet. It can be converted to dextrins (like the browning of toast) by heat

tests for carbohydrates
Tests for Carbohydrates

Starch can be detected by the iodine test, since it will make a blu-black complex with starch. Dextrins will cause a reddish color, but monosaccharides will give no reaction

The Molisch test will detect any kind of carbohydrate. The positive is a red-violet ring

The Seliwanoff test will differentiate between aldoses and ketoses. Ketoses give a red color and aldoses give a pink color.


Glycogen—the storage form of glucose in animals and man, similar to starch, but heavily branched in a 1,6-linkage

Your body stores glycogen in the liver for release of glucose from it when needed


Cellulose—similar to starch in that it is made entirely of glucose, but instead of the a-form, the b-form is 1,4 linked in what looks like an up-down sequence:

This makes a long fibrous molecule; very strong and rigid; this makes up the strength of plants.

Cellulose is the wrong shape to be digested by our digestive system, so it makes up fiber. Some microorganisms in ruminant animals can digest it so that those animals can get glucose by that process.


The energy stored in carbohydrates comes from the sun by way of photosynthesis. It is a very complex series of reactions which builds the monosaccharide glucose from water and carbon dioxide:

This reaction is part of the carbon cycle by which carbon is reused throughout the environment

homework 15b
Homework 15b

p. 397 CYU all

p. 400ff 25, 26, 27, 29, 30, 33, 34