24.2 carbohydrates > 1 copyright © pearson education, inc., or its affiliates. all rights...

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved..

24.2 Carbohydrates 24.2 Carbohydrates >>

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Chapter 24The Chemistry of Life

24.1 A Basis for Life

24.2 Carbohydrates

24.3 Amino Acids and Their Polymers24.4 Lipids24.5 Nucleic Acids24.6 Metabolism



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A cow’s diet is rich in cellulose, which belongs to a class of organic molecules known as carbohydrates.

CHEMISTRY & YOUCHEMISTRY & YOU

Why does a cow chew all day?



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Classifying Carbohydrates

What is the general formula of carbohydrates?

Classifying CarbohydratesClassifying Carbohydrates



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Long-distance runners often prepare for a race by eating a great deal of bread and pasta, a process called carbohydrate loading.




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• Breads and pastas are excellent sources of the family of important molecules called carbohydrates.



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• Breads and pastas are excellent sources of the family of important molecules called carbohydrates.

• Carbohydrates are monomers and polymers of aldehydes and ketones that have numerous hydroxyl groups attached; they are made up of carbon, hydrogen, and oxygen.



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Most carbohydrates have the general formula Cn(H2O)n.




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• The name carbohydrate comes from the early observation that because of the formula, Cn(H2O)n, the compounds appear to be hydrates of carbon.

• But carbohydrates are not true hydrates.



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• They are found in most foods, including fruits, breads, pastas, and legumes.

• Carbohydrates are also in many sweets, such as cookies and pies.

Carbohydrates are the main source of energy for the body.



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The simplest carbohydrate molecules are called simple sugars, or monosaccharides.

Monosaccharides


• Glucose and fructose are examples of simple sugars.



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Glucose is abundant in plants and animals.

Monosaccharides


• Glucose is the primary source of energy for our bodies.

• Depending on the source, glucose has also been called corn sugar, grape sugar, or blood sugar.



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Fructose occurs in a large number of fruits and in honey.

Monosaccharides


• Glucose and fructose both have the molecular formula C6H12O6.

• However, glucose has an aldehyde functional group, whereas fructose has a ketone functional group.



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Glucose and fructose are constitutional isomers.

Monosaccharides


• Both undergo many of the same reactions as ordinary aldehydes and ketones.



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In aqueous solution, simple sugars such as glucose and fructose exist in a dynamic equilibrium between straight-chain and cyclic forms.

Monosaccharides


• The cyclic form predominates.



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The structures for each sugar in both forms are below.

Monosaccharides


Straight-chain and cyclic forms of glucose Straight-chain and cyclic forms of fructose



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Monosaccharides


O

Note the aldehyde functional group (—CHO) on the straight-chain form of glucose and the ketone functional group (—C—) on the straight-chain form of fructose.

Straight-chain and cyclic forms of glucose Straight-chain and cyclic forms of fructose



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Simple sugars form the building blocks of more complex carbohydrates.

Disaccharides


• The cyclic forms of two simple sugars can be linked by means of a condensation reaction.



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The linking of glucose and fructose with the loss of a water molecule produces sucrose—common table sugar.


Glucose+

Fructose Sucrose

–H2O

Disaccharides



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Sugarcane plants are a major source of sucrose.

Disaccharides


• A sugar such as sucrose that forms from the condensation of two monosaccharides is known as a disaccharide.



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The formation of a disaccharide is sometimes the first step in a condensation polymerization reaction that produces extremely large molecules.

Polysaccharides


• The polymers produced by the linkage of many monosaccharide monomers are called polysaccharides.



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Starches, the major storage form of glucose in plants, are polysaccharide polymers that consist of glucose monomers.


Polysaccharides



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A typical linear starch molecule contains hundreds of glucose monomers.


• Other starches are branched molecules, each branch containing about a dozen glucose units.

Polysaccharides



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Glycogen, the energy source stored in the liver and muscles of cells of animals, is more highly branched than plant starches.


• Glycogen, too, consists of glucose monomers.

Polysaccharides



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Cellulose is probably the most abundant biological molecule on Earth.


• Cellulose is also a polymer of glucose.

Polysaccharides



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The orientation of the bond that links the glucose monomers in cellulose is different from the bond orientation in glycogen.


Polysaccharides

• Starch can be digested by most animals and is partially soluble in water.

• Cellulose, however, can be digested by only a few kinds of microorganisms, such as those that live in the digestive tracts of cattle and termites.



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Cellulose is insoluble in water and is an important structural polysaccharide that provides form, hardness, and rigidity in plants.


Polysaccharides

• Plant cell walls are made of cellulose. • Cotton is 80 percent cellulose.



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A cow’s diet consists mostly of cellulose. A cow’s stomach contains several parts. Food enters the first part of the stomach and is then regurgitated so the cow can chew it again. The regurgitated food then goes to a later part of the stomach where special bacteria live. Why is chewing and regurgitating necessary for a cow to digest cellulose?




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Why is chewing and regurgitating necessary for a cow to digest cellulose?

Cellulose is hard to break down. It is digested by the bacteria that live in part of a cow’s stomach. Chewing, regurgitating, and chewing again breaks down the plants cows eat into small particles with lots of exposed surface area that can be digested by the bacteria.




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What are some common examples of the three classes of carbohydrates discussed in this lesson?



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What are some common examples of the three classes of carbohydrates discussed in this lesson?

Glucose and fructose are common monosaccharides. Sucrose is a common disaccharide. Starch and cellulose are common polysaccharides.



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Key ConceptsKey Concepts



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• carbohydrate: the name given to monomers and polymers of aldehydes and ketones that have numerous hydroxyl groups; sugars and starches are carbohydrates

• monosaccharide: a carbohydrate consisting of one sugar unit; also called a simple sugar

Glossary TermsGlossary Terms



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• disaccharide: a carbohydrate formed from two monosaccharide units; common table sugar (sucrose) is a disaccharide

• polysaccharide: a complex carbohydrate polymer formed by the linkage of many monosaccharide monomers; starch, glycogen, and cellulose are polysaccharides

Glossary TermsGlossary Terms



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Most carbohydrates are polymers that release energy when broken down.

BIG IDEABIG IDEA

Chemistry as the Central Science



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END OF 24.2END OF 24.2

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