05 An Introduction to Carbohydrates

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2011 Pearson Education, Inc.

Lectures by Stephanie Scher Pandolfi

BIOLOGICAL SCIENCEFOURTH EDITION

SCOTT FREEMAN

5An Introduction to

Carbohydrates


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Key Concepts

Sugars and other carbohydrates are highly variable in structure.

Monosaccharides are monomers that polymerize to form polymers

called polysaccharides, and are joined by different types of

glycosidic linkages.

Carbohydrates perform a wide variety of functions in cells:

serving as raw material for synthesizing other molecules,

providing structural support, indicating cell identity, and storing

chemical energy.


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Monosaccharides Vary in Structure

Monosaccharide monomers are simple sugars that structurally varyin four primary ways:

1. Location of the carbonyl group

Aldose: found at the end of the monosaccharide

Ketose: found in the middle of the monosaccharide

2. Number of carbon atoms present

Triose: three

Pentose: five

Hexose: six

3. Spatial arrangement of their atoms

Different arrangement of the hydroxyl groups

4. Linear and alternative ring forms

Sugars tend to form ring structures in aqueous solutions.


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Summary of Monosaccharide Structure

Many distinct monosaccharides exist because so many aspects of

their structure are variable: aldose or ketose placement of the

carbonyl group, variation in carbon number, different

arrangements of hydroxyl groups in space, and alternative ring

forms. Each monosaccharide has a unique structure and function.



The Structure of Polysaccharides

Polysaccharides, orcomplex carbohydrates, are polymers of

monosaccharide monomers.

The simplest polysaccharides are disaccharides, comprised of two

monosaccharide monomers.

The monomers can be identical or different.

Simple sugars polymerize when a condensation reaction occurs

between two hydroxyl groups, resulting in a covalent bond called

a glycosidic linkage.



Glycosidic Linkages

The glycosidic linkages can form between any two hydroxyl

groups; thus, the location and geometry of these bonds vary widely.


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Types of Polysaccharides

1. Plants store sugar as starch.

Mixture of branched (amylopectin) and unbranched(amylose) -glucose polymer

2. Animals store sugar as glycogen.

Highly branched -glucose polymer

3. Cellulose is a structural polymer found in plant cell walls. Polymer of-glucose monomers

4. Chitin is a structural polymer found in fungi cell walls, some

algae, and many animal exoskeletons.

Comprised of N-acetylglucosamine (NAc) monomers

5. Bacterial cell walls get structural support from peptidoglycan.

Backbones of alternating monosaccharides


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How Do Carbohydrates Provide Structure?

Cellulose, chitin, and peptidoglycan form long strands with bonds

between adjacent strands.

These strands may then be organized into fibers or layered in sheets

to give cells and organisms great strength and elasticity.

Unlike the -glycosidic linkages in the storage polysaccharides, the

-1,4-glycosidic linkages of structural carbohydrates are very

difficult to hydrolyzevery few enzymes have active sites that

accommodate their geometry or have the reactive groups necessary.


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Carbohydrate Structure and Function

Web Activity: Carbohydrate Structure and Function
http://localhost/var/www/apps/conversion/tmp/scratch_2/CarbohydrateStructandFunc.html


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Carbohydrates and Chemical Evolution

Most monosaccharides are readily synthesized under conditions

that mimic early conditions; thus, it is likely that the prebiotic soup

contained a wide diversity of monosaccharides.

Polysaccharides, however, despite their current relative abundance

on Earth, probably played little to no role in the origin of life.

Monosaccharide polymerization requires specialized enzymes.

Polysaccharides do not catalyze any known reactions.

Polysaccharide monomers cannotprovide the information

required for themselves to be copied.


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What Do Carbohydrates Do?

Carbohydrates have diverse functions in cells: In addition to

serving as precursors to larger molecules, they provide fibrous

structural materials, indicate cell identity, and store chemical

energy.


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Glycoproteins: Cell Identity

Although polysaccharides are unable to store information, they do

display information on the outer surface of cells in the form of

glycoproteins

proteins joined to carbohydrates by covalent

bonds.

Glycoproteins are key molecules in cell-cell recognition and cell-

cell signaling.

Each cell in your body has glycoproteins on its surface that identify

it as part of your body.


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C S


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Carbohydrates and Energy Storage

Carbohydrates store and provide chemical energy in cells.

In chemical evolution, the kinetic energy of sunlight and heat were

converted into chemical energy stored in the bonds of H2CO and

HCN.

Today, most sugars are produced via photosynthesis, a key

process that transforms the energy of sunlight into the chemical

energy of CH bonds in carbohydrates.

Carbohydrates have more free energy than CO2

because the

electrons in CH bonds and CC bonds are shared more equally

and held less tightly than they are in CO bonds.


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St h d Gl A H d l d t R l Gl


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Starch and Glycogen Are Hydrolyzed to Release Glucose

The hydrolysis of-glycosidic linkages in glycogen is catalyzed by

the enzyme phosphorylase.

Most animal cells contain phosphorylase so they can readily

break down glycogen to provide glucose.

The -glycosidic linkages in starch are hydrolyzed by amylase

enzymes.

Amylases play a key role in carbohydrate digestion.

E St d i Gl I T f d t ATP


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Energy Stored in Glucose Is Transferred to ATP

When a cell needs energy, carbohydrates participate in exergonic

reactions that synthesize adenosine triphosphate (ATP):

CH2O + O2 + ADP + Pi CO2 + H2O + ATP

The free energy in ATP is used to drive endergonic reactions andperform cell work.

Carbohydrates contain a large number of CH bonds, which have

high free energy.

Fatty acids have even more CH bonds and consequently more free

energy than carbohydrates.

H D C b h d t St E ?


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How Do Carbohydrates Store Energy?

Starch and glycogen are efficient energy-storage molecules because

the -linkages are readily hydrolyzed, whereas the -linkages of

structural carbohydrates resist enzymatic degradation.

The enzymes amylase and phosphorylase catalyze the hydrolysis of

-glycosidic linkages in glycogen and starch, respectively. The

released glucose subunits can then be used in the production of

ATP.

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05 An Introduction to Carbohydrates