Brother Gregory speaks to his class,

The subject for today is the structure and function of carbohydrates and polysaccharides. You must follow the lesson, answer the questions, then complete the research investigation, if required.

"Let us begin .......

"Carbohydrates and polysaccharides are one of the four major groups of macromolecules that are found in all living organisms. These molecules carry out many of the vital functions needed by cells.

"Carbohydrate are involved in many cellular processes as diverse as cell identification, movement, energy storage and much more. They are complex molecular structures that appear in a variety of forms from simple saccharides (sugars) to large, energy containing polymers which can also be used as versatile building materials.

"Getting high value food into our mouths and thus into our bodies helps our survival. One of the most valued type of organic molecule in food is carbohydrate. Our tongues contain specialized receptors that fire off signals of pleasure when we put sweet, carbohydrate containing foods into our mouths.

"Sweet foods give us pleasure and so we search out and consume more of them than almost any other kind. Carbohydrates, with their high energy content, give us the fuel we need to drive our bodies.

What are you made of?

Find out how much carbon
is in your body - right now!

"One of the main types of atoms (elements) found in the molecular structures of all carbohydrates is carbon. These atoms have had a very long journey. All the carbon atoms in the human body were created in dying stars.

"Today, pure, elemental carbon is found widely on our planet and can be found in 3-5 allotropic forms, depending on how you count them.

"Allotropy is when an element (type of stable atom) exists in two or more variations. The constituent atoms vary in either the way they are arranged into solids (crystals) or in the number of atoms found in basic molecular arrangement(s).

As you proceed through this lesson, test yourself as you go by answering some of these "true/false" questions.

Where it all starts

"The formation of carbohydrates starts with the conversion of 'light energy' into 'chemical energy' (that stored in the covalent bonds of linked atoms). Specialized solar receptors in plant cells trap the light, convert it to 'electron energy' and then finally to bond energy as carbon, hydrogen and oxygen atoms are linked together to form complex, organic molecules.

"One of the first stable organic molecules created in this way is carbohydrate. This is a very abundant type of organic molecule found in one form or another in all living organisms. They perform a variety of roles, but one very important function is to act as an "energy reserve", for fuel, for the cell or creature.

"All carbohydrate molecules, large or small, have one of two different chemical reactive groups as part of their structure. Aldehydes and Ketones are molecules of carbon, oxygen and hydrogen that contain a reactive group called a carbonyl group, typically written C=O.

"The carbon atom in a carbonyl group can still form two additional covalent bonds. If this carbon atoms is joined with two other carbon atoms then the compound formed is a ketone, but if this carbon is joined with a hydrogen, then the compound is an aldehyde.

"Without doubt, the most common and abundant carbohydrate is glucose, which is manufatured, stored and then used by all plants. It is also consumed, stored and used by many animals as a source of carbon and energy.

"Glucose is a monosaccharide ("single sugar") that contains carbon, hydrogen and oxygen in amounts that conform to the general carbohydrate formula CnH2nOn, but this does not tell us much about its chemical, physical and structural properties.

" These were worked out by subjecting glucose to a number of chemical reagents, observing what happened, and then deducing something about the original glucose molecule from the changes that were seen taking place.

"There are two different forms of the glucose molecule which crystallized from two different versions of the molecule. Both have the same chemical properties, but the only way to explain their other differences is to draw the molecule in the form of a ring.

"The arrangement of the atoms attached to the first carbon atom (C1) can take two different configurations. This explains why the two forms of glucose each have a different way of rotating polarized light.

"Glucose is a good example of an organic molecule that has one, simple, molecular formula (C6H12O6) but can be shown to have multiple different structural formulae.

The Shape of Glucose - a ring and a boat

"The German chemist Emil Fischer showed that a molecule such as glucose could have the same atoms connected with the same bonds, to the same neighboring atoms, and yet have very different physical, biological and chemical properties depending on how the bonds and atoms were arranged in three-dimensional space.

"These different versions of the same molecule are called stereoisomers.

"Chiral objects have the property of "handedness", or two versions that are not identical in all respects, even though they may be designed to do the same job or basically have the same properties (like the golf clubs).

"A test of a chiral object is to see if you can superimpose the original object on its mirror image. If you cannot superimpose the object on its mirror image, it is a chiral object (like a hand or a shoe).

"Glucose molecules form rings. The first carbon atom (C1), which is an aldehyde group (-CHO), joins with the fifth carbon atom (C5) to make a 6-membered-ring (termed a pyranose). The atoms in this cyclic molecule then arrange themselves in space to minimize the amount of strain on each of the covalent bonds.

"In this shape, the glucose molecule is at its most stable when it is arranged into a chair shape or conformation.

Polysaccharides - joining glucose together

"Glucose is a reactive molecule. In the presence of acid it will combine with any other molecule that has an alcohol (-OH) group as part of its structure. The resulting, larger, molecule is called a glycoside.

"Monosaccharide molecules have a number of alcohol (-OH) reactive groups as part of their structure, so it possible for two monosaccharide molecules to react in the way shown above so as to form a "double-saccharide" or disaccharide.

"Polysaccharides are very large, high molecular weight biological molecules that are almost pure carbohydrate.

"They are constructed by animals and plants from simpler, monosaccharide molecules, by joining together large numbers of the simpler molecules using glycosidic bonds (-O-). In some of the largest polysaccarhide structures there can be 10,000 individual units joined together.

"There is a large diversity of polysaccharide forms; they can differ in the type of sugar, the connections between the sugars and the complexity of the overall molecule.

"There are three common and principal types of polysaccharide, cellulose, starch and glycogen, all made by joining together molecules of glucose in different ways.

"Cellulose is a polymer of glucose monosaccharides that plants use as their primary building material. Threads of cellulose are bound by hydrogen bonds into bundles of great strength and flexibility. These are used by plants to surround each cell in a way that protects them from the effects of osmosis and also gives them shape and form.

"Starch is a polymer of the alternate anomer of glucose and is used by plants as a way of storing glucose. It is a major reserve of energy that can be quickly mobilized as necessary.

Biological Role

"Life on this planet needs a constant supply of energy in order to fight the effects of entropy and the second law of thermodynamics. A tiny part of the sun's energy reaching this planet is absorbed by plants and converted from light energy into chemical energy. This is the process called photosynthesis.

"Two important molecular products are produced in this process; oxygen, which is released into the atmosphere, and 3-phosphoglyceric acid, which is kept inside the cells. Monosaccharide sugars are made by combining and recombining all those carbon atoms first trapped as 3PG. The most abundant and versatile of these monosaccharides is glucose.

"Although plant and animal cells make a large number of different polysaccharides, for all kinds of roles, the dominant ones are those made from glucose. Cellulose is a polymer of glucose monosaccharides that plants use as their primary building material. Starch is a polymer of the alternate anomer of glucose and is used by plants as a way of storing glucose. It is a major reserve of energy that can be quickly mobilized as necessary.

"Mono-. Di-, and Polysaccharides have a large and diverse set of properties and serve many different functions in plants, animals and single-celled organisms. They are very versatile molecules."

Research investigation
check your schedule to see if this is required
The Curse of Amun
Carbohydrate research Investigation
Concept questions
for the lesson

check your schedule to see if this is required
Carbohydrate Set
Concept Questions and personal question page
Required Readings
for the lesson
Table of Contents -|- Sweetness -|- Carbon Creation

Forms of Carbon -|- Allotropes -|- Types of Carbon

Carbohydrates -|- Aldehydes -|- Glucose

Anomers -|- Stereoisomers -|- Symmetry

Rings -|- Disacharides -|- Polysaccharides

Biological role -|_ Polarized light

Science@a Distance
© 2004, Professor John Blamire