Download 1.1 - cloudfront.net

Carbohydrates - Advanced
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both
in the U.S. and worldwide. Using an open-content, web-based
collaborative model termed the FlexBook®, CK-12 intends to
pioneer the generation and distribution of high-quality educational
content that will serve both as core text as well as provide an
adaptive environment for learning, powered through the FlexBook
Platform®.
Copyright © 2013 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/terms.
Printed: September 9, 2013
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Carbohydrates - Advanced
1
Carbohydrates - Advanced
• Describe the structure and function of carbohydrates.
Sugar. Does this look like biological energy?
As a child, you may have been told that sugar is bad for you. Well, that’s not exactly true. Essentially, carbohydrates
are made of sugar, from a single sugar molecule to thousands of sugar molecules all attached together. Why? One
reason is to store energy. But that does not mean you should eat it by the spoonful.
Carbohydrates
Carbohydrates are organic compounds that contain only carbon (C), hydrogen (H), and oxygen (O). They are the
most common of the four major types of organic compounds. There are thousands of different carbohydrates, but
they all consist of one or more smaller units called monosaccharides.
Monosaccharides and Disaccharides
The general formula for a monosaccharide is:
(CH2 O)n ,
where n can be any number greater than two. For example, if n is 6, then the formula can be written:
C6 H12 O6 .
This is the formula for the monosaccharide glucose. Another monosaccharide, fructose, has the same chemical
formula as glucose, but the atoms are arranged differently. Molecules with the same chemical formula but with
atoms in a different arrangement are called isomers. Compare the glucose and fructose molecules in Figure 1.1.
Can you identify their differences? The only differences are the positions of some of the atoms. These differences
affect the properties of the two monosaccharides.
1
www.ck12.org
Monosaccharides can be classified by the number of carbon atoms they contain: diose (2), triose (3), tetrose (4),
pentose (5), hexose (6), heptose (7), and so on.
In addition to glucose, other common monosaccharides include fructose ("fruit sugar"), galactose, xylose ("wood
sugar") and ribose (in RNA) and deoxyribose (in DNA).
If two monosaccharides bond together, they form a carbohydrate called a disaccharide. Two monosaccharides
will bond together through a dehydration reaction, in which a water molecule is lost. A dehydration reaction is a
condensation reaction, a chemical reaction in which two molecules combine to form one single molecule, losing a
small molecule in the process. In the dehydration reaction, this small molecule is water.
An example of a disaccharide is sucrose (table sugar), which consists of the monosaccharides glucose and fructose
(Figure 1.1). Other common disaccharides include lactose ("milk sugar") and maltose. Monosaccharides and
disaccharides are also called simple sugars. They provide the major source of energy to living cells.
FIGURE 1.1
Sucrose Molecule. This sucrose molecule
is a disaccharide. It is made up of two
monosaccharides: glucose on the left and
fructose on the right.
Sucrose forms
through a condensation reaction: glucose
(C6 H12 O6 ) + fructose (C6 H12 O6 ) sucrose
(C12 H22 O11 ).
Oligosaccharides
An oligosaccharide is a saccharide polymer containing a small number (typically two to ten) of monosaccharides.
Oligosaccharides can have many functions; for example, they are commonly found on the plasma membrane of
animal cells where they can play a role in cell–cell recognition. In general, they are found attached to compatible
amino acid side-chains in proteins or to lipids.
Oligosaccharides are often found as a component of glycoproteins or glycolipids. They are often used as chemical
markers on the outside of cells, often for cell recognition. An example is ABO blood type specificity. A and B
blood types have two different oligosaccharide glycolipids embedded in the cell membranes of the red blood cells,
AB-type blood has both, while O blood type has neither.
Polysaccharides
Polysaccharides are long carbohydrate molecules of repeated monomer units joined together by glycosidic bonds.
A polysaccharide may contain anywhere from a few monosaccharides to several thousand monosaccharides. Polysaccharides are also called complex carbohydrates. Polysaccharides have a general formula of Cx (H2O)y , where x is
usually a large number between 200 and 2500. Considering that the repeating units in the polymer backbone are
often six-carbon monosaccharides, the general formula can also be represented as (C6 H10 O5 )n , where 40≤n≤3000.
2
www.ck12.org
Concept 1. Carbohydrates - Advanced
Starches are one of the more common polysaccharides. Starch is made up of a mixture of amylose (15–20%) and
amylopectin (80–85%). Amylose consists of a linear chain of several hundred glucose molecules and amylopectin
is a branched molecule made of several thousand glucose units. Starches can be digested by hydrolysis reactions,
catalyzed by enzymes called amylases, which can break the glycosidic bonds. Humans and other animals have
amylases, so they can digest starches. Potato, rice, wheat, and maize are major sources of starch in the human diet.
The formations of starches are the ways that plants store glucose. Glycogen is sometimes referred to as animal
starch. Glycogen is used for long-term energy storage in animal cells. Glycogen is made primarily by the liver and
the muscles.
The main functions of polysaccharides are to store energy and form structural tissues. Examples of several other
polysaccharides and their roles are listed in the Table 1.1. These complex carbohydrates play important roles in
living organisms.
TABLE 1.1: Complex Carbohydrates
Complex Carbohydrate
Starch
Function
Stores energy
Organism
Plants
Amylose
Glycogen
Stores energy
Stores energy
Plants
Animals
Cellulose
Forms cell walls
Plants
Chitin
Forms an exoskeleton
Some animals
KQED: Biofuels: From Sugar to Energy
For years there’s been buzz – both positive and negative – about generating ethanol fuel from corn. But thanks
to recent developments, the Bay Area of California is rapidly becoming a world center for the next generation of
green fuel alternatives. The Joint BioEnergy Institute is developing methods to isolate biofeuls from the sugars in
3
www.ck12.org
cellulose. See Biofuels: Beyond Ethanol at http://www.kqed.org/quest/television/biofuels-beyond-ethanol for further
information.
MEDIA
Click image to the left for more content.
Vocabulary
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
amylase: An enzyme that catalyses the breakdown of starch into sugars; present in human saliva.
carbohydrate: Organic compound such as sugar or starch; major source of energy to living cells.
complex carbohydrate: Carbohydrates with three or more monosaccharides bonded together.
condensation reaction: A chemical reaction in which two molecules combine to form one single molecule,
together with the loss of a small molecule, often water.
disaccharide: A carbohydrate composed of two monosaccharides.
glycogen: A carbohydrate used for long-term energy storage in animal cells; human muscle and liver cells
store energy in this form.
glycolipid: A lipid with a carbohydrate attached; provides energy and serves as markers for cellular recognition.
glycoprotein: A protein that contain oligosaccharide chains (glycans) covalently attached to polypeptide sidechains; often important integral membrane proteins, where they play a role in cell–cell interactions.
glycosidic bond: A covalent bond that joins a carbohydrate molecule to another group, which may or may not
be another carbohydrate.
hydrolysis reaction: A chemical process in which a molecule of water is split, resulting in the separation of a
large molecule into two smaller molecules.
isomers: Molecules with the same chemical formula but with differently arranged atoms.
monosaccharide: A simple sugar such as glucose; the building block of carbohydrates.
oligosaccharide: A saccharide polymer containing a small number (typically two to ten) of monosaccharides.
polysaccharide: A chain of monosaccharides; a complex carbohydrate such as starch or glycogen.
starch: A carbohydrate used for long-term energy storage in plant cells.
Summary
• Carbohydrates are organic molecules that consist of carbon, hydrogen, and oxygen. They are made up of
repeating units called saccharides. They provide cells with energy, store energy, and form structural tissues.
Practice
Use this resource to answer the questions that follow.
• http://www.hippocampus.org/Biology → Biology for AP* → Search: Structure and Function of Polysaccharides
1.
2.
3.
4.
4
Describe a polysaccharide. How many monomers may make a polysaccharide?
How do polysaccharides form?
What determines the function of a polysaccharide?
Describe 3 properties of cellulose.
www.ck12.org
5.
6.
7.
8.
Concept 1. Carbohydrates - Advanced
What is chitin?
What is the main function of starch?
What is the main structural difference between starch and glycogen?
Compare the structures of glycogen and starch to cellulose and chitin.
Review
1.
2.
3.
4.
5.
What are carbohydrates?
State the function of monosaccharides, such as glucose and fructose.
Compare and contrast simple sugars and complex carbohydrates.
What are glycoproteins and glycolipids?
Give examples of polysaccharides.
References
1. User:Booyabazooka/Wikimedia Commons. . Public Domain
5