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B3  CARBOHYDRATES
B.3.4  List the major functions of carbohydrates in the human body.
Major functions of carbohydrates in the human body include:
 Immediate energy source (glucose)
 Short term energy reserves (glycogen)
 Precursors for other biologically important molecules
B.3.1  Describe the structural features of monosaccharides.
*Structural features of Monosaccharides:
-
Emperical Formula = CH2O
Contain a carbonyl group (C=O)
Contain at least two hydroxyl groups (–OH)
Between 3 and 6 carbons [e.g. C5H10O5  pentoses ; C6H12O6  hexoses]
Many structural/optical isomers possible
Ring and open chain structures possible
B.3.2  Draw the straight-chain and ring structural formulas of glucose and fructose.
[Students should be made aware of the structural difference between alpha and beta isomers.]
Alpha-fructose
Beta-fructose
B.3.3  Describe the condensation of monosaccharides to form disaccharides and polysaccharides.
Examples include:
- disaccharides  lactose, maltose and sucrose
- polysaccharides  starch (alpha-glucose), glycogen (alpha-glucose) and cellulose (beta-glucose).
B.3.5  Compare the structural properties of starch and cellulose, and explain why humans can digest
starch but not cellulose.]
[Both are polymers of glucose units. Starch has two forms: amylase, which is a straight-chain polymer
(alpha-1,4 linkage), and amylopectin, which is a branched structure with both alpha 1-4, and alpha 1,6
linkages. Cellulose has a beta 1,4 linkage; this can be hydrolysed by the enzyme cellulose, which is absent in
most animals, including mammals.]
Polysaccharide Role
amylose
(in starch)
Energy
reserve in
plants
α-glucose
amylopectin
(in starch)
glycogen
cellulose
Monomer
Energy
reserve in
animals
Structural
material in
plants
β-glucose
Structural Features
Properties
Straight chains
with α1-4
linkages
Branched chains
with α1-4
linkages and a
few α1-6
linkages
Straight chains
with β1-4
linkages
Chains coil
to give a
globular
structure
Slightly
soluble and
readily
hydrolyzed by
enzymes in
humans
Chains pack
side by side
to give fibres
Insoluble and
cannot be
hydrolyzed by
enzymes in
humans
Role in
Human
Nutrition
Source of
glucose for
energy
Dietary
fibre
B.3.6  State what is meant by the term dietary fibre.
[Dietary fibre is mainly plant material that is not hydrolysed by enzymes secreted by the human digestive
tract but may be digested by microflora in the gut. Examples include cellulose, hemicellulose, lignin and
pectin.]
B.3.7  Describe the importance of a diet high in dietary fibre.
[Aim 8: Dietary fibre may be helpful in the prevention of conditions such as diverticulosis, obesity, Crohn’s
disease, hemorrhoids and diabetes mellitus.]
Dietary Fibre  mainly plant material that is part of fruits, grains and vegetables that the human body cannot digest
due to the absence of the needed enzymes; however, may be digested by bacteria in the gut
Examples:
- Cellulose  plant cell walls are composed of multiple layers of cellulose; about 33% of all plant material; the
most common organic compound on earth; a polysaccharide
- Hemicelluloses  another component of cell walls; a polysaccharide
- Lignin  commonly derived from wood; provides mechanical support for stems and leaves; supplies the
strength and rigidity of cell walls; a biopolymer
- Pectin  major binding component of the cell walls of plants and fruits; a polysaccharide
Lignin
Importance of diet high in dietary fibre:
Type of Dietary Fibre
A. Water Insoluble
B. Water Soluble
Examples
- Cellulose
- Lignin
- Pectins
Sources
- whole grain foods
- fruits & vegetable
- whole grain foods
- fruits & vegetable
Function
- absorb water to provide bulk; move food through
the digestive system
- undergo fermentation in large intestine by
bacteria to produce short-chain fatty acids (e.g.
propanoic/butanoic acid)
- these fatty acids stabilize lipid and blood glucose
levels; may therefore help prevent diabetes
- they also stimulate the production of antibodies
and therefore provide immune protection
Dietary Fibre may also help prevent:
1. Irritable Bowel (Intestine)Syndrome(IBS)
 includes constipation, bloating, abdominal pain etc...all arise from malfunctioning bowels
(↑ dietary fibre ↓ these symptoms)
2. Obesity
 excess body mass leading to many, many health issues
(↑ fibre = feeling full on a diet with ↓ carbs and fats = ↓ weight)
3. Crohn’s Disease
 inflammatory bowel disease; cause unknown
(↑ dietary fibre may help in its prevention)
4. Hemorroids
 enlarged, weak, swollen blood vessels in and around the anus and rectum that can burst; can be caused by pressure
in the abs as a result of constipation
[↑ dietary fibre makes the bulk move through the large intestine more easily (i.e. prevents constipation)]
5. Constipation & Diverticulosis
 ↓ fibre = hard stool that does not pass easily = constipation
 ↑ pressure required to move stool causes diverticulosis; that is, bulges in the colon leading to pain