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9.7 The Biochemistry of Movement: 2. Carbohydrates
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2. Carbohydrates are an important part of
an athlete's diet
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Students learn to:
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Students: |
Extract from Chemistry Stage 6 Syllabus (Amended
October 2002) © Board of Studies, NSW.
[Edit: 7 Jul 09]
Prior Learning: Preliminary modules
8.4.5, 8.5.4 and 8.5.2.
HSC module 9.2.2.
choose resources and perform first-hand investigations to compare the structures of glycogen and glucose from diagrams or models
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Choose information from books, CD-ROMs
or the internet to find out different representations of
the structure of the glucose molecule. The information
needed is the elements present, the molecular formula and
the structural formula. Any numbering system for the C
atoms should be noted.
- You may need to ask your teacher to help with the convention of drawing sugar molecules. The C atoms at the corners in the ring are often omitted and hydrogen atoms may also be omitted, their positions indicated by lines.
An extension activity
Analyse the structure of glucose and try to predict its
solubility from its structure. The presence of OH
(hydroxyl) groups and their hydrogen bonding potential may
need to be considered. Is glucose soluble because of
hydrogen bonding between the glucose molecules and water
molecules?
- Perform first hand investigations to
model a glucose molecule, choosing appropriate resources to
represent each atom. Using molecular model kits is an easy
way of doing it.
- Keeping the glucose models until later, research and analyse the way the glucose molecules are put together to form glycogen molecules. This can be modelled initially using small plasticine balls to represent glucose units and toothpicks to join them together. Alternately, ball and stick models can be made with each ball one glucose molecule. Note the many branches in glycogen, about one in every ten glucose molecules, making an open structure with many ends.
Extension
Discuss whether the open structure, with OH groups readily
accessible to water and hydrogen bonding, affects
solubility of glycogen.
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Construct a modelof glycogen. A class can use all of their glucose models and join them together at C1 and C4 OR at C1 and C6 positions, eliminating a molecule of water each time a bond is formed. C1 to C6 links should occur as a branch point on an average of about every tenth glucose unit that is joined C1 to C4. The C atoms are joined by oxygen atoms (-C-O-C-).
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Record a section of the structure of glycogen in a diagram and compare it to glucose by writing down similarities and differences between glucose and glycogen. The elements present are the same, both have OH groups which can H-bond with water and both are soluble. Would you expect glucose or glycogen to have the greater solubility in water?
For your information
Three-quarters of the glycogen in a human body is in
muscles. About 1% of a muscle cell is glycogen.
Approximately half of your body weight could be muscle. You
might be able to use this information to estimate the mass
of glycogen in your body.
identify glucose as the monomer which forms the polymer glycogen and describe the process of bond formation between the glucose molecules which produces the polymer
From your model, identify that glycogen is a condensation polymer formed from glucose monomers.
Describe the reaction between two glucose monomers as between two hydroxyl groups leading to the condensation of a water molecule out of the structure.
An oxygen atom joins each monomer.
The bonds joining glucose monomers are called either 1,4 glycosidic links or 1,6 glycosidic links. Can you work out why there are two sorts of links?
A glycogen polymer consists of thousands of glucose monomers joined to form an open structure with many ends, allowing for removal of many monomer units at a time from the many ends when required. Enzymes can work at each of the ends simultaneously, releasing one glucose unit from each end. This allows for a rapid supply of glucose when required.
identify
that carbohydrates are composed of carbon, hydrogen and
oxygen according to the formula:
Cx
(H2O)y
- From the glucose formula (C6H12O6), you can identify that glucose contains the elements C, H and O and the proportion is C:H:O 1:2:1. This would give a general formula Cx(H2O) x. However when glucose units are joined and a water is eliminated, the proportion of H2O decreases. Therefore the general formula for all carbohydrates is Cx(H2O)y. In the case of glucose x=y, why doesn’t x=y in glycogen?
explain that humans store carbohydrates as glycogen granules in our muscles and liver
- Glycogen is the carbohydrate store in animals. (Compare to starch as the storage form in plants).
- After a meal, the rising blood glucose level is controlled by the excess glucose being taken into liver and muscle cells and converted by enzymes to glycogen. The glycogen forms granules in the cytoplasm of these cells.
- Between meals, the blood glucose levels are constantly replenished from the liver glycogen stores so that there is a reasonably constant level of glucose in the blood. The liver stores do not usually last much longer than about 12 hours.
Further information
Muscle glycogen is used within the muscle cells (called
muscle fibres) during exercise. As exercise demands energy,
glycogen is broken down to supply glucose for respiration.
The branched structure of glycogen allows the molecule to
grow readily at each chain end or to be broken down to
glucose quickly by simultaneous removal of glucose from
each of the branched ends. Thus the structure is suited to
the function of the molecule.
Use a role play or a model to compare the speed of removal
of the units from a straight chain as against a branched
chain.
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