Transcript Slide 1
Friday, May 1, 2020
Unit 1: Enzymes and the digestive system Title: Carbohydrates – disaccharides and polysaccharides
Learning Objectives:
We are learning….
• How are monosaccharides linked to form disaccharides?
• How are alpha-glucose molecules linked to form starch?
• What is the test for non-reducing sugars?
• What is the test for starch?
Keywords: • disaccharide • polysaccharide • non-reducing sugar
Starter: Odd one out – which word is the odd one out?
1. hexose, pentose, sucrose, triose 2. carbon, nitrogen, hydrogen, oxygen 3. maltose, glucose, galactose, glyceraldehyde 4. lactose, galactose, monomer, monosaccharide
How do alpha-glucose monomers combine?
When two monomers or monosaccharides (single-sugars) combine a disaccharide (double-sugar) is produced.
This combination involves the loss of a single water molecule and is therefore a
condensation reaction
.
maltose The bond formed is known as a
glycosidic bond.
It is usually formed between carbon atom 1 of one monosaccharide and carbon atom 4 of the other (1,4 glycosidic bond).
Maltose is a disaccharide.
How do glucose molecules combine to form starch?
Starch is a polysaccharide which is found in most parts of the plant in the form of small granules.
It is a reserve food formed from any excess glucose produced during photosynthesis.
Alpha glucose monomers combine to form starch.
The alpha-glucose is usually arranged in a helix.
Amylose and amylopectin
Starch is made up of two glucose polymers – amylose amylopectin .
and Amylose makes up about 20% of starch and forms helices with mainly 1,4 glycosidic bonds.
Amylopectin has many 1,6 glycosidic bonds and is highly branched.
Why is starch so suitable for energy storage?
• Insoluble and does not need to draw water into the cells by osmosis (does not affect water potential of cells) • Insoluble so does not easily diffuse out of cells • Compact, so a lot can be stored in one place • When hydrolysed (split – enzymes) it forms alpha-glucose, which is easily transported and readily used during respiration.
Glycogen – ‘animal starch’
Glycogen is the major polysaccharide storage material of animals and fungi and is often called ‘animal starch’.
It is stored mainly in the liver and muscles.
Like starch, it is made up of alpha-glucose molecules and exists as granules in the cytoplasm of cells.
However, glycogen has shorter chains and is more highly branched than starch, meaning it can be hydrolysed even more quickly for a rapid supply of glucose fuel.
Cellulose
Cellulose typically comprises up to 50% of a plant cell wall and in cotton makes up 90%.
It is a polymer of around 10,000 beta-glucose molecules forming a long unbranched chain.
The chains run parallel to one another and have links between them (hydrogen bonds forming cross-bridges) Cellulose is also permeable to CO 2
Cellulose and hydrogen cross-bridges
Structural differences – starch and cellulose
Differences between cellulose and starch
The monomers which make up starch and cellulose provide a major difference in the structure and function of both polysaccharides.
In Beta-glucose, the –OH group is above, rather than below, the ring.
This means to form glycosidic links, each Beta-molecule must be rotated 180 degrees compared to its neighbour.
This means that the –CH 2 OH group on each Beta-glucose molecule alternates between being above and below the chain.
The chain is straight and unbranched, rather than coiled, increasing the strength of the cellulose molecule.
Bond cannot form unless molecule is rotated Bond can now form between OH groups (releasing water)
True Not Sure False
Polysaccharides are polymers
Starch is mainly found in liver and muscle cells
Long chains of starch provide structural support
Cellulose is a polymer of β glucose
This diagram shows β-glucose:
Chains of β-glucose are cross-linked to form microfibrils which form cellulose fibres
The diagram below shows the structure of starch
Glycogen is more branched than starch and has longer chains
Quick activity: Complete the gap fill ‘Biological molecules – carbohydrates’
What are non-reducing sugars?
A reducing sugar is a type of sugar that reduces certain chemicals through an oxidation reaction.
Reducing sugars donate electrons (which are negatively charged) and become oxidised . The other chemical which gains electrons is reduced. In Benedict’s reagent copper sulphate is reduced to copper sulphide.
Non-reducing sugars do not change the colour of Benedict’s reagent when they are heated with it.
In order to detect a non-reducing sugar it must first be broken down into its monosaccharide components by hydrolysis.
How can disaccharides be broken down into monosaccharides? (e.g. How do we get glucose from maltose?)
When water is added to a disaccharide under suitable conditions…… …….it breaks the glycosidic bond releasing the monosaccharides.
This is a
hydrolysis reaction
and is how many enzymes break up large molecules into smaller ones.
Hydrolysis reactions in the lab
To break the bonds between monosaccharides (e.g. glucose and fructose) in the lab we first use hydrochloric acid (HCl). This is the hydrolysis reaction (addition of water).
We then add sodium hydroxide to neutralise the acid and return the solution to pH 7.
To test for non-reducing sugars...
.
We then add Benedict’s reagent and once again
heat strongly
in a water bath.
What do I need to remember?
First of all we test for reducing sugars – 1. Dissolved food sample (sucrose) + Benedict’s reagent 2. Heated strongly in water bath 3. Blue = negative (not a reducing sugar) So, to see if it’s a non-reducing sugar.....
1. Dissolved food sample ( 4 cm 3 sucrose) + dilute HCl (2 cm 3 ) 2. Heat strongly in water bath (this hydrolyses bonds in disaccharide – forms monosaccharides) 3. Add sodium hydrogencarbonate to neutralise hydrochloric acid (Benedict’s won’t work in acidic conditions). Test with pH paper (pipette) for alkaline solution – green = neutral 7.0
4. Re test with Benedict’s (heat strongly again) 5. If non reducing sugar present in original sample, Benedict’s will turn from blue -green -yellow -orange /red.
6. No change in Benedict’s = there are no non-reducing sugars present (may be protein)
(ii) No change is observed if sucrose, a non-reducing sugar, is tested for in this way. The bond between the glucose and fructose units must first be broken. The test for a reducing sugar can then be carried out.
Describe how this bond can be broken chemically before carrying out the test for a reducing sugar.
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(add) hydrolytic enzyme / sucrase / invertase
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Plenary:
If I am the Answer, what is the Question?
Complete a question for each of the answers on the sheet
How successful were we this lesson?
Learning Objective