Carbohydrates Flashcards
Give examples of Monomers
monosaccharides,
amino acids
and
nucleotides
What is a saccharide?
In carbohydrates, the basic monomer unit is a sugar, otherwise known as a saccharide.
What is a monosaccharide?
A single monomer is called a monosaccharide
Monosaccharides are sweet-tasting, soluble substances that have the general formula (CHO), where n can be any number from three to seven.
What is a disaccharide?
A pair of monosaccharides can be combined to form a disaccharide.
How can Polysaccharides be made?
Monosaccharides can also be combined in much larger numbers to form polysaccharides.
Give examples of monosaccharides
glucose, galactose and fructose
What is glucose?
Glucose is a hexose (6-carbon) sugar and has the formula C6H12O6.
glucose has two isomers - a-glucose and ẞ-glucose.
Give examples of reducing sugars
All monosaccharides and some disaccharides (e.g., maltose) are reducing sugars
What is reduction (in terms of electrons), and what does it mean in the context of reducing sugar?
(so what is a reducing sugar?)
Reduction is a chemical reaction involving the gain of electrons or hydrogen.
A reducing sugar is therefore a sugar that can donate electrons to (or reduce) another chemical, in this case Benedict’s reagent.
The test for a reducing sugar is therefore known as the Benedict’s test.
What is Benedicts reagent? (the chemical)
Benedict’s reagent is an alkaline solution of copper(II) sulfate
What happens whena reducing sugar is heated with Benedict’s reagent?
When a reducing sugar is heated with Benedict’s reagent it forms an insoluble red precipitate of copper(I) oxide
Describe the process of testing for reducing sugars.
- Add 2 cm3 of the food sample to be tested to a test tube. If the sample is not already in liquid form, first grind it up in water.
- Add an equal volume of Benedict’s reagent.
- Heat the mixture in a gently boiling water bath for five minutes.
Maltose
Glucose + Glucose
Sucrose
Glucose + Fructose
Lactose
Glucose + Galactose
Give 3 examples of Disaccharides
Maltose
Sucrose
Lactose
What happens when monosaccharides join?
A molecule of water is removed, which is called condensation reaction.
The bond that is formed is called glycosidic bond
What does hydrolysis do to a disaccharide?
When water is added, the glycosidic bond breaks releasing the constituent monosaccharides
What is hydrolysis?
Additional of water that causes a breakdown
What are polysaccharides?
Polymers formed by combining many monosaccharides.
These are joined by glycosidic bonds which were formed in condensation reactions.
Why are polysaccharides suitable for storage?
Polysaccharides are very large molecules, which makes them insoluble.
This feature makes them suitable for storage.
What happens when polysaccharides are hydrolysed?
They break down into disaccharides or monosaccharides
Name a polysaccharide that is not used to storage but another function.
Hint: it’s in plants
Cellulose - to give structural strength/support in plant cells
What is starch?
A polysaccharide, found in the form of starch granules/grains
Eg starch grains in chloroplasts
It formed by joining 200 - 100,000 a-glucose molecules by glycosidic bonds in a series of condensation reactions
Some disaccharides (e.g. maltose) are reducing sugars. To detect these we use the Benedict’s test.
Other disaccharides, such as sucrose, are known as non-reducing sugars because they 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 hydrolysed into its monosaccharide components by hydrolysis.
Describe this process.
- If the sample is not already in liquid form, it must first be ground up in water.
- Add 2 cm3 of the food sample being tested to 2 cm3 of Benedict’s reagent in a test tube and filter.
- Place the test tube in a gently boiling water bath for 5 minutes. If the Benedict’s reagent does not change colour (the solution remains blue), then a reducing sugar is not present.
- Add another 2 cm3 of the food sample to 2 cm3 of dilute hydrochloric acid in a test tube and place the test tube in a gently boiling water bath for five minutes. The dilute hydrochloric acid will hydrolyse any disaccharide present into its constituent monosaccharides.
-Slowly add some sodium hydrogencarbonate solution to the test tube in order to neutralise the hydrochloric acid. (Benedict’s reagent will not work in acidic conditions.) Test with pH paper to check that the solution is alkaline.
- Re-test the resulting solution by heating it with 2 cm3 of Benedict’s reagent in a gently boiling water bath for five minutes.
- If a non-reducing sugar was present in the original sample, the Benedict’s reagent will now turn orange-brown. This is due to the reducing sugars that were produced from the hydrolysis of the non-reducing sugar.
Starch is easily detected by its ability to change the colour of the iodine
in potassium iodide solution from yellow to blue-black.
Explain the test for starch.
The test is carried out at room temperature.
Steps:
- Place 2 cm3 of the sample being tested into a test tube (or add two drops of the sample into a depression on a spotting tile).
- Add two drops of iodine solution and shake or stir.
- The presence of starch is indicated by a blue-black coloration.
Where does starch occur mainly and why is it important?
Starch is a polysaccharide that is found in many parts of a plant in the form of small grains.
Especially large amounts occur in seeds and storage organs, such as potato tubers.
It forms an important component of food and is the major energy source in most diets.
Starch is never found in animal cells.
Describe the structure of the polysaccharide Starch.
Starch is made up of chains of a-glucose monosaccharides linked by glycosidic bonds that are formed by condensation reactions.
The chains may be branched or unbranched.
The unbranched chain is wound into a tight coil that makes the molecule very compact.
The main role of starch is energy storage.
Describe how starch is structure is suited for this.
- it is insoluble and therefore doesn’t affect water potential, so water is not drawn into the cells by osmosis
- being large and insoluble, it does not diffuse out of cells
- it is compact, so a lot of it can be stored in a small space
- when hydrolysed it forms a-glucose, which is both easily transported and readily used in respiration
- the branched form has many ends, each of which can be acted on by enzymes simultaneously meaning that glucose monomers are released very rapidly.
Where is Glycogen found?
What is it’s structure like?
Glycogen is found ONLY in animals and bacteria
- shorter chains (than starch)
- more highly branched (than starch)
Where is glycogen STORED in animals?
In animals it is stored as small granules mainly in the muscles and the liver.
The mass of carbohydrate that is stored is relatively small because fat is the main storage molecule in animals.
How is glycogen structure suited for storage?
- it is insoluble and therefore does not tend to draw water into the cells by osmosis
- being insoluble, it does not diffuse out of cells
- it is compact, so a lot of it can be stored in a small space
- It is more highly branched than starch and so has more ends that can be acted on simultaneously by enzymes. It is therefore more rapidly broken down to form glucose monomers, which are used in respiration. This is important to animals which have a higher metabolic rate and therefore respiratory rate than plants because they are more active.
How is cellulose different for starch/glycogen?
Cellulose differs from starch and glycogen in one major respect: it is made of monomers of ẞ-glucose rather than a-glucose.
This seemingly small variation produces fundamental differences in the structure and function of this polysaccharide.
Describe the structure of a Cellulose molecule
- has straight, unbranched chains.
- These run parallel to one another, allowing hydrogen bonds to form cross-linkages between adjacent chains.
While each individual hydrogen bond adds very little to the strength of the molecule, the sheer overall number of them makes a considerable contribution to strengthening cellulose, making it the valuable structural material that it is.
What are microfibrils?
The cellulose molecules are grouped together to form microfibrils which, in turn, are arranged in parallel groups called fibres
Why is cellulose important in plant cells?
Cellulose is a major component of plant cell walls and provides rigidity to the plant cell.
The cellulose cell wall also prevents the cell from bursting as water enters it by osmosis. It does this by exerting an inward pressure that stops any further influx of water.
As a result, living plant cells are turgid and push against one another, making non-woody parts of the plant semi-rigid.
This is especially important in maintaining stems and leaves in a turgid state so that they can provide the maximum surface area for photosynthesis.
The structure of cellulose is suited to its function of providing support and rigidity because:
-cellulose molecules are made up of B-glucose and so form long straight, unbranched chains
- these cellulose molecular chains run parallel to each other and are crossed linked by hydrogen bonds which add collective strength
- these molecules are grouped to form microfibrils which in turn are grouped to form fibres all of which provides yet more strength.
