3.1.2 Carbohydrates Flashcards
What is a carbohydrate?
A substance which contains carbon, hydrogen and oxygen atoms only
What is a monosaccharide?
The name given to the monomers from which larger carbohydrates are made
-include glucose, fructose and galactose
What is a disaccharide?
Two monosaccharides joined together via a condensation reaction. They are a form of dimers
-include sucrose, maltose and lactose
What is the formula for glucose?
C6H12O6
What type of sugar is glucose and why?
A hexose sugar as it has 6 carbon atoms in its structure
(Note that different monosaccharides form from molecules with different numbers of carbon atoms)
What is an isomer?
Organic molecules that have the same molecular formula but different structural formulas, which therefore result in different properties
-include alpha and beta glucose
Describe the difference between alpha and beta glucose
The OH and H groups on the carbon 1 have been swapped
How are dissacharides formed?
When two monomers/monosaccharides join via a condensation reaction, which forms a glycosidic bond between the two.
Describe the process of forming a glycosidic bond
-2 hydroxyl groups interact via a condensation reaction (the H from the first monosaccharide and the OH from the second)
-this forms a strong covalent bond called a glycosidic bond
-this also produces a water molecule as a product
Word equation for the reaction of alpha glucose and fructose
A-glucose + fructose -> sucrose + water
C6H12O6 + C6H12O6 -> C12H22O11 + H20
Word equation for the reaction of two alpha glucose molecules
a-glucose + a-glucose -> maltose + water
C6H12O6 + C6H12O6 -> C12H22O11 + H20
Word and symbol equation for the reaction of alpha glucose and galactose
A-glucose + galactose -> lactose + water
C6H12O6 + C6H12O6 -> C12H22O11 + H20
How are polysaccharides broken back down into monosaccharides?
-via hydrolysis reactions
-this addition of a water molecule breaks the glycosidic bonds
-the bonds are hydrolysed which breaks the polysaccharide back into monosaccharides
-for example, the breakdown of glycogen into glucose
What is a polysaccharide?
Polymers formed by many monosaccharides joined together by glycosidic bond formed via a condensation reaction to form long chains.
Which polysaccharides are formed from alpha-glucose?
Starch and glycogen
What polysaccharide is formed from beta-glucose?
Cellulose
What is a reducing sugar?
A sugar which is able to donate electrons, meaning it is a reducing agent.
-it is able to reduce copper (ii) sulphate into copper oxide
-copper sulphate gains electrons so is reduced, and the reducing sugar is therefore oxidised
Why do reducing sugars give a positive result for the Benedict’s test?
Because Benedict’s reagent contains copper (ii) sulphate ions, which are reduced to copper oxide, the brick red precipitate.
What is a non-reducing sugar?
A sugar that cannot donate electrons, so therefore cannot reduce.
-it is unable to reduce copper (ii) sulphate into copper oxide, as the reduction group is involved in a glycosidic bond
What is acid-hydrolysis?
The breaking of a glycosidic bond using an acid, to expose the reducing group of a non-reducing sugar.
Give an example of a non-reducing sugar
Sucrose
Give 5 examples of reducing sugars
Glucose
Fructose
Galactose
Maltose
Lactose
Describe how to conduct the iodine test
-add iodine solution to the food sample
-observe the colour change
Colour change for the iodine test if starch is present
Orange/brown to blue-black
Describe how to conduct the Benedict’s test
-add Benedict’s reagent to the sample
-heat this in a boiling water bath for 5 minutes
-observe the colour change
What is the positive result for the Benedict’s test?
-if a reducing sugar is present, the colour will change from blue to brick red
(A scale of colour changes may occur from green, yellow or orange depending on the concentration of the reducing sugar present).
Describe the test for a non-reducing sugar
-following a negative result for the Benedict’s test
-add dilute HCL to the sample
-heat in a boiling water bath for 5 minutes
-add sodium hydrogencarbonate (an alkali) to neutralise
-carry out the Benedict’s test as normal
What happens during the acid hydrolysis test?
The acid hydrolyses the glycosidic bonds in the carbohydrate molecules, which exposes the reducing group, which will then give a positive result in the Benedict’s test
Describe how to find the concentration of glucose in an unknown substance
-use serial dilution to create a concentration gradient in the 5 test tubes, and add the unknown solution to a 6th test tube
-conduct the Benedict’s test on each solution
-use a colourimeter to measure the percentage absorbance of light through each one
-plot a graph and use the calibration curve to find the unknown concentration of reducing sugar
Briefly outline how we can find the concentration of reducing sugar in an unknown substance
By setting up standard solutions of known concentrations of a reducing sugar, and conducting the Benedict’s test to compare results.
What is a colourimeter?
An instrument that beams specific wavelengths of light through a sample and measures how much is absorbed
Name 3 examples of polysaccharides
starch, cellulose and glycogen
What is a polysaccharide?
polysaccharides are polymers formed by many monosaccharides joined by glycosidic bonds in a condensation reaction to form long chains.
Describe the structure of starch
-formed from monomers of alpha glucose
-formed from two different polymers: amylose and amylopectin (based on where the glycosidic bond is)
Describe the structure of amylose
-created by condensation reactions that form glycosidic bonds between carbon-1 and carbon-4 (1-4)
-a straight long chain that coils into a helical structure
Describe the structure of amylopectin
-Created by condensation reactions that form glycosidic bonds between carbon-1 and carbon-4 (1-4)
-it also forms 1-6 glycosidic bonds which give it a branched structure
What is the function of starch?
It is an insoluble store of glucose in plants, stored in seeds and storage organs eg. chloroplasts
what is the link between the structure and function of starch?
-it is insoluble so does not affect water potential (water is not drawn into the cell by osmosis so will not burst when turgid)
-large and insoluble so cannot diffuse out of cells
-helix shape is compact so lots of glucose can be stored in a small space
-branched structure increases surface area to be accessed by enzymes for rapid hydrolysis back to glucose
-glucose stored can be readily transported and used in respiration in plants
Describe the structure of cellulose
-formed by monomers of beta-glucose
-formed by many condensation reactions joining beta-glucose by glycosidic bonds
-glycosidic bonds between carbon-1 and carbon-4 (1-4)
-only 1-4 glycosidic bonds creates long, straight polymer chains of beta-glucose
-chains lined up parallel to each other are held together in a fibril by many hydrogen bonds
What is the function of cellulose?
It is found in the cell wall of plant cells, where it provides structural strength to the cell wall. It prevents the cell from bursting when it becomes turgid under osmotic pressure.
Describe the links between the structure and function of cellulose?
-Many hydrogen bonds/cross links provide collective strength
-insoluble so does not affect water potential (water is not drawn into the cell by osmosis)
-long unbranched chains provide support for the plant
-prevents the cells from bursting when they become turgid under osmotic pressure, which helps the cell wall to provide the maximum surface area for photosynthesis.
Describe the structure of glycogen
-formed from monomers of alpha-glucose that undergo many condensation reactions
-these form glycosidic bonds between carbon-1 and carbon-4 (1-4)
-they also form glycosidic bonds between carbon-1 and carbon-6 (1-6)
-this leads to a highly branched structure (even more 1-6 glycosidic bonds than amylopectin)
What is the function of glycogen?
A insoluble store of glucose in the liver and muscle cells of animals.
How is the structure of glycogen linked to its function?
-insoluble so does not affect water potential (water is not drawn into the cell by osmosis)
-branched structure increases surface area for rapid hydrolysis back to glucose, as there is increased surface area to be accessed by enzymes
-rapid hydrolysis enables energy to be released rapidly as glucose is readily available for respiration
-highly branched structure makes it compact so lots of glucose can be stored in a small space
