Carbohydrates Flashcards
What elements make up carbohydrates?
C H O
What are the tests for reducing and non-reducing sugars?
- benedict’s test
- reagent strips test
Explain benedict’s test (reducing sugars)
- glucose solution in test tube, add a few drops of benedict’s solution, stand tube in 100 degrees C water bath
- positive result = blue to brick-red precipitate
- reducing sugar reduces copper (II) ions in Benedict’s solution to copper (I) oxide
Explain the test for non-reducing sugars
- sucrose solution in test tube
3 drops of dilute hydrochloric acid
shake
place in 100 degrees C water bath for 5 mins
allow to cool
3 drops of dilute sodium hydroxide + mix (neutralise)
repeat reducing sugar test - positive result = blue to brick-red precipitate
- acid hydrolyses sucrose into glucose + fructose (positive)
Explain the test for starch
- drop of starch solution in depression on spotting tile
add drop of iodine solution - positive result = orange to blue-black precipitate
- coloured polyiodide complex formed in presence of starch
Explain reagent strip test for reducing sugars
- quantitative measure of reducing sugars in a substance
- colour-coded chart used to approximate the concentration of sugars
Define monosaccharide
small organic molecules used as building blocks for more complex substances (carbohydrates)
What is the general formula for a monosaccharide?
(CH2O)n
Define disaccharide
two monosaccharides joined together by a glycosidic bond
How is a disaccharide synthesised?
CONDENSATION REACTION
- two monosaccharides bond together
- 2 alpha glucose next to each other = 2 hydroxyl groups react
- OH from 1 and OH from 4 join to produce water molecule
- leftover O atom bonds two monosaccharides together and this is the 1- 4 glycosidic bond (covalent)
How is a disaccharide broken down?
HYDROLYSIS REACTION
- addition of water molecules
- breaks glycosidic bond and O joins with one of the H from water
- 2 individual monosaccharides created
What is the difference between a hexose and a pentose monosaccharide?
- hexose contains 6 carbon atoms and pentose contains 5 carbon atoms
- hexose has more stereoisomers than pentose
What is sucrose?
a disaccharide formed by 2 monosaccharides:
- glucose and fructose
- glycosidic bond at carbon 1 and carbon 2
- known as cane sugar or just sugar
What is maltose?
a disaccharide formed by 2 monosaccharides:
- 2x alpha glucose
- glycosidic bond at carbon 1 and carbon 4
- wheat, cornmeal, barley and other ancient grains
What is lactose?
a disaccharide formed by 2 monosaccharides:
- alpha glucose and beta galactose
- glycosidic bond at carbon 1 and carbon 4
- commonly found in milk and milk products
What is ribose and deoxyribose?
pentose monosaccharides that are important components of biological molecules:
- ribose is present in RNA nucleotides
- deoxyribose is present in DNA nucleotides
What is starch?
- found in plants
- alpha glucose joined by glycosidic bonds form two slightly different types of polysaccharide - collectively known as starch
- glucose = product of photosynthesis + stored as starch
What is amylose?
- type of starch
- formed by alpha glucose molecules by 1,4 glycosidic bond only
- bond is angled; long chain forms helix, stabilised by by H bonds
- polysaccharides = more compact + less soluble than glucose molecules alone
- preferable use of energy storage in plants (grains in chloroplast + nucleus & lysosomes in chloroplasts)
What is amylopectin?
- type of starch
- formed by alpha glucose molecules with 1,6 glycosidic bond or 1,4 glycosidic bonds
- branched-chain polymer
- branching takes place every 25 glucose subunits
- compact + insoluble
- energy store for grains in chloroplast + nucleus & lysosomes in chloroplasts
What is glycogen?
- secondary long-term energy store in animals + fungi
- found in liver, muscle and kidney cells
- formed by condensation reactions of glucose molecules, glycosidic bond between carbons 1,4
- branching takes place approximately every 10 glucose subunits via 1,6 glycosidic linkages
- less branching than amylopectin = more compact = less space needed for storage (good for mobile animals)
- branching = lots of free ends for glucose molecules to be added / removed = speeds up storing / releasing glucose molecules required by the cell
- insoluble, compact and branched
What is cellulose?
- formed by beta glucose molecules, which cannot join together as easily as alpha molecules can - alternate beta molecule is flipped upside down
- unable to branch / coil = straight-chain polymer
- H bonds with each other = microfibrils = fibres
- strong and insoluble
- used in cell walls
- part of our diet - hard to break down into monomers + forms ‘fibre’ / ‘roughage for a healthy digestive system
