carbohydrates Flashcards
what are carbohydrates made from
monosaccharides eg glucose, fructose and galactose
what is glucose
a hexose sugar - six carbon atoms in each molecule
two types of glucose, alpha and beta
what type of reaction joins monosaccharides together and what happens
a condensation reaction is when two molecules join together with the formation of a new chemical bond, and a water molecule is released when the bond is formed
monosaccharides are joined together by condensation reactions
a glycosidic bond forms between the two monosaccharides as a molecule of water is released
a disaccharide is formed when two monosaccharides join together
sucrose is a disaccharide formed from a condensation reaction between a glucose molecule and a fructose molecule
lactose is another disaccharide formed from a glucose molecule and a galactose molecule
what breaks polymers apart and what happens during it
hydrolysis reactions
a hydrolysis reaction breaks the chemical bond between monomers using a water molecule - it’s basically the opposite of a condensation reaction
for example, carbohydrates can be broken down into their constituent monosaccharides by hydrolysis reactions
what test
benedict’s test for sugars (general term for monosaccharides and disaccharides)
reducing sugars test
reducing sugars include all monosaccharides and some disaccharides (eg maltose and lactose)
you add benedict’s reagent (blue) to a sample and heat it in a water bath that’s been brought to the boil
if the test’s positive it will form a coloured precipitate (brick red)
the higher the conc, the further the colour change goes - blue - green - yellow - orange - brick red
non-reducing sugars test
if reducing sugars test is negative eg sucrose - need to break them down into monosaccharides
get a new sample of the test solution, add dilute hydrochloric acid and carefully heat it in a water bath that’s been brought to the boil. Then neutralise it with sodium hydrocarbonate and then carry out the benedict’s test
if test is positive it will form a coloured precipitate
if stays blue - no sugars are present
what is a polysaccharide
loads of sugars joined together
when more than two monosaccharides are joined together by condensation reactions
eg lots of a-glucose molecules are joined together by glycosidic bonds to form amylose
starch
main energy storage material in plants
cells get energy from glucose - plants store excess glucose as starch
starch is a mixture of two polysaccharides of a-glucose - amylose and amylopectin
starch is insoluble in water and doesn’t affect water potential so it doesn’t cause water to enter cells by osmosis, which would make them swell
This makes it good for storage
amylose
long, unbranched chain of a-glucose
the angles of the glycosidic bonds give it a coiled structure, almost like a cylinder. This makes it compact so it’s really good for storage because you can fit more in to a small space
amylopectin
a long, branched chain of a-glucose
its side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily
this means that the glucose can be released quickly
test for starch
iodine test
add iodine dissolved in potassium iodide solution to the test sample
if there is starch present, the sample changes from browny-orange to a dark, blue-black colour
glycogen
main energy storage material in animals
animal cells get energy from glucose too
but animals store excess glucose and glycogen - another polysaccharide of a-glucose
its structure is very similar to amylopectin, except that it has loads more side branches coming off it. Loads of branches means that stored glucose can be released quickly, which is important for energy release in animals
its also a very good compact molecule, so it’s good for storage
cellulose
major component of cell walls in plants
made of long, unbranched chains of beta-glucose
when b-glucose molecules bond, they form straight cellulose chains
the cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibils - the strong fibres mean cellulose provides structural support for cells
sucrose
disaccharide formed from a condensation reaction between a glucose molecule molecule and a fructose molecule
