3.1.2 Carbohydrates

As mentioned in 3.1.1, monosaccharides come together in a condensation to form either a disaccharide/polysaccharide - the bond that forms is a glycosidic bond. There are three disaccharides we need to be aware of:

Maltose is formed from the condensation of two glucose molecules

Sucrose is formed from the condensation of a glucose molecule and a fructose molecule (think like, sucrose is a lot in fruit so FRUctose for FRUit)

Lactose is formed from the condensation of a glucose molecule and a galactose molecule (think gaLACTOSE makes LACTOSE).

There are two isomers of glucose we need to be able to draw,  α-glucose and β-glucose. Here’s what they look like (pretty similar):

There are also three types of polysaccharide we need to know (these are formed by the condensation of many glucose units). Glycogen and starch are formed by the condensation of many α-glucose molecules, whilst cellulose is formed from the condensation of many β-glucose molecules. The structure and function of each polysaccharide is as follows…

Starch

Does not affect the water potential of cells (is insoluble)

Very compact

Branched

1-4 and 1-6 glycosidic bonds

α-glucose

does not diffuse out of cells (large and insoluble)

Glycogen

Does not affect water potential (insoluble)

does not diffuse out of cells (insoluble)

1-4 and 1-6 glucosidic bonds

α-glucose

branched (more than starch)

Cellulose

1-4 glycosidic bonds

inverted β-glucose molecules

straight unbranched chains that run parallel

chains cross-linked by hydrogen bonds forming microfibrils and fibrils add collective strength

Testing for reducing sugars:

Add 2cm^3 of the sample to a test tube (crush in water if not liquid)

Add an equal volume of benedicts reagent

Heat the mixture gently in a water bath for 5 minutes (gently boiling)

clear/blue = trace/none

green = very low

yellow = low

orange = medium

red = high

Testing for non-reducing sugars:

Following a negative benedicts test…

Add 2cm^3 of the sample to a test tube (crush in water if not already liquid)

Add 2cm^3 dilute HCl

place the test tube in a gently boiling water bath for 5 minutes

Slowly add sodium hydrogen carbonate

Test with pH paper to ensure the solution is alkaline

re-test with benedicts reagent.

If reducing sugars are present it is because they were produced from the hydrolysis of non-reducing sugars.