4.1. Carbohydrates, fats and proteins & Food tests

Chemical elements that make up:

• Carbohydrates: C, H, O
• Fats: C, H, O (lower O content than carbs)
• Proteins: C, H, O and N; sometimes S and P

1. Carbohydrates

one glucose molecule

Carbohydrates are built up from simple sugars, such as glucose, C6H12O6, usually represented in this ring shape.

• monosaccharides: sugars with 1 carbon ring
• e.g.: fructose, glucose
• disaccharides: sugars with 2 carbon rings
• e.g.: maltose, sucrose
• polysaccharides: many glucose molecules joined together by chemical bonds
• glycogen: food storage in animal cells
• starch: food storage in plant cells
• cellulose: chains of glucose --> microscopic fibers --> form cell wall of plant cells

Image created by BYU-I student Hannah Crowder, 2013

2. Proteins

• formed from long chains of amino acids held together by chemical bonds
• C, H, N, O + P, S (chnops)
• 20 types of amino acids

• different sequences of amino acids give different shapes to protein molecules
• the shape and structure of proteins are specific to themselves, and is very important on its reaction with other substances
• active site of enzymes, is specific to its substrate
• binding site of antibodies, is specific to antigens (on pathogen)

Proteins come in many shapes and sizes

3. Fats

Lipids = fats + oils

• lipid in solid form = fat
• lipid in liquid form = oil
• C, H, O

1 molecule of lipid = 3 molecules of fatty acid + 1 molecule of glycerol.

5 Food tests to know!

Here's a summary table of the main ideas. For more details and explanations for each test, be sure to click its links!

Food tested	Name of test	Method	Positive result
Starch	Starch test	add a few drops of iodine to a solution of food	blue/black color
Reducing sugar	Benedict’s test*	- add equal of amount of Benedict’s solution to solution of food - boil carefully over Bunsen flame	a succession of color changes - turquoise à pale green à pea green à orange à brick red * the further the color change, the more reducing sugar is present
Protein	Biuret test	- add equal amounts of NaOH to solution of food à mix - add a few drops of 1% CuSO4; no shaking!	violet halo
Fats	Emulsion test	- dissolve food in ethanol - pour solution into clean test tube	milky white emulsion
Vitamin C	DCPIP test	- DCPIP is a deep blue color - measure out 2.0 cm3 DCPIP into test tube - add fruit juice drop by drop, counting shake mixture after each drop	DCPIP decolorises

*Benedict's test is the ONLY test that involves heating.

All tests except DCPIP

Definitely check out Crash Course's video on biological molecules!

For past papers, click here.