Food Tests

Benedict's Test for Reducing Sugars

• Pour 2cm³ of solution sample solution into a test tube and add an equal volume of Benedict's solution. 
• Shake the mixture.
• Heat test tube in a boiling water bath for 5 minutes.
• Record observation after 5 minutes.

Colour of precipitate and the corresponding amount of reducing sugar present

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Biuret's Test for Proteins

• Add 2cm³ of sample solution to a clean test tube and add 2cm³ of dilute sodium hydroxide solution.
• Shake the mixture.
• Add 1% copper (II) sulphate solution, drop by drop. Shake mixture after every drop and observe the colour change. 
• If mixture remains blue, no protein is present.
• If mixture turns violet/purple, protein is present. 

Iodine Test for Starch

• Add 2cm³ of sample solution to a clean test-tube, followed by a few drops of iodine solution.
• Observe the colour change.
• If mixture remains brown, no starch is present.
• If mixture turns blue black, starch is present.

Ethanol Emulsion Test for Fats

• Add 2cm³ of sample solution to a clean test tube and add 2cm³ of ethanol.
• Shake mixture thoroughly.
• Add 2cm³ of water into the test tube and shake. 
• Record the observation.
• If mixture remains clear, no lipid is present.
• If a white emulsion was obtained, lipid is present. 

Condensation and Hydrolysis

Condensation and hydrolysis are reverse processes of each other.

Condensation (or dehydration synthesis) is a chemical process which by 2 molecules are combined to make a larger molecule through the loss of water. It is the process which by biomolecules are formed.

Hydrolysis is a chemical process which by a macromolecule is broken down into smaller molecules by the addition of water. Often, enzymes are needed to carry out hydrolysis, as water alone is insufficient.

image taken from: http://classconnection.s3.amazonaws.com/311/flashcards/1406311/jpg/polymers1334535675092.jpg

Biomolecules — Lipids

What are lipids?

• lipids are insoluble in water
• contain C, H and O
• contain less oxygen than carbohydrates
• made up of fatty acids and glycerol

What are fatty acids?

• contain hydrophobic "tail" of a long hydrocarbon chain, and hydrophilic "head" which is a carboxyl group
• can be saturated (no double bonds, no kinks) or unsaturated (double bonds and kinks in fatty acid tail)

Classification of lipids

• simple lipids
• compound lipids
• steroids and sterols

Simple Lipids

• formed by joining fatty acid with an alcohol (eg. glycerol)
• fats (eg. triglyceride) are simple lipids
• triglyceride structure: glycerol + 3 fatty acid tails

Compound Lipids (eg. phospholipids)

• found in phospholipid bilayer of cell membrane
• phospholipid structure: phosphate head + 2 fatty acid tails

Biomolecules — Carbohydrates

Carbohydrates are the most abundant biomolecule in nature. 

Function of carbohydrates

• building blocks for larger molecules 
• energy storage — starch (plant) and glycogen (animal)
• structural — cellulose cell wall (plant) and chitin (insects, crabs, shrimps)

The bond between monomers of carbohydrates is known as glycosidic bond.

Monosaccharides

• monomers of carbohydrates
• eg. glucose, fructose, galactose
• all monosaccharides are reducing sugars

Disaccharides

• made up of two monosaccharides

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Polysaccharides

• made up of many monosaccharides

What is starch? 

• polymer of glucose molecules
• has two components; amylose and amylopectin
• both fit together to form a complex 3-dimensional structure which is insoluble in water
• amylose helices are entangled in the branches of amylopectin molecules
• each amylose chain is coiled into a helix, with six glucose residues for every complete turn of the helix — compact shape making it a complex structure for storage
• amylopectin have many branches

What is glycogen?

• animal equivalent of starch
• found in liver and skeletal muscles of vertebrate animals 

Starch and glycogen (energy stores)

• their molecules have many side branches where glucose molecules can be removed from their tips (by enzymes)
• their insolubility stops them interfering with osmosis
• their compactness provides an efficient way to store lots of glucose for future cellular respiration

Cellulose (long, unbranched chain)

• most abundant organic molecule on Earth
• major component of cell wall in plants
• made from long, straight unbranched chains of glucose
• chains cross-linked by H-bonds which holds them tightly together (excludes water)
• chemically very inert and insoluble 
• many molecules form strong fibrils
• only some bacteria, fungi and a very small number of animals can secrete cellulase enzymes

Biomolecules — Proteins

Amino Acids:

The basic building blocks of proteins are amino acids. Amino acids are the monomers which form polypeptide and proteins.

Properties of Proteins:

• sensitive to pH and heat
• shape determines function
• enzymes are special group of proteins
• contain H, N, C and S
• made up of amino acids

There are about 20 essential amino acids found in proteins, and they differ in their R group. 

Amino acids join together to form a polypeptide through the process called condensation.

Two amino acids are joined by a peptide bond to form a dipeptide via condensation. Water is formed as a by product. 

The reaction is between an amino group and a carboxyl group. 

Continued condensation leads to the addition of more amino acids and thus resulting in a polypeptide chain.

 

Structure of proteins:

1. Primary structure --> specific linear order which amino acids join to form a polypeptide chain
2. Secondary structure --> regular folding of segments by hydrogen bonds (α-helix and β-pleated sheet)
3. Tertiary structure --> (only for globular proteins) overall shape of polypeptide resulting from interactions* between the R group 
4. Quaternary structure --> association of 2 or more polypeptide chains 

*Bonds that form tertiary structures:

• hydrogen bonds
• ionic bonds
• disulfide bonds
• hydrophobic interaction

Classification of proteins (based on structure):

• Globular --> compact and spherical structure which is soluble
• Fibrous --> long polypeptide chains twisted around each other, forming long fibres that provide tensile strength, insoluble 

Biological Molecules - You Are What You Eat: Crash Course Biology #3

A helpful video on biomolecules!

 ← screengrab from the video!