Proteins are complex biomolecules composed of amino acids linked by peptide bonds. They perform many essential functions in the body. During digestion, proteins are broken down into amino acids which can then be absorbed and used to form new proteins containing three-dimensional structures essential for their functions. The biuret test detects proteins and peptides in a solution by turning purple in the presence of copper(II) ions, indicating the presence of peptide bonds.

2. Protein
Proteins are complex, specialized
molecules composed of carbon,
hydrogen, oxygen, and nitrogen. Many
proteins also contain sulfur. The building
blocks of proteins are the amino acids.

3. Proteins are always present in protoplasm. Their
molecules are the largest and most
complicated of all the food substances.

4. Every protein molecules is built up from simpler
compounds known as amino acid. A amino acid is
made up of an amino group (-NH2), an acidic group
(-COOH) and a side chain (donated by R). the
general formula for an amino acid can be written
as:

5. • Amino acids link up to form
polypeptides.
• Two amino acid may link up in
condensation reaction. The bond
formed between the two amino acids is
strong and is called a peptide bond.
• Many amino acids linked up by peptide
bonds form a polypeptide or peptone.

6. • In most proteins, the chains are coiled or
folded, giving the protein a more complex,
three- dimensional shape. The coils are
held in place by weak bonds. These weak
bonds are easily broken by heat and
chemicals such as acids, and alkalis. When
these bonds are broken, the protein loses
its three-dimensional shape. We say the
protein has been denatured. Denaturation
usually involves a loss of function of the
protein.

8. • Animals cannot directly absorb the proteins
it takes in, because protein molecules are
too large to pass through living cell surface
membranes.
• These protein molecules must be broken
down by enzymes during digestion.

10. • Amino acids are simpler and much
smaller molecules than proteins. They
are soluble in water and are small
enough to diffuse through living
membrane. Hence, they can easily be
absorbed into an animal’s body.
• When amino acids enter the body
cells, they are linked up again to form

11. • Proteins can be found in both
animals and plants foods. Milk,
eggs, seafood and meat such as
chicken and lean beef are some
animal foods rich in proteins. Plant
foods rich in proteins include
soya. Beans, nuts, grains, and
vegetables such as French beans.

12. • Proteins are used:
 In the synthesis of new protoplasm,
for growth and repair of worn-out
body cell;
 In the synthesis of enzymes and some
hormones; and
 In the formation of antibodies to

13. • The average adult human being needs
50-100g of proteins a day. Children
who do not get enough protein may
end up with a protein deficiency
disease (disease caused by lack of
proteins) called KWASHIORKOR. Such
children usually have:
 Swollen stomachs.
 Their skins crack and become scaly.

16. • Biuret solution is a blue solution made
up of sodium hydroxide and copper (II)
sulphate. It turns violent (deep purple)
when the proteins are present and
turns pink when short-chain
polypeptides are present.

17. •To detect the presence
of peptide bonds in
molecule.

18. • Test tubes
• Test tube rack
• Egg white
• Beaker
• 10% Sodium Hydroxide(NaOH)
• 0.5% Copper Sulfate(CuSO4)

19. • Add 1 cm3 of sodium hydroxide solution (
40% or bench solution) to 2cm3 of egg white
solution in a test tube and shake it
thoroughly.
• Add 1% copper (II) sulphate solution, drop
by drop, shaking after every drop.
(Alternatively, to 20cm3 of egg white
solution add an equal volume of biuret
solution. Shake well and allow the mixture
to stand for 5 minutes). What color do you

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