Biological molecules & human diet, O-Level biology IGCSE

2. 
DNA
A DNA molecule looks like a twisted ladder,
with two long strands parallel to one another
with bases in between forming cross links;
we call this shape a double helix.
There are four types of bases, we give them
letters A, T, C and G.
Base A always pairs with T while base C
always pairs with G. The sequence of these
bases codes for the sequence of amino acids
that make up proteins, different sequences
code for different characteristics.

3.  Carbohydrate molecules contain: C, H, O
 H:O is 2:1 like in water molecule
 Made up of smaller molecules : sugars or monosaccharides.
e.g. glucose, fructose … we call them simple sugars
 Two sugars bonded together are called disaccharides e.g. maltose,
lactose, sucrose.
 Many sugars bonded together make a polysaccharide
e.g. starch, cellulose, glycogen.
 Carbohydrates are a source of energy to our bodies.
Carbohydrates
Plant cells
for storage
Plant cell walls
For support
Animal cells
for storage

4. 
Starch granules in cells

5. 
Food rich in carbohydrates

6. 
 We use Iodine solution (I2/KI), which is a yellowish
brown solution that turns to a blue-black colour
when it reacts with starch
To test for starch

7. 
 We use Benedict’s solution to test for reducing
sugars. It is a Blue solution, when a reducing sugar is
mixed with Benedict’s solution and boiled it forms a
precipitate that is green, yellow, orange or brick red.
To test for reducing sugars

8.  Protein molecules contain elements: C,H,O,N (S,P)
 Proteins are made of smaller molecules called amino acids.
 Two amino acids bonded together are called dipeptides.
 Many amino acids bonded together make a polypeptide.
 Proteins are needed to make:
1. Enzymes
2. Haemoglobin
3. Muscles fibers
4. Hair and nails (Keratin)
5. Cell membranes of cells
6. As a source of energy during starvation.
Proteins

9. 
Food rich in protein

10. 
 We use NaOH and CuSO4
(Biuret solution) that has a pale
blue colour, when it reacts with
proteins it changes its colour to
lilac, or purple or mauve.
To test for proteins

11. 
 Fat molecules contain elements: C,H,O
 Each fat molecule contains one glycerol and three fatty
acids
 Fats are needed to:
1. Make cell membranes of cells
2. Good energy storage
3. Act as a heat insulator under the skin
4. Protect some organs
Fats

12. 
Foods rich in fat

13. 
 Add Ethanol to the sample
then shake vigorously to
dissolve the fat, then add
water. A milky/cloudy
appearance is formed.
To test for fats

14.  There are many vitamins known to us by now, we will
study vitamin C and vitamin D as examples.
 Vitamin C, the first discovered vitamin, is needed by
the body to make an important protein called collagen
that toughens the skin, walls of blood vessels and
bones.
 Lack of vitamin C causes inability to form collagen
fibres which gives strength to blood vessels and skin.
 Deficiency symptoms: bleeding gums, skin ulcers.
 Deficiency disease: scurvy
Vitamins

15. 
 Citrus fruits
 Bell peppers
 Dark leafy vegetables
 Tomatoes, guava
 Broccoli, grapes
Foods rich in vit. C

16. 
 Use DCPIP a blue stain that becomes colourless
when reduced.
 To compare the amount of ascorbic acid (vit. C) we
can count the number of drops (or …) needed to change
the blue DCPIP into colourless.
To test for Vitamin C

17.  Your body must have vitamin D to
absorb calcium and promote bone
growth. Too little vitamin D results in
soft bones in children (rickets) and
fragile bones in adults (osteoporosis).
Vitamin D

18. 
 Fish especially fish oil
 Milk and milk products
 Egg yolk
 Mushrooms
 Our skin can make vit. D
when exposed to
sunlight
Vitamin D