1. Topic 3.2
Carbohydrates,
Lipids and Proteins

2. 3.2.1 Carbon Makes Organic Molecules
Why Carbon?
1. Carbon is the second most abundant element in living organisms.
Carbon can share four electrons, therefore it can bond to four additional atoms.
Carbon establishes covalent bonds (stable, high energy bonds)
Carbon molecules have strength, flexibility, and great versatility to chemically react with other atoms and molecules.
Any molecule which contain carbon is defined as organic.
There are few exceptions:
Oxides of carbon (CO, CO2) and
Hydrogencarbonates HCO3¯

3. Macromolecules
Macromolecules are constituted by hydrocarbon backbones, which mainly provide structural stability, and by one or several functional groups.
Functional groups are involved in many and diverse chemical reactions,
establishing bonds with other atoms and molecules.

4. 3.2.1 Structure of some important
macromolecules

5. 3.2.3/4 Examples of Monosaccharides and Function
Monosaccharides are the simplest form of carbohydrate.
They provide the building blocks for larger carbohydrate molecules.
They also act as a respiratory substrate, providing cells with an energy source.
Glucose is the main energy source for most living cells. It is one of the carbohydrates produced in photosynthesis and forms the building blocks of many carbohydrates.
Fructose is a very sweet sugar. It is the main component of flower nectar and the sugar found in the fruits.
Galactose is found in milk. It combines with glucose to form the disaccharide milk sugar molecule lactose

7. 3.2.3 Examples of Disaccharides
A disaccharide consists of two monosaccharides joined by a glycosidic linkage, a covalent bond formed between two monosaccharides through a condensation reaction.
The hydroxyl group of one sugar and a hydroxyl of another sugar can join together, splitting out water to form a glycosidic bond:
R-OH + HO-R' R-O-R' + H2O

8. 3.2.3/4 Examples of Disaccharides and Function
Disaccharides are relatively small molecules.
They are water-solubes and taste sweet.
Disaccharides are more suitable for transport and storage than monossacharides.
Sucrose is stored in sugar beet and sugar cane. It is the main form in which carbohydrates are transported in the phloem tubes of plants. OH H
Lactose 1 2 3 4 5 6
Sucrose

9. 3.2.3 Examples of Polysaccharides
The majority of sugars found in nature exist in the form of polysaccharides.
Large chains of sugar units
Plants store glucose as starch.
The major constituent of plant cell walls, Cellulose, it consists of long linear chains of glucose with b(1  4) linkages.

10. 3.2.3 Polysaccharides in animals: Glycogen
Glycogen, the glucose storage polymer in animals, is similar in structure to starch, but glycogen has a(16) branches.
The highly branched structure permits rapid glucose release from glycogen stores, e.g., in muscle during exercise.

11. 3.2.5 Lipids structure
Lipids are organic molecules insoluble in water.
They constitute the main reservoir of stored energy.
The three main categories of lipids are:
Fats (fatty acids and triglycerides),
Phospholipids
Steroids
Fats also make cell membranes and coatings (i.e. fruit coats)
The basic structure of fats is a hydrocarbon backbone with a carboxyl group attached

12. 3.2.5 Lipids structure: Fatty acids
• A fatty acid molecule:
Hydrocarbon chain = Hydrophobic
Carboxylic acid group =Hydrophilic
Amphipathic
Types of fatty acids:
A fatty acid molecule has two distinct regions:
a long, not very reactive, hydrophobic hydrocarbon chain,
and a carboxylic acid group, extremely reactive and hydrophilic
• Molecules such as fatty acids — with two distinct hydrophobic and hydrophilic regions — are termed amphipathic.
Saturated: single C-C bond
Unsaturated: Double C=C bond

13. 3.2.5 Lipids structure: Triglycerides
Fatty acids are very efficient sites of energy storage
They are stored in cytoplasm of many cells as Triglycerides:
3 fatty acid chains
bonded to a glycerol molecule.
+ H2O
• Fatty acids are very efficient
sites of energy storage; they
are stored in the cytoplasm of
many cells in the form of
droplets of triacylglycerol
molecules — compounds
made of three fatty acid chains
bonded to a glycerol molecule.
• When a carboxylic acid and an
alcohol react, a water molecule
is removed, and an ester
linkage is formed
• Triglycerides make “the fat” of
our bodies. In animals, they
are stored as droplets in fat
cells or adipocytes.

14. 3.2.5 Lipids structure: Phospholipids
Phospholipids are similar to triglycerides but one of the fatty acids molecules is replaced by a phosphate group (PO4³¯).
The lipid part = hydrophobic
The phosphate part = hydrophilic

15. 3.2.5 Protein structure: peptide bond
• Amino acids are joined together when a condensation reaction removes a hydroxyl group from the carboxyl end of one amino acid and a hydrogen from the amino group of another amino acid
• The resulting covalent bond is called a peptide bond (C-N)

16. 3.2.6 Function of lipids

18. 3.2.7 Compare the use of carbohydrates and lipids in energy storage
1. Long time energy storage
2. Lipids contain twice as much energy per gram as carbohydrates
3. Lipids are insoluble in water – do not cause problems with osmosis cell.
1. Short time energy storage because are more easily digested so the energy stored can be released more rapidly
2. Soluble in water – easier to transport to and from the store.