3.2.1 Carbon Makes Organic Molecules
Why Carbon?
1. Carbon is the second most abundant
element in living organisms.
2. Carbon can share four electrons, therefore it
can bond to four additional atoms.
3. Carbon establishes covalent bonds (stable,
high energy bonds)
4. 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¯
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.
3.2.1 Structure of some important
macromolecules
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
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
3.2.3/4 Examples of Disaccharides and
Function
Disaccharides are relatively small molecules.
They are water-solubes and taste sweet.
5
H
4
3
Disaccharides are more suitable for transport and storage
H
than monossacharides.
H
H
H
1
4
HOH
5
OH
H
3
H
phloem tubes of plants.
O
H
OH
H
OH
H
maltose
H
1
OH
6 CH2OH
H
2
OH
H
H
1
4
5
O
H
OH
H
O
1
2
3
Lactose
maltose
H
H
OH
6
5
4
1
1
3
OH
2
3
O
H
OH
5
O
2
6 CH2OH
4
main form in which carbohydrates are transported in the
O
H
OH
OH
Sucrose is stored in sugar beet and sugar cane. It is the
6 CH2OH
6 CH2OH
2
2
5
3
Sucrose
4
6
OH
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.
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.
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
O
OH
CH2OH
H
H
OH
H
H
OH
H
H
OH
CH2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
glycogen
H
1
O
6 CH2
5
H
OH
3
H
CH2OH
O
H
2
OH
H
H
1
O
CH2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
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
3.2.5 Lipids structure: Fatty acids
• A fatty acid molecule:
Types of fatty acids:
•
Hydrocarbon chain = Hydrophobic
•
Carboxylic acid group =Hydrophilic
Amphipathic
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
+ H2O
+ H2O
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
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)
3.2.6 Function of lipids
3.2.7 Compare the use of carbohydrates
and lipids in energy storage
Lipids
Carbohydrates
1. Long time energy storage
1. Short time energy storage because are
2. Lipids contain twice as much energy
more easily digested so the energy stored
per gram as carbohydrates
can be released more rapidly
3. Lipids are insoluble in water – do not
2. Soluble in water – easier to transport
cause problems with osmosis cell.
to and from the store.