Organic compounds include carbohydrates, lipids, proteins, and nucleic acids (DNA and RNA).
Carbohydrates –
groups of molecules that contain carbon, oxygen, and hydrogen. Hydrogen and oxygen occur in the
same ratio as in water (2:1) Carbohydrates include sugars and starches.
Carbohydrates are classified according to their size and solubility.
Monosaccharides (=simple sugars) –
single-chain or single-ring molecules. Ratio of carbon : hydrogen : oxygen is usually 1:2:1.
Examples: glucose (6 carbons); ribose (5 carbons).
Isomers – molecules with the same molecular formula, but different arrangement of atoms
which gives that molecule different chemical properties. Example: isomers of glucose are galactose and
fructose.
Disaccharides (=double sugars) –
contains 2 monosaccharide molecules joined together by dehydration synthesis.
Examples of disaccharides: sucrose (glucose+glucose), lactose (glucose+galactose), and maltose
(glucose+glucose).
2C H O → C H O + H O
6
12
2
12
22
11
2
or
C H O +C H O →C H O +H O
6
12
6
6
12
6
12
22
11
2
glucose + fructose → sucrose + water
Hydrolysis and dehydration synthesis
The decomposition reaction in which two molecules are split by addition of water molecule is called
hydrolysis. The reverse reaction is called dehydration synthesis.
Polysaccharides –
long chains of simple sugars formed in dehydration synthesis. Polysaccharides are large, fairly
insoluble molecules ideal for energy storage. Also, due to their poor solubility, they do not taste sweet.
Example of polysaccharides important for the body: starch (formed in plants) and glucogen (storage
carbohydrate in animal tissues: skeletal muscles and liver). Cellulose in another example of plant
polysaccharide (we are unable to digest it); is a source of fiber (essential for digestion).
Functions of carbohydrates –
to provide readily available energy for cellular metabolism. When glucose is broken down and oxidized
within the cell, its energy is used to synthesize ATP molecule.
ATP (adenosine triphosphate) molecule–
adenine-containing RNA nucleotide (see Nucleic acids below) with two additional phosphate groups.
Energy released in glucose catabolism is captured and stored in phosphate bonds of ATP.
Lipids –
molecules that are insoluble in water, but dissolve in other lipids and alcohol. Lipids contain hydrogen,
oxygen, and carbon, but oxygen occurs in lower quantities in lipids than in carbohydrates. Some lipids
also contain phosphorus.
Triglycerides (neutral fats) –
composed of glycerol and fatty acids. The ratio of fatty acids to glycerol is 3:1. Solid triglycerides are
known as fats, liquid triglycerides are known as oils. Triglycerides are non-polar molecules and do not
mix with polar water molecule. Triglycerides provide an efficient and compact form of stored energy;
their deposits are found under the skin (also serve as insulation from heat loss and physical trauma).
Phospholipids –
molecules composed of glycerol backbone, two fatty acid chains, and a phosphorus-containing group.
They are amphipathic molecules (contain polar end – a phosphate group, and non-polar end – fatty acid
chains.) Phospholipids form cell membrane (together with proteins).
Steroids –
flat molecules composed of four hydrocarbon rings; found in animal products (milk, eggs, cheese). An
example of steroids is cholesterol: important component of cell membrane, part of vitamin D structure,
steroid hormones (cortisol, for example: a hormone necessary for maintaining normal blood sugar).
Proteins –
contain carbon, hydrogen, oxygen, and nitrogen. Some proteins also contain phosphorus and sulfur.
Amino acids –
building blocks of proteins. Amino acids contain a basic amino group (-NH2) and acidic carboxyl
group (-COOH); they may act as a base (proton acceptor) or acid (proton donor).
Amino acids are different due to the presence of unique R group.
Structural levels of proteins
Primary structure –
linear chain of amino acids (=polypeptide chain)
Secondary structure –
-alpha helix: polypeptide chain is coiled and held together by hydrogen bonds between NH and CO
groups.
-beta-pleated sheet: polypeptides do not coil but linked by hydrogen bonds to form ribbon-like
structure.
Tertiary structure –
alpha-helix and/or beta-pleated sheets fold upon one another to make globular protein.
Quaternary structure –
two or more polypeptides aggregate to form a complex protein.
Fibrous proteins –
strand-like proteins; provide structural framework, mechanical support, or movement; insoluble in
water, very stable. Also called structural proteins. Examples: collagen in bones, keratin in hair, actin
and myosin in muscle tissues.
Globular proteins –
spherical proteins that have at least tertiary structure; water soluble; chemically active; sensitive to
large temperature and pH swings. Also called functional proteins. Examples: hemoglobin, albumin;
enzymes; immunoglobulins (antibodies).
Protein denaturation
The activity of a protein depends on its specific 3D structure, which is maintained by hydrogen
bonds. Whenever there is a rise in temperature above physiological levels or drop in pH, fragile
hydrogen bonds break, protein loses its shape, and is said to be denatured.
Active site of a protein – specific arrangement of atoms where other atoms or molecule will
bind. If protein denatures, it loses its active site
Molecular chaperones –
participate in folding of proteins; prevent premature or incorrect folding; help in breakdown of
damaged proteins.
Enzymes –
globular proteins that act as catalysts. Catalysts are substances that increases the rate of chemical
reactions without changing their own chemical composition.
(More on enzymes in Enzymes homework; see Forum.)
Nucleic acids (DNA and RNA) –
composed of carbon, hydrogen, oxygen, nitrogen, and phosphorus.
Nucleotides –
structural units of nucleic acids; include nitrogenous base, a 5-carbon sugar (deoxyribose in DNA,
ribose in RNA), and a phosphate group.
Nitrogenous bases of DNA: adenine (A); guanine (G); cytosine (C ); thymine (T).
Nitrogenous bases of RNA: adenine (A); guanine (G); cytosine (C ); uracil (U).
DNA has two strands of sugar-phosphate backbones with complementary nitrogenous bases.
Nitrogenous bases complement each other by binding in specific way: A binds to T; C binds to G.
Complementary bases are held together by hydrogen bonds. The molecule is coiled onto itself creating
a double helix, a structure that resemble spiral staircase.
RNA has only one strand of sugar-phosphate backbone with nitrogenous bases.
The role of DNA
--a blueprint of protein synthesis;
--directs its own replication.
The role of RNA
--decoding;
--communication;
--translation.