Download a) Water is a good solvent – all molecules in a living things are

Cell Elemental Composition
Cells are 90-70% of water. Of the remaining molecules present, the dry weight is approximately:
o 50% protein
o 15% carbohydrate
o 15% nucleic acid
o 10% lipid
o 10% miscellaneous
Total approximate composition by elements:
65% O
18% C
10% H
3% N
1.5%Ca
1.0% P
0.35% K
0.25% S
0.17% Na
0.05% Mg …
Molecular Structure of Cells
All molecules in living things are classified into two groups: inorganic and organic.
Inorganic molecules – they are water and mineral salts (ions).
1.1.
Functions of water in living things:
a) Water is a good solvent – all molecules in a living things are dispersed in water forming a
colloid suspensions in cytoplasm,
b) A colloid may changes from a fluid (sol) to a semi-fluid ( gel),
c) Water participates in some chemical reactions,
d) Water is a temperature moderator.
1.2.
2.
2.1.
Ions are necessary for enzymes activity and for the buffer systems of cytoplasm.
Organic molecules
Polymers and monomers
2.1.1. Monomers are similar unit structures (organic molecules) of polymers. The monomers
link with each other by the covalent bonds to form the chains of oligomers and polymers.
The oligomers contain small number of monomers (from two to twenty), the polymers
contain from hundreds to millions monomers in the chain.
2.1.2. The monomers for different types of polymers are: monosaccharides (glucose), amino
acids and nucleotides.
2.1.3. Polymers are the largest organic molecules consist from the chain of monomers. The
polymers are polysaccharides, proteins and nucleic acids.
3.
Protein structure
3.1. Proteins are linear polymers constructed from the amino acids. The amino acids sequence
in the protein chain is encoded in the DNA of the genome. All living organisms use the
same 20 amino acids types and the same genetic code.
3.2. Amino acids are the monomers of proteins. The amino acids linking together by the
peptides bonds to form the polypeptide chain.
4. The levels of organization in protein molecules
4.1. Primary structure of proteins is formed by the specific amino acids sequence in the
polypeptide chain. This sequence is encoded for in DNA.
4.2. Secondary structure is the relationships of neighboring amino acids residues in the
polypeptide chain. It may be:
 a helical structure (- helix),
 -pleated sheet ( - configuration).
4.3. Tertiary structure refers to the relationships of more distant amino acids residues. It is a
globular structure. The conformation results from:
H-bonding,
non - covalent bonds,
disulfide bridges.
4.4. Quaternary structure refers to the relationships between the individual polypeptides chains
in complex protein. A single molecule of hemoglobin consists of four polypeptide chains: two
of them are -chains and two of them are -chains.
Lipids
1. The lipids are non polar and hydrophobic. They are insoluble in water, but
soluble in ether, chloroform, alcohols.
2. Lipids are not polymers: they contain long chains of hydrocarbons in their
molecules.
3. Lipids are classified as follows:
a) Simple lipids (oils, waxes)
b) Compound lipids:
 Sterols (cholesterols)
 Steroids (sex hormones)
 Phospholipids are the main components of the cell membranes. Their
molecules have two parts: two hydrophobic “tails” and hydrophilic “head”
 Glycolipids contain in their molecules carbohydrates and lipids
 The matrix of animal cells contains two kinds of glycolipids:
 Cerebrosides (in the cells of brain)
 Ganglioside (in the membrane of erythrocytes and brain).