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Summary Cell chemistry and proteins
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Summary of lecture and corresponding literature of cell chemistry and proteins.
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Introduction Cell Chemistry andProteins (BOOK)
Chapter 2 pp 43-50; Ch.3 pp 109-121 (+ Fig. 3-38)
CH2. pp 43-50
The chemical components of a cell
Organic chemistry is the study of carbon compounds. Living organisms are made of only a mall
selection of all 92 elements. Carbon (C), hydrogen (H), nitrogen (N) and oxygen (O) make up 96,5% of
an organism’s weight. The atoms are linked through covalent bonds to form molecules. Covalent
bonds are about 100 times stronger then the thermal energy within a cell and therefore they resist
being pulled apart by thermal motions.
Two different molecules can be hold together by noncovalent bonds which are weaker.
Hydrogen bonds
In each water molecule the two H atoms are linked to the O atom by covalent bonds. These covalent
bonds are highly polar because the O has a larger electronegativity than does the H atom. This
means that there is an unequal distribution of electrons in the water molecule; the O atoms are more
negative, and the H atom more positive. So when a H atom from one water molecule meets an O
atom from another water molecule, there is an electrical attraction between them which results in a
hydrogen bond. Hydrogen bonds are weaker than covalent bonds and only last a short time.
Hydrophilic molecules are molecules that dissolve in water because they can form hydrogen bonds
or carry charges. Hydrophobic molecules in contrast are uncharged and form almost no hydrogen
bonds and do also not dissolve in water.
Four types of noncovalent attractions
- Electrostatic attractions (ionic bonds)
o These forces occur both between fully charged groups (ionic bond) and between
practically charged groups on polar molecules.
- Hydrogen bonds
o Strongest when the three atoms are in a straight line.
- Van der Waals attractions
o Caused by correlations in the fluctuating polarisations of nearby particles.
- Hydrophobic force
o Water forces hydrophobic groups together because doing so minimises their
disruptive effects on the hydrogen-bonded water network.
, Acids and bases in water
When a molecule containing a highly polar covalent bond
between hydrogen and another atom dissolves in water, the
hydrogen atom gives up its electron almost entirely to the
other atom and and becomes a proton H+. When the proton
becomes surrounded by water molecules, it will easily
become attracted to the negative charge on the O atom
generating a hydronium ion (H3O+). The reverse reaction also
take place very readily. These substances that release
protons when dissolved in water are called acids.
The higher the concentration H3O+ the more acidic the solution. The H 3O+ concentration is usually
referred to as the H+. The concentration of H3O+ is expressed using a logarithmic scale called the pH
scale. Pure water has a pH of 7 and is said to be neutral (pH<7 is acidic and pH>7 basic). Strong acids
lose their protons quickly, while weaker acids lose their protons less quickly. Strong bases accept
protons quickly while weaker bases don’t.
Weak bases and weak acids are the most important in living cells.
The interior of the cell is kept close to neutrality by the presence of buffers which are weak acids and
bases that can release or take up protons near pH 7.
A cell is formed from carbon compounds
Nearly all the molecules in a cell are based on carbon. Carbon is outstanding among all the elements
in its ability to form large molecules. Because carbon is small and has four electrons and four
vacancies in its outermost shell, it can form four covalent bonds with other atoms.
Carbon compounds made by cells are called organic molecules, all other molecules are called
inorganic.
Cells contain four major families of small organic molecules: sugars, fatty acids, nucleotides and
amino acids.
Macromolecules
Macromolecules are the most abundant carbon-containing molecules in a living cell. They are the
principal building blocks form which a cell is constructed and also the components that confer the
most distinctive properties of living things.
Proteins, nucleic acids and polysaccharides share an important feature which is that all polymers
grow by the addition of a monomer onto the and of a growing chain —> condensation reaction in
which one molecule of water is lost.
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