Geoscience Reference
In-Depth Information
O
O
Adenosine-triphosphate
(ATP)
P
O
O
O
P
O
O
O
P
O
N
N
O
C
C
C
C
O
N
C
C
N
C
C
C
N
C
O
O
Figure 8.5
Structure and composition of adenosine-triphosphate (ATP). Hydrolysis of this compound to form
adenosine-diphosphate (ADP) and phosphoric acid liberates the energy needed by the cell.
3. Lipids include the fats and the glycerides, or esters of glycerol esters with one or
more fatty acids. When PO
3
4
groups attach themselves to one chain or more of car-
boxylic acids, they are known as phosphoglycerides. Tetracyclic compounds made of
four pyrrole units (a five-membered ring with a nitrogen atom opposing two double
carbon bonds) are biologically important as they form pigments known as porphyrines:
chlorophylls (Mg) and hemoglobin (Fe) are closely related compounds. Terpenes are
a broad class of lipids based on the isoprene chain (C
5
H
8
), which has one unsaturated
bond at each end. Many form important compounds with multiple cyclic bonds (sterols,
hopanes).
4. Aromatic alcohols (phenols) condense to form lignin, the second most abundant
component in plants.
5. Nucleotides contain nitrogen-bearing compounds, a pentose sugar (ribose or deoxyri-
bose), and a phosphate. Nucleic acids are polymers of nucleotides.
The universal energy currency of cellular metabolism is ATP, a compound which com-
bines an amino acid known as adenine, a sugar called ribose, and phosphate groups
(
Fig. 8.5
). Hydrolysis of ATP gives adenosine-diphosphate (ADP) and phosphoric acid
by releasing a very large amount of energy.
A distinction should be made between living organisms and fossil organic matter. For
all practical purposes, the mass inventory of living cells is dominated by an outer plas-
mic membrane and an inner medium called cytoplasm, which can be either a gel or a
sol depending on environmental conditions (
Fig. 8.6
). Vegetal cells are also wrapped in
an additional film of cellulose and some plants even develop a rigid framework of lignin.
Lignin and cellulose are the main initial constituents of coal. Genetic material dominated
by polynucleotides (RNA
deoxyribonucleic acid) is either
loose in the cytoplasm of prokaryotic cells (e.g. bacteria) or hosted in the nucleus, in
mitochondria (animals), or in chloroplasts (plants) of eukaryotic cells. In general, mem-
branes are composed of two layers of phospholipids made at one end of hydrophilic
phosphoglycerides and at the other end of hydrophobic fatty acids, which mostly con-
=
ribonucleic acid and DNA
=
