Page 15 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN002H-54 May 17, 2001 20:22

Bioenergetics 111

and are synthesized in the cytoplasm. Proteins destined carbon sugar) on the third position of the glycerol. Galac-
for mitochondria and chloroplasts have an extension on tosyldiglycerides are absent in the membranes of animal,
their N-terminal end that targets the proteins to the cor- yeasts, and fungi but are present in the photosynthetic
rect organelle and to the correct place within the organelle. membranes of all organisms that carry out oxygenic pho-
These extensions, which, like the remainder of the pro- tosynthesis. The lipid compositions of mitochondrial and
teins, are composed of amino acids, are usually cleaved chloroplast membranes are consistent with the engulfment
off as the proteins find their proper place within the or- hypothesis for the origin of these organelles.
ganelle. Remarkably, some proteins composed of more
than one polypeptide may contain a polypeptide coded
for by nuclear DNA and synthesized in the cytoplasm and IV. ILLUSTRATIONS OF THE USES
another polypeptide that is coded for by mitochondrial OF ATP: ION TRANSPORT,
or chloroplast DNA. Ribulose 1,5-bisphosphate carboxy- BIOSYNTHESIS, AND MOTILITY
lase/oxygenase is a prominent example of such a protein
in chloroplasts. ATP powers most of the endergonic processes in cells.
The discovery that mitochondria and chloroplasts con- How the potential energy of the phosphoanhydride bond
tain DNA, coupled with a wealth of sequence information of ATP may be used to drive otherwise unfavorable reac-
about both DNA and proteins, added credence to the no- tions (Fig. 11) is discussed in this section. This discussion
tion that these organelles arose from the engulfment of focuses on three major uses of ATP: the generation of ion
unicellular organisms by a primitive nucleated cell. Mi- gradients, biosynthesis, and movement.
tochondria may have been derived from a bacterium, and
chloroplasts, from a unicellular alga. After the engulfment A. Ion Transport
events, genes in the bacterium and alga coding for proteins
that duplicated those in the nuclear genomes of the hosts The plasma membrane is the barrier that separates the
were lost and other genes were transferred from the bac- cytoplasm of cells from the exterior medium. All cells
terial and algal genomes to the genomes of the hosts. maintain a membrane potential that is negative. There is
The distribution of proteins and lipids within biological an excess of positive charge in the external medium in
membranes is asymmetric. Thus, one side of a membrane comparison with that in the cytoplasm. The membrane
is distinct from the other. The coupling membranes of mi- potential in plant cells can be as high as −200 mV. En-
tochondria and chloroplasts are opposite to each other. ergy is required to generate and maintain the membrane
Protons are ejected from mitochondria during respiratory potential.
electron transport but are taken up by thylakoids during All cells maintain gradients in ions across the plasma
light-driven electron transport. The catalytic portion of the membrane. The intracellular K concentration is higher
+
ATP synthase is located on the outside of the thylakoid than that of the extracellular medium, and the concentra-
membranes, whereas that of the mitochondrial ATP syn- tion of Na , much lower. The free Ca 2+ concentration
+
thase is present on the inside of the inner membrane. As in the cytoplasm is maintained at very low levels, 1000-
seen in Fig. 7, the orientation of the coupling membranes fold or more below the extracellular Ca 2+ concentration.
of mitochondria and chloroplasts is consistent with the Often the intracellular proton concentration can be quite
hypothesis that these organelles are of bacterial and algal different from that in the medium. The pH in the cyto-
origin. plasm of plant cells is close to 7.0, whereas that in the
Each membrane in a cell has its distinct set of proteins medium is about 5.0. Energy is needed to generate and
and lipids. The most common membrane lipids are phos- maintain these ionic disequilibria. For example, the en-
pholipids.Phospholipidsarediglycerides.Twoofthethree ergy cost to generate a pH gradient of two pH units is
+
hydroxyls of glycerol are linked to long-chain fatty acids equal to RT ln([H ]/[H ]), where the subscripts o and i
+
o
i
by ester bonds. The third position is occupied by phos- stand for outside and inside the cell, respectively. At 25 C,
◦
phate. A number of different polar substituents are linked the G for a 100-fold proton activity (pH 7.0 in versus

to the phosphate by anhydride bonds. The phospholipid pH 5.0 out) gradient is 2.7 kcal/mol.
composition of the mitochondrial inner membrane is vir- Plasma membranes of all higher organisms contain en-
tually the same in plant mitochondria as in animal mito- zymes that are embedded in the membrane that act as ion
chondria and resembles that in the plasma membrane of pumps. That is, they catalyze the transport of ions against
some bacteria. The lipids in chloroplast membranes are their electrochemical potential. In physiology, transport
very distinctive. The phospholipid content is unusually that is thermodynamically uphill is termed active trans-
low and about 80% of the membrane lipids in thylakoids port to distinguish it from the spontaneous flow of ions
are diglycerides that have one or two galactose (a six- down their electrochemical potential. The energy needed