Description given on Youtube:
The green colour of algae, and of cabbages, pine trees and grasses, comes
from small green bodies called chloroplasts within their cells. Chloroplasts are
distant descendants of once free-living green bacteria. They still have their own
DNA, and they still reproduce by asexual division, building up to a substantial
population within each host cell. As far as a chloroplast is concerned, it is a
member of a reproducing population of green bacteria. The world in which it
lives and reproduces is the interior of a plant cell. From time to time its world
suffers a minor upheaval when the plant cell divides into two daughter cells.
Roughly half the chloroplasts find themselves in each daughter cell, and they
soon resume their normal existence of reproducing to populate their new world
with chloroplasts. All the while, the chloroplasts use their green pigment to
trap photons from the sun and channel the sun’s energy in the useful direction
of synthesising organic compounds from carbon dioxide and water supplied by
the host plant. The oxygen wastes are partly used by the plant and partly exhaled
into the atmosphere through holes in the leaves called stomata (singular
‘stoma’). The organic compounds synthesised by the chloroplasts are ultimately
made available to the host plant cell.
Interestingly reminiscent of the Mixotrich’s Tale, some chloroplasts show
evidence of having entered plant cells indirectly, by piggybacking inside other
eukaryotic cells, which would presumably have been called algae. The evidence
is that some chloroplasts have a double membrane. Presumably the inner one is
the wall of the original bacterium, the outer one the wall of the alga. As with
Mixotricha, we can see recent re-enactments in the many examples of singlecelled
green algae being incorporated in the cells or tissues of fungi and animals.
for example the algae that inhabit corals. Those chloroplasts that have a single
membrane presumably entered directly, not on the coat-tails of algae.
All the free oxygen in the atmosphere comes from green bacteria, whether
free-living or in the form of chloroplasts. And, as mentioned before, when it first
appeared in the atmosphere oxygen was a poison. Indeed, some people colourfully
say it still is a poison, which is why doctors advise us to eat ‘anti-oxidants’.
In evolution, it was a brilliant chemical coup to discover how to use oxygen to
extract (originally solar) energy from organic compounds. This discovery, which
can be seen as a sort of reverse photosynthesis, was entirely made by bacteria,
but a different kind of bacteria. As with photosynthesis itself, bacteria still have
a monopoly on the technology except that, again as with photosynthesis,
eukaryotic cells like ours give house room to these oxygen-loving bacteria, who
now travel under the name of mitochondria. We have become so dependent on
oxygen, via the biochemical wizardry of mitochondria, that the statement that
it is a poison makes sense only when uttered in a tone of self-conscious paradox.
Carbon monoxide, the deadly poison in car exhausts, kills us by competing with
oxygen for the favours of our oxygen-carrying haemoglobin molecules. Depriving
somebody of oxygen is a swift way to kill them. Yet our own cells, unaided,
wouldn’t know what to do with oxygen. It is only mitochondria, and their bacterial
cousins, that do.
As with chloroplasts, molecular comparison tells us the particular group of
bacteria from which mitochondria are drawn. Mitochondria sprang from the socalled
alpha-proteo bacteria and they are therefore related to the rickettsias
that cause typhus and other nasty diseases. Mitochondria themselves have lost
much of their original genome, and have become completely adapted to life
inside eukaryotic cells. But, like chloroplasts, they still reproduce autonomously
by division, making populations within each eukaryotic cell. Although mitochondria
have lost most of their genes, thay haven’t lost all of them, and this is
fortunate for molecular geneticists, as we have seen throughout this book.