Page 245 - The Manga Guide to Biochemistry

The Origin of the Cell

No doubt you’ve heard the phrase “origin of life.” We’re not going to tackle the intimidating
question of what life actually is here. Instead, we’re concerned with the first living organisms
or, in other words, “the origin of cells.”

So how did cells originate on Earth?
By now, you should understand that a cell is the location of many vital chemical reac-
tions in living organisms and that cells can divide and multiply. These are two major traits
that define life.
Chemical reactions that transform one substance into another substance, which are
performed inside of cells, and networks of many of these chemical reactions, which occur
across many cells, are collectively referred to as metabolism.
Cells must constantly undergo metabolism in order to produce the energy required to
maintain their organization and to reproduce.
In addition to metabolism, reproduction, also called self-replication, is a major char-
acteristic of living organisms. Unicellular organisms can often simply replicate their cellular
contents and split in two; this process is called binary fission. For multicellular organisms,
reproduction is more complex and involves specialized reproductive cells, which initiate the
development of offspring.
(Although the way in which cells replicate themselves can certainly be called self-­
replication, the expression is a little awkward. Let’s just call it plain old “replication” from
now on.)
Cells perform both metabolism and replication, both of which are extremely important
when considering the origin of life. But which came first, metabolism or replication?
This is one of the most difficult questions confronting scientists who are conduct-
ing research concerning the origin of life. When the “pouch wrapped in a membrane” (the
cell) originated, what had been occurring inside that enclosure? Some scholars think that
the “pouch” had been performing metabolism after many diverse molecules had collected
together. Some time later, the “pouch” absorbed molecules capable of causing replication,
so the entire assembly became able to divide and multiply. This idea is known as metabo-
lism first.
Other scholars think that the original “pouch” had enclosed replicating molecules and
had been dividing. Later, it became able to perform metabolism, and it eventually obtained
a more advanced replication method, causing it to evolve into a cell. This idea is known as
replication first.
Many other scholars think that posing the question of “which came first” is nonsense
and that metabolism and replication coevolved cooperatively.
In any case, the biochemical process of metabolism is a very large-scale phenomenon.

Molecular Biology and the Biochemistry of Nucleic Acids 231