I have previously explained how complex eukaryotic cells, the cells that make up the tissues in our bodies, developed. They arose in an early stage of the evolution of life from simpler bacteria cells called prokaryotes.
This was one of the major transitions in the history of life and would pave the way for other accelerations in life’s development.
Eukaryotes would prove to be more adaptable and more efficient at extracting energy from food sources. Unlike prokaryotes, these cells could associate with one another to form specialized tissues and they made sexual reproduction possible.
The earliest forms of life were prokaryotic cells, essentially bacteria. They were good at perhaps one task, but had little potential for more complex activities.
At some point, prokaryotic cells hijacked other prokaryotes by engulfing them within their external membrane. This is the process called endosymbiosis.
Evidence strongly suggests the hijacked bacteria became so intertwined in the workings of the host cell that they became part of the host’s internal structure. These structures are called organelles (small organs).
The ability of the once free-living bacteria to perform a specialized task was incorporated into the operations of the host cell. At some point, it became structurally and functionally indistinguishable from the architecture and workings of the host cell.
These transformed more robust cells are called eukaryotes. They have elaborate structures and new capabilities, all because of their various specialized organelles.
Two organelles are of particular note in the saga of life’s increasing complexity: chloroplasts and mitochondria.
Two vital functions, photosynthesis and cellular respiration, are performed by these two organelles. Without them complex life forms would be impossible.
Chloroplasts reside in the cytoplasm of plant cells and algae. They are the sites where nearly all the energy required for life on Earth is captured from sunlight.
The process begins with sunlight being absorbed by chlorophyll imbedded in the structural membranes of chloroplast. The light’s energy is used to produce ATP (adenosine triphosphate), a molecule that serves as the currency of energy transport in living organisms.
This is followed by a second phase, called the Calvin Cycle, where the chemical energy stored in ATP is used in a chemical reaction that combines six molecules of carbon dioxide and 12 of water to make the sugar glucose. The energy of the sunlight now resides in the energy-rich carbon bonds of this carbohydrate.
In the cells of both plants and animals energy is derived from glucose in a process called glycolysis. The initial stages of the process occur in the cytoplasm of both prokaryotic and eukaryotic cells. Its products are pyruvate, something called NADH, and 2 ATP molecules.
This anaerobic respiration isn’t very efficient. Much of the stored energy remains untapped in the pyruvate molecule. Better efficiency was achieved once metabolic pathways developed that used oxygen.
Atmospheric oxygen concentrations had been very low early in Earth’s history. But some bacteria developed metabolic pathways capable of extracting the additional energy in pyruvate molecules. These bacteria would become, through endosymbiosis, the mitochondrial organelles discussed earlier.
In eukaryotic cells, pyruvate is oxidized (caused to react with oxygen) in the cell’s mitochondria. There are many steps in this process, called the Krebs cycle, but the end result is six molecules of carbon dioxide, six of water and nearly 30 molecules of ATP.
This is aerobic cellular respiration. It is nearly 15 times more efficient than the anaerobic respiration that predominated in the early stages of life’s history when prokaryotes prevailed. It is the reason eukaryotic cells were so much more resourceful in extracting their greater energy needs from food sources.
With more readily available energy, cells developed better means for moving about and the ability to move molecules across cell membranes. This later function would become important for the signaling that occurs in multicellular organisms, especially those having specialized tissues.
Other organelles provided internal scaffolding in the cytoplasm of eukaryotes. These structures are filaments that form elaborate webs. They serve to support a larger cell volume and contort the cell membrane into adaptable shapes.
Filaments and other structures within the cell’s cytoplasm are classified as organelles. Their origins are thought to be similar to those of mitochondria and chloroplast.
But, unlike mitochondria and chloroplast, they did not retain their own DNA from their prokaryotic past.
Filaments and other structures are typically attached to the outer cell membrane or to the membrane of the nucleus. They act as guards, regulating what passes through membranes and is transported within the cytoplasm. This facilitates passing instructions to and from the cell’s cytoplasm and nuclear DNA.
Similarly, by orchestrating transmission of signals across the outer membrane, they control interactions with other nearby or distant cells. Membrane structures also facilitate cells adhering to one another, allowing cells to congregate together and form tissues.
Finally, in sexual reproduction, a cell divides itself, creating two daughter cells. In the process, the cell’s two strands of DNA are separated and segments of each become reshuffled. Filaments and internal structures facilitate the entire process.
Fertilization brings contributions of DNA from daughter cells (gametes) contributed from each parent. The result is a unique individual having traits that may or may not promote its survival and eventual reproduction.
It is upon this blend of attributes that nature acts, favoring propagation of the fittest within a population.
These major transitions exemplify life’s constant innovation through unrelenting cobbling of simple things into ever greater complexity.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.