October 8, 2011
Life as we (and as we don’t) know it: 2

All life on Earth can be divided into two domains. Eukaryotic and Prokaryotic. These domains describe the two types of cell which inhabit our planet. To those out of the loop, or yet to enter said loop - the cell is smallest unit of life. The most basic entity that can currently be considered as a living system.

Prokaryotes are the phylogenetically older of the two cell types, meaning they existed first. Other cells then evolved from them. Before the prokaryotes, would have been some kind of proto-cellular organism, which would have been semi autonomous, relying on the energetics of the primordial soup around it to power its longevity. Prokaryotes are much simpler organisms than eukaryotes and understanding their biology, as well as being crucial to medical knowledge, is also a glimpse into the past, and at our origins. Their structure is, in essence, a shell containing a soup of chemicals and some circular DNA. All bacteria are prokaryotes.



Eukaryotes are much more structured (hence have a much more negative entropy if you remember PART 1 to this post). They contain organelles - which are like organs in a body, but instead, these are mini organs inside cells. These organelles are separate compartments within the cell, encased in membranes, to separate the particular mix of macromolecules and chemicals in them from other separate mixtures in other parts of the cell. This means that they can eacn carry out specialised functions in cellular metabolism, basically ensuring each region does its function properly and in relative isolation. The DNA is kept in an organelle called a nucleus, and the DNA is a linear strand, not circular like in a prokaryote. The DNA is also bound by loads of proteins and contains genetic switches. Eukaryotes thus require more energy to maintain this increased structure (or massively negative entropy), hence why they respire by an oxidative metabolism mediated by MITOCHONDRIA - the energy powerhouses of eukaryotic cells. All animals, plants, protists and fungi are eukaryotes Eukaryotic organisms are everything from amoebae to trees, to whales and mushrooms, to you and I.

Plants, animals, protists and fungi. The eukaryotic kingdoms.

Billions of years ago, mitochondria were once prokaryotic cells which were engulfed by primitive eukaryotic cells (which would only have been capable of anaerobic respiration via process known as glycolysis), and they formed a symbiotic union. The early mitochondrial precursor cell would have been able to use atmospheric oxygen to help in the production of massive quantities of ATP (the energy currency of all life on earth), and the eukaryotic cell would have provided it with stability, safety and nutrients. They would have reproduced in tandem, producing more eukaryote-prokaryote hybrids to the point they would have been undergoing symbiotic evolution. These days, mitochondria only function semi autonomously. They have their own genome, but most of their function is coded for by the main DNA in the nucleus if the host eukaryote cell. The process of mitochondrial-eukaryotic union is known as the Endosymbiosis event.

Endosymbiosis theory

This extra energy input made it possible for organisms to maintain larger more robust structures, become multicellular, and thus the entropy of life on earth decreased and decreased to the point we have us! Us. We require a lot of energy just to maintain our brain. The human brain is only 2% of our body mass, yet uses a staggering 20% of our oxygen intake. On top of this, We require a lot of energy to help build up the physically ordered societies and cities we see around us, and this comes at an energetic expense of the environment. We’re decreasing our entropy, and if you remember, there must be a corresponding increase in entropy in the surroundings from which the energy was taken (every action has a reaction… Newton!). This means more disorder in our surroundings. We use a lot of fossil fuels and throw away a lot of junk. Our massive energy dependence, and poor waste disposal is physically and biologically unsustainable.

Damn you physics.

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