Last update: 6 V 2018, Luxor t|c
This particular phenomenon surrounds us wherever we are, yet we still struggle to make a working definition for it, as it is a working process. Many were proposed, even more denied, though we have some kind of idea about what life really is. Some approach the problem from the point of view of a physicist, stating:
"Living beings are thermodynamic systems with an organized molecular structure that can reproduce itself and evolve as survival dictates."
Or to put it in different words:
"Life is an open system which makes use of gradients in its surroundings to create imperfect copies of themselves."
Let’s stick to these for a moment. These definitions make use of the physical processes that govern the Universe. As everything that exists within it, Life is a subject to the laws of nature. Every living being uses energy conveyed to its environment to maintain its low entropy (high complexity), increasing the total entropy of the environment (low complexity). For example, bacteria living in a pond feed off the organics dissolved in water, excreting non-organic, basic compounds to their environment. Every living thing does that – even plants, by absorbing matter from the ground, decrease its complexity.
Life has an ability to reproduce, what will be discussed in detail later on. Of all things in the universe, living organisms can create working copies of itself, which maintain low entropy and high functionality. Literally every cell on the Earth can do that, with no exceptions. That’s where another question becomes important: how did the very first lifeform look?
Well, we don’t know. We have nothing but assumptions, yet we are certain about what it must have been able to do. The very first living thing on Earth must have had the ability to:
- reproduce, presumably by fission,
- maintain its basic functions after reproducing,
- catalytically conduct certain chemical reactions.
Before we’ll get back to the topic of covering the planet with the offspring, let’s take look at some different definitions.
"Life is considered a characteristic of something that preserves, furthers or reinforces its existence in the given environment."
Life is very good at surviving no matter the odds. So far, life on Earth has survived 5 global extinction events, with the 6th one just over the horizon, though this time, we might be the baddies. What’s the Life’s trick? It is very resilient, which is determined by a crucial ability of living beings to adapt to their environment. Organisms can make changes in their bodies that enable them to thrive in less pleasant environment, yet this applies to all populations and the entire biosphere. How does it do that? We’ll find out in the next chapter.
"Life is a general term for the presence of the typical enclosures found in organisms; the typical closures are a membrane and an autocatalytic set in the cell."
This definition touches upon the chemistry of life, which will be discussed in detail in the latter part of the article. Living organisms cannot persist without some kind of a barrier, shielding their interior, “guts”, from the environment. Without some kind of such structure, such beings would be prone to immediate loss of homeostasis, a property of Life that is crucial for its very survival. As an example – pouring droplets of water onto a concrete causes them to soak into the ground; if you drop a balloon filled with water on the ground, there is some chance that it will not fracture. Similarly, ink droplets added to a glass of water will not hold tight together; instead they will disperse in water, creating a coloured, dark solution.
This brings us to a clear conclusion: defining Life is very, very hard. Not because it ditches our senses or knowledge, though because it does so much, in so many different ways, that it is simply hard to put in a few meaningful words. Life is so diverse, that describing it would not really impact what we know about it. In spite of that, we know a lot about terrestrial life. We have managed to distinguish several basic properties. Nearly all manifestations of Life on our planet have the ability to grow, reproduce, maintain homeostasis and their metabolism, respond to stimuli, hand over their traits to new generations of organisms and are characterized by an universal type of structure – all terrestrial organisms are made of cells – should they be unicellular or multicellular. Where do these traits come from? How do they impact how living organisms look and behave? To answer these questions, we must travel nearly 4 billion years into the past.
Life’s journey begins with the abiogenesis – the gradual process of diversification of organic compounds, leading to birth of the first living organisms. For centuries, humankind was puzzled by the origin of life – where did it emerge? Primary sources, that being fossils and various deposits found around the world. The oldest of these leftovers were found near hydrothermal vents – Earth’s wrinkles found on the bottom of the oceans. These structures, formed in volcanic activity, are stable sources of protons (H+ ions) and various inorganic compounds vital for life. These energy vents are proposed to be the objects which hosted first life forms, feeding on the abundance of minerals and stable acidity gradient sustained by these vents.
The Gurgling Broth Edit
Even the most primitive lifeforms needed building blocks to survive though, so even if we work out where life appeared, we still need to understand why sugars, fats, proteins and nucleic acids came from. In an attempt to solve this question, Stanley Miller and Harold Urey have conducted the famous “Miller–Urey experiment”, which has shown, that complex organic compounds can spontaneously form from basic substances like methane, ammonia or carbon dioxide, under certain conditions. These reactions have most likely occurred in the young atmosphere of Earth, which has led to permeation of first oceans with organics, which often reacted with each other with a little help from inorganic catalysts, e.g. transition metal oxides.
Terrestrial life is mostly composed of 4 essential elements: carbon, oxygen, nitrogen and hydrogen (CONH). These elements can form an incredibly wide variety of molecules, including simple amino acids and more advanced globular proteins. These elements are abundant on earth – hydrogen and oxygen are widely spread on the surface of the planet in the form of water, which together with oxygen and nitrogen molecules is also present in the atmosphere. Carbon however, not only makes up a large family of salts known as carbonates, but is also found in organic compounds. Generally, compounds widely spread among the living organisms on Earth can be classified into 4 main groups: carbohydrates (= saccharides), fats, proteins and nucleic acids.
Among these groups, carbohydrates are the most widespread organic polymer on Earth, found mainly in the walls of plant cells. Monosaccharides, e.g. glucose or fructose, are the main source of energy for living organisms, and, unsurprisingly, the most common substances in the biosphere of the planet. This property arises from the properties of these compounds – they are relatively small, easy to modify and contain a lot of energy which can be utilized.
Fats are particularly important for living organisms because of their chemical properties. These compounds are not polar and barely interact with water. This characteristics makes them very unlikely to mix with it, rather staying in larger aggregates, sometimes forming micelles. Most fats found in living organisms are lipids – glycerol esters, containing one polar group and two long, hydrocarbon chains. This structure causes these molecules to form two-ply membranes in water, with water-friendly (hydrophilic), polar “heads” contacting with the water and water-unfriendly (hydrophobic) hydrocarbon chains hidden within the structure.
[PH Proteins & Nucleic acids]