Have you ever found yourself sitting out under the stars, gazing up and wondering ‘What is life?’? Well, Paul Nurse, a Nobel Prize winner, has thought about it pretty deeply, and has written a book called just that. I thought it might be useful to report back on his conclusions.
Paul Nurse started as a ‘lowly’ lab assistant, who was encouraged by his brewery employers to do some research into yeast. It turns out that Nurse had a talent for research, but not the qualifications he needed to get into university, despite taking the exam six times. Finally he was ushered into Birmingham University ‘by the back door,’ and went on to make several fundamental discoveries about how life is organised - and how it organises itself. By looking at cell biochemistry and cell division in yeast, he was able to elucidate the basic mechanics of life.
He starts off by saying that ‘life is chemistry.’ On Earth, life is always based on cells - the membrane-bounded ‘smallest particle of life.’ Each cell is a microscopic chemical factory, which enacts multiple chemical reactions many times per second.
He points out that cells make the energy that they need for their chemical reactions through a process called cellular respiration. This takes place in tiny reactors called mitochondria that occur within each cell (sometimes one to a cell, sometimes many thousands, such as in heart muscles), an arrangement which may have evolved after one early cell ‘swallowed’ another (about 1.5 billion years ago) and both found benefits. Cellular respiration is the process in the mitochondria whereby sugars (derived from the sugars, fats and proteins that our bodies eat) react with oxygen to create water and carbon dioxide, releasing energy.
The exact mechanism, as related by Nurse, is quite extraordinary. Each mitochondrion has a double membrane surrounding it. Protons, single atoms of hydrogen that have been stripped of an electron, are pushed out of the centre of the mitochondrion into the gap between the two membranes. They then physically rush back into the centre of the reactor through channels made of protein, pushing past tiny molecular turbines, which capture the energy of the rushing protons and transform it into high-energy chemical bonds in the form of adenosine triphosphate, or ATP, which is the universal molecule of life. Practically all of the food that you eat is processed in your mitochondria, and this is a process that occurs so frequently (around 150 times per second) in each of your body’s 30-trillion or so cells, that your body’s cells produce your body-weight in ATP every day. ATP is used in turn by the cell to power its myriad chemical reactions, being broken down and having to be recreated in the next round of synthesis.
Whether a cell is powered by cellular respiration, by photosynthesis (plants), or chemosynthesis (hydrothermal-vent dwellers), whether it is a single-celled life-form (a bacterium, for example) or one with 100 quadrillion cells (a blue whale), all of these cells rely on chemical reactions to sustain themselves. Cells working together form multicellular organisms, such as ourselves.
It seems that the prime directive of all life boils down to two things - finding nutrition to power the mitochondria and ensuring reproduction - ‘food and sex.’ The manufacture and use of building materials - incuding gypsum wallboard, insulation and roofing - are higher-level behaviours that enable us to do everything else. Nurse also points out several other factors that are common to all life on Earth:
- The ability to evolve through natural selection;
- Being a bounded, physical entity, separated from but in communication with its environment;
- Life is composed of chemically-driven, physically-active and information-based organisms that construct their own metabolism and use it to maintain themselves, to grow and to reproduce.
Paul Nurse states that the similarity of ATP-reliant chemistry in cells on Earth - being the same in a yeast cell as in a human and a whale - suggests that life evolved just once on the planet, over 3.5 billion years ago - before the earliest fossils of unarguably ‘alive’ cyanobacterial colonies.
However, Paul Nurse also goes further: he suggests that the information-dense and ultra-stable DNA molecule, which allows the passing-on of information from cell to cell and from generation to generation, and which allows cells to synthesise numerous life-critical chemicals using RNA and ribosomes (macromolecular biological machines that perform protein synthesis) - may be a general pattern that is followed by organisms throughout the universe.
So, when staring up at the stars, wondering what life is, you might be startled by the thought that something up there - based on the same or similar chemistry - might be looking at us through interstellar space, wondering the same thing: ‘What is life?’
