The following is a 600-word essay which I wrote for Physics of life School at National Center for Biological Sciences-TIFR, Bangalore, India.
Biological cells, that all living organisms are made of, singly or collectively, are open dissipative machines, in a complicated assemblage of sub-machines in a hierarchical organization of soft matter, which is a perfect system showing emergent properties. They are ever-evolving, self-replicating machines that utilize energy stored in the chemical bonds and perform tasks with incredible organization and coordination, at all levels, for self-sustenance.
Machines require energy to work, be it mechanical or electromagnetic or chemical energy and a program i.e. a set of instructions to accomplish tasks. Some cells get their energy by converting electromagnetic energy to low energy covalent bonds in glucose by photosynthesis using chlorophylls like photovoltaic cells and some have been seen feeding on inorganic molecules directly. They then extract the energy from the glucose in the mitochondria (power source) to produce high-energy ATP molecules (energy currency), which drives all the activities in the cell from DNA replication, RNA transcription, protein synthesis by Ribosomes to active transportation and locomotion.
The cell as a machine is called alive because at every moment delicate, elaborate and sophisticated chemical reactions-Metabolism takes place efficiently, that’s why it is necessary to confine them to a region, by a selectively permeable boundary-Plasma membrane, which can discriminate between the materials transported in and out of the cell. A more sophisticated cell requires more sophisticated membrane in general. The metabolic pathways in advanced cells are regulated by compartmentalizing the cell-Organelles to make the reactions accurate, targeted, less interfering and coordinated by creating optimal conditions for heavy molecular engines-Enzymes to work. Cells are so robust that they have evolved from extreme environments on earth. From hydrothermal vents to freezing cold temperatures to acidic environments.
To make these kinds of machines the raw materials required are amino acids, lipids, and carbohydrates. Amino acids form proteins. Protein can give structure-Cytoskeleton to the system and are very specialized materials so together with lipids they form the membrane system and most importantly they work as engines-Enzymes that catalyze the metabolic reactions and play a major role in cell’s defense system e.g. antibodies. The carbohydrates majorly used as fuel.
To run this system we need instructions coded in a sophisticated material which must be readable with high fidelity, replicable so that it can be passed on to the next generation and editable so that variation can be introduced (Mutation).
A measure its complexity can be estimated by:
So it is fair to say that even a liver cell is much more complicated than the most complicated machine like Large Hadron Collider. We exploit this very nature of machine of cell in genetic engineering for producing vaccines and agricultural products.