The Cellular Nature of Life
Before microscopes revealed the 'granular' nature of life, it was not obvious how large organisms were constructed. An animal could be taken apart and shown to have bones, skin, organs and fluids, but an insect had no bones, and a plant had no obvious internal structure at all. It was all very confusing. Any search for a common pattern or theme that would link together the structures of living things seemed doomed to failure.
That was why the discovery of cells, and the universal occurrence of cells in all living things was so exciting to Mendel and contemporary scientists. Life was granular. These tiny blocks, called cells, could be found in plant roots and human brains, they functioned perfectly well in pond scum as they did in fermenting beer casks. Cells, it seemed, had the fundamental property of life, and the diversity of amazing degrees of specialization.
As the studies of the nineteenth century slowly revealed, all cells share a set of properties and basic functions that combine to produce the complex almost indefinable phenomenon we call life; cells are alive in a way that even complex machines such as computer driven cars are not. Each cell is a tiny, unique entity separated from the outside world by a boundary membrane (and sometimes a wall); they need a constant supply of chemical energy, which they absorb and convert to their own needs; they are creative synthesizers of amazingly complex macromolecules; they grow; they reproduce and have the ability to respond to rapid changes in their environment. In short they have all of the "signs of life" see at much higher levels of organization.
All cells share, to some extent, common physical structures that perform the same role for the cell no matter what the cell is doing. Cells in pond scum have the same type of cell membrane found in panda intestines and in both cases the cell membrane is carrying out exactly the same tasks.
But at the next level, cells are capable of a remarkable degree of specialization. Some cells, like those found in plant leaves, specialize in harvesting light energy and converting it to chemical energy. Others, like the cells found in adipose tissue, are specialized for lipid storage. Under the microscope, these two types of cells can appear, at first glance, to be totally different and share nothing in common. It takes careful examination and analysis to show that both type of cells have the same fundamental structures, but either one type has an organelle not seen in the other (chloroplasts), or grossly exaggerated structures (such as the vacuoles that store the lipids).
Despite the enormous range of specializations, all the individual cells within a single multicellular organism must be integrated and coordinated. Without this integration, a multicellular organism would be nothing more than a collection of cells in a soup of liquid, like the algae in a pond. Cells must communicate with one another and coordinate their activities, so, although life is granular, the individual granules are more than just selfish 'building blocks', they are partners and shareholders in the corporation we call an organism.
Quite a role for a tiny globule of life to play.