One of the simplest cells found on earth is an obligate parasite called the pleuro
pneumonia like organism or PPLO
in short and that lives in some animal lungs. It is a very small almost spherical cell, about a tenth of micrometer in diameter. Its envelope consists of a plasma membrane only. In its hyaloplasm (salt water rich of organic molecules including enzymes) the only organelles found are ribosomes. The genetic center is a nucleoid (a single circular molecule of DNA in a zone without ribosomes).
And that is enough to carry on the many chemicals reactions that make up what we call life. Indeed, with the enzymes not only in the cytoplasm but also in plasma membrane and in the nucleoid, the PPLO
continually transforms its matter/energy molecules into new molecules more suitable for its current needs. But to grow it has to add more matter/energy from the outside into its system and also get rid of waste products not needed inside. Thus as a living organism it takes its food (water and organic molecules) from the lung cell it is attached to, digests it its cytoplasm, retains simple molecules like amino acids, simple sugar molecules (glucose and ribose for example), organic acids, nucleotides, etc. use them to create its own more complex organic molecules on demand (proteins, lipids, carbohydrates, RNA, DNA, etc.) and release in the animal lungs waste products that may cause pneumonia in the host.
To import and export as a way of living the PPLO uses the selective permeability of its plasma membrane that filters what needs to go out and what needs to enter into the cell.
To do all this work some form of energy is needed. It is a chemical based energy necessary to link atoms that make up molecules and released when molecules break down into smaller molecules they were made from. There is a molecule called ATP (adenosine tri phosphate) that is used in living cells to move chemical energy from place to place to place as needed. When a chemical reaction releases energy by breaking down a molecule that energy is used to create ATP from its source molecules (AMP, ADP and phosphate). When energy is needed somewhere in the cell to create new molecules, existing ATP is broken down to release needed energy for the process.
So the PPLO lives by assimilating external matter/energy as its own, by using enzymes to import, export and transform existing matter/energy into more useful forms of matter/energy and in the process accumulates ATP when existing molecules are broken down and consumes ATP when creating new needed molecules. Since enzymes are the active molecules and are short living, they must be replaced so that life could last longer than the life span of existing enzymes. That is where the DNA in the genetic center comes in.
DNA carries on genetic information in the form of genes. A gene is a portion of DNA with a specific place on the DNA molecule and having a specific function, the translation of that portion of DNA into a certain form of RNA (mRNA, tRNA, rRNA, etc.). The different types of RNA molecules combine their actions to allow protein synthesis at the ribosome level in the cytoplasm and among proteins are enzymes that play the key role of sustaining chemical reactions combined to make up all those cell activities called life.
By doing this the PPLO can grow, reach the highest size it can reach and the divide into two smaller cells that go the same process.
That is how one of the simplest cells, the PPLO, lives. By nature life is complex and cannot be described in a few words. A living cell has some personal goals to meet (existence, growth, adaptation to the environment, reproduction, etc.) and work to meet them. That is why life is not simply a work. Life is a useful work. That useful work is based on assimilating external matter/energy, so it can be also called a useful assimilation.
What does the PPLO to live is what all living organisms do but with more tools and more outstanding result. We can see animals and their many signs of living organisms just because they each are huge societies of cells working in harmony. To understand their very complex live we need to go to the simplest forms of life like that of the PPLO.