1. All organisms are made of cells
Cells are the smallest unit of life. Each cell is a membrane of semi-permeable phospholipids wrapped around cytosol or a solution of water and dissolved solutes. All cells rely on DNA to hold the information necessary to produce the molecules they use to obtain energy. Although the methods for obtaining energy vary widely, all organism obtain energy to grow and reproduce. This first tenet of the cell theory is mostly true, but the discovery of viruses lead to complications. Viruses, while they use DNA or RNA to reproduce, do not have cells or cellular membranes. Viruses typically use a host cell to replicate. In this case, the virus appears to be living, but does not create its own cell. Some scientist argue that viruses are not living, thus cell theory is not violated.
2. Cells are the most fundamental unit of life
Organisms can be single cells, which hold all of the components necessary for a metabolism, or they can be more complex. More complex organisms divide the various metabolic tasks into different groups of cells, called tissues. These tissues are arranged in compartments with membranes that separate them from other tissues. These groups of tissues are called organs. A group of organs functioning together is an organism, or an individual creature. Each cell is distinct from the cells next to it, and each functions independently, while contributing to the output of the organism as a whole. Again, modern cell theory is a bit more complicated because advances in science have revealed many different organelles within cells. These organelles are bound in membranes themselves, and serve different functions for eukaryotic cells. Some scientists argue that these are more fundamental units, but other scientists argue that like an organ outside of an organism, they could not function without the cell.
3. Cells come from other cells
As far as we know, no cell on Earth currently has arisen spontaneously. All cells are the result of cell division. When a cell is large enough, it replicates its DNA and important components. These components can then be divided into two daughter cells, which are copies of each other. Variations in the DNA in each cell can lead to changes in how they function, which can result in them dividing at different rates. The cell that reproduces more than the other cell will pass on more of its DNA. The purpose of every cell or organism is to reproduce the DNA in cells.
This third tenet of the cell theory has yet to be disproven. No scientist has ever created a functioning cell without replicating another cell, although some scientists are trying. If they were successful, it would give proof to how life could have evolved. It is thought that a self-replicating molecule mutated, developed the ability to produce a membrane, and thus the first cell was born. The cell was such a successful form of life that all life since has used the same basic template.