Origins and Energy of Life
It is believed that life first emerged more than 3.8 billion years ago, about 750 million years after the formation of Earth.1 Since the 1800s, scientists have theorized how cells have come to be.2 While uncertain of how exactly life first arose, scientists have since postulated theories based on laboratory experiments that simulate steps in the process of forming a cell. In the 1920s, it was first suggested that under the conditions of Earth’s primitive atmosphere, simple organic molecules could form spontaneously and polymerize into macromolecules.3 The lack of oxygen in the primitive atmosphere allowed for reduction reactions, allowing the spontaneous production of organic molecules with an energy source such as sunlight or electrical discharges. Stanley Miller first demonstrated the spontaneous production of H2, CH4, and NH3 in a laboratory in the 1950s, providing some credibility to the theories of how life could have come to exist as it does today.4 Because of RNA’s self-replicative nature, it is believed that once macromolecules formed, nucleic acids were then capable of replicating themselves. RNA is thought to have been the first genetic system, later being replaced by a DNA system to form the modern genetic code.1
The first cell is thought to have formed by the spontaneous enclosure of RNA in a phospholipid membrane.5 The nature of the phospholipid membrane, as discussed later in the chapter, creates an internal environment separate from external stimuli allowing for isolated reactions to occur. It is believed that the production of energy, in the form of adenosine 5’-triphosphate as all cells require today, evolved in three stages. The major metabolic pathways of glycolysis, photosynthesis, and oxidative metabolism are all modern-day cells’ primary methods of creating energy.1 These reactions will be discussed later in the chapter.
Function of Cell Architecture
The small size of a cell allows for the efficiency necessary to respond to external stimuli in a timely manner to maintain homeostasis. This basic principle that smaller size allows for optimal functioning is explained by four key points: fast reactions, size matters, concentration, and protect and serve.6
Fast reactions: For living organisms to survive, the speed of synthesis must overcome the speed of entropy, or disintegration. Chemical reactions are the result of random collisions between reactants, and there are mechanisms of increasing the frequency of collisions in a biological system. These include increasing the speed of the reactants (can be achieved by increasing temperature) and bringing reactants closer together by either increasing the concentration of molecules in the solution, or by decreasing the distance between the reactants. Maintaining a small cell size assures that reactants are close enough to allow for efficient reactions.
Size matters: As the diameter of a cell increases, the volume of a cell disproportionately increases relative to the cell’s ...