Читать книгу Oceans For Dummies - Joseph Kraynak - Страница 47

However life started, for it to become more complex, cells were required. (A cell is a membrane-bound entity containing molecules to sustain life.) The first cell is thought to have consisted of a membrane composed of phospholipids surrounding self-replicating RNA. Phospholipids are fatty acids consisting of two hydrophobic (water-fearing) tails and a hydrophilic (water-loving) head that function as the building blocks of all biological membranes. When placed in water, phospholipids naturally aggregate with their heads facing out and tails facing in, forming a two-layer barrier. A cell membrane composed of phospholipids functions like the “skin” of a cell, separating its contents from what’s outside it. In early cells, the membrane functioned as an enclosure for the RNA and other molecules, enabling them to operate as a unit with the capacity to reproduce and evolve.

The first true single-cell organisms to enter the picture were the prokaryotes — bacteria and archaea. The distinguishing characteristic of prokaryotes is that they lack a membrane-bound nucleus (control center), mitochondria (power plant), or other membrane-bound organelles (organized structures within a cell). Early prokaryotes are thought to have been chemoautotrophs — creating their own energy by oxidizing inorganic compounds. Later, approximately 3.5 billion years ago, cyanobacteria evolved, deriving their energy from photosynthesis — using sunlight to synthesize foods from carbon dioxide and water.

Even later, about 1.8 billion years ago, more complex single-cell organisms called eukaryotes appeared. A eukaryote is any organism consisting of one or more cells containing membrane-bound organelles including a distinct nucleus that contains DNA in the form of chromosomes. (You still with us?) Eukaryotes include all living organisms except prokaryotes. In other words, you’re a eukaryote and probably didn’t even know it. However, you do have numerous prokaryotes living on and inside you, most of which are beneficial, and some of which perform essential functions (such as aiding in digestion and synthesizing vitamins your body needs but doesn’t get in its diet or produce on its own).