Modern Cells

Today, living cells can be divided into three major groups: bacteria (prokaryotes), archaebacteria (archaea), and modern cells (eukaryotes), which include all animals and plants. Eukaryotic cells are composed of parts derived from cells of the first two groups. The exact process of the fusion of precursor cells is the subject of various theories. The most widely accepted theory is the endosymbiotic theory, which suggests that eukaryotes originated when prokaryotic precursor organisms entered into a symbiotic relationship. Over time, these endosymbionts evolved into organelles within their host cells. The organelles, which still retain many prokaryotic features, include mitochondria and chloroplasts. These organelles enable eukaryotic cells to perform cellular respiration and photosynthesis.