Over the past century, several scientists around the world have made the following observations: new mitochondria and plastids can only be generated by old mitochondria and plastids through a process similar to binary fission. both mitochondria and plastids contain dna that is similar in shape and size to that of bacteria. mitochondria and plastids are surrounded by two or more membranes whose composition is similar to a prokaryotic cell membrane. mitochondria and plastids are about the size as bacteria. these observations led scientists to conclude that

That this an endosymbiotic relation that has evolved over long time that has made the relationship now obligatory. These organelles represent formerly free-living prokaryotes taken one inside the other in endosymbiosis, around 1.5 billion years ago. According to the theory, bacteria received shelter from the protist cell while it generated energy for use by the protist cell.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.