B 5B Cellular Complexity on-level – 03
“How did a cell become complex if it began as simple unicellular organism?” a student asked. Mrs. Jackson explained that complexity can arise in different ways. One of the simplest ways would be to combine simple cells. One example of this is the mitochondrion and the chloroplast. There is evidence to show that they were once primitive bacterial cells. The endosymbiotic theory discusses how larger host cells ingested these bacteria and over time became dependent on each other for survival. Mitochondria and chloroplasts, unlike other organelles, have their own DNA and reproduce independently. Over millions of years of evolution, mitochondria and chloroplasts have become specialized and cannot survive outside the cell. The result of this endosymbiosis is the rise of a complex cell with specialized organelles.
We are made up of billions of cells that have different structures and functions. Differentiation also leads to complexity. Differentiation enables cells to modify to perform to the requirement of the specific type of cell. Evolution supports complexity only if it’s beneficial for survival of the organism. Accumulation of mutations over time can produce very complex organisms. If a mutation is not beneficial, then natural selection would select against these mutations, and the characteristic would be eliminated from the population.

“But,” paused Mrs. Jackson, “where there are rules, there can be exceptions.” As a rule, animals cannot photosynthesize, but pea aphids use carotenoids to use the sun’s energy to make ATP using genes that they copied from a fungus. The pea aphid’s body color ranges from white to orange to green. Researchers have found that the green aphids have significantly more ATP.
Some sea anemones, clams, sponges, worms, and the spotted salamander also have photosynthetic endosymbionts. The chloroplasts are found near the mitochondria in salamander cells. This could mean that the mitochondria directly consume the oxygen and carbohydrates created during photosynthesis.
