The big question for this segment is, what way do eukaryotes and multicellularity represent greater levels of complexity than prokaryotes? [MUSIC] Today I want to talk about the way in which complex multicellular organisms arose from the first forms of life on Earth, the prokaryotes. But first, where did prokaryotes come from? They appeared on Earth almost 4 billion years ago from inorganic origins, bonds formed between simple inorganic atoms, most especially between carbon, nitrogen, phosphorous and hydrogen. As a result, small molecules and eventually, chains of small molecules called polymers evolved. This is the first stage in the evolution of living cells, the first steps from chemistry to metabolism. Metabolism requires the input of energy to support catalytic reactions that give rise to stable chemical bonds. Ultimately, this energy comes from the sun. So, harnessing energy features in the earlier steps in life on Earth. Let's think of living organisms as conforming to the laws of thermodynamics. Energy must be invested in entropy randomness and decreases creating an internal order in the form of polymers, compartments and metabolic control. However, two other vital components will be needed for basic forms of life to evolve, a replicating genetic code based on chemical information and a means of generating chemical energy to catalyze reactions. Let us explore these two aspects of living cells. They probably arose about 4 billion years ago in prokaryotes, an early life form, but one that persists today in the form of bacteria and in the domain of the Archaea. First, the chemical code, it is stored in the pairing pattern of four basic molecules called nitrogenous bases within long chains of sugar phosphates called nucleotides. RNA, the earliest form of nucleic acid, can still be found as the primary repository of information in retroviruses, such as human immunodeficiency virus. DNA then evolved to become the dominant storage molecule for genetic information, even in prokaryotes. Prokaryotes have a tiny circular DNA complement that can be thousands of times smaller than in eukaryotes. Hence, prokaryotes generally have fewer genes and lack the non-coding DNA typical of the eukaryotic cells. This DNA is not bound by a membrane such as the nucleus of the eukaryotic cell. Instead, it is in a nucleoid in the matrix of the prokaryotic cell. We will discuss circular DNA again when we consider organelles in the eukaryotes. Because prokaryotes do not have any internal membrane system, they do not have internal compartments in which to concentrate metabolites. However, prokaryotes are living cells and therefore they make metabolic energy and can use it to power their movement, often by flagellin and swim towards resources such as light and food. You see them here swimming towards a sugar crystal. These bacteria act as a coordinated population by detecting diffusion gradients. How can this energy supply be achieved in such a primitive structure? Prokaryotes adopt many life forms. They can be heterotrophs, deriving their energy from inorganic resources such as volcanic extrusions like hydrogen sulfide, or autotrophic, using solar energy for photosynthesis. Prokaryotes achieve these energy transformations by exploiting proton gradients across the plasma membrane that bounds the entire cell. The linkage between proton gradients and energy as ATP is achieved through the ATP synthase enzyme complex. On the other hand, eukaryotes make most of their energy in a complex series of electrical and chemical events in organelles, specifically, the mitochondria. As with prokaryotes, eukaryotes can be heterotrophs that consume organic compounds or autotrophs that photosynthesize. The common feature is that all living things must acquire resources to produce chemical energy. This energy, in the form of ATP, is used to synthesize macromolecules, that is, polymers, generate new cells, grow, and support many other metabolic processes like defense. Eukaryotic cells are complex and emerged from a world of prokaryotes more than 2 billion years ago. They probably arose in a series of events involving prokaryotic cells being engulfed by other cells. This process enslaved these prokaryotes to form what we recognize as the organelles of modern eukaryotes. So maybe it is not surprising that organelles have internal membranes, a circular DNA molecule, and provide new metabolic capabilities in eukaryotes. We call this process of enslavement, endosymbiosis. Among the organelles that arose from these endosymbiotic events are the mitochondria and chloroplast and, possibly, the nucleus. For example, mitochondria probably arose from a single endosymbiotic event, we think from the bacterium that causes typhus. And chloroplast probably arose from capture of the cyanobacterium commonly called blue green algae. Compartmentation by internal membranes enable powerful biophysical forces to develop. In particular, it enabled proton gradients to be created, driving ATP generation for an exquisite molecular scale pump, ATP synthase. You'll recall that I mentioned this pump in prokaryotes. So we should not be surprised to see it appear as a product of endosymbiosis. This is nature's smallest and most important nanomotor. These functional advances and a much increased genome size have conferred great metabolic flexibility on multicellular organisms and a capacity to acclimate quickly to changing environments. So are eukaryotes functionally superior to prokaryotes? This is a moot point. Prokaryotes have very short generation times, small surface area to volume ratios, and less capacity to gain resources quickly, and the ability of populations to mutate, and thereby respond to external conditions. But eukaryotes have perfected the internal division of labor. Many eukaryotes are single cells, principally the protists that caused malaria and brought the potato blight to Ireland. Eukaryotes assemble into multicellur organisms through cell-cell interaction. The most elegant example is the amoeba, Dictyostelium discoideum, which lives as free cells for part of its life, then assembles into a multicellular structure for the reproductive stage. Some cells die to allow the generation of new spores. Being complex multicellular bodies, eukaryotes have evolved a dazzling array of functional specializations. Imagine a mushroom growing out of the hyphae of a fungus, a field of flowering plants, or the cells that animals allow to smell transport oxygen or defend against disease. These specializations have enabled even larger and better adapted organisms to develop whether they be trees, whales, or elephants. One must also mention the single most important step that allowed the largest of these organisms to evolve. They required sophisticated transport mechanisms, specifically, circulatory systems in animals, blood and lymphatic, and long-distance transport elements in plants, xylem and phloem. These enabled optimal delivery of resources, such as carbohydrates, hormones for regulation, and most critically, oxygen and carbon dioxide exchange for oxygenic respiration. And speaking of elephants and trees, and thousands of other organisms that are land dwellers, terrestrial life required another notable set of specializations. They had to control their physiology in a hostile world of wildly oscillating water supply and temperature and intense solar radiation. Waxy coatings on plants, thick skins on animals and nocturnal behavior are just a few essential adaptations to cope with terrestrial life. Some organisms can dehydrate altogether to survive dry periods. That is a story for another day. 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