Question

How can we use the structures that are involved with photosynthesis and cellular respiration to determine the earliest forms of life and how they developed? Why is the study of endosymbiosos vital for biology?

Answer 1

Connecting Cellular Respiration and Photosynthesis

Photosynthesis and cellular respiration are connected through an important relationship. This relationship enables life to survive as we know it. The products of one process are the reactants of the other. Notice that the equation for cellular respiration is the direct opposite of photosynthesis:

•             Cellular Respiration: C6H12O6 + 6O2 ? 6CO2 + 6H2O

•             Photosynthesis: 6CO2 + 6H2O ? C6H12O6+ 6O2

Photosynthesis makes the glucose that is used in cellular respiration to make ATP. The glucose is then turned back into carbon dioxide, which is used in photosynthesis. While water is broken down to form oxygen during photosynthesis, in cellular respiration oxygen is combined with hydrogen to form water. While photosynthesis requires carbon dioxide and releases oxygen, cellular respiration requires oxygen and releases carbon dioxide. It is the released oxygen that is used by us and most other organisms for cellular respiration. We breathe in that oxygen, which is carried through our blood to all our cells. In our cells, oxygen allows cellular respiration to proceed. Cellular respiration works best in the presence of oxygen. Without oxygen, much less ATP would be produced.

Cellular respiration and photosynthesis are important parts of the carbon cycle. The carbon cycle is the pathways through which carbon is recycled in the biosphere. While cellular respiration releases carbon dioxide into the environment, photosynthesis pulls carbon dioxide out of the atmosphere. The exchange of carbon dioxide and oxygen during photosynthesis and cellular respiration worldwide helps to keep atmospheric oxygen and carbon dioxide at stable levels.

Overwhelming evidence indicates that eukaryotic photosynthesis originated from endosymbiosis of cyanobacterial-like organisms, which ultimately became chloroplasts . So the evolutionary origin of photosynthesis is to be found in the bacterial domain. Significant evidence indicates that the current distribution of photosynthesis in bacteria is the result of substantial amounts of horizontal gene transfer, which has shuffled the genetic information that codes for various parts of the photosynthetic apparatus, so that no one simple branching diagram can accurately represent the evolution of photosynthesis . However, there are some patterns that can be discerned from detailed analysis of the various parts of the photosynthetic apparatus, so some conclusions can be drawn. In addition, the recent explosive growth of available genomic data on all types of photosynthetic organisms promises to permit substantially more progress in unraveling this complex evolutionary process.

While we often talk about the evolution of photosynthesis as if it were a concerted process, it is more useful to consider the evolution of various photosynthetic subsystems, which have clearly had distinct evolutionary trajectories.

All animals and most microorganisms rely on the continual uptake of large amounts of organic compounds from their environment. These compounds are used to provide both the carbon skeletons for biosynthesis and the metabolic energy that drives cellular processes. It is believed that the first organisms on the primitive Earth had access to an abundance of the organic compounds produced by geochemical processes, but that most of these original compounds were used up billions of years ago. Since that time, the vast majority of the organic materials required by living cells have been produced by photosynthetic organisms, including many types of photosynthetic bacteria.

The most advanced photosynthetic bacteria are the cyanobacteria, which have minimal nutrient requirements. They use electrons from water and the energy of sunlight when they convert atmospheric CO2 into organic compounds—a process called carbon fixation. In the course of splitting water [in the overall reaction nH2O + nCO2 (CH2O)n + nO2], they also liberate into the atmosphere the oxygen required for oxidative phosphorylation. As we see in this section, it is thought that the evolution of cyanobacteria from more primitive photosynthetic bacteria eventually made possible the development of abundant aerobic life forms.

In plants and algae, which developed much later, photosynthesis occurs in a specialized intracellular organelle—the chloroplast. Chloroplasts perform photosynthesis during the daylight hours. The immediate products of photosynthesis, NADPH, and ATP, are used by the photosynthetic cells to produce many organic molecules. In plants, the products include a low-molecular-weight sugar (usually sucrose) that is exported to meet the metabolic needs of the many non photosynthetic cells of the organism.

Biochemical and genetic evidence strongly suggests that chloroplasts are descendants of oxygen-producing photosynthetic bacteria that were endocytosed and lived in symbiosis with primitive eukaryotic cells. Mitochondria are also generally believed to be descended from an endocytosed bacterium. The many differences between chloroplasts and mitochondria are thought to reflect their different bacterial ancestors, as well as their subsequent evolutionary divergence.

It is only due to endosymbiosis that today photosynthesis and respiration is possible.