16.5Topology of Mitochondria and Chloroplasts
Mitochondria and chloroplasts are thought to have originated from certain eubacteria*18 that lived symbiotically within the cells of eukaryotic organisms (endosymbiosis theory). That is why, unlike other organelles, mitochondria and chloroplasts are enclosed by double membranes, an inner and an outer membrane*19 . The inner membrane of mitochondria forms cristae, through which H+ is transported outside the membrane by electron transport in the respiration chain. In chloroplasts, on the other hand, the photosynthetic system (including chlorophyll) does not exist in the inner envelope. Rather, it is found in the thylakoid membrane further inside the chloroplast, and H+ is transported into thylakoid by photosynthetic electron transport. The directions of this transport seem opposite to each other in mitochondria and chloroplasts; however, because the matrix of mitochondria and the stroma of chloroplasts are homologous compartments, the two directions are in fact the same, when it is considered that H+ is emitted from them (i.e., they are topologically homologous). The matrix of mitochondria (in which citric-acid-cycle enzymes work) and the stroma of chloroplasts (in which Calvin-cycle enzymes work) are homologous, and H+ movement in electron transport is directed outward from these compartments. Coupling with H+ transport based on this H+ concentration gradient, F-ATP synthase therefore synthesizes ATP in the same compartment as the citric acid cycle and the Calvin cycle.
*18 The ancestor of chloroplast is cyanobacteria (blue-green algae), whereas some of the prospective candidate ancestors of mitochondria are intracellular pathogen rickettsia, belonging to the alpha proteobacteria group. However, due to the intense specialization mitochondria have undergone, the ancestral relationship has not been determined.
*19 Outer and inner membranes are called outer and inner envelopes in chloroplasts. Both chloroplasts and mitochondria have a transporter called non-specific porin in their outer membranes, and therefore does not block material transport. According to the endosymbiosis theory, the outer membrane can be considered to have originated from host cells, but another widely accepted theory is that the outer membrane is homologous to a special outer membrane of cyanobacteria, which are ancestors of chloroplasts.
Regulation of ATP synthase by coupling and light
Since ATP synthesis reactions involving kinase (phosphotransferase) are coupled with metabolic reactions at a ratio of 1:1, the ATP synthesis efficiency is 100%. On the other hand, ATP synthesis reactions involving F-ATP synthase are indirectly coupled with electron transport via H+ transport, making the story a little more complex. As an example, the state of chloroplasts differs between daytime and nighttime. During the day, when light is available, chloroplasts are in a state of high energy, with the thylakoid membrane maintaining a H+ gradient by electron transport; they are in a state ready to supply ATP when it is consumed. However, at night, it is difficult for the chloroplasts to maintain this high-energy state. Although the thylakoid membrane has low permeability for H+ and is therefore suited for maintaining the concentration gradient, it is still unable to stop H+ from gradually leaking through the membrane during the night. Generally, enzymatic reactions are reversible, and when the concentration gradient of H+ decreases, the gradient is restored by breaking down ATP, which is a waste of energy. In the F-ATP synthase of chloroplasts, an activity-regulation mechanism involving thioredoxin (an oxidation-reduction protein) has thus evolved. When thioredoxin with cysteine residue is reduced by photosynthetic electron transport, it reduces the cysteine residue of a protein in the stalk of ATP synthase, thereby activating the enzyme. On the other hand, in dark places, the activity of the enzyme is inhibited through the oxidation of the cysteine residue, thereby reducing the wasteful use of energy.
Photosynthesis and changes in carbon dioxide concentration in the Earth’s atmosphere
Column Figure 16-6 Rise in CO2 concentration in Earth’s atmosphere
Data observed at Mt. Mauna Loa in Hawaii. Seasonal changes are due to photosynthesis by plants whereas a secular increase in CO2 is mainly due to the burning of fossil fuel. It can be seen that the concentration is being accelerated every year.
Source: Scripps Institution of Oceanography and the NOAA Earth System Research Laboratory
When the first organisms appeared on earth 3.8 billion years ago, it is believed that the carbon dioxide concentration in the atmosphere was much higher than it is today. However, most of this was fixed as organic substances with the emergence of phototrophs, dropping to just a few percent in the Paleozoic era, and even further reduced with the growth of terrestrial plants and the emergence of C4 plants to today’s levels. Furthermore, significant short-term changes have occurred in connection with human activity and climate change. For instance, carbon dioxide levels stood at 270 ppm in the 17th century, but rose sharply after the Industrial Revolution, to the current figure of 390 ppm (Column Figure 16-6). It is still being debated whether this sudden rise is the direct cause of global warming, which has been an impending concern in recent years.