Here we are starting a chapter that is nothing but plants...this chapter covers that pathway distinctly plant-related. No animals carry out this process (yes, yes, there are zooxanthellae in corals that do photosynthesis, but zooxanthellae are "plants").
Your chapter starts with a wonderful historical account of the study of photosynthesis. This is good reading because it shows you how scientists think, and how slowly (sometimes) ideas develop. It also clearly shows how induction works; note the thinking of Van Niel in comparing H2S and H2O. Photosynthesis was studied scientifically in the 1600s, yet we still have not got all the steps completely understood. In fact most of it was a complete mystery until the 1950s (the ancient era when I was born).
It is also worthy to note, that bee vision extends from near UV (340 nm = beyond our vision) to orange (600 nm) but excludes red. Plants respond physiologically to wavelengths of light including blue (450 nm) up to far-red (750 nm = beyond our vision).
Thus, while chlorophyll clearly reflects and cannot use green light (Fig 7.7), plants can use some green light to do photosynthesis (Fig 7.8). Carotenoid and other antenna pigments (pg 140-142) absorb these wavelengths and pass the energy to chlorophyll. The diagram 7.14 (page 144) shows how this energy transfer is an energetically down-hill process (2nd Law of Thermodynamics). The excess energy is lost as heat. Since antenna pigments pass energetically down-hill, then it is no surprise that they absorb at shorter wavelengths (higher energy) than the reaction center pigments. It is no surprise that the reaction center pigments absorb optimally at 680 and 700 nm respectively!
Please note the error in Figure 7.14. One of the transfers shown is uphill! This cannot occur without absorption of additional light. That is not shown...therefore the diagram violates the second law of thermodynamics. I think the lack of yellow shading on that circle is evidence of an artist's error in adding arrows.
The electron excited by light or light energy in the reaction center pigment is transferred down an electron transport system (Fig 7.16) Does this look vaguely like mitochondrial electron transport? It is very similar...including the coupled ATP synthesis. We call this photophosphorylation (vis a vis oxidative phosphorylation) but it is very similar. The critical difference is that the energy comes directly from light, rather than a molecule such as NADH + H+. In fact, notice that NADPH (a compound similar to NADH) is generated as the ultimate electron receptor.
The ATP and NADPH + H+ are the main products of these light reactions. The electrons lost from the first chlorophyll molecule (P680) are replaced by taking them from H2O. The two H+ are attached to NADP+ and the O is assembled into O2. Thank you plants for the oxygen we breathe!
Since plants might not want a fixed ratio of NADPH + H+ to ATP, plants can also do cyclic flow (Fig 7.18). This allows electrons to cycle through the carriers and make lots of ATP without splitting water or making any NADPH+ H+. The famous Z-scheme is shown in Fig 7.19 A. Cyclic flow is not shown there.
Maybe the best name for these is the Calvin Cycle (in honor of Melvin Calvin who discovered many of the steps in the 1950s--see Fig 7.20 and text). You should study this figure to appreciate the level of biochemistry possible just a few decades ago. The work was considered sufficient to earn Calvin a Nobel prize. Unfortunately after 40 years, study of this biologically most-important pathway has been eclipsed by the pathway of the central dogma (DNA->RNA->protein) of genetics [In fact at ECSU we offer two courses that deal directly in this (DNA->RNA = Biotechnology, RNA->protein = Research Methods)]. We do not offer a course on Photosynthesis, let alone separate courses on the light reactions and light-independent reactions...time passes...foci change.
The Calvin Cycle, like the Krebs Cycle, is a wonderful cyclic set of enzymatic steps. It is the source of 3-carbon sugars for assembling 6-carbon sugars for respiration or polymerization into starch and cellulose, etc. It is also a biosynthetic source for four-, five-, and seven-carbon sugars for nucleic acids and other cell wall monomers. Unfortunately, while the diagram (7.21) shows the intermediates, it does not show any possible exit of these intermediates for other uses by the cell. Indeed this happens, but this is "glossed over" by the book. Your book does not name the enzymes in the cycle, unlike its more thorough coverage of glycolysis...more's the pity. Considering the thorough coverage of the light reactions, this is an area of the book needing more balance.
What has evolved is a carbon-dioxide pump, to "swamp out" any oxygen and thereby strongly inhibit photorespiration. This pump attaches carbon dioxide to phosphoenolpyruvate (remember this 3-carbon acid from glycolysis?) to form a 4-carbon acid. That is why these reactions are called a C4 pathway. The enzyme, PEPCO, does not have the "oxygen problem." Exactly which 4-carbon product is made and how it is processed varies among plants that have C4 photosynthesis.
The C-4 pathways studied to date can be divided into two separate types: the C-4 and CAM pathways.
In the daytime, the guard cells collapse so the stomata are closed to gas exchange. The 4-carbon acid is released from the vacuole and the CO2 is cleaved off to supply RUBISCO with a high concentration of CO2 relative to O2. The Light Reactions and Calvin Cycle then operate in the daylight to refix that CO2 into sugars to drive the night respiration...and so on.
Of these two solutions, the regular C-4 plants are very efficient because of their simultaneous operation of the C-4 and Calvin Cycle. CAM plants are not as efficient because of the constraints caused by their time-based separation of C-4 and Calvin cycle paths. Of course, an organism evolving by selection in a desert environment has very limited options. The harshness of the environment may compromise growth rate in favor of a kind of photosynthesis that can conserve water. The CAM solution certainly does that magnificently, explaining why it is found in most desert species, if not in an obligate way, then in a facultative way. You should read carefully the section on why C-4 plants don't dominate the planet.
Notice how the author (page 160) reconnected this chapter to our first lecture. Learning is to see the web-work of our real world, rather than just the linear pathways we show you in text books and lectures!
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