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The first electron source for plants was H2S, but now most modern plants use H2O as an electron source.
What is the advantage of using H2O instead of H2S?
The major reason for this is because H20 can cause hydrogen bonding. Hydrogen bonding is what allows for plants to transport "this electron source" from their roots through their stems an further.
The stronger polarity of H20 also allows for stronger interactions. Such as ionic interactions (not fully bound but 2 polar atoms interacting). This kind of ionic interaction also allows for other types of material interactions such as cohesion and adhesion.
Overall, aside from the discussion of abundance as mentioned above (which is a very good point), it also allowed the plants to grow in directions that were more advantageous (darwinian) for their survival.
Antenna Complexes for Photosynthesis
The capture of light energy for photosynthesis is enhanced by networks of pigments in the chloroplasts arranged in aggregates on the thylakoids. These aggregates are called antennae complexes. Evidence for this kind of picture came from research by Robert Emerson and William Arnold in 1932 when they measured the oxygen released in response to extremely bright flashes of light. They found that some 2500 molecules of chlorophyll was required to produce one molecule of oxygen, and that a minimum of eight photons of light must be absorbed in the process.
The model that emerges is that of some 300 chlorophyll molecules and 40 or so beta carotenes and other accessory pigments acting as a light harvesting antenna surrounding one chlorophyll a molecule that is a part of an action center. A photon is absorbed by one of the pigment molecules and transfers that energy by successive flourescence events to neighboring molecules until it reaches the action center where the energy is used to transfer an energetic electron to an electron acceptor.
The fluorescence model would suggest that each transferred photon has a longer wavelength and lower quantum energy with some energy being lost to heat.
When a photon reaches the chlorophyll a in the reaction center, that chlorophyll can receive the energy because it absorbs photons of longer wavelengths than the other pigments. Two types of chlorophyll centers have been identified, and are associated with two protein complexes identified as Photosystem I and Photosystem II.
H₂S as a source of electrons for plants - Biology
Which statement about carbohydrate biosynthesis during the dark reactions of photosynthesis (i.e. the Calvin cycle reactions) is NOT TRUE?
A. RUBISCO is a an enzyme required for carbon dioxide fixation. B. NADPH is the source of electrons for glucose biosynthesis. C. ATP is the energy source for glucose biosynthesis. D. The reactions occur in the photosynthetic membranes of chloroplasts. E. Oxygen is not required.
Features of the equation for dark reactions
- RUBISCO is the CO 2 fixing enzyme for carbohydrate biosynthesis.
- 6 CO 2 are fixed by RUBISCO per glucose synthesized.
- Oxygen is not required for carbohydrate biosynthesis.
- Remainder of reactions used to synthesize glucose and regenerate RuBP.
- NADPH is used to reduce 3-phosphoglycerate to a 3-carbon sugar. 2 NADPH are required.
- 3 ATP are required for each RuBP regenerated.
Pathway for carbohydrate biosynthesis
The reactions are catalyzed by soluble enzymes of the chloroplast stroma.
Water oxidation by photosystem II is the primary source of electrons for sustained H 2 photoproduction in nutrient-replete green algae
The unicellular green alga Chlamydomonas reinhardtii is capable of photosynthetic H2 production. H2 evolution occurs under anaerobic conditions and is difficult to sustain due to 1) competition between [FeFe]-hydrogenase (H2ase), the key enzyme responsible for H2 metabolism in algae, and the Calvin-Benson-Bassham (CBB) cycle for photosynthetic reductants and 2) inactivation of H2ase by O2 coevolved in photosynthesis. Recently, we achieved sustainable H2 photoproduction by shifting algae from continuous illumination to a train of short (1 s) light pulses, interrupted by longer (9 s) dark periods. This illumination regime prevents activation of the CBB cycle and redirects photosynthetic electrons to H2ase. Employing membrane-inlet mass spectrometry and [Formula: see text], we now present clear evidence that efficient H2 photoproduction in pulse-illuminated algae depends primarily on direct water biophotolysis, where water oxidation at the donor side of photosystem II (PSII) provides electrons for the reduction of protons by H2ase downstream of photosystem I. This occurs exclusively in the absence of CO2 fixation, while with the activation of the CBB cycle by longer (8 s) light pulses the H2 photoproduction ceases and instead a slow overall H2 uptake is observed. We also demonstrate that the loss of PSII activity in DCMU-treated algae or in PSII-deficient mutant cells can be partly compensated for by the indirect (PSII-independent) H2 photoproduction pathway, but only for a short (<1 h) period. Thus, PSII activity is indispensable for a sustained process, where it is responsible for more than 92% of the final H2 yield.
Keywords: carbon dioxide green algae hydrogen production hydrogenase water splitting.
Copyright © 2020 the Author(s). Published by PNAS.
Conflict of interest statement
The authors declare no competing interest.
H 2 photoproduction ( A…
H 2 photoproduction ( A ) and O 2 exchange ( B )…
The effect of DCMU and the psbA deletion (FuD7) on H 2 (…
Long-term H 2 photoproduction by…
Long-term H 2 photoproduction by pulse-illuminated algae. The cultures of the wild-type (CC-124)…
The release of ambient ( m/z 44) CO 2 and 18 O-labeled (…
The effect of light pulse…
The effect of light pulse duration in the pulse-illumination sequence on CO 2…
H₂S as a source of electrons for plants - Biology
Supplements To Biology 101 Cell Unit
1. Fluorescence In A Chlorophyll Solution
|Left: A transparent-green chlorophyll solution of ground up spinach leaves and acetone. Right: Beam of light directed at the chlorophyll solution producing a reddish glow called fluorescence.|
|A transparent-green chlorophyll solution can be made by grinding up spinach leaves or grass with acetone in a mortar and pestle. The solution is then filtered through cheesecloth and coarse filter paper to remove the impurities and debris. Chlorophyll molecules impart the green color to the solution however, the actual chloroplasts and thylakoid membranes have been dissolved. When a bright beam of light is directed at the chlorophyll solution in the test tube, it gives off a reddish glow. This phenomenon is known as fluorescence. The chlorophyll electrons become excited by the light energy, but have no cytochrome transport system to flow along because the chloroplast thylakoid membranes have been dissolved away. Therefore, the chlorophyll electrons give up their excited energy state by releasing energy in the form of a reddish glow. This is essentially the same phenomenon as a neon light, except the electrons of neon gas molecules in the glass tube become excited and then release their energy as a white glow.|
2. Simplified Illustration Of A Mitochondrion
3. ATP Structure & Function
T he structure of adenosine monophosphate, an RNA nucleotide containing the purine base adenine, is very similar to ATP (adenosine triphosphate), except that ATP has three phosphates (PO 4 ) instead of one. ATP is synthesized in all living cells by the addition of a phosphate to ADP (adenosine diphosphate). ATP is the vital energy molecule of all living systems which is absolutely necessary for key biochemical reactions within the cells. The terminal (3rd) phosphate of ATP is transferred to other molecules in the cell, thereby making them more reactive. For example, the monosaccharide glucose is very stable at ordinary body temperatures and would require a great amount of heat (such as from a flame) to break it down into carbon dioxide and water. After receiving a phosphate from ATP (a process called phosphorylation), glucose becomes glucose-phosphate and can be enzymatically broken down within seconds.
M ost of the ATP in eukaryotic cells of animals is made inside cellular organelles called mitochondria from the oxidation of glucose, a process called cellular respiration. Glucose combines with oxygen (oxidation), forming carbon dioxide, water and 38 molecules of ATP. During the oxidation process, electrons from glucose are shuttled through an iron-containing cytochrome enzyme system on the inner mitochondrial membranes (called cristae). The actual synthesis of ATP from the coupling of ADP (adenosine diphosphate) with phosphate is very complicated and involves a mechanism called chemiosmosis. The electron flow generates a higher concentration (charge) of positively-charged hydrogen (H+) ions (or protons) on one side of the membrane. When one side of the membrane is sufficiently "charged," these protons recross the membrane through special channels (pores) containing the enzyme ATP synthetase, as molecules of ATP are produced. The detailed, step-by-step breakdown of glucose during cellular respiration is called the Krebs Cycle or Citric Acid Cycle.
4. Simplified Illustration Of A Chloroplast
Light Reactions Of Photosynthesis