Plants and all other organisms that perform cellular respiration do not specifically recycle hydrogen in an isolated manner; rather, hydrogen atoms are involved as part of the biochemical processes that convert glucose and oxygen into ATP (adenosine triphosphate), water, and carbon dioxide.
Here's a simplified version of what happens during cellular respiration:
1. **Glycolysis:** The process starts in the cytosol with glycolysis, where one molecule of glucose (which contains hydrogen atoms as part of its chemical structure) is broken down into two molecules of pyruvate (pyruvic acid). This process also releases a small amount of ATP and transfers hydrogen atoms to the carriers NAD+ to form NADH.
2. **Citric Acid Cycle (Krebs Cycle):** The pyruvates enter the mitochondria (in eukaryotic cells) and are converted into acetyl CoA, which contains hydrogen. During the citric acid cycle, more hydrogen atoms are transferred to NAD+ and another carrier called FAD (flavin adenine dinucleotide), forming NADH and FADH2.
3. **Oxidative Phosphorylation and Electron Transport Chain:** The NADH and FADH2 then donate the electrons from these hydrogen atoms to the electron transport chain in the inner mitochondrial membrane. As the electrons move through the chain, the energy from the electrons is used to pump protons (which are hydrogen ions, H+) across the membrane, creating a proton gradient.
4. **Chemiosmosis and ATP Synthesis:** The protons flow back into the mitochondrial matrix through the enzyme ATP synthase due to the gradient created, and this flow of protons generates the energy needed to synthesize ATP from ADP and inorganic phosphate.
5. **Water Formation:** At the end of the electron transport chain, the electrons, protons, and molecular oxygen (O2) combine to form water (H2O). This is the step where hydrogen is "recycled" in the sense that the hydrogen atoms from glucose and other substrates end up combining with oxygen to form water, which is a byproduct of cellular respiration.
Thus, hydrogen from the breakdown of glucose and other molecules is effectively transferred and utilized throughout the cellular respiration process, ending up as part of water molecules. Plants release some of this water back into the environment via transpiration, continuing the natural cycles of elements.