What Four Substances Are Recycled During Photosynthesis And Respiration

The predominant forms of visible life on Earth, plants and animals, operate in a complementary way, which is definitely no accident.

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A substance vital for the nourishment of plants is no more than a waste product in humans and other animals, and a substance discarded as waste by plants is needed by animals (and different parts of the same plant cell) for aerobic respiration. Other molecules are “conserved” in this way, too.

The four substances recycled during photosynthesis and respiration are: carbon dioxide (CO2), which is emitted as waste in cellular respiration and used by plants to make glucose, oxygen (O2), which is emitted as waste by plants and taken in by animals to allow cellular respiration to proceed, glucose (C6H12O6), which is consumed in cellular respiration and made from CO2 in photosynthesis and water (H2O), which is a waste product of cellular respiration but required for photosynthesis and a host of other reactions.

In some forms of cellular respiration, though, substances are not recycled in the reactions and are thus considered waste, although this doesn”t necessarily mean that humans have not found uses for this “disposable” material.

Photosynthesis is how plants, lacking mouths and digestive systems in general, get their food. By taking in carbon dioxide gas through openings in their leaves called stoma, they incorporate the raw material they need to build glucose. Some of that glucose is used by the plant itself in cellular respiration, while the rest may become food for animals.

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The first part of photosynthesis consists of the light reactions and requires a light source to proceed. Light strikes structures inside plant cells called chloroplasts, which contain thylakoids, which in turn contain a group of pigments called chlorophyll. The end result is the harvesting of energy for the second part of photosynthesis and the release of oxygen gas as waste.

In the dark reactions, which do not require sunlight (but are not adversely affected by it), carbon dioxide is combined with a five-carbon compound called ribulose-1,5-biphosphate to make a six-carbon intermediate, some of which ultimately becomes glucose. The energy for this phase comes from ATP and NADPH made in the light reactions.

It includes four steps: glycolysis, the oxygen-independent conversion of glucose to pyruvate; the bridge reaction, which is the oxidation of pyruvate to acetyl coenzyme A, the Krebs cycle, which combined acetyl CoA with oxaloacetate to make a six-carbon compound that is eventually converted to oxaloacetate again, yielding electron carriers and ATP and the electron transport chain, which is where most of the ATP of cellular respiration is generated.

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The last three of these steps, comprising aerobic respiration, occur in the mitochondria, whereas glycolysis occurs in the cytoplasm. A common misconception is that plants undergo photosynthesis instead of cellular respiration; in fact, they use both, using the former process to make glucose as an input for the latter process.


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