The predominant forms of visible life on Earth, plants and animals, operate in a complementary way, which is definitely no accident.
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.
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.
The photosynthesis equation is:
6 CO2 + 6 H2O + Light Energy → C6H12O6 + 6 O2
Cellular respiration is the complete oxidation of glucose in eukaryotic cells.
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.
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.
The complete equation for cellular respiration is
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 36 (or 38) ATP
Waste Products of Cellular Respiration
When pyruvate cannot be processed via the aerobic reactions of cellular respiration, either because not enough oxygen is present or the organism lacks the enzymes to make use of it, fermentation is one alternative. This is what happens when you run an all-out sprint or lift heavy weights and go into "oxygen debt" from this anaerobic exercise.
In this process of lactic acid fermentation, which also occurs in the cytoplasm, pyruvate is converted to lactic acid in a reduction reaction that generates NAD+ from NADH. This makes more NAD+ available for glycolysis, which, along with removing pyruvate from the environment, tends to drive glycolysis forward. Lactate can be used by some animal cells, but it is considered a waste product generally.
In yeast, fermentation produces the two-carbon product ethanol instead of lactate. While still waste, it is undeniable that human societies would look vastly different were there no ethanol, the active ingredient in alcoholic beverages the world over.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.
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