| MadSci Network: Botany |
Autotrophic plants use light energy to make all the organic compounds they require from carbon dioxide, water, and about 14 mineral nutrients they absorb from their environment. Therefore, they don't really produce metabolic wastes to the extent that heterotrophs do. Heterotrophs digest food. Plants do not need to digest with the exception of a few species of carnivorous plants that obtain mineral nutrients by digesting insects and small animals. The term food is not even a term that should be applied to autotrophic plants. Food is something that is digested and provides energy. Plants are food for animals and other heterotrophs but they do not produce food for themselves. Heterotrophs get energy and organic nutrients (amino acids, fatty acids, carbohydrates, vitamins) from other organisms (food). Autotrophic plants get energy from light and synthesize their organic nutrients from carbon dioxide and water. Probably the only metabolic waste product of autotrophic plants is oxygen, which readily diffuses out of the stomata on leaves or lenticels on photosynthetic stems with secondary growth. Nonphotosynthetic plant parts and photosynthetic plant parts at night produce excess carbon dioxide which might be considered a waste product. However, carbon dioxide is essential to plants for photosynthesis. Overall plants absorb much more carbon dioxide than they excrete. Plants do sometimes accumulate mineral nutrients or salts that they put in their cell's central vacuole. Certain salt-tolerant plants (called halophytes) sometimes excrete salt onto their leaves. Plant cytoplasm maintains a low calcium level by transporting calcium into the central vacuole. The calcium often forms calcium oxalate crystals in the central vacuole. However, those sharp crystals may function in protecting the plant from herbivores so they probably aren't waste products. Plants produce thousands of secondary compounds such as caffeine, latex, and nicotine, that were once thought to be waste products. However, the current view is that secondary compounds have functions in the plant. Many secondary compounds deter herbivores. References Anatomy of a Halophyte Calcium oxalate crystals in spinach Photo of calcium oxalate crystal from Peperomia plant Photo of calcium oxalate crystal from Lotus leaf Plant Secondary Compounds
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