Biology Keypoints: Excretion; Excretion is the biological process of eliminating metabolic waste products, toxins, and excess substances from an organism’s body. This process is essential for maintaining homeostasis and preventing the buildup of harmful substances. In this biology class note, we will discuss the different types of excretory structures, excretory mechanisms, and excretory products of plants.
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a. Types of excretory structures:
i. Contractile Vacuole: Contractile vacuoles are present in some freshwater protozoans and some lower aquatic animals like Amoeba, Paramecium, and Euglena. The contractile vacuole is an organelle that contracts and expels excess water and waste products from the cell.
ii. Flamecell: Flame cells are the excretory structures of flatworms. These are specialized cells that are lined up in a row, forming a tubule. They use cilia to create a current that moves waste products from the body cavity into the tubule, and then out of the body.
iii. Nephridium: Nephridia are excretory organs found in many invertebrates such as annelids, mollusks, and arthropods. They consist of a tubule system that filters waste products and excess fluids from the coelom and releases them outside the body.
iv. Malpighian Tubule: Malpighian tubules are found in insects and other arthropods. These are blind-ended tubes that are attached to the digestive system and filter waste products from the hemolymph, the insect’s circulatory fluid.
v. Kidney: The kidneys are the primary excretory organs in vertebrates. They filter waste products from the blood and excrete them as urine. The kidneys also play a vital role in maintaining the body’s fluid and electrolyte balance and regulating blood pressure.
vi. Stoma and Lenticel: Stomata are tiny pores found on the surface of plant leaves and stems that allow for gas exchange. They also play a role in excretion by releasing excess water and oxygen. Lenticels are found on the surface of woody stems and roots, and they allow for gas exchange and the release of excess water.
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b. Excretory mechanisms:
i. Kidneys: As mentioned earlier, the kidneys are the primary excretory organs in vertebrates. They filter waste products from the blood and excrete them as urine. The kidneys also regulate the body’s fluid and electrolyte balance and blood pressure.
ii. Lungs: The lungs play a vital role in the excretion of carbon dioxide, a waste product of cellular respiration. During respiration, oxygen is taken in and carbon dioxide is released through the lungs.
iii. Skin: The skin is the largest organ in the human body and plays a crucial role in excretion. It excretes excess water and salt through sweat and eliminates small amounts of waste products through sebum, the skin’s natural oil.
c. Excretory products of plants:
Plants excrete waste products through various mechanisms, including through the roots, leaves, and stems. Some examples of excretory products of plants include oxygen, carbon dioxide, and excess water. Plants also release organic compounds such as tannins and alkaloids, which can be toxic to other organisms.
Plants have several ways of excreting waste products. One of the most important ways is through the roots. As plants absorb water and nutrients from the soil, they also take in toxins and excess substances. These substances are then transported to the roots and excreted back into the soil.
Another way plants excrete waste products is through the leaves. During photosynthesis, plants produce oxygen as a byproduct. Oxygen is released through tiny pores on the leaves called stomata. Carbon dioxide, which is a waste product of respiration, is taken in through the same pores.
Plants also excrete waste products through the stems. The stem is responsible for transporting water and nutrients throughout the plant. As water and nutrients are transported, toxins and excess substances are also carried along and excreted through the stem.
In addition to these basic excretory functions, plants also produce a variety of organic compounds that can be considered excretory products. These compounds can be toxic to other organisms and serve as a form of chemical defense for the plant.
For example, tannins are a class of compounds that are produced by many plants. Tannins bind to proteins and other organic molecules, making them difficult for other organisms to digest. This can be an effective defense mechanism against herbivores.
Similarly, alkaloids are another class of organic compounds produced by many plants. Alkaloids can be toxic to other organisms, and some have medicinal properties for humans. Examples of alkaloids include caffeine, nicotine, and morphine.
