JAMB Biology Mote- Excretion
Sub Topics:
a. Types of excretory structures: contractile vacuole, flamecell, nephridium, Malpighian tubule, kidney, stoma and lenticel.
b. Excretory mechanisms:
i. Kidneys
ii. lungs
ii. skin
c. Excretory products of plants
Table of Contents
- Sub Topics:
A. Types of Excretory Structures
Excretion is the process by which waste products of metabolism are removed from the body. Different organisms have evolved specialized structures for this function.
1. Contractile Vacuole
- Organism: Protists (e.g., Amoeba, Paramecium).
- Function: The contractile vacuole helps in osmoregulation by collecting excess water from the cytoplasm and expelling it out of the cell. It prevents the cell from bursting due to excess water intake in a hypotonic environment.
2. Flame Cells
- Organism: Flatworms (Platyhelminthes).
- Function: Flame cells are involved in excretion and osmoregulation. They have cilia that beat to create a current that draws waste products and excess water into excretory tubules.
3. Nephridium
- Organism: Annelids (e.g., earthworms).
- Function: Nephridia are tubular excretory organs that remove nitrogenous waste (such as ammonia) and regulate water and ion balance. They consist of a ciliated funnel that collects waste from the coelom and excretes it through pores.
4. Malpighian Tubules
- Organism: Insects (e.g., grasshoppers, cockroaches).
- Function: Malpighian tubules are involved in the excretion of nitrogenous waste (mainly uric acid). These tubules absorb waste from the hemolymph and expel it as a solid paste along with water in the digestive tract.
5. Kidney
- Organism: Vertebrates (e.g., humans, birds, amphibians).
- Function: Kidneys are the primary organs for excretion and osmoregulation in many animals. They filter blood, remove nitrogenous wastes (urea), regulate water and electrolyte balance, and excrete urine.
6. Stoma and Lenticel
- Organism: Plants.
- Function: Stomata are openings in the leaves through which gas exchange occurs. Lenticels are pores in stems and roots that allow for the exchange of gases. Both play a role in the excretion of excess oxygen and water vapor through transpiration.
B. Excretory Mechanisms
i. Kidneys
- Structure: The kidney consists of several key parts, including the renal cortex, renal medulla, nephrons, and renal pelvis. Each nephron consists of a Bowman’s capsule, glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.
- Excretory and Osmoregulatory Functions:
- Filtration: Blood enters the kidney through the renal artery, and plasma is filtered in the Bowman’s capsule, where waste products like urea, glucose, and ions are removed.
- Reabsorption: Useful substances like glucose, water, and ions are reabsorbed in the proximal convoluted tubule, loop of Henle, and distal convoluted tubule.
- Secretion: Excess ions, drugs, and other waste products are secreted into the tubules.
- Excretion: The final urine, containing nitrogenous waste (urea), excess water, and ions, is excreted via the ureters to the bladder.
ii. Lungs
- Excretory Products: The lungs play a role in the excretion of carbon dioxide (a waste product of respiration) and water vapor.
- Mechanism: Oxygen is inhaled into the lungs for cellular respiration, while carbon dioxide and water vapor are diffused from the blood into the alveoli and exhaled through the nose or mouth.
iii. Skin
- Excretory Products: The skin excretes sweat, which contains water, salts, and small amounts of urea.
- Mechanism: Sweat glands in the skin secrete sweat to regulate body temperature and remove waste products such as urea and excess salts through evaporation.
C. Excretory Products of Plants
Plants excrete various waste products, and some of these products have significant economic and ecological importance.
i. Carbon Dioxide (CO2)
- Excreted by: All green plants during respiration.
- Economic Importance: While CO2 is generally considered a waste product, it is essential for photosynthesis and used by plants to create glucose.
ii. Oxygen (O2)
- Excreted by: Plants during photosynthesis.
- Economic Importance: Oxygen is essential for aerobic respiration in animals and humans. It is also a by-product of photosynthesis, contributing to the oxygen supply in the atmosphere.
iii. Tannins
- Excreted by: Certain plants (e.g., oak, tea).
- Economic Importance: Tannins are used in leather production, as antioxidants in the food industry, and in some traditional medicines.
iv. Resins
- Excreted by: Plants (e.g., pine trees).
- Economic Importance: Resins are used in making varnishes, adhesives, and in the production of essential oils and medicines.
v. Gums
- Excreted by: Various plants (e.g., acacia, guar).
- Economic Importance: Gums are used in the food industry as thickening agents, in pharmaceuticals, and in cosmetics.
vi. Mucilage
- Excreted by: Some plants (e.g., flax, aloe).
- Economic Importance: Mucilage is used as a thickening agent, in wound healing, and in some medicinal applications.
vii. Alkaloids
- Excreted by: Some plants (e.g., poppies, tobacco, coffee).
- Economic Importance: Alkaloids have pharmacological effects and are used in drugs (e.g., morphine, caffeine) for medicinal and recreational purposes.
Summary
Excretion is a vital biological process that removes waste products from the body, maintaining homeostasis. Different organisms have developed a variety of excretory structures and mechanisms to meet their metabolic needs. These include contractile vacuoles in protozoans, flame cells in flatworms, nephridia in annelids, Malpighian tubules in insects, kidneys in vertebrates, and stomata in plants. Plants also excrete waste products such as oxygen, carbon dioxide, tannins, resins, and alkaloids, which have various ecological and economic uses.
Conclusion
Excretion is a critical function that helps maintain the internal environment of organisms. Understanding the different excretory structures and mechanisms, as well as the excretory products of plants, enhances our knowledge of physiology and the role of organisms in ecosystems.
