Lesson Notes By Weeks and Term v4 - SHS 3
PLANT SYSTEMS
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PLANT SYSTEMS
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Biology
Class: SHS 3
Term: 2nd Term
Week: 20
Grade code: 3.4.2.LI.3
Strand code: 4
Sub-strand code: 2
Content standard code: 3.4.2.CS.1
Indicator code: 3.4.2.LI.3
Theme: SYSTEMS OF LIFE
Subtheme: PLANT SYSTEMS
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This lesson explores the fascinating and often overlooked process of excretion in plants. While we are familiar with excretion in animals, plants also produce waste products from their metabolic activities. However, their methods of removal are very different and much simpler. Understanding this topic is important for Ghanaian learners as it connects directly to our environment, agriculture, and economy. We see it every day: in the oxygen we breathe, the rubber tapped from trees on plantations, the medicinal properties of leaves and bark, and even the "itchiness" of `kontomire` (cocoyam leaves).
A. What is Excretion in Plants? Excretion is the process by which living organisms remove metabolic wastes from their bodies. Metabolic wastes are unwanted or toxic by-products of the chemical reactions that occur within cells (metabolism).
It is crucial to distinguish this from egestion, which is the removal of undigested food (faeces), a process that does not occur in plants as they make their own food. B. Why is Plant Excretion Different from Animal Excretion? Plants lack sophisticated excretory organs like kidneys, lungs, or skin found in animals. This is because: Slower Metabolism: Plants have a much lower metabolic rate than animals. They produce waste products far more slowly. Autotrophic Nature: Plants are producers. They can reuse some of their own waste products. Example: Carbon dioxide (CO₂) produced during respiration at night can be used for photosynthesis during the day. Nature of Waste: The primary waste products are less toxic than in animals. Animals produce a lot of nitrogenous waste (like urea and ammonia) from protein breakdown, which is highly toxic and must be removed quickly. Plants produce very little of this. Storage Capability: Many plant wastes are stored in forms that are harmless to the plant, often in specific parts like leaves, bark, or even vacuoles within cells, which are then shed. C. Major Plant Waste Products and Their Removal Gaseous Wastes Oxygen (O₂): This is a by-product of photosynthesis. While plants use some oxygen for their own respiration, the excess is a waste product. It is removed primarily through the stomata (tiny pores on leaves) and lenticels (pores on woody stems) by diffusion. Carbon Dioxide (CO₂): This is a waste product of respiration. During the day, it is mostly reused for photosynthesis. At night, when photosynthesis stops, excess CO₂ diffuses out of the plant through the stomata and lenticels. Water Vapour (H₂O): Excess water is removed mainly through transpiration, the evaporation of water from the leaf surface via the stomata. While transpiration's main roles are cooling the plant and pulling water up from the roots, it also serves as an excretory process for excess water. Water (in liquid form) Guttation: This is the loss of water in liquid form (as droplets) from the tips or margins of leaves. It occurs through special pores called hydathodes. Guttation usually happens at night or in very humid conditions when transpiration is low, and the roots are still absorbing water, creating pressure that forces water out. You can often see these droplets on the tips of grass or cocoyam leaves in the early morning. Solid and Liquid Stored Wastes (Secondary Metabolites) These are complex chemical compounds that are not directly involved in the primary processes of growth and development (like photosynthesis or respiration). They are often stored in specific parts of the plant to render them harmless or are used for defence. Tannins: These are bitter-tasting compounds. Storage: Stored in bark, old leaves, and heartwood (the central, non-living part of a tree trunk). Function: Protects the plant from herbivores and microbial attacks. Example: The dark colour of the bark of trees like the Odum or Mahogany is due to tannins. When leaves containing tannins fall and decompose, they enrich the soil. Resins: These are sticky, waterproof substances. Storage: Secreted into special tubes called resin ducts. Function: They seal wounds to prevent infection and water loss. Example: The sticky substance from pine trees or other conifers. Gums: These are sticky substances produced when cellulose is broken down, often at a site of injury. Function: They help to seal the wound and prevent infection. Example: Gum Arabic from the Acacia tree. Many local trees exude gum when their bark is cut. Latex: A milky or clear, often sticky fluid. Storage: Found in a network of tubes called laticifers. Function: It is believed to be a defence mechanism against insects and helps in wound healing. Ghanaian Example: The white, milky latex from the Rubber Tree (*Hevea brasiliensis*) is of huge economic importance. The Pawpaw plant also produces latex. Alkaloids: These are nitrogen-containing compounds, often with physiological effects on animals. Storage: Stored in leaves, seeds, or bark. Function: They are toxic or bitter, acting as a powerful defence against herbivores. Examples: Quinine (from the Cinchona tree), nicotine (from tobacco), and theobromine (in cocoa). Crystals: Plants can convert toxic soluble substances like oxalic acid into insoluble crystals, which are harmless. Storage: Stored in the vacuoles of cells, especially in leaves, stems, and fruits. Type: The most common are calcium oxalate crystals, which can be needle-shaped (called raphides). Ghanaian Example: The itching sensation you feel when handling or eating uncooked `kontomire` (cocoyam leaves) or taro is caused by thousands of these tiny needle-like raphides piercing your skin or mouth. D. Summary of Excretory Methods Diffusion: For gaseous wastes (O₂, CO₂) through stomata and lenticels. Transpiration & Guttation: For removing excess water. Storage: Storing waste in vacuoles, dead tissues (heartwood), or organs that will be shed. Shedding (Abscission): Dropping of leaves, bark, flowers, and fruits that have accumulated waste products. This is a very effective way to remove solid waste permanently. Root Exudation: Some waste substances are passed out of the roots into the surrounding soil.
Guided Practice (With Solutions)
Question 1: A student observes small droplets of water on the edges of a cocoyam leaf early in the morning. (a) Name this phenomenon. (b) Explain why this process is considered a form of excretion. (c) Through which structures does this water exit the leaf?