Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Describe different support systems in plants.
• Explain how xylem and phloem transport substances.
• Describe the components and roles of blood.
• Compare open and closed circulatory systems.
• Relate the structure of blood vessels to their function.

Lesson summary

The focus of this week is on understanding how plants and animals maintain their structure and efficiently transport essential substances throughout their bodies. This is crucial for their survival and growth. Think about the massive baobab tree, which stands tall for centuries, or a giraffe reaching leaves high in the air – these are amazing examples of natural support systems. In animals, a constant supply of oxygen and nutrients is vital for energy production and waste removal, highlighting the importance of efficient transport. Understanding these systems provides a deeper appreciation for the complex biological processes that sustain life.

Lesson notes

2.1 Support Systems in Plants Plants, unlike animals, don't have bones. They rely on various support systems to maintain their shape and withstand environmental stresses.

Hydrostatic Support: This involves maintaining turgor pressure within cells. Plant cells have a large central vacuole filled with water. When this vacuole is full, it pushes against the cell wall, making the plant firm. Wilting occurs when water is lost, and turgor pressure decreases. A simple example is a freshly watered lettuce compared to one left out overnight. The turgidity is lost. Imagine a balloon; it’s firm when inflated (like a plant cell with turgor pressure) and floppy when deflated (like a wilted plant cell).

Collenchyma: This is a flexible support tissue found in young stems and petioles (leaf stalks). Collenchyma cells have unevenly thickened cell walls, providing support while allowing for growth.

Think of the celery stalk: it's somewhat flexible but still provides support. Collenchyma cells are often found just beneath the epidermis (outer layer) of stems and leaves.

Sclerenchyma: This is a rigid support tissue found in mature plants. Sclerenchyma cells have thick, lignified cell walls, making them very strong. There are two main types of sclerenchyma cells: fibers (long and slender, found in flax and hemp) and sclereids (short and irregular, found in the gritty texture of pears). Imagine the hard shell of a peanut or the strong fibers used to make rope – these are examples of sclerenchyma. Lignin is a complex polymer that adds strength and rigidity to cell walls.

Xylem: While primarily responsible for water transport, xylem also provides significant structural support. Xylem vessels are essentially dead cells that form long, hollow tubes. The walls of xylem vessels are thickened with lignin, providing considerable strength to the plant. Imagine xylem as the "skeleton" of the plant's vascular system. 2.2 Transport Systems in Plants: Xylem and Phloem Plants have two main transport tissues: xylem and phloem.

Xylem: Transports water and dissolved minerals from the roots to the rest of the plant (shoots, leaves, flowers, fruits). This movement is driven by transpiration, the evaporation of water from leaves, which creates a "pull" that draws water up the xylem vessels.

Transpiration: This process involves several steps: Water enters the plant through the roots via osmosis. Water moves up the xylem vessels, aided by capillary action and cohesion (water molecules sticking to each other) and adhesion (water molecules sticking to the xylem walls). Water evaporates from the leaves through stomata (small pores on the leaf surface). This evaporation creates a tension (transpirational pull) that draws more water up from the roots.

Example: Imagine a tall tree like a Blue Gum (Eucalyptus) in Mpumalanga. Water is absorbed from the soil, travels up the trunk against gravity, and reaches the leaves at the top. This is all driven by transpiration.

Phloem: Transports sugars (produced during photosynthesis) from the leaves to other parts of the plant where they are needed for growth, storage, or respiration. This movement is called translocation. Phloem transport is bidirectional, meaning sugars can move up or down the plant depending on where they are needed.

Translocation: Sugars (mainly sucrose) are actively transported from the source (e.g., leaves) into the phloem sieve tubes. This increases the solute concentration in the phloem, causing water to move in from the xylem via osmosis. The influx of water creates a pressure gradient that pushes the sugar-rich sap towards the sink (e.g., roots, fruits, developing shoots). Sugars are actively unloaded at the sink.

Example: Consider a sugar cane plant in KwaZulu-Natal. The leaves produce sugars through photosynthesis. These sugars are transported via the phloem to the stem, where they are stored as sucrose. Farmers harvest the sugar cane stems to extract this stored sugar. 2.3 Transport Systems in Animals: Circulatory Systems Animals have circulatory systems to transport oxygen, nutrients, hormones, and waste products throughout their bodies.

Open Circulatory System: Found in some invertebrates like insects and most mollusks. Blood (called hemolymph in this case) is pumped by the heart through vessels that open into body cavities (sinuses). The hemolymph directly bathes the tissues and organs.

Example: A grasshopper. The heart pumps hemolymph into sinuses where it directly contacts the organs.

Closed Circulatory System: Found in vertebrates (including humans) and some invertebrates like earthworms. Blood is always contained within vessels (arteries, veins, capillaries). This allows for more efficient delivery of oxygen and nutrients to tissues.

Example: A human. The heart pumps blood through a network of vessels, ensuring that blood does not directly contact the organs.