Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Describe the structure and function of xylem and phloem.
• Explain how roots absorb water.
• Understand transpiration and the factors affecting it.
• Describe the cohesion-tension theory for water transport.
• Explain translocation of food in phloem.

Lesson summary

South Africa's diverse ecosystems, from the Fynbos to the Savannah, depend heavily on efficient plant life. Understanding how plants transport water and nutrients is crucial to appreciating their survival and our dependence on them. Consider the succulent Karoo plants surviving with limited water, or the towering trees of the Knysna forest drawing water meters up dozens of metres. This topic explains the "plumbing" that allows all of this to happen. Deforestation impacts these transport systems, causing erosion and desertification. Sustainable agriculture also relies on understanding how plants access and utilize water and minerals.

Lesson notes

2.1 Xylem: The Water Highway Xylem is the vascular tissue responsible for transporting water and dissolved mineral salts from the roots to the rest of the plant. It's essentially the plant's "water highway." Structure: Xylem consists of two main types of cells: tracheids and vessel elements. Both are dead at maturity, forming hollow tubes.

Tracheids: These are long, thin cells with tapered ends. Water moves between tracheids through pits, small openings in their cell walls. They provide structural support to the plant.

Vessel elements: These are wider and shorter than tracheids. They are stacked end-to-end to form long tubes called vessels. The end walls of vessel elements are either perforated (have many holes) or completely absent, allowing for efficient water flow.

Lignin: Both tracheids and vessel elements are reinforced with lignin, a complex polymer that provides rigidity and prevents the xylem vessels from collapsing under the negative pressure (tension) created by transpiration. The lignin allows the xylem walls to withstand pressure. 2.2 Phloem: The Food Distributor Phloem is the vascular tissue responsible for transporting organic compounds (mainly sugars) produced during photosynthesis from the leaves (source) to other parts of the plant (sink), such as roots, fruits, and developing buds. This movement is called translocation.

Structure: Phloem consists of sieve tube elements and companion cells.

Sieve tube elements: These are living cells, but they lack a nucleus and other organelles. They are arranged end-to-end to form sieve tubes. The end walls between sieve tube elements are perforated, forming sieve plates, which allow for the flow of phloem sap.

Companion cells: These are closely associated with sieve tube elements and provide them with metabolic support. They contain a nucleus and other organelles and are connected to sieve tube elements by plasmodesmata (small channels through the cell walls). Companion cells play a critical role in loading and unloading sugars into the sieve tubes. 2.3 Water Uptake by Roots: From Soil to Xylem Water enters the plant through root hairs, which are extensions of epidermal cells. The root hairs greatly increase the surface area available for water absorption.

Osmosis: Water moves from the soil (high water potential) to the root cells (lower water potential) by osmosis. Osmosis is the movement of water across a semi-permeable membrane from a region of high water concentration to a region of low water concentration.

Active Transport: Mineral ions are absorbed into root cells by active transport. This process requires energy (ATP) because minerals are often present in lower concentrations in the soil than in the root cells. Special carrier proteins in the cell membrane bind to mineral ions and transport them into the cell against their concentration gradient.

Pathways of Water Movement: Once inside the root cells, water can move to the xylem via two pathways: Apoplast pathway: Water moves through the cell walls and intercellular spaces without entering the cells. This pathway is relatively fast but is blocked by the Casparian strip.

Symplast pathway: Water moves from cell to cell through the cytoplasm via plasmodesmata. This pathway is slower but allows the plant to control which substances enter the xylem.

Casparian Strip: The Casparian strip is a band of waterproof material (suberin) in the cell walls of endodermal cells. It forces water and minerals to enter the symplast pathway, allowing the plant to selectively control which substances enter the xylem. This is a protective measure. 2.4 Transpiration: The Driving Force Transpiration is the loss of water vapor from the aerial parts of the plant, mainly through the stomata (small pores on the leaves). It creates a pulling force that draws water up the xylem.

Mechanism: Water evaporates from the mesophyll cells (cells inside the leaf) into the air spaces within the leaf. This increases the water potential of the air spaces. Because the water potential in the leaf air spaces is now greater than the water potential outside the leaf, water diffuses out of the leaf through the stomata.

Factors Affecting Transpiration Rate: Temperature: Higher temperatures increase the rate of evaporation, increasing transpiration.

Humidity: Higher humidity decreases the water potential gradient between the leaf and the air, decreasing transpiration.

Wind: Wind removes water vapor from the leaf surface, increasing the water potential gradient and increasing transpiration.

Light Intensity: Higher light intensity causes the stomata to open (for photosynthesis), increasing transpiration.

Importance of Transpiration: Water Transport: Transpiration creates the "pull" that draws water up the xylem.

Nutrient Transport: Water carries dissolved mineral salts from the roots to the leaves.

Cooling: Transpiration helps to cool the plant, preventing overheating.