Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Explain transpiration, cohesion, adhesion, and root pressure.
• Describe the structure and function of xylem and phloem.
• Explain translocation and the source-sink relationship.
• Discuss factors affecting transpiration rate.
• Explain plant adaptations for water conservation (e.g., xerophytes).

Lesson summary

Welcome to Week 3 of our exciting journey into the world of plants! This week, we delve deeper into the intricate transport systems that allow plants to thrive. Understanding how plants move water, minerals, and food is crucial, not just for acing your exams, but also for appreciating the vital role plants play in our environment and economy here in South Africa. From the drought-resistant succulents of the Karoo to the towering trees of our indigenous forests, understanding plant transport helps us understand how these diverse species survive and contribute to our ecosystems.

Lesson notes

2.1 Water Transport (Xylem): The Ascent of Sap The upward movement of water and dissolved minerals from the roots to the leaves, against the force of gravity, is a remarkable feat. This process, known as the ascent of sap, relies on several key mechanisms: Transpiration: This is the loss of water vapor from the aerial parts of the plant, primarily through the stomata in the leaves. Transpiration creates a "pull" or tension at the top of the plant, drawing water upwards. Think of it like sucking on a straw – the evaporation of water from the leaves is like the sucking action.

Why it matters: Transpiration is essential for cooling the plant (like sweating in humans) and for transporting minerals.

However, excessive transpiration can lead to water stress, particularly in arid regions.

Cohesion: Water molecules are attracted to each other through hydrogen bonds. This attraction, called cohesion, creates a continuous column of water within the xylem vessels, extending from the roots to the leaves. Imagine a long chain of water molecules, pulling each other upwards.

Adhesion: Water molecules are also attracted to the walls of the xylem vessels, a phenomenon called adhesion. This helps to counteract the force of gravity and prevents the water column from breaking. Think of it as the water molecules "sticking" to the sides of the straw.

Root Pressure: This is the pressure exerted by the roots that helps to push water up the xylem. It is generated by the active transport of ions into the root xylem, creating a water potential gradient that draws water into the roots. Root pressure is more significant in smaller plants and plays a lesser role in tall trees. You can observe root pressure in action by cutting off a plant stem close to the ground; you'll notice water oozing out.