Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Describe transpiration and the transpiration stream.
• Explain the role of cohesion, adhesion, and root pressure.
• Relate the structure of xylem and phloem to their function.
• Discuss factors affecting transpiration rate.
• Explain translocation and the source-sink relationship.

Lesson summary

This week, we delve deeper into the fascinating world of plant transport systems, focusing specifically on the movement of water and minerals (the transpiration stream) and the transport of manufactured food (translocation). Understanding these processes is crucial. In South Africa, where agriculture plays a significant role in our economy and food security, knowing how plants effectively transport nutrients and water is essential for optimizing crop yields and managing water resources, especially in drought-prone areas. Improper irrigation techniques or understanding of plant needs leads to crop failure and economic hardship for many farmers.

Lesson notes

2. 1. Transpiration and the Transpiration Stream Transpiration is the process by which water is lost from a plant in the form of water vapor. Most of this water loss (about 90%) occurs through the stomata on the leaves. The remaining 10% occurs through the cuticle. While it may seem like a negative process, transpiration is vital for several reasons: Transport of Water and Minerals: As water evaporates from the leaves, it creates a "pull" or tension that draws water up from the roots through the xylem. This upward movement of water also carries dissolved mineral salts from the soil to all parts of the plant. This is the transpiration stream.

Cooling: Evaporation of water from the leaves has a cooling effect on the plant, preventing it from overheating, especially during hot South African summers.

Turgor Pressure: Water uptake maintains turgor pressure in cells, which is essential for keeping the plant upright and supporting growth. How does the transpiration stream work? The transpiration stream relies on several forces: Root Pressure: The active transport of mineral ions into the root cells creates a higher concentration of solutes in the root cells than in the surrounding soil water. This causes water to move into the root cells by osmosis, generating a positive pressure that pushes water up the xylem. Root pressure is more effective at night when transpiration rates are low.

Capillarity: The narrow xylem vessels act like capillary tubes. The forces of adhesion (attraction between water molecules and the xylem walls) and cohesion (attraction between water molecules to each other) contribute to water rising in the xylem due to capillarity.

Cohesion-Tension Theory: This is the most widely accepted explanation for the movement of water in the xylem.

Transpiration Pull: As water evaporates from the leaves, it creates a tension or negative pressure in the xylem.

Cohesion: Water molecules are attracted to each other due to hydrogen bonding (cohesion). This means that as one water molecule is pulled up the xylem, it pulls the water molecule behind it, creating a continuous column of water from the roots to the leaves.

Adhesion: Water molecules are also attracted to the walls of the xylem vessels (adhesion). This helps to counteract the force of gravity and prevents the water column from breaking. Imagine a group of people holding hands (cohesion) while climbing a rope (xylem). As the person at the top pulls (transpiration pull), everyone else is pulled up too, because they are holding hands. Their feet sticking slightly to the rope (adhesion) also helps. 2.

2. Xylem and Phloem Structure and Function Xylem: The xylem is the vascular tissue responsible for transporting water and mineral salts from the roots to the rest of the plant.

Xylem consists of two main types of cells: Tracheids: These are long, narrow, tapered cells with pits in their walls, which allow water to move between adjacent tracheids.

Vessel Elements: These are wider and shorter than tracheids. They are joined end-to-end to form long continuous tubes called vessels. The end walls of vessel elements have perforations, which allow for more efficient water flow. Xylem also includes xylem parenchyma cells (for storage) and xylem fibres (for support). The lignin in the walls of xylem cells provides structural support to the plant.

Phloem: The phloem is the vascular tissue responsible for transporting manufactured food (mainly sucrose) from the leaves (source) to other parts of the plant (sink), such as roots, stems, fruits, and developing leaves.

Phloem consists of: Sieve Tube Elements: These are long, cylindrical cells that are arranged end-to-end to form sieve tubes. Sieve tube elements lack a nucleus and other organelles. The end walls of sieve tube elements have pores, forming sieve plates, which allow for the passage 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 the sieve tube elements by plasmodesmata. 2.

3. Factors Affecting Transpiration Rate The rate of transpiration is influenced by several environmental factors: Temperature: Higher temperatures increase the rate of transpiration because warmer water evaporates faster.

Humidity: High humidity decreases the rate of transpiration because the air is already saturated with water vapor, reducing the concentration gradient between the leaf and the air.

Wind: Wind increases the rate of transpiration by removing water vapor from the leaf surface, maintaining a steep concentration gradient.

Light Intensity: Higher light intensity increases the rate of transpiration because it stimulates the opening of the stomata, allowing for more water to evaporate.

Water Availability: Limited water availability reduces the rate of transpiration, as the plant conserves water by closing its stomata.