Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Describe support systems in plants like turgor pressure and woody tissue.
• Explain the function of xylem and phloem for transport.
• Understand the role of osmosis and diffusion in water movement.
• Identify plant adaptations for support and transport in South Africa.

Lesson summary

The study of support and transport systems in plants and animals is fundamental to understanding how organisms function and survive. In South Africa, where diverse ecosystems range from the Karoo desert to lush forests and coastal regions, understanding how plants and animals adapt their support and transport mechanisms to their environments is crucial. For example, understanding water transport in drought-resistant plants is critical to addressing food security challenges in arid regions. Similarly, understanding circulatory systems in animals helps us understand health issues like heart disease, prevalent in our communities.

Lesson notes

A. Support Systems in Plants: Plants, unlike animals, cannot move to find support.

Therefore, they rely on internal and external support structures.

Turgor Pressure: This is the main supporting factor in herbaceous (non-woody) plants. Plant cells have vacuoles that fill with water. The water exerts pressure against the cell wall, making the cells turgid and the plant upright. If a plant loses water (e.g., during a drought), turgor pressure decreases, and the plant wilts.

Example:* A wilted lettuce leaf shows loss of turgor pressure.

Cellulose in Cell Walls: Plant cell walls are made of cellulose, a strong and rigid polysaccharide. This provides structural support to individual cells and, collectively, to the entire plant. The thickness of the cell wall varies depending on the cell's function and location in the plant.

Example:* The sclerenchyma cells in plant stems and roots have thickened cell walls, providing significant strength.

Lignin: This complex polymer is deposited in the cell walls of woody plants, making them even more rigid and resistant to decay. Lignin is a key component of wood.

Example: The trunk of a Vachellia (Acacia)* tree, common in South Africa, owes its strength to lignin.

Support Roots: Many plants have specialized roots that provide additional support, particularly in unstable environments.

Example:* Mangrove trees, found along the South African coastline, have prop roots that help anchor them in the soft mud.

B. Transport Systems in Plants: Plants have two main vascular tissues for transport: xylem and phloem.

Xylem: Transports water and dissolved minerals from the roots to the rest of the plant (unidirectional flow). Xylem consists of dead cells, primarily tracheids and vessel elements, which form continuous tubes.

Mechanism:* Water moves up the xylem through transpiration pull (evaporation of water from leaves), cohesion (water molecules sticking together), and adhesion (water molecules sticking to the xylem walls). This is known as the cohesion-tension theory.

Example:* Imagine a straw. You suck from the top, and water rises through the straw. Transpiration pull is similar - the 'sucking' force created by water evaporating from the leaves.

Why it matters:* Xylem function is directly related to plant survival, especially in drought-prone areas of South Africa. Water scarcity affects crop yields and natural ecosystems.

Phloem: Transports sugars (produced during photosynthesis) from source (e.g., leaves) to sinks (e.g., roots, fruits, developing leaves) (bidirectional flow).

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

Mechanism:* Sugars are actively loaded into sieve tubes at the source, decreasing water potential. Water then moves into the sieve tubes by osmosis, increasing pressure. This pressure drives the sugar-rich sap towards the sink, where sugars are unloaded, and water moves out. This is called the pressure flow hypothesis.

Example:* Imagine a factory (leaf) producing sweets (sugars) and sending them to different shops (roots, fruits) across the country. The phloem is like the transport network.

Why it matters:* Phloem function is essential for delivering energy to all parts of the plant, including roots for nutrient uptake and fruits for reproduction.

C. Support Systems in Animals: Animals have various support systems, ranging from hydrostatic skeletons to exoskeletons and endoskeletons.

Hydrostatic Skeletons: Found in soft-bodied invertebrates like earthworms. They use fluid-filled cavities to provide support and facilitate movement.

Example:* Earthworms use their hydrostatic skeleton to burrow through the soil.

Exoskeletons: Found in arthropods like insects and crustaceans. These are external, hard coverings made of chitin that provide protection and support.

Example:* The exoskeleton of a beetle provides protection from predators and physical damage. They must moult (shed their exoskeleton) to grow.

Endoskeletons: Found in vertebrates, including humans. These are internal skeletons made of bone and cartilage. Bone is a living tissue containing calcium phosphate for strength. Cartilage is more flexible and found in areas like joints and the nose.

Function:* Provides support, protects internal organs, and allows for movement via muscles attached to the bones.

Example:* The human skeleton supports the body, protects the brain (skull), and allows for movement (limbs).

D. Transport Systems in Animals: Animal transport systems (circulatory systems) deliver oxygen, nutrients, and hormones to cells and remove waste products.

Open Circulatory System: Blood (hemolymph) is not always confined to vessels and flows directly over the tissues. Found in arthropods and some molluscs.

Example:* Insects have an open circulatory system where hemolymph circulates within the body cavity, delivering nutrients and removing wastes.

Closed Circulatory System: Blood is always confined to vessels. Found in vertebrates and some invertebrates (e.g., earthworms).