Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Differentiate between major plant tissues (parenchyma, collenchyma, sclerenchyma, xylem, phloem).
• Differentiate between major animal tissues (epithelial, connective, muscle, nervous).
• Relate the structure of specific tissues to their functions.
• Explain how cell modifications lead to specialized tissue functions.
• Compare and contrast plant and animal tissues.

Lesson summary

This week, we revisit and extend our understanding of plant and animal tissues. A strong grasp of tissue structure and function is fundamental to understanding how multicellular organisms work. In South Africa, understanding plant tissues is particularly relevant to agriculture, a cornerstone of our economy and food security. Knowing how plants transport water and nutrients, or how animals adapt to different environments, is crucial for tackling challenges like drought resilience, optimizing crop yields, and conserving our diverse animal life. Animal tissues are equally important in understanding human physiology, health, and disease.

Lesson notes

Plant Tissues: Meristematic Tissue: These are plant tissues capable of continuous cell division and are responsible for plant growth. Found at root and shoot tips (apical meristems) and in lateral buds (axillary meristems). Apical meristems cause primary growth (lengthening). Lateral meristems cause secondary growth (widening). This is particularly important in woody plants.

Permanent Tissues: These tissues have differentiated from meristematic tissue and have specific functions.

Parenchyma: These are the most common plant cells. They are generally thin-walled and involved in various functions, including photosynthesis (in chlorenchyma), storage of food and water, and secretion. Found in leaves, stems, and roots.

Example: the fleshy part of an apple consists largely of parenchyma cells storing sugars.

Collenchyma: These tissues provide flexible support to young stems and leaves. They have unevenly thickened cell walls.

Example: The strings in celery stalks are made of collenchyma.

Sclerenchyma: These provide rigid support and strength. They have thick, lignified (containing lignin, a strengthening polymer) cell walls and are often dead at maturity.

Two main types: fibres (long and slender) and sclereids (short and irregular).

Example: The hardness of a peach pit is due to sclereids.

Xylem: This is a vascular tissue responsible for transporting water and minerals from the roots to the rest of the plant. Xylem cells are dead at maturity and form long, hollow tubes called tracheids and vessel elements. Lignin in their cell walls provides support. Xylem Transport – Transpiration-Cohesion-Tension Mechanism: Transpiration: Water evaporates from leaves through stomata, creating a negative pressure (tension).

Cohesion: Water molecules are attracted to each other through hydrogen bonds.

Adhesion: Water molecules are attracted to the xylem walls. These properties allow water to be pulled up the xylem column.

Example: Consider a mealie (maize) plant during a hot summer day in KwaZulu-Natal. The high temperatures increase transpiration, placing a high demand on the xylem to supply water to the leaves.

Phloem: This is a vascular tissue responsible for transporting sugars (produced during photosynthesis) from the leaves to other parts of the plant. Phloem cells are living but require companion cells for support. Phloem consists of sieve tube elements (conduct sugars) and companion cells (provide metabolic support). Phloem Transport – Pressure Flow Hypothesis: Sugars are actively loaded into sieve tubes at the source (e.g., leaves) and unloaded at the sink (e.g., roots, developing fruits). This creates a pressure gradient that drives the flow of phloem sap.

Example:* During the mango season in Limpopo, the phloem actively transports sugars from the leaves of the mango tree to the developing fruits, resulting in the sweet taste we enjoy.

Animal Tissues: Epithelial Tissue: This tissue covers surfaces, lining organs and cavities. Functions include protection, absorption, secretion, and excretion. Epithelial tissue can be classified based on its shape (squamous, cuboidal, columnar) and the number of layers (simple, stratified).

Simple Squamous Epithelium:* Single layer of flattened cells, allowing for diffusion. Found in air sacs of lungs (alveoli).

Stratified Squamous Epithelium:* Multiple layers of flattened cells, providing protection. Found in the epidermis of skin.

Columnar Epithelium:* Tall, column-shaped cells, often with cilia or microvilli. Found lining the small intestine (for absorption).

Connective Tissue: This tissue provides support, connects other tissues, and transports substances. Characterized by cells embedded in an extracellular matrix.

Loose Connective Tissue:* Loosely arranged fibres, providing support and cushioning. Found under the skin.

Dense Connective Tissue:* Densely packed fibres, providing strength and support. Found in tendons (muscle to bone) and ligaments (bone to bone).

Cartilage:* Provides support and flexibility. Found in the ear and joints.

Bone:* Provides rigid support and protection. The skeleton.

Blood:* Transports oxygen, nutrients, and waste. Composed of red blood cells, white blood cells, and platelets in a liquid matrix (plasma).

Muscle Tissue: This tissue is responsible for movement.

There are three types: skeletal, smooth, and cardiac.

Skeletal Muscle:* Striated (banded) and voluntary (consciously controlled). Attached to bones and responsible for movement.

Example: biceps muscle.

Smooth Muscle:* Non-striated and involuntary. Found in the walls of internal organs (e.g., stomach, intestines).

Cardiac Muscle:* Striated and involuntary. Found only in the heart.

Nervous Tissue: This tissue is responsible for communication and control. Composed of neurons (nerve cells) and glial cells (support cells).

Neurons:* Transmit electrical signals.

Glial cells:* Support and protect neurons.

Worked example

Example: Leaf Structure and Function: Consider the leaf of a boabab tree. The upper epidermis is a single layer of cells covered by a waxy cuticle, reducing water loss in the dry savanna. The mesophyll contains parenchyma cells (chlorenchyma) with chloroplasts for photosynthesis. The lower epidermis has stomata, controlled by guard cells, allowing for gas exchange. Xylem and phloem are found in the vascular bundles (veins), transporting water and nutrients.

Example: Skin Structure and Function: The skin is composed of multiple layers. The epidermis is the outer layer, made of stratified squamous epithelium, providing protection against abrasion and infection. The dermis contains connective tissue (collagen and elastic fibers), blood vessels, nerve endings, and hair follicles. The hypodermis is a layer of adipose tissue (fat), providing insulation and energy storage.

Guided Practice (With Solutions)

Question 1: Describe the structure of xylem tissue and explain how its structure is related to its function.

Solution: Xylem tissue consists of tracheids and vessel elements, which are dead cells at maturity. These cells have thick, lignified cell walls, providing strength and support. The cells are arranged end-to-end, forming long, hollow tubes that allow for efficient water transport from roots to leaves. The absence of cytoplasm reduces resistance to water flow.

Commentary: This question tests your understanding of structure-function relationships. Mentioning lignin and the hollow tube structure is crucial.

Question 2: Compare and contrast parenchyma and sclerenchyma cells in terms of their structure and function.