1. What are Tissues?
A tissue is a group of cells with similar structure and function, working together to perform specific tasks. In multicellular organisms like plants and animals, tissues show division of labour. For example, muscle cells in humans cause movement, while phloem in plants transports food. Plants have more dead supportive tissues as they are stationary, while animals have mostly living tissues for movement.
Fun Fact: In Amoeba, a single cell does everything, but in humans, millions of cells team up as tissues!
2. Plant Tissues
Plant tissues are divided into meristematic (dividing) and permanent (non-dividing) tissues, reflecting their limited growth regions.
2.1 Meristematic Tissue
Meristematic tissues are found in growing regions like root and shoot tips. Their cells are active, with dense cytoplasm, thin walls, and prominent nuclei, but no vacuoles. They divide to help plants grow.
2.2 Permanent Tissue
Formed from meristematic tissue through differentiation, permanent tissues have fixed roles and cannot divide.
Simple Permanent Tissues
- Parenchyma: Living cells with thin walls, loosely arranged for food storage. Chlorenchyma (with chlorophyll) photosynthesizes, and aerenchyma (with air cavities) helps aquatic plants float.
- Collenchyma: Living, elongated cells with thick corners, providing flexibility and support in leaf stalks.
- Sclerenchyma: Dead, lignified cells (e.g., coconut husk), making plants hard and strong.
Epidermis
A single layer of flat cells covering plant surfaces, protecting against water loss and fungi. In dry habitats, it may have a waxy cutin layer. Stomata (pores with guard cells) allow gas exchange and transpiration. Root epidermal cells have hair-like extensions for water absorption.
Cork
In older plants, cork (dead cells with suberin) forms a protective, waterproof layer, replacing epidermis in stems and roots.
Complex Permanent Tissues
Made of multiple cell types:
- Xylem: Includes tracheids, vessels (dead, tubular for water transport), xylem parenchyma (stores food), and xylem fibres (support).
- Phloem: Includes sieve tubes (food transport), companion cells, phloem parenchyma (storage), and phloem fibres (support, dead).
Activity: Observe onion root growth in water. Cut the root tip of one and compare growth!
3. Animal Tissues
Animal tissues are classified into epithelial, connective, muscular, and nervous tissues, each with specialized functions.
3.1 Epithelial Tissue
Covers body surfaces (e.g., skin, lung alveoli) with tightly packed cells and minimal intercellular spaces. Types include:
- Squamous: Thin, flat cells (e.g., blood vessels) for transport.
- Stratified Squamous: Layered for protection (e.g., skin).
- Columnar: Tall cells for absorption (e.g., intestine), some with cilia (respiratory tract).
- Cuboidal: Cube-shaped for support (e.g., kidney tubules).
- Glandular: Secretes substances (e.g., sweat glands).
3.2 Connective Tissue
Connects body parts with loosely spaced cells in a matrix:
- Blood: Fluid matrix (plasma) with RBCs, WBCs, and platelets for transport.
- Bone: Hard matrix with calcium for support.
- Cartilage: Flexible matrix in ears, joints.
- Ligaments: Elastic, connect bones.
- Tendons: Strong, connect muscles to bones.
- Areolar: Supports organs, repairs tissues.
- Adipose: Stores fat, insulates.
3.3 Muscular Tissue
Contains contractile proteins for movement:
- Striated (Skeletal): Voluntary, cylindrical, multinucleate, with striations (e.g., limb muscles).
- Smooth: Involuntary, spindle-shaped, uninucleate (e.g., digestive tract).
- Cardiac: Involuntary, branched, uninucleate, in heart for rhythmic contractions.
3.4 Nervous Tissue
Made of neurons (nerve cells) with a cell body, axon, and dendrites for rapid signal transmission in the brain, spinal cord, and nerves.
Activity: Observe a blood smear under a microscope to identify RBCs and WBCs!
Next Chapter
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Go to Chapter 7