Introduction
A tissue is a group of cells that are similar in structure and work together to perform a specific function. Multicellular organisms develop tissues because division of labour among specialised cells is more efficient than every cell doing everything. This chapter covers the major types of plant and animal tissues.
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Key Concepts
1. Plant Tissues
Plants have two main categories of tissues:
- A. Meristematic Tissue (Dividing Tissue)
- Made of actively dividing cells with large nuclei and thin cell walls.
- Apical meristem: found at root and shoot tips; responsible for increase in length.
- Lateral meristem (cambium): found along the sides; responsible for increase in girth.
- Intercalary meristem: found at internodes; helps in re-growth after grazing.
B. Permanent Tissue (Non-dividing)
Formed when meristematic cells stop dividing and become specialised.
- · Simple permanent tissues · (all cells of one type):
- Parenchyma: loosely packed cells with large vacuoles; stores food and water; can photosynthesise (chlorenchyma) or provide buoyancy (aerenchyma).
- Collenchyma: cells with thickened corners; provides flexible mechanical support to young stems and leaf stalks.
- Sclerenchyma: cells with thick, lignified, dead cell walls; provides rigid support (e.g., fibres in jute, stone cells in coconut shell).
- · Complex permanent tissues · (more than one cell type working together):
- Xylem: transports water and minerals from roots to leaves. Contains tracheids, vessels, xylem fibres, and xylem parenchyma.
- Phloem: transports sugars (food) made in leaves to all other parts. Contains sieve tubes, companion cells, phloem fibres, and phloem parenchyma.
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- 2. Animal Tissues
- A. Epithelial Tissue
- Covers body surfaces and lines cavities. Types:
- Simple squamous (flat cells): lines blood vessels and alveoli; allows diffusion.
- Cuboidal: lines kidney tubules; secretion and absorption.
- Columnar: lines intestine; absorption and secretion. If cilia are present: ciliated columnar (e.g., trachea).
- Stratified squamous: multiple layers; found in skin; protection.
- B. Connective Tissue
- Connects, supports, and binds other tissues. Types:
- Blood: fluid connective tissue; transports substances.
- Bone: hard matrix (calcium and phosphorus); supports and protects.
- Cartilage: flexible matrix; found at joints, nose, ear.
- Areolar: loose connective tissue around organs.
- Adipose: stores fat for energy and insulation.
- Tendons: connect muscle to bone (dense regular connective tissue).
- Ligaments: connect bone to bone.
- C. Muscular Tissue
- Striated (skeletal) muscle: voluntary; attached to bones; striped appearance.
- Smooth (unstriated) muscle: involuntary; found in walls of hollow organs (stomach, blood vessels).
- Cardiac muscle: involuntary, striated; found only in the heart; never fatigues.
D. Nervous Tissue
Made of neurons (nerve cells). A neuron has a cell body, dendrites (receive signals), and an axon (sends signals). Neurons transmit electrical impulses throughout the body.
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Worked Examples
Why do plants need meristematic tissue but adult animals do not have equivalent regions?
Meristematic tissue allows plants to grow throughout their life. Animals grow to a fixed size — their growth zone cells differentiate early. Plants need continuous growth to reach sunlight and water.
Identify the tissue: thick lignified walls, dead cells, provides rigid support in coconut shells.
This is sclerenchyma. The lignified dead cell walls provide maximum rigidity, making it suitable for protection in hard shells.
What is the difference between xylem and phloem?
Xylem conducts water and minerals upward from roots, using dead cells (tracheids, vessels) through a process involving transpiration pull. Phloem conducts sugars in both directions using living sieve tubes powered by pressure flow.
Why is blood classified as a connective tissue?
Connective tissue has cells scattered in a matrix. Blood has cells (RBCs, WBCs, platelets) in a liquid matrix called plasma. It connects organs by transporting oxygen, nutrients, hormones, and waste. Hence it is connective tissue.
A student examines a tissue under a microscope and sees cells with striations (stripes) that are branched and connected at intercalated discs. What tissue is this?
This is cardiac muscle tissue. It is striated, branched, and connected by intercalated discs — features unique to the heart muscle. It is involuntary.
Why does the lining of the trachea have ciliated epithelium?
The cilia sweep dust, mucus, and microbes upward toward the throat to be expelled. This protects the lungs from infection. Without cilia, particles would reach the alveoli.
Differentiate between tendons and ligaments.
Tendons connect muscle to bone and are made of dense regular connective tissue (parallel collagen fibres). Ligaments connect bone to bone and are slightly elastic, allowing controlled movement at joints.
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Common mistakes
- Confusing xylem and phloem: xylem = water (up); phloem = food (both directions).
- Saying smooth muscle is striated — only skeletal and cardiac muscles are striated.
- Forgetting that sclerenchyma cells are dead at maturity.
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Summary
Tissues are organised groups of specialised cells. Plants have meristematic (dividing) and permanent (non-dividing) tissues; complex tissues like xylem and phloem are vital for transport. Animals have four tissue types: epithelial (covering), connective (support), muscular (movement), and nervous (communication). Each tissue's structure is perfectly suited to its function.