6. Tissues | connecting-the-dots

1. Introduction to Tissues

Definition: A tissue is a group of cells that are similar in structure and/or work together to achieve a particular function.
Division of Labour: In multi-cellular organisms, millions of cells are specialised to carry out specific functions. Cells specialising in one function are grouped together to form a tissue, which ensures the highest possible efficiency of function.
Examples of Tissues: Blood, phloem, and muscle are examples of tissues.

2. Comparison of Plant and Animal Tissues

Feature	Plants	Animals
Movement	Stationary or fixed (do not move).	Move around in search of food, mates, and shelter.
Energy	Consume less energy.	Consume more energy.
Tissue Type	Mostly supportive tissues, many of which are dead.	Most tissues are living.
Growth Pattern	Growth is limited to certain regions (localised dividing tissues).	Cell growth is more uniform, with no clear demarcation of dividing and non-dividing regions.
Organisation	Less specialised and localised organ systems.	Far more specialised and localised structural organisation of organs and organ systems.

3. Plant Tissues

Plant tissues are classified based on the dividing capacity of the cells: Meristematic Tissue and Permanent Tissue.

A. Meristematic Tissue (Dividing Tissue)

Function: Responsible for the growth of plants as it divides throughout the plant's life.
Cell Characteristics: Very active cells with dense cytoplasm, thin cellulose walls, prominent nuclei, and they lack vacuoles.
Classification by Location:
- Apical Meristem: Found at the growing tips of stems and roots, increasing their length.
- Lateral Meristem (Cambium): Found in the girth of the stem or root, increasing its thickness.
- Intercalary Meristem: Found at the base of the leaves or internodes on twigs.

B. Permanent Tissue

Formation: Formed when cells of meristematic tissue undergo differentiation, losing the ability to divide and taking up a permanent shape, size, and function.

i. Simple Permanent Tissue (Made of one type of cell)

Parenchyma (Basic Packing Tissue):
- Cells: Relatively unspecialised, live cells with thin cell walls. They are usually loosely packed with large intercellular spaces.
- Functions: Provides support to plants, stores food, nutrients, and water.
- Variants:
  - Chlorenchyma: Parenchyma that contains chlorophyll and performs photosynthesis.
  - Aerenchyma: Parenchyma with large air cavities found in aquatic plants, providing buoyancy.
Collenchyma:
- Cells: Living, elongated, and irregularly thickened at the corners, with very little intercellular space.
- Functions: Provides flexibility, allowing easy bending of plant parts (leaf, stem) without breaking. Also provides mechanical support.
- Location: Found in leaf stalks below the epidermis.
Sclerenchyma:
- Cells: Dead cells that are long and narrow. Walls are greatly thickened due to lignin, often leaving no internal space.
- Functions: Makes the plant hard and stiff, providing strength to the plant parts.
- Location: Present in stems, around vascular bundles, in the veins of leaves, and in the hard covering of seeds and nuts (e.g., husk of a coconut).
Protective Tissues:
- Epidermis: The outermost layer of cells, typically a single layer, that forms a continuous sheet without intercellular spaces. It protects all parts of the plant, aiding against water loss, mechanical injury, and invasion by parasitic fungi.
  - Stomata: Small pores in the epidermis of the leaf, enclosed by two kidney-shaped guard cells. They are necessary for exchanging gases and for transpiration (loss of water vapour).
  - Root Epidermal Cells: Bear long hair-like parts that increase the total absorptive surface area for water absorption.
- Cork (Bark): As plants grow older, a strip of secondary meristem replaces the epidermis, forming the cork. Cork cells are dead, compactly arranged without intercellular spaces, and have a chemical called suberin in their walls that makes them impervious to gases and water.

ii. Complex Permanent Tissue (Made of more than one type of cell)

Vascular Bundle: Xylem and phloem are conducting tissues that together form a vascular bundle.
Xylem (Water and Mineral Transport):
- Elements: Consists of tracheids, vessels, xylem parenchyma, and xylem fibres.
- Characteristics: Cells have thick walls, and many of them are dead cells.
- Function: Tracheids and vessels are tubular structures that transport water and minerals vertically. Xylem parenchyma stores food, and fibres are mainly supportive.
Phloem (Food Transport):
- Elements: Made up of four types of elements: sieve tubes, companion cells, phloem fibres, and phloem parenchyma.
- Characteristics: Sieve tubes are tubular cells with perforated walls. Except for phloem fibres, phloem cells are living.
- Function: Transports food from leaves to other parts of the plant, and materials can move in both directions.

4. Animal Tissues

Animal tissues are classified into epithelial, connective, muscular, and nervous tissue.

A. Epithelial Tissue

Function: The covering or protective tissues in the animal body. Forms a continuous sheet and a barrier between different body systems.
Cell Characteristics: Tightly packed cells with little cementing material and almost no intercellular spaces. Always separated from underlying tissue by a fibrous basement membrane.
Types:
- Simple Squamous Epithelium: Extremely thin, flat cells forming a delicate lining, found where transportation of substances occurs (e.g., lining of blood vessels, lung alveoli, oesophagus, mouth).
- Stratified Squamous Epithelium: Cells arranged in many layers to prevent wear and tear (e.g., skin).
- Columnar Epithelium: Tall, pillar-like cells facilitating movement across the epithelial barrier, found where absorption and secretion occur (e.g., inner lining of the intestine).
  - Ciliated Columnar Epithelium: Columnar tissue with hair-like projections (cilia) that move to push mucus forward (e.g., respiratory tract).
- Cuboidal Epithelium: Cube-shaped cells that provide mechanical support (e.g., lining of kidney tubules, ducts of salivary glands).
- Glandular Epithelium: Epithelial tissue folded inward to form a multicellular gland, specialised to secrete substances.

B. Connective Tissue

Function: To connect various body parts. Cells are loosely spaced and embedded in an intercellular matrix (jelly-like, fluid, dense, or rigid).
Types:
- Blood: A type of fluid connective tissue.
  - Matrix: Fluid matrix called plasma (contains proteins, salts, hormones).
  - Suspended Cells: Red blood cells (RBCs), white blood cells (WBCs), and platelets.
  - Function: Transports gases, digested food, hormones, and waste materials.
- Bone: Forms the framework, supports the body, anchors muscles, and supports main organs. Cells are embedded in a hard matrix of calcium and phosphorus compounds.
- Ligament: Connects two bones to each other. It is very elastic and has considerable strength but very little matrix.
- Tendon: Connects muscles to bones. It is a fibrous tissue with great strength but limited flexibility.
- Cartilage: Has widely spaced cells and a solid matrix of proteins and sugars. It smoothens bone surfaces at joints and is present in the nose, ear, trachea, and larynx.
- Areolar Connective Tissue: Found between the skin and muscles, around blood vessels and nerves, and in bone marrow. It fills space inside organs, supports internal organs, and helps in tissue repair.
- Adipose Tissue (Fat-storing): Found below the skin and between internal organs. Cells are filled with fat globules. Stores fats and acts as an insulator.

C. Muscular Tissue

Function: Responsible for movement in the body due to special contractile proteins that contract and relax.
Types:
- Striated Muscle (Voluntary/Skeletal):
  - Control: Voluntary (move when we want).
  - Structure: Cells are long, cylindrical, unbranched, and multinucleate. Show alternate light and dark bands (striations).
  - Location: Mostly attached to bones (limbs).
- Smooth Muscle (Unstriated/Involuntary):
  - Control: Involuntary (cannot be started or stopped by conscious will).
  - Structure: Cells are long with pointed ends (spindle-shaped) and uninucleate (single nucleus). They lack striations.
  - Location: Movement of food in the alimentary canal, contraction/relaxation of blood vessels, iris of the eye, ureters, and bronchi of the lungs.
- Cardiac Muscle:
  - Control: Involuntary, showing rhythmic contraction and relaxation throughout life.
  - Structure: Cells are cylindrical, branched, and uninucleate. Show striations.
  - Location: Heart.

D. Nervous Tissue

Function: Highly specialised for being stimulated and then transmitting the stimulus very rapidly from one place to another within the body.
Components: The brain, spinal cord, and nerves are composed of nervous tissue.
Neuron (Nerve Cell): The unit of nervous tissue.
- Structure: Consists of a cell body with a nucleus and cytoplasm, from which hair-like parts arise.
- Parts:
  - Axon: Usually a single long part.
  - Dendrites: Many short, branched parts.
Nerve Impulses: Allow muscles to move in response to stimuli, a functional combination of nerve and muscle tissue fundamental to most animals.