Tissues

A tissue is a group of cells that are similar in structure and work together to perform a specific function. The cells of a tissue are derived from the same origin and coordinate their activities to carry out a common role.

In multicellular organisms, different tissues are organized together to form organs. Examples of tissues include:

(i) Muscle tissue — cells that contract to produce movement.
(ii) Nervous tissue — cells that transmit electrical signals (impulses).
(iii) Blood — a connective tissue that transports materials throughout the body.

In multicellular organisms, different tissues are specialized to perform specific functions efficiently. This specialization brings about a division of labour among tissues, which ensures that different functions — digestion, respiration, movement, coordination, reproduction — can be carried out simultaneously and more efficiently.

This is in contrast to unicellular organisms (like Amoeba) where a single cell must perform all life functions, making the organism less efficient. In multicellular organisms:

(i) Each tissue is highly specialized for its function, increasing efficiency.
(ii) Energy expenditure per cell is reduced as it focuses on one role.
(iii) Tissues allow complex body organization, making larger and more complex organisms possible.

Simple permanent tissues are composed of only one type of cell. There are three types:

(i) Parenchyma — Living cells with thin cellulose cell walls and large central vacuoles. Cells are loosely packed with intercellular spaces. Functions: store food (starch, water), provide turgidity/support to soft parts, in leaves they contain chloroplasts (chlorenchyma) and perform photosynthesis, in aquatic plants they have air spaces (aerenchyma) for buoyancy.

(ii) Collenchyma — Living cells with unevenly thickened corners (deposition of pectin and cellulose). Found in leaf stalks (petioles) and edges of leaves. Functions: provide mechanical support and flexibility to young stems and leaves; allow bending without breaking.

(iii) Sclerenchyma — Dead cells with very thick, lignified (woody) cell walls. No living contents at maturity. Found in seed coats, nut shells, husk of coconut. Functions: provide mechanical strength, hardness, and stiffness to plant parts. Make the plant rigid.

Apical meristem is found at the growing tips (apices) of roots and stems. It is present at:

(i) Root tip — the apical meristem here is protected by the root cap and is responsible for the growth of the root downward (primary root elongation).
(ii) Shoot tip (terminal bud) — responsible for the growth of the stem upward and the formation of new leaves and branches.

Apical meristem is responsible for primary growth — the increase in the length of the plant body. The cells of the apical meristem are small, compactly arranged, have thin walls, dense cytoplasm, and large nuclei; they divide rapidly and continuously.

Sclerenchyma tissue makes up the husk of coconut. The coir fibres of the coconut husk consist of long, thick-walled dead sclerenchyma cells (sclerenchyma fibres) whose walls are heavily impregnated with lignin.

These cells are dead at maturity and provide:

(i) Exceptional mechanical strength and rigidity to the husk.
(ii) Protection to the coconut seed from mechanical damage.
(iii) The hard, fibrous texture characteristic of the coconut husk (coir).

Phloem is a complex permanent tissue that transports food (mainly sucrose) made by photosynthesis from leaves to other parts of the plant. It consists of four constituents:

(i) Sieve tubes — Long, thin-walled tubular cells arranged end to end. Their end walls have perforations called sieve plates, which allow food substances to flow through. They lack a nucleus at maturity.

(ii) Companion cells — Small, nucleated cells lying alongside sieve tubes. They are derived from the same mother cell as the sieve tube element. They assist sieve tubes in loading and unloading of food materials.

(iii) Phloem fibres (Bast fibres) — Sclerenchymatous fibres that provide mechanical strength and support to the phloem tissue.

(iv) Phloem parenchyma — Living parenchyma cells that store food materials and assist in lateral conduction of food to surrounding tissues.

Muscle tissue is responsible for movement in our body. Muscle cells (also called muscle fibres) contain special proteins called contractile proteins (actin and myosin) that contract and relax to produce movement.

There are three types of muscle tissue:

(i) Striated (Skeletal / Voluntary) Muscle — Attached to bones. Cells are long, cylindrical, multinucleated and show alternating dark and light bands (striations). Under voluntary (conscious) control. Responsible for movement of body parts like limbs.

(ii) Smooth (Unstriated / Involuntary) Muscle — Found in walls of internal organs (digestive tract, uterus, blood vessels). Cells are spindle-shaped with a single nucleus. No striations. Involuntary control. Responsible for movements like peristalsis in the gut.

(iii) Cardiac Muscle — Found only in the walls of the heart. Cells are branched and interconnected forming a network. Involuntarily controlled but striated. Responsible for the rhythmic beating of the heart throughout life.

A neuron (nerve cell) is the structural and functional unit of the nervous system. It is a highly specialized cell designed to receive, process, and transmit electrical impulses. A neuron has the following parts:

(i) Cell body (Cyton / Soma) — The central part containing the nucleus and cytoplasm (neuroplasm) with Nissl's granules (rough ER). It carries out most metabolic activities of the cell.

(ii) Dendrites — Short, highly branched extensions of the cell body. They receive nerve impulses from other neurons or sensory receptors and carry them toward the cell body.

(iii) Axon — A single long fibre (can be up to 1 metre long) that carries impulses away from the cell body to target cells (muscles, glands, or other neurons). At the end, the axon branches into axon terminals (synaptic knobs) that release neurotransmitters to pass signals to the next cell.

(iv) Myelin sheath — A fatty (lipid) covering around the axon that acts as electrical insulation and speeds up nerve impulse transmission. Gaps in the sheath are called Nodes of Ranvier.

A neuron is the longest cell in the human body — the axon of a motor neuron can stretch from the spinal cord all the way to the toes.

Cardiac muscle is found exclusively in the walls of the heart. Its three main features are:

(i) Involuntary — Cardiac muscles are not under conscious (voluntary) control. They contract and relax rhythmically and automatically, regulated by the heart's own pacemaker (SA node) and the autonomic nervous system.

(ii) Striated (cross-banded) — Like skeletal muscle, cardiac muscle cells show striations (alternating dark A-bands and light I-bands) due to the regular arrangement of actin and myosin filaments.

(iii) Branched and interconnected cells — Cardiac muscle cells are cylindrical and branched. They are connected to adjacent cells at specialized junctions called intercalated discs, which allow rapid and coordinated transmission of electrical impulses, ensuring the heart contracts as a single unit.

(iv) Fatigue-resistant — Cardiac muscles never fatigue; they contract approximately 70 times per minute throughout a person's entire life without tiring (rich in mitochondria for continuous ATP production).

Areolar connective tissue is a loose connective tissue found beneath the skin (subcutaneous tissue) and between organs. It is made of a matrix containing collagen fibres, elastic fibres, and various cells (fibroblasts, mast cells, macrophages). Its functions are:

(i) Packing and support — Fills the spaces between organs and provides packing material, holding organs in place and maintaining their structural relationships.

(ii) Tissue repair — Fibroblasts in areolar tissue produce collagen fibres that help repair damaged tissues and heal wounds.

(iii) Defence against infection — Contains macrophages (that engulf pathogens by phagocytosis) and mast cells (that release histamine during allergic reactions and inflammation), providing a first line of immune defence.

(iv) Connecting skin to muscles — The subcutaneous areolar tissue connects the skin (dermis) to the underlying muscles, allowing the skin to move freely over the muscles.

Meristematic Tissue:

(i) Cells are actively and continuously dividing (mitosis).
(ii) Cells are undifferentiated — they have not taken up any specialized form or function yet.
(iii) Cells are small, isodiametric, with dense cytoplasm and a prominent nucleus.
(iv) Cell walls are thin (primary cell walls only).
(v) No intercellular spaces between cells.
(vi) Located at specific regions: root tip, shoot tip (apical), nodes (intercalary), lateral regions — vascular cambium and cork cambium (lateral).
(vii) High metabolic rate; large amounts of ATP consumed.

Permanent Tissue:

(i) Cells have stopped dividing (non-dividing or rarely dividing).
(ii) Cells are differentiated — specialized in structure and function (e.g., elongated for transport, thick-walled for support).
(iii) Cells may be large, irregular in shape; cytoplasm may be reduced or absent (as in sclerenchyma).
(iv) Cell walls may be thin or thick (secondary cell walls, lignified).
(v) May have intercellular spaces (as in parenchyma).
(vi) Found throughout the plant body — in epidermis, cortex, pith, vascular bundles.
(vii) Lower metabolic rate (except in living parenchyma and phloem).

Cork (also called phellem) is the outermost layer of the bark found in older stems and roots of plants. It is produced by a lateral meristem called the cork cambium (phellogen). Cork acts as an effective protective tissue due to the following reasons:

(i) Suberin impregnation — The cell walls of cork cells are impregnated with suberin, a waxy, waterproof substance. This makes the walls impermeable to water and gases, preventing water loss (desiccation) and blocking entry of gases.

(ii) Dead cells — Cork cells are dead at maturity and have no living contents. This creates a compact, non-reactive layer that is resistant to mechanical damage and chemical attack by pathogens.

(iii) Protection from pathogens — The waterproof, dead layer prevents fungi, bacteria, and other pathogens from entering the plant body.

(iv) Insulation — Cork is a poor conductor of heat, helping protect the plant from extremes of temperature.

(v) Multi-layered — Cork forms multiple layers, providing a thick protective barrier. Lenticels in the cork allow limited gas exchange between the atmosphere and internal tissues.

The epidermis is the outermost single layer of cells that covers the entire surface of the plant body — roots, stems, leaves, flowers, and fruits. It is derived from the protoderm (outer layer of apical meristem). Its roles include:

(i) Protection — Acts as a protective barrier against mechanical injury, excessive water loss, and entry of pathogens.

(ii) Cuticle — In aerial parts (stem, leaves), epidermal cells secrete a waxy layer called the cuticle on their outer surface, which greatly reduces water loss by evaporation.

(iii) Gas exchange and transpiration — In leaves, the epidermis contains specialized pores called stomata, each bordered by two bean-shaped guard cells. Stomata regulate the exchange of gases (CO₂ and O₂) and water vapour (transpiration) between the plant and the atmosphere.

(iv) Water and mineral absorption — Root epidermal cells have long, hair-like extensions called root hairs that greatly increase the surface area for absorption of water and dissolved minerals from the soil.

(v) Trichomes — Some epidermal cells form hair-like outgrowths (trichomes) that reduce water loss, reflect light, and protect against insect attack.

Smooth (Unstriated / Involuntary) Muscle:

(i) Cells are spindle-shaped (narrow at both ends) and relatively small.
(ii) No cross-striations (bands) visible — actin and myosin filaments are not arranged in regular sarcomeres.
(iii) Under involuntary control (not under conscious will) — controlled by the autonomic nervous system.
(iv) Each cell has a single, centrally placed oval nucleus.
(v) Contractions are slow and sustained; not easily fatigued.
(vi) Location: walls of internal (visceral) organs — stomach, intestine, urinary bladder, uterus, blood vessels.

Striated (Skeletal / Voluntary) Muscle:

(i) Cells (fibres) are very long and cylindrical (can be up to 30 cm in length).
(ii) Show distinct alternating dark (A) and light (I) bands — striations — due to regular arrangement of actin and myosin in sarcomeres.
(iii) Under voluntary (conscious) control.
(iv) Each fibre has multiple nuclei at the periphery (multinucleated / syncytium).
(v) Contractions are fast and powerful but fatigue quickly.
(vi) Location: attached to bones via tendons; responsible for body movements like walking, lifting, facial expressions.

Location:

Stratified squamous epithelium is found where surfaces are subject to wear and abrasion:

(i) Skin (epidermis) — the outermost covering of the body (keratinized stratified squamous epithelium).
(ii) Lining of the mouth (buccal cavity) and oesophagus (non-keratinized).
(iii) Lining of the vagina and cervix (non-keratinized).
(iv) Cornea of the eye.

Function:

The primary function of stratified squamous epithelium is protection against physical, chemical, and biological damage:

(i) Its multiple layers (stratification) allow surface cells to be continuously shed and replaced by cells dividing in the deeper germinal (basal) layer. This makes it an excellent wear-resistant barrier.
(ii) In the skin, cells are filled with the protein keratin (keratinization), making the tissue tough, waterproof, and resistant to abrasion, dehydration, and pathogen entry.
(iii) Protects underlying tissues from mechanical abrasion during activities such as chewing, swallowing, and locomotion.