Tissues

Why can't a single type of cell do everything? In a complex organism, different functions — support, transport, sensation, contraction — require specialisation. Groups of similar cells performing a common function form tissues, and tissues organised together form organs. This chapter is directly relevant to UPSC questions on organ donation policy, blood as a connective tissue (blood transfusion, bone marrow donation), stem cell therapy, and tissue banking — all of which have appeared in GS2 and GS3 papers.

PART 1 — Quick Reference Tables

Overview: Types of Tissues

Plant Simple Tissues — Comparison

Xylem vs Phloem — Complex Tissues

Animal Tissues — Connective Tissue Types

Muscle Tissue — Three Types

PART 2 — Detailed Notes

1. Why Do Multicellular Organisms Need Tissues?

Division of labour — just as a society functions better when people specialise in different trades, a multicellular organism functions more efficiently when cells specialise. Specialised cells group together into tissues. The degree of specialisation increases from simpler to more complex organisms.

2. Plant Tissues

Meristematic Tissues (meristos = divisible) contain actively dividing cells. They are responsible for growth.

• Apical meristem: Located at root and shoot tips. Responsible for primary growth (increase in length).
• Lateral meristem (cambium): Located on the sides of stems and roots. Responsible for secondary growth (increase in girth). The cork cambium produces bark; vascular cambium produces wood (secondary xylem).
• Intercalary meristem: Located at the base of leaves or internodes. Allows regrowth after grazing (important in grasses).

Permanent Tissues are formed when meristematic cells differentiate and lose the ability to divide. They may be living or dead.

Simple permanent tissues are made of a single cell type:

Parenchyma — the most common plant tissue. Thin-walled, living cells with large vacuoles. Functions include: photosynthesis (chlorenchyma — parenchyma with chloroplasts), storage of starch, water, oils and other substances, and wound healing through dedifferentiation. Aerenchyma (parenchyma with large air spaces) is found in aquatic plants for buoyancy.

Collenchyma — living cells with unevenly thickened cell walls (thickening at corners). Provides mechanical support while allowing flexibility — hence found in young growing stems, leaf stalks (petioles), and leaf ribs. Unlike sclerenchyma, collenchyma can grow with the plant.

Sclerenchyma — cells dead at maturity with heavily lignified (lignin — a complex polymer) cell walls. Two forms: fibres (long, narrow cells for support — linen from flax, jute fibres) and sclereids/stone cells (irregularly shaped, found in hard seed coats and nut shells). Provides rigidity.

Complex permanent tissues are made of more than one cell type:

Xylem components:

• Tracheids — elongated, dead cells with pits in walls. Water moves from tracheid to tracheid through pits. Found in gymnosperms and primitive angiosperms.
• Vessel elements — larger, dead cells that join end-to-end to form continuous vessel tubes (the dominant water-conducting tissue in flowering plants).
• Xylem parenchyma — living, stores food and water.
• Xylem fibres — dead, provide support.

Phloem components:

• Sieve tube elements — living cells (lose nucleus at maturity) stacked to form sieve tubes; perforated sieve plates at ends allow phloem sap to move.
• Companion cells — living cells with nucleus that support sieve tubes metabolically (since sieve tubes lose their nucleus).
• Phloem parenchyma — living, stores food.
• Phloem fibres (bast fibres) — dead, provide support. Commercial fibres like jute come from phloem fibres.

🎯 UPSC Connect: Plant Tissues and Agriculture

Xylem and phloem explain how water, minerals and food move through plants. This directly connects to:

• Drip irrigation efficiency — delivers water directly to root zone where xylem uptake occurs
• Plant grafting — scion and rootstock must have compatible vascular (xylem+phloem) alignment
• Fibre crops — jute fibres are phloem fibres; cotton fibres are seed trichomes; both are GI-tagged products important for agricultural trade

3. Animal Tissues

Epithelial Tissues cover body surfaces, line organs, and form glands. Key types:

• Squamous epithelium — flat cells, forms skin surface, lining of blood vessels and alveoli (enables diffusion)
• Cuboidal epithelium — cube-shaped, kidney tubules and salivary glands
• Columnar epithelium — tall cells, lining of the intestine with microvilli for absorption
• Ciliated epithelium — columnar cells with cilia; lines respiratory tract to sweep mucus and dust upward (mucociliary clearance — impaired in smokers and COVID-19 patients)
• Glandular epithelium — secretes hormones and enzymes

Connective Tissues — scattered cells in an extracellular matrix (ECM). The matrix may be solid (bone), semi-solid (cartilage), or liquid (blood).

Blood — the only liquid connective tissue. Plasma (55%) is the liquid matrix; formed elements (45%) are RBCs (erythrocytes), WBCs (leukocytes), and platelets (thrombocytes). RBCs carry O2 via haemoglobin; WBCs are immune cells; platelets are involved in clotting.

🎯 UPSC Connect: Blood and Policy

• Blood transfusion: ABO and Rh blood group compatibility — basis of India's National Blood Policy
• Anaemia: Low RBC/haemoglobin — India has among the highest anaemia prevalence rates globally; addressed by POSHAN Abhiyaan
• Bone marrow donation: Bone marrow contains haematopoietic stem cells that produce all blood cells — transplants used for leukaemia treatment; DKMS-BMST India is the national registry
• Blood as connective tissue — a frequent MCQ trap (students expect blood to be a fluid, not a tissue)

Bone — calcified connective tissue. Osteocytes (bone cells) sit in lacunae within a hard matrix of collagen fibres and calcium phosphate (hydroxyapatite). Bone is not inert — it is remodelled constantly by osteoblasts (build bone) and osteoclasts (break down bone). Bone marrow inside long bones produces blood cells.

Cartilage — flexible connective tissue. Matrix is chondroitin sulphate. Found in: ear pinna, tip of nose, intervertebral discs, tracheal rings (C-shaped, allow oesophagus to expand during swallowing), and covering the ends of bones at joints (articular cartilage). Unlike bone, cartilage has no blood vessels — this is why cartilage heals slowly after injury.

Muscular Tissue:

• Striated muscle fibres are multi-nucleated (formed by fusion of many cells) — a syncytium.
• Cardiac muscle is unique: it is involuntary like smooth muscle but striated like skeletal muscle. Cardiac cells are connected by intercalated discs (gap junctions) that allow electrical signals to spread rapidly, ensuring the heart contracts as one unit.

Nervous Tissue: Composed of neurons (nerve cells) and glial cells (support cells). A neuron has a cell body (soma), dendrites (receive signals), and an axon (transmits signals to next neuron or muscle). Myelin sheath (from Schwann cells) insulates the axon and speeds up conduction (saltatory conduction). Loss of myelin occurs in multiple sclerosis — an autoimmune disease.

🎯 UPSC Connect: Organ Donation and Tissue Banking

The Transplantation of Human Organs and Tissues Act (THOTA) 1994, amended in 2011, governs organ and tissue donation in India. Key tissues that can be donated include:

• Cornea (epithelial tissue derived) — most donated tissue in India; eye banks operated by NOTTO
• Bone and cartilage — orthopaedic reconstruction
• Skin (epithelial) — burn treatment
• Heart valves (cardiac muscle tissue)
• Blood vessels (smooth muscle + connective tissue)

India's organ donation rate remains among the lowest globally (0.52 per million population in 2023 vs Spain at 46 per million), making this a perennial Mains topic.

PART 3 — Frameworks & Analysis

Framework: Tissue → Organ → Organ System

Tissues do not function in isolation:

• Epithelial + connective + muscular + nervous tissues combine to form the stomach (an organ)
• Multiple organs working together form organ systems (digestive system)
• This hierarchy — cell → tissue → organ → organ system → organism — is foundational for all biology-related GS questions

Framework: UPSC-Relevant Tissue Connections

Exam Strategy

Prelims traps:

• Blood is a connective tissue — not a fluid tissue separate from connective tissue.
• Xylem transport is unidirectional (up); phloem is bidirectional.
• Cardiac muscle is involuntary AND striated — the only such combination.
• Sclerenchyma cells are dead at maturity unlike collenchyma which remains living.
• Companion cells support sieve tube elements because sieve tubes lack a nucleus.

Mains frameworks (tissue → policy):

• Organ donation: tissue science → THOTA Act → NOTTO → international comparisons
• Anaemia policy: blood tissue science → iron deficiency → POSHAN Abhiyaan → National Nutrition Mission
• AMR and tissue infection: how pathogens invade epithelial barriers → antibiotic policy

Previous Year Questions

Q1 (Prelims 2020): With reference to blood, which of the following statements is/are correct? (Tests: blood as connective tissue, components of blood)

Q2 (Prelims 2018): Consider the following statements about stem cells... (Tests: understanding of undifferentiated cells and their potential)

Q3 (Mains GS2 2019): "India's organ donation rate is abysmally low." Examine the reasons and suggest measures to improve it. Tissue science link: requires understanding of what organs and tissues can be transplanted, legal framework.

Q4 (Prelims 2015): In the context of the human body, which one of the following is correctly matched? (Tests: classification of tissues — blood/bone/cartilage as connective tissue)