Tissues

1. What is a Tissue?

Unicellular organisms (like Amoeba) do everything — digestion, movement, respiration — within a single cell. In multicellular organisms, different cells become specialised for different functions. A tissue is a group of cells that share a similar structure and origin and work together to perform a specific function.

The study of tissues is called histology. Tissues reduce the burden on individual cells by division of labour — for example, muscle cells handle movement and nerve cells transmit signals, so neither has to do both.

Why do plants have more dead tissue than animals? Plants are stationary; they need mechanical strength more than speed. Dead cells with thick walls (like sclerenchyma) give great strength at low metabolic cost. Animals move and respond quickly, so they need living, active tissue (muscle, nerve) more.

2. Plant Tissues — Overview

Plant tissues are classified based on the ability of cells to divide:

Once meristematic cells stop dividing and take on a specific role, they are said to have undergone differentiation and become permanent tissue.

3. Meristematic Tissue

Meristematic tissue consists of cells that are actively dividing. Their key features are:

• Cells are small, isodiametric (equal sides), with dense cytoplasm and a large prominent nucleus.
• Cell wall is thin (primary wall only).
• No vacuoles (or very small ones).
• No intercellular spaces — cells are tightly packed.

Classification by location:

4. Permanent Tissue — Simple Permanent Tissue

Simple permanent tissues are made of one type of cell. They arise when meristematic cells stop dividing and differentiate. There are three types:

4a. Parenchyma

• Cells are living, isodiametric (roughly spherical or loosely packed polygonal shapes).
• Thin cell walls made of cellulose.
• Large central vacuole present.
• Intercellular spaces are present (loosely packed).

Functions:

• Storage of food (starch, fats, water) — e.g., potato tuber, carrot root.
• Photosynthesis when chloroplasts are present (then called chlorenchyma).
• Buoyancy when large air spaces are present (then called aerenchyma) — helps aquatic plants float.
• Packing tissue filling spaces between organs.
• Lateral conduction of water in stems.

4b. Collenchyma

• Cells are living, elongated.
• Cell walls are unevenly thickened with deposits of cellulose and pectin, especially at the corners.
• Very few or no intercellular spaces.
• May contain chloroplasts.

Function: provides mechanical support with flexibility — the plant can bend without breaking. Found in the epidermis and cortex of dicot stems and in the petioles (leaf stalks) of leaves.

4c. Sclerenchyma

• Cells are dead at maturity.
• Cell walls are uniformly and heavily thickened with lignin (a very hard substance).
• No intercellular spaces — cells fit together tightly.
• No living protoplasm — the lumen (interior) is often empty or very small.

Function: provides great tensile strength and rigidity. Protects the plant from mechanical stress. Found around vascular bundles, in the husk of coconut, seed coats, and the hard shell of nuts.

Quick Comparison — the three simple permanent tissues:

5. Complex Permanent Tissue — Xylem and Phloem

Complex tissues are made of more than one type of cell, all working together for a common function. There are two types: xylem (conducts water) and phloem (conducts food). Together they form the vascular bundle, the plant's transport system.

5a. Xylem

Xylem conducts water and dissolved minerals from roots to leaves. The conduction is unidirectional (upward). It has four elements:

1. Tracheids: elongated, dead cells with tapering ends and lignified walls. Water moves through pits (thin spots) in the walls. Found in gymnosperms (e.g., pine) and lower vascular plants. They are the evolutionarily older conducting element.
2. Vessels (Vessel members): cylindrical, dead cells arranged end to end forming long continuous tubes. Perforation plates at the ends allow water to flow freely. More efficient than tracheids. Common in angiosperms (flowering plants).
3. Xylem fibres (wood fibres): dead cells with very thick lignified walls. Provide mechanical strength to the plant. Do not conduct water.
4. Xylem parenchyma: the only living cells in xylem. Store food (starch) and help in lateral conduction of water to other parts of the plant.

5b. Phloem

Phloem conducts food (sucrose and amino acids) made in leaves to all other parts of the plant. Unlike xylem, conduction is bidirectional (upward and downward). It has four elements:

1. Sieve tubes: long, cylindrical, living cells placed end to end, but without a nucleus at maturity. The cross-walls (sieve plates) have tiny pores that allow food to pass through. They are the main food-conducting elements.
2. Companion cells: small, living cells with a prominent nucleus, found alongside each sieve tube. They control the activity of the adjacent enucleate sieve tube — essentially acting as the nucleus for the sieve tube.
3. Phloem fibres (bast fibres): dead cells with thick walls. Provide mechanical support to phloem. Commercially important — jute, flax, and hemp fibres used in textiles are phloem fibres.
4. Phloem parenchyma: living cells that store food and help in slow lateral conduction. Absent in most monocots.

Xylem vs Phloem — quick comparison:

6. Animal Tissues — Overview

Animals are mobile and need rapid response, so their tissues are designed differently from plant tissues. There are four main types of animal tissues:

1. Epithelial tissue — covers surfaces (inside and outside).
2. Connective tissue — links and supports other tissues.
3. Muscular tissue — enables movement.
4. Nervous tissue — receives and transmits signals.

Animal cells are generally living, do not have a cell wall (only a cell membrane), and have varying shapes depending on their function. Unlike plant cells, animal cells can change shape — a key requirement for movement.

7. Epithelial Tissue

Epithelial tissue forms a protective covering on all body surfaces — the skin (outer body surface), lining of the digestive tract, lungs, blood vessels, and body cavities. Key features:

• Cells are tightly packed with almost no intercellular matrix (material between cells).
• Cells rest on a basement membrane — a thin non-cellular layer that anchors them to underlying tissue.
• Blood vessels do not enter the epithelium — it is avascular (nutrients diffuse from below).
• Epithelial cells divide rapidly to replace worn-out cells.

Types of epithelial tissue:

7a. Squamous Epithelium

Cells are extremely flat (like floor tiles), with a centrally placed nucleus. Also called pavement epithelium. Found in the lining of the mouth, oesophagus, skin surface (dead keratinised form), alveoli (air sacs of lungs), and blood vessel walls. Function: protection and filtration.

7b. Cuboidal Epithelium

Cells are roughly cube-shaped, as wide as they are tall, with a central round nucleus. Found in kidney tubules, salivary glands, thyroid gland follicles, and pancreatic ducts. Functions: absorption, secretion, and excretion.

7c. Columnar Epithelium

Cells are taller than they are wide (column-shaped), with an oval nucleus near the base. Found lining the inner surface of the intestine and stomach. Functions: absorption and secretion. When the free surface of columnar epithelial cells has tiny finger-like projections called microvilli, it is called brush-bordered columnar epithelium (intestinal villi) — greatly increases surface area for absorption.

7d. Ciliated Epithelium

Columnar or cuboidal cells with cilia (hair-like vibrating projections) on their free surface. Found lining the respiratory tract (trachea, bronchi), fallopian tubes (oviducts), and ventricles of the brain. Function: the cilia beat rhythmically to move mucus, dust, eggs, or fluid in a specific direction. In the respiratory tract, cilia sweep mucus (with trapped dust and bacteria) away from the lungs toward the throat.

7e. Glandular Epithelium

Certain epithelial cells are specialised to produce and secrete substances. These form glands. Unicellular glands (e.g., goblet cells in the intestine) secrete mucus directly. Multicellular glands (e.g., salivary glands, sweat glands) form organised secretory structures. Some glands are exocrine (secrete through ducts, e.g., sweat glands) and some are endocrine (secrete hormones directly into the blood, e.g., thyroid).

8. Connective Tissue

Connective tissue binds, supports, cushions, and connects other tissues and organs. Its hallmark is a large amount of non-living intercellular matrix (ground substance + fibres) secreted by the cells. The matrix may be fluid (blood), semi-solid (cartilage), or solid (bone). There are several types:

8a. Blood

Blood is a fluid connective tissue. Its matrix is called plasma (55% of blood volume), a straw-coloured liquid of water, proteins, hormones, and dissolved substances. Cells in blood:

• Red blood cells (RBCs / erythrocytes): contain haemoglobin; carry oxygen from lungs to body cells and help carry CO2 back. Mature RBCs in humans lack a nucleus.
• White blood cells (WBCs / leucocytes): nucleated cells that fight infection and are part of the immune response.
• Platelets (thrombocytes): tiny cell fragments that help in blood clotting (coagulation) at a wound site.

Functions of blood: transport of gases, nutrients, hormones, waste products; defence against infection; temperature regulation; clotting to prevent blood loss.

8b. Bone

Bone is a rigid connective tissue. Its matrix is hardened by deposits of calcium salts (calcium phosphate) and collagen fibres, making it very hard. Bone cells (osteocytes) sit in small cavities called lacunae. Functions:

• Forms the skeleton — gives shape and framework to the body.
• Protects internal organs (skull protects brain; ribs protect heart and lungs).
• Acts as a lever for muscles to work on, enabling movement.
• Stores calcium and phosphorus and produces blood cells (in the red bone marrow).

8c. Cartilage

Cartilage is a flexible connective tissue (softer than bone). Its matrix is made of chondrin (a protein-carbohydrate complex) and is slightly elastic. Cells are called chondrocytes, housed in lacunae. Found at the tips of long bones (articular cartilage), in the nose and outer ear (elastic cartilage), between vertebrae of the spine (fibrocartilage — very tough), in the trachea and larynx. Functions: smooth joint movement, shape without rigidity, shock absorption.

8d. Tendon

Tendons connect muscle to bone. Made of dense regular connective tissue with bundles of collagen fibres running parallel (very strong in one direction). Fibres are non-elastic — they transmit pulling force without stretching. The Achilles tendon connecting the calf muscle to the heel bone is the largest tendon in the body.

8e. Ligament

Ligaments connect bone to bone at joints. Made of dense regular connective tissue rich in collagen and some elastic fibres — slightly more flexible than tendons. They stabilise joints and prevent dislocation while permitting normal movement. Spraining a joint (e.g., ankle sprain) usually means overstretching or tearing a ligament.

8f. Areolar Connective Tissue

A loose, unorganised connective tissue found beneath the skin (between skin and muscles), around blood vessels, nerves, and between organs. Its matrix contains collagen fibres, elastic fibres, and various cells (fibroblasts, mast cells, macrophages). Functions: fills space, provides cushioning, allows passage of nutrients and waste between blood and tissues, first line of defence (mast cells and macrophages attack pathogens).

8g. Adipose Tissue

Specialised connective tissue where cells (adipocytes) are filled with large fat droplets that push the nucleus to one side. Found beneath the skin (subcutaneous layer), around kidneys, behind the eyes, in bone marrow, and around the heart. Functions:

• Insulation — conserves body heat.
• Energy reserve — fat stores provide long-term energy.
• Cushioning — protects organs from mechanical shock.

9. Muscular Tissue

Muscular tissue is specialised for contraction and relaxation, enabling movement of the body and its parts. Muscle cells are also called muscle fibres because they are long and fibre-like. They contain a protein called myosin and a specialised cytoplasm called sarcoplasm. There are three types:

9a. Striated (Skeletal / Voluntary) Muscle

• Cells are long, cylindrical, unbranched, with many nuclei (multinucleate).
• Show alternating light and dark bands (striations) under a microscope — caused by the regular arrangement of proteins actin and myosin.
• Voluntary — controlled consciously by the will.
• Attached to bones. Used for body movements (walking, lifting, writing).
• Contract powerfully but fatigue quickly.

9b. Smooth (Unstriated / Involuntary / Visceral) Muscle

• Cells are spindle-shaped (pointed at both ends), with a single central nucleus.
• No striations — no regular banding pattern.
• Involuntary — not under conscious control.
• Found in walls of the alimentary canal (gut), blood vessels, uterus, urinary bladder, iris of the eye, bronchi.
• Contract slowly and rhythmically without fatigue — suited for sustained, slow movements like peristalsis pushing food through the gut.

9c. Cardiac Muscle

• Cells are cylindrical (not as long as skeletal), with one or two central nuclei, and are branched.
• Show faint striations (intermediate between striated and smooth).
• Cells are joined end to end by special junctions called intercalated discs that allow rapid spread of electrical signals.
• Involuntary — the heart beats without conscious control.
• Found only in the heart.
• Contract rhythmically throughout life without fatigue — extremely resistant to fatigue because of their abundant mitochondria and rich blood supply.

Muscle tissue comparison:

10. Nervous Tissue

Nervous tissue is the most specialised tissue in the body. It is designed to receive stimuli, process information, and transmit impulses at high speed from one part of the body to another. It forms the brain, spinal cord, and nerves.

The structural and functional unit of nervous tissue is the neuron (nerve cell). The human brain alone has about 100 billion neurons.

Structure of a Neuron

A neuron has three parts:

1. Cell body (Cyton / Soma): the main body of the neuron, contains the nucleus and most organelles. Has granular masses called Nissl's granules (rough ER) in the cytoplasm.
2. Dendrites: short, branched extensions from the cell body. They receive signals (impulses) from other neurons or sense organs and carry them toward the cell body.
3. Axon: a single, long fibre that carries impulses away from the cell body to other neurons, muscles, or glands. In large nerve fibres, the axon is covered by a fatty sheath called the myelin sheath (produced by Schwann cells), which insulates the fibre and greatly speeds up impulse conduction. The gap between two neurons is called a synapse — signals cross this gap using chemical messengers called neurotransmitters.

Types of neurons (by function):

• Sensory neurons (afferent): carry signals from sense organs (eyes, skin) to the brain and spinal cord.
• Motor neurons (efferent): carry signals from the brain and spinal cord to muscles and glands.
• Relay neurons (interneurons): connect sensory and motor neurons within the CNS.

Glial cells: besides neurons, nervous tissue contains neuroglia (glial cells) — supporting cells that protect, nourish, and maintain neurons. They outnumber neurons about 10 to 1.

11. Common Mistakes and Exam Tips

• Sclerenchyma cells are dead — do not say "living cells with thick walls." Only collenchyma are living with thick walls.
• Xylem parenchyma is the only living part of xylem. Everything else in xylem is dead.
• Sieve tubes are living but lack a nucleus — companion cells provide nuclear control for them.
• Blood is a connective tissue, not merely a body fluid. A very common exam trick question.
• Tendons join muscle to bone; ligaments join bone to bone. Mix-up is one of the most common errors.
• Cardiac muscle is involuntary — even though it looks striated (banded), it is NOT skeletal muscle.
• The axon carries impulses away from the cell body; dendrites carry them toward the cell body. Memory aid: Away = Axon (both start with A).
• Aerenchyma = parenchyma with large air spaces. Chlorenchyma = parenchyma with chloroplasts. Both are still parenchyma.