Tissues in Action
From a single cell to a complete organism — understanding how life organises itself
Why do we study Tissues?
Life begins when a single cell divides itself several times to give rise to a large number of cells. These cells gradually form the skin, muscles, bones, nerves, and all other organs. This process is so intricate that it is considered one of nature's greatest engineering marvels.
- How is the study of cells and tissues significant for understanding life processes and human welfare?
- How are tissues in plants and animals different, and why?
- How is the division of labour at various levels of organisation in multicellular organisms correlated with their structure and function?
In all multicellular organisms, there is a hierarchy of organisation:
Definition
A tissue is a group of cells (similar in structure) that work together to perform a specific function. The formation of different types of tissues leads to division of labour, which increases the efficiency of the body and enables it to carry out complex life processes.
Why are Plant and Animal Tissues Different?
Fixed & Supported
Plants are fixed in one place. They have a cell wall that provides rigidity and strength. Their mode of nutrition uses solar energy (photosynthesis). Growth patterns differ — tissues for growth are structurally unique.
Flexible & Mobile
Animals can move. Without a rigid cell wall, animal cells can change shape easily. This cellular flexibility helps make bodies suitable for locomotion. They have tissues that digest food from different sources.
Tissues for Growth in Plants
Plants grow in different ways — increase in length (height of stem and depth of roots), increase in girth (thickness of stem), and regrowth after cutting or grazing. All this growth requires actively dividing cells that form a tissue called meristematic tissue.
Types of Meristematic Tissue
Apical Meristem
Located at the tips of roots and shoots. Helps plants grow in length. Contains continuously dividing cells — confirmed by onion root tip experiment.
Lateral Meristem
Arranged in a ring in the stem. Produces new cells inside and outside — leading to increase in girth (diameter). Forms visible annual growth rings.
Intercalary Meristem
Located at the base of internode / above the node. Helps plants regrow after cutting — seen when grass grows back after mowing.
🔬 Characteristics of Meristematic Cells
- Small cells with thin cell walls
- Large, prominent nucleus and dense cytoplasm with many organelles
- Vacuoles are generally absent
- Cells are tightly packed with little or no intercellular space
- These characteristics allow continuous and rapid cell division
Key Concept: Differentiation
Some newly formed meristematic cells lose the ability to divide. They undergo changes in structure and function and become permanent tissues. This process is called differentiation.
Permanent Tissues in Plants
Permanent tissues can be simple (composed of only one type of cell) or complex (composed of more than one type of cell).
(i) Protective Tissue — Epidermis
The epidermis forms the outermost layer of the plant body. It consists of a tightly packed, single layer of flat and rectangular cells. Covered with a waxy layer of cutin called cuticle which reduces water loss and protects against mechanical injury. In leaves, epidermis contains pores called stomata — for gaseous exchange and transpiration. In roots, epidermal projections called root hair increase surface area for absorbing water and minerals.
(ii) Supporting Tissue — Simple Permanent Tissues
Food Storing
- Living cells with thin walls
- Loosely packed with intercellular spaces
- Stores food; performs photosynthesis in green parts
- In aquatic plants: forms air spaces to help float
Flexible Support
- Living cells with unevenly thickened corners (pectin)
- Provides support and flexibility
- Allows stems and tendrils to bend without breaking
- Found in young stems, leaf stalks
Hard & Strong
- Cells have thick walls due to lignin
- Most cells are dead
- Forms woody structure — hard and strong
- Found in stems, leaf veins, coconut husk, walnut shell
(iii) Conducting Tissues — Complex Permanent Tissues
| Feature | Xylem | Phloem |
|---|---|---|
| Function | Transports water and minerals from roots upward | Transports food (sugars) from leaves to other parts |
| Living cells? | Mostly dead (xylem parenchyma is the only living component) | Mostly living cells |
| Components | Tracheids, vessels, xylem parenchyma, xylem fibres | Sieve tubes, companion cells, phloem parenchyma, phloem fibres |
| Additional Role | Provides strength to the plant | Companion cells regulate loading/unloading of sugars in sieve tubes |
Plant Tissue Systems
Dermal Tissue System
Forms the outer covering. Protects inner parts and reduces water loss.
Ground Tissue System
Forms the main body between dermal and conducting tissues. Includes parenchyma, collenchyma, sclerenchyma.
Vascular Tissue System
Conducting tissues — xylem and phloem.
⏸ Pause & Ponder
- Why is a thick cuticle advantageous for a desert plant but disadvantageous for an underwater plant?
- How do the 'dead' xylem cells work together with living leaf cells to keep water moving upward against gravity?
- What would happen if there were no stomata in the epidermis of leaves?
Animal Tissues
Like plants, animal cells group together and specialise in performing different functions. Animal tissues are mainly of four types:
1. Epithelial Tissue
Forms the outer covering of the body (skin) and lines internal organs (mouth, lungs, blood vessels, intestine). Composed of closely packed cells with very little space between them.
- Prevents entry of germs
- Reduces water loss
- Helps in absorption, secretion, and exchange
2. Connective Tissue
Connects and supports other tissues. Includes blood, bone, cartilage, tendon, and ligament.
- Blood — fluid matrix, transports substances
- Bone — rigid matrix (calcium & phosphorus)
- Cartilage — soft jelly-like matrix, flexibility
- Tendon — connects muscle to bone
- Ligament — connects bone to bone
3. Muscular Tissue
Produces voluntary and involuntary movements. Three types based on structure and control.
4. Nervous Tissue
Forms the body's control and coordination network. Cells called neurons receive, process, and transmit messages. The brain is the control centre.
Types of Epithelial Tissue
| Function | Structure | Location in Body |
|---|---|---|
| Exchange | Single layer of thin, flat cells | Lining of blood vessels and lungs |
| Protection | Many layers; outer cells flat and tightly packed | Skin, mouth, oesophagus |
| Secretion | Cuboidal or columnar cells specialised for releasing substances | Salivary glands, sweat glands, stomach lining |
| Sensory | Specialised receptor cells with hair-like cilia | Nostrils, taste buds, inner ear |
| Absorption | Single layer of tall, pillar-like cells, often with hair-like structures | Lining of small intestine |
Types of Muscular Tissue
Skeletal Muscle
Long, cylindrical, unbranched, multinucleate (many nuclei), striated (light and dark bands). Attached to skeleton. Controlled consciously — voluntary.
VoluntarySmooth Muscle
Spindle-shaped, single nucleus, no striations. Found in stomach, intestines. Slow, continuous movements like digestion — involuntary.
InvoluntaryCardiac Muscle
Cylindrical, branched, single nucleus, faint striations. Found only in the heart. Works tirelessly and rhythmically without fatigue — involuntary.
InvoluntaryStructure of a Neuron
🧠 Parts of a Neuron
- Cell Body — contains the nucleus; controls cell activities
- Dendrites — receive signals from other neurons
- Axon — a long fibre that carries messages from the cell body
- Axon Terminals — transmit messages to other cells
Components of Blood
Red Blood Cells
Red colour due to haemoglobin, an iron-rich protein. Live for about 4 months and are replaced regularly. Carry oxygen to cells.
White Blood Cells
Collect at infected areas. Cause pus formation and inflammation (redness, swelling). Defence mechanism of the body against infection.
Platelets
Help in blood clotting at the site of an injury. Prevent excessive blood loss from cuts and wounds.
Plasma
Watery, fluid part. 55% of total blood volume. Transports dissolved nutrients, hormones, gases, and waste products.
The Musculoskeletal System & Skeletal System
The musculoskeletal system is made up of bones, muscles, joints, cartilage, tendons and ligaments. It helps us stand upright, move, maintain posture and protect delicate organs — all under control of the nervous system.
Key Facts about the Skeleton
- The adult human skeleton makes up about 12–15% of body weight
- The skull protects the brain, eyes and ears
- The vertebral column (spine) is made up of small bones called vertebrae — supports the body and helps us stand upright
- Between each vertebra is a cartilage disc — acts as a cushion, allows flexibility
- We have 12 pairs of ribs forming the rib cage — protects heart and lungs
- The rib cage is joined by flexible cartilage allowing it to expand and contract during breathing
🔗 How Muscles & Bones Work Together
Muscles pull on bones to produce movement. They are attached to bones by tendons. When a muscle contracts, the tendon transmits this force to the bone, resulting in movement at a joint.
Types of Joints
A joint is a junction between two or more bones. The type of joint determines the type of movement possible.
Ball & Socket Joint
Rounded end of one bone fits into a hollow of another. Allows movement in all directions — forward, backward, sideways, circular.
Examples: Shoulder, HipHinge Joint
Works like a door hinge. Allows movement in one direction only — bending and straightening.
Examples: Elbow, KneePivot Joint
Allows rotation/turning side to side like a doorknob. The skull connects to backbone via this joint.
Example: NeckFixed Joint
Bones are joined together with no movement. Keeps the brain safe even when the body moves.
Example: Skull bones| Connective Tissue | Connects | Function |
|---|---|---|
| Bone | — | Gives strength, support and protection; rigid matrix of calcium & phosphorus |
| Cartilage | Between bone ends | Soft, jelly-like matrix — provides flexibility and cushions bones; shock absorption |
| Tendon | Muscle → Bone | Transmits force from muscle to bone, brings about movement |
| Ligament | Bone → Bone | Provides stability, limits excessive movement, prevents dislocation |
Pioneers in Tissue Research
B. G. L. Swamy
Renowned Indian botanist known for contributions to plant morphology and anatomy. His book Hasuru Honnu (in Kannada) is a blend of science, satire and culture describing botanical excursions in the Western Ghats. Won the Kendra Sahitya Akademi Award in 1978.
Sipra Guha Mukherjee & S. C. Maheshwari
Made a breakthrough discovery in plant tissue culture — developed a complete plant through anther culture using artificial nutrient medium under controlled conditions. This greatly contributed to crop improvement and modern agriculture.
F. C. Steward (1958) — Totipotency
Demonstrated that single cells from vascular phloem of carrot can regenerate whole plants. Phloem cells dedifferentiated → formed a mass → redifferentiated into a complete plant. This property is called totipotency. Best results were obtained with light + air + liquid nutrient medium (20% increase in weight). This led to advances in plant tissue culture and genetic engineering.
Crown Gall Disease & Agrobacterium
Scientists observed that Agrobacterium tumefaciens causes tumour-like swellings on plant stems (crown gall disease) due to rapid, uncontrolled cell division. Instead of just finding a cure, scientists studied how the bacterium transfers genetic material into plant cells. Today, Agrobacterium is used as a tool in genetic engineering to introduce useful genes into plants for improved crops and disease-resistant varieties.
Test Yourself — MCQs
Click an option to check your answer!
Short Answer Questions
The process by which meristematic cells lose their ability to divide and undergo changes in structure and function to become specialised is called differentiation. Meristematic tissue becomes permanent tissue through differentiation. Permanent tissues are specialised to perform specific functions like support (collenchyma, sclerenchyma), storage (parenchyma), or transport (xylem, phloem).
Meristematic cells need to divide rapidly and continuously. Large vacuoles take up space within the cell and would push the nucleus to the periphery, making cell division harder. Since meristematic cells need dense cytoplasm with many organelles and a large, central, prominent nucleus for active division, vacuoles (which store water and mature the cell) are absent.
- Xylem transports water and minerals from roots upward; mostly dead cells; provides strength to plant.
- Phloem transports food (sugars) from leaves to other parts; mostly living cells.
- Xylem components: tracheids, vessels, xylem parenchyma, xylem fibres.
- Phloem components: sieve tubes, companion cells, phloem parenchyma, phloem fibres.
Grass regrows because of the presence of intercalary meristem at the nodes of its stem. When the grass is grazed or mowed, the apical portion is cut off, but the intercalary meristematic cells at the base of internodes continue to divide and regenerate the plant. This is why grass appears again after some time of being mowed or grazed.
Annual growth rings are ring-like patterns visible on the cross-section (T.S.) of a tree trunk. They are formed by the lateral meristem. Each ring represents one year of growth. Wide rings indicate favourable growth conditions (good climate, rainfall) during that year. Narrow rings indicate unfavourable conditions. By counting these rings, scientists can estimate the age of a tree and understand past climatic conditions (dendrochronology).
Totipotency is the ability of a mature plant cell to undifferentiate, divide, and redifferentiate to develop into a new complete plant under specific conditions.
Steward's Experiment (1958): He grew phloem cells of carrot in a liquid nutrient medium (containing sugars and hormones) with light and air. The cells dedifferentiated → divided to form a mass of unspecialised cells → redifferentiated to form roots, shoots → complete plant. Best results: light + air + liquid medium (20% weight increase). No growth occurred without air, even with light + liquid medium.
Cardiac muscle cells have a high number of mitochondria and an abundant blood supply, providing a constant supply of oxygen and ATP. This enables them to contract rhythmically without accumulating fatigue-causing products, allowing the heart to beat continuously throughout life.
Collenchyma is responsible. It has unevenly thickened corners due to pectin deposits, providing support with flexibility.
If replaced by sclerenchyma: the stem would become hard, rigid and brittle (lignified dead cells). It would no longer bend but would instead break under force, damaging the plant severely.
Key Terms Glossary
📋 Chapter At a Glance
- Tissues are groups of similar cells that work together to perform specific functions.
- Plant tissues are broadly classified into meristematic (dividing) and permanent (non-dividing) tissues.
- Three types of meristematic tissues: apical (length), lateral (girth), intercalary (regrowth).
- Simple permanent tissues: parenchyma, collenchyma, sclerenchyma.
- Complex permanent tissues: xylem (water transport) and phloem (food transport).
- Plant tissues are organised into three systems: dermal, ground, and vascular.
- Animal tissues are of four types: epithelial, connective, muscular, and nervous.
- Epithelial tissue covers and lines body surfaces; connective tissue connects and supports.
- Three muscle types: skeletal (voluntary, striated), smooth (involuntary, no striations), cardiac (involuntary, branched, in heart only).
- Nervous tissue consists of neurons; brain is the control centre coordinating all activities.
- The musculoskeletal system consists of bones, muscles, joints, cartilage, tendons and ligaments.
- Joints: ball & socket (all directions), hinge (one direction), pivot (rotation), fixed (no movement).
- F.C. Steward demonstrated totipotency — single carrot phloem cells regenerated whole plants.
