Tissues — NCERT Solutions, AI Tutor & Practice

Understand: A tissue is a group of cells that are similar in structure and origin and work together to perform a specific function.

Analyze: In multicellular organisms, cells become specialised to carry out particular tasks. When cells are grouped to do one job, the body works more efficiently.

• Different groups of cells (tissues) perform different functions — this is called division of labour.
• For example, muscle cells contract for movement, nerve cells carry messages, and xylem cells conduct water.

Conclude: Division of labour allows each tissue to specialise and become efficient at one task, so the survival efficiency of the whole organism increases.

Meristematic tissue:

• Cells actively divide.
• Cells are small, thin-walled with dense cytoplasm.
• Vacuoles are absent or very small.
• Found at growing regions — root and shoot tips (apical), sides (lateral/cambium), and near nodes (intercalary).

Permanent tissue:

• Cells have lost the ability to divide.
• Cells take up a definite shape, size and function.
• Vacuoles are usually large.
• Formed when meristematic cells differentiate.

Conclude: Meristematic tissue produces new cells by division; these cells then differentiate to form permanent tissues that carry out specific functions.

1. Apical meristem — present at the tips of roots and stems. It increases the length of the plant (primary growth).

2. Lateral meristem (cambium) — present on the lateral sides of stems and roots. It increases the girth/thickness of the plant (secondary growth).

3. Intercalary meristem — present at the base of leaves or internodes (e.g., in grasses). It also helps in increasing length of the plant.

Conclude: Apical and intercalary meristems contribute to length; lateral meristem contributes to thickness.

These are three types of simple permanent tissues in plants.

Parenchyma:

• Living cells with thin cell walls; loosely packed with intercellular spaces.
• Functions: stores food and nutrients; provides support. When it contains chlorophyll (chlorenchyma) it does photosynthesis; when it has air cavities (aerenchyma) it helps aquatic plants float.

Collenchyma:

• Living cells with cell walls unevenly thickened at the corners.
• Little or no intercellular space.
• Function: provides mechanical support and flexibility, allowing bending of stems and leaves without breaking.

Sclerenchyma:

• Dead cells with thick, lignified cell walls; no intercellular space.
• Function: makes the plant hard and stiff; provides strength (e.g., husk of coconut).

Conclude: Parenchyma stores, collenchyma gives flexible support, and sclerenchyma gives rigid strength.

Complex permanent tissues are made of more than one type of cell working together. They are the conducting/vascular tissues: xylem and phloem.

1. Xylem:

• Components: tracheids, vessels, xylem parenchyma, and xylem fibres.
• Tracheids and vessels are tubular and mostly dead.
• Function: conducts water and dissolved minerals upward from roots to the rest of the plant; also gives mechanical support.

2. Phloem:

• Components: sieve tubes, companion cells, phloem parenchyma, and phloem fibres.
• Most phloem cells are living (except phloem fibres).
• Function: transports food (prepared in leaves) to all parts of the plant. This transport is called translocation and occurs in both directions.

Conclude: Xylem carries water (mostly one-way, upward); phloem carries food (both ways).

Epithelial tissue is the covering or protective animal tissue that lines the surfaces of the body, organs, and cavities. Its cells are tightly packed with little intercellular material and rest on a basement membrane.

Main types:

1. Squamous epithelium — flat, thin cells; e.g., lining of mouth, blood vessels, and lung alveoli.
2. Cuboidal epithelium — cube-shaped cells; e.g., lining of kidney tubules and salivary ducts.
3. Columnar epithelium — tall pillar-like cells; e.g., inner lining of the intestine (helps in absorption).
4. Ciliated epithelium — columnar cells with cilia; e.g., lining of respiratory tract.
5. Glandular epithelium — cells specialised to secrete substances; e.g., glands.

Conclude: Epithelium protects, absorbs, secretes, and filters depending on where it is located.

Understand: Connective tissues have cells loosely spaced in an intercellular matrix. In blood, the matrix is fluid (plasma), so blood is a fluid connective tissue.

Analyze — components of blood:

• Plasma: the liquid matrix containing water, proteins, salts, hormones, and nutrients.
• Red blood cells (RBCs): carry oxygen using haemoglobin.
• White blood cells (WBCs): fight infection and provide immunity.
• Platelets: help in blood clotting.

Conclude: Because blood consists of cells suspended in a fluid matrix and functions to connect and transport substances between body parts, it is classified as a connective tissue.

Striated (skeletal/voluntary) muscle:

• Cells are long, cylindrical, unbranched with many nuclei (multinucleate).
• Show light and dark bands (striations).
• Under our control (voluntary); e.g., muscles of limbs.

Smooth (unstriated/involuntary) muscle:

• Cells are long, spindle-shaped with a single central nucleus.
• No striations.
• Not under our control (involuntary); e.g., muscles of stomach, intestine, blood vessels.

Cardiac muscle:

• Cells are cylindrical, branched, and uninucleate.
• Show striations.
• Involuntary; found only in the heart and contract rhythmically throughout life without tiring.

Conclude: Striated = voluntary; smooth = involuntary; cardiac = involuntary + branched + found only in heart.

Nervous tissue is made up of nerve cells called neurons. It responds to stimuli and conducts impulses rapidly.

Structure of a neuron:

• Cell body (cyton): contains the nucleus and cytoplasm.
• Dendrites: short branched projections that receive impulses.
• Axon: a single long fibre that carries the impulse away from the cell body to the next cell.
• Neurons can be very long (up to a metre).

Function: Nervous tissue detects stimuli, generates and conducts nerve impulses very quickly, and coordinates the activities of the body.

Conclude: The neuron's structure — dendrites receiving, axon transmitting — makes rapid coordination possible.

Epidermis:

• Usually a single layer of cells covering the entire surface of the plant.
• Often coated with a waxy layer called the cuticle which reduces water loss.
• Contains stomata (guarded by guard cells) for gaseous exchange and transpiration.
• Root epidermis bears root hairs for water absorption.

Cork (phellem):

• As older stems and roots grow thicker, the outer layer forms cork.
• Cork cells are dead, have thick walls containing suberin (a waterproof substance).
• Cork prevents water loss and protects against mechanical injury and infection.

Conclude: Epidermis protects young plant parts and controls exchange; cork protects the older, woody parts.

Ligament:

• Connects bone to bone.
• Very elastic and strong, yet contains very little matrix.
• Allows and stabilises movement at joints.

Tendon:

• Connects muscle to bone.
• Made of tough, fibrous tissue with great strength but limited flexibility (inelastic).
• Transmits the pull of muscle to bone.

Conclude: Ligaments are elastic (bone–bone); tendons are strong and non-elastic (muscle–bone).

| Basis | Plant Tissues | Animal Tissues | |-------|--------------|----------------| | Growth | Growth is limited to certain regions (meristems) | Growth is uniform throughout the body | | Structure | Many tissues are made of dead cells (e.g., sclerenchyma, xylem) | Most tissues are made of living cells | | Energy need | Require less maintenance energy (plants are mostly stationary) | Require more energy as animals move about | | Organisation | Simpler organisation; less specialisation | More complex, highly specialised for movement and coordination |

Conclude: Plant tissues are adapted for a stationary life with regional growth and many dead supporting cells, while animal tissues are adapted for movement with living, specialised, energy-consuming cells.