NCERT Class 9 Science Chapter 6: Tissues - A Detailed Guide for Students

Introduction to Tissues: The Building Blocks of Life

Welcome to this comprehensive guide on Chapter 6, 'Tissues,' from the NCERT Class 9 Science textbook. In the previous chapter, we learned that the cell is the fundamental structural and functional unit of all living organisms. From the single-celled Amoeba to the giant blue whale, cells are the foundation of life. But have you ever wondered how complex organisms like us perform so many different functions simultaneously? How do we breathe, digest food, move, and think all at once? The answer lies in the organisation of cells into tissues.

In unicellular organisms, a single cell performs all essential functions like movement, intake of food, gaseous exchange, and excretion. However, in multicellular organisms, millions of cells are organised to carry out specialised functions. This organisation leads to a division of labour. Cells that are specialised in one function are often grouped together in the body. This cluster of cells, which work together to achieve a particular function, is called a tissue. For example, muscle cells contract and relax to cause movement, nerve cells carry messages, and blood flows to transport oxygen, food, hormones, and waste materials.

Therefore, a tissue can be defined as a group of cells that are similar in structure and/or work together to achieve a particular function. This chapter will delve into the fascinating world of tissues, exploring their types, structures, and functions in both plants and animals. Understanding tissues is crucial as it forms the bridge between the microscopic world of cells and the macroscopic world of organ systems and the organism as a whole.

Are Plants and Animals Made of the Same Types of Tissues?

While both plants and animals are complex multicellular organisms, their lifestyles and functional requirements are vastly different. These differences are reflected in the types of tissues they possess. Let's explore the key distinctions:

• Mobility: Animals are motile; they move around in search of food, mates, and shelter. This requires a significant amount of energy and tissues that can support movement, like muscle and nervous tissues. Plants, on the other hand, are stationary or sessile. They are fixed in one place. Their primary needs are structural support to stand upright and a transport system to move water and food.
• Structural Support: Since plants are immobile, they need tissues that provide immense structural strength. A large proportion of their tissue is supportive, which is often composed of dead cells. These dead cells provide mechanical strength just as effectively as live ones and require less maintenance. In animals, most tissues are composed of living cells that support active locomotion and other life processes.
• Growth: The pattern of growth is another key differentiator. In animals, cell growth is more or less uniform throughout the body, and it stops after reaching a certain age or size. In plants, growth is limited to certain regions. These regions, known as meristems, contain actively dividing cells throughout the plant's life. This is why plants can grow new leaves or branches continuously.
• Energy Consumption: The active lifestyle of animals demands more energy compared to the sedentary existence of plants. Consequently, the cellular and tissue organisation in animals is geared towards higher metabolic rates.

These fundamental differences in lifestyle and function have led to the evolution of distinct types of tissues in plants and animals, which we will now explore in detail.

Plant Tissues

Plant tissues can be broadly categorised into two main types based on their ability to divide: Meristematic Tissues and Permanent Tissues.

Meristematic Tissue

The growth of plants occurs only in specific regions. This is because the dividing tissue, known as meristematic tissue (also called meristems), is localised in these regions. These tissues are responsible for the growth in the length and girth of the plant.

Characteristics of Meristematic Cells:

• They are very active and have dense cytoplasm.
• They possess thin cellulose walls and prominent nuclei.
• They lack vacuoles, as vacuoles are primarily for storage and providing turgidity, which are functions of mature cells, not rapidly dividing ones.

Based on their location in the plant body, meristematic tissues are classified into three types:

• Apical Meristem: Found at the growing tips (apices) of stems and roots. It is responsible for the increase in the length of the stem and the root. This is known as primary growth.
• Lateral Meristem (Cambium): Found along the sides of the stem and root. It is responsible for the increase in the girth or diameter of the plant. This is known as secondary growth, which is prominent in woody plants.
• Intercalary Meristem: Located at the base of leaves or internodes (the area between two nodes on a stem). It is responsible for the longitudinal growth of organs like leaves and stems, especially in grasses where it helps regenerate parts eaten by herbivores.

Permanent Tissue

What happens to the cells formed by meristematic tissue? They grow, mature, and gradually lose their ability to divide. They take up a specific role and form a permanent tissue. This process of taking up a permanent shape, size, and function is called differentiation. Cells of permanent tissues are differentiated to perform various functions like protection, support, storage, and conduction.

Permanent tissues are further divided into two categories: Simple Permanent Tissue and Complex Permanent Tissue.

Simple Permanent Tissue

These tissues are composed of cells that are structurally and functionally similar, meaning they consist of only one type of cell. They primarily provide support and store food. There are three main types:

• Parenchyma: This is the most common and unspecialised simple permanent tissue. The cells are living, have thin cell walls, and are typically loosely packed with large intercellular spaces.
  • Function: Its primary function is food storage.
  • Modifications: In some situations, parenchyma is modified to perform other functions. When it contains chlorophyll and performs photosynthesis, it is called chlorenchyma (found in leaves). In aquatic plants, parenchyma has large air cavities to provide buoyancy, and it is then called aerenchyma.
• Collenchyma: This tissue provides flexibility to various parts of the plant, like the leaf stalk (petiole) and stem, allowing them to bend without breaking. The cells are living, elongated, and are irregularly thickened at the corners due to the deposition of pectin. They have very little intercellular space. It is found in the leaf stalks below the epidermis.
• Sclerenchyma: This tissue is responsible for making the plant hard and stiff. The cells of this tissue are dead. They are long and narrow, and their walls are thickened due to lignin, a chemical substance that acts like cement and hardens them. There are no internal spaces inside the cells. Sclerenchyma is found in stems, around vascular bundles, in the veins of leaves, and in the hard covering of seeds and nuts (e.g., the husk of a coconut is made of sclerenchymatous tissue).

Complex Permanent Tissue

Complex tissues are made of more than one type of cell. All these cells coordinate to perform a common function. The two main types of complex permanent tissue in plants are xylem and phloem. Both are conducting tissues and together constitute a vascular bundle.

• Xylem: Xylem is the water-conducting tissue of the plant. It transports water and minerals vertically from the roots to the leaves. It also provides mechanical support. Xylem is composed of four different types of elements:
  • Tracheids and Vessels: These are tubular structures that form a continuous channel for efficient water transport. They are dead cells with thick, lignified walls.
  • Xylem Parenchyma: These are the only living cells in the xylem. They store food and help in the lateral conduction of water.
  • Xylem Fibres: These are sclerenchymatous fibres that are mainly supportive in function.
• Phloem: Phloem is the food-conducting tissue. It transports food (primarily sucrose) from the leaves, where it is made during photosynthesis, to other parts of the plant. This transport is bidirectional, meaning it can move up or down the stem to reach all parts of the plant. Phloem is composed of four types of elements:
  • Sieve Tubes: These are tubular cells with perforated walls (sieve plates). Unlike xylem vessels, the cell walls are not lignified. Mature sieve tubes have a peripheral cytoplasm and a large vacuole but lack a nucleus.
  • Companion Cells: These are specialised parenchyma cells associated with sieve tubes. They have a nucleus and other organelles and help regulate the functions of the sieve tube elements.
  • Phloem Parenchyma: These cells are involved in the storage of food and other substances like resins and latex.
  • Phloem Fibres: Made of sclerenchymatous cells, these provide mechanical strength. Jute and hemp fibres are examples of phloem fibres.

Protective Tissue

Plants also have protective tissues that cover their entire surface. The two main types are the epidermis and cork.

• Epidermis: The epidermis is usually a single layer of cells that forms the outermost covering of the entire plant body, including leaves, flowers, stems, and roots. It protects the plant from water loss, mechanical injury, and invasion by parasitic fungi. The cells are flat and have no intercellular spaces. In dry habitats, the epidermis may be thicker to prevent water loss. The epidermis of a leaf is not continuous; it is interrupted by small pores called stomata. Each stoma is guarded by two kidney-shaped cells called guard cells, which regulate the exchange of gases and transpiration (loss of water vapour).
• Cork: As plants grow older, the outer protective tissue of the stem undergoes changes. A strip of secondary meristem, called the cork cambium, replaces the epidermis of the stem. It forms a multi-layered outer covering called cork or bark. Cells of cork are dead and compactly arranged without intercellular spaces. They have a chemical called suberin in their walls, which makes them impervious to gases and water.

Animal Tissues

In animals, cells are organised into four fundamental types of tissues based on their structure and function: Epithelial Tissue, Connective Tissue, Muscular Tissue, and Nervous Tissue.

Epithelial Tissue

Epithelial tissue is the covering or protective tissue in the animal body. It covers most organs and cavities within the body and also forms a barrier to keep different body systems separate. The skin, the lining of the mouth, the lining of blood vessels, lung alveoli, and kidney tubules are all made of epithelial tissue. The cells are tightly packed and form a continuous sheet with a small amount of cementing material between them and almost no intercellular spaces. All epithelium is usually separated from the underlying tissue by an extracellular fibrous basement membrane.

Based on the shape and function of the cells, epithelial tissue is classified as:

• Simple Squamous Epithelium: Composed of a single layer of thin, flat, irregularly shaped cells which fit together like floor tiles. It forms a delicate lining, such as in the lining of blood vessels and lung alveoli, where transport of substances occurs through a selectively permeable surface.
• Stratified Squamous Epithelium: Cells are arranged in a pattern of layers (strata). This provides protection against mechanical and chemical stress. The skin, which protects the body, is made of this tissue.
• Cuboidal Epithelium: Composed of a single layer of cube-like cells. It is commonly found in the lining of kidney tubules and ducts of salivary glands, where it provides mechanical support and is involved in secretion and absorption.
• Columnar Epithelium: Composed of cells that are taller than they are wide, appearing like columns. It is found where absorption and secretion occur, such as in the inner lining of the intestine. In the respiratory tract, these cells may have hair-like projections on their outer surfaces called cilia. This is known as ciliated columnar epithelium, and the movement of the cilia pushes mucus forward to clear it.
• Glandular Epithelium: Sometimes, a portion of the epithelial tissue folds inward, and a multicellular gland is formed. This is called glandular epithelium, and it is specialised for secretion (e.g., sweat glands, salivary glands).

Connective Tissue

Connective tissues are the most abundant and widely distributed tissues in the body of complex animals. As the name suggests, they are involved in connecting and supporting other tissues and organs. The cells of connective tissue are loosely spaced and embedded in an intercellular matrix. The matrix may be jelly-like, fluid, dense, or rigid. The nature of the matrix determines the function of the tissue.

The main types of connective tissue include:

• Blood: This is a fluid connective tissue. The fluid matrix is called plasma, in which red blood corpuscles (RBCs), white blood corpuscles (WBCs), and platelets are suspended. Plasma contains proteins, salts, and hormones. Blood flows and transports gases, digested food, hormones, and waste materials to different parts of the body.
• Bone: This is a rigid connective tissue that forms the framework that supports the body. It anchors the muscles and supports the main organs. It is a strong and non-flexible tissue. Bone cells (osteocytes) are embedded in a hard matrix that is composed of calcium and phosphorus compounds.
• Ligament: This is a fibrous connective tissue that connects one bone to another. It is very elastic and has considerable strength. It contains very little matrix.
• Tendon: This fibrous tissue connects muscles to bones. Tendons are tough and have great strength but limited flexibility.
• Cartilage: This is another type of supportive connective tissue with widely spaced cells. The solid matrix is composed of proteins and sugars. It is firm but flexible. It smoothens bone surfaces at joints and is also present in the nose, ear, trachea, and larynx.
• Areolar Tissue: This is a loose connective tissue found between the skin and muscles, around blood vessels and nerves, and in the bone marrow. It fills the space inside organs, supports internal organs, and helps in the repair of tissues.
• Adipose Tissue: This tissue is specialised for fat storage. It is found below the skin and between internal organs. The cells of this tissue are filled with fat globules. Storage of fat also lets it act as an insulator.

Muscular Tissue

Muscular tissue is responsible for movement in our body. It consists of elongated cells, also called muscle fibres. These cells contain special proteins called contractile proteins (actin and myosin), which contract and relax to cause movement.

We can move some muscles by conscious will. These are called voluntary muscles. Other movements, like the pumping of the heart or the movement of food in the alimentary canal, are not under our conscious control and are performed by involuntary muscles.

Based on their structure, location, and function, muscular tissues are of three types:

Comparison of Muscle Types

Nervous Tissue

All cells possess the ability to respond to stimuli. However, cells of the nervous tissue are highly specialised for being stimulated and then transmitting the stimulus very rapidly from one place to another within the body. The brain, spinal cord, and nerves are all composed of nervous tissue.

The fundamental unit of the nervous tissue is the nerve cell, or neuron. A neuron consists of a cell body with a nucleus and cytoplasm, from which long, thin, hair-like parts arise. Each neuron has a single long part, called the axon, and many short, branched parts called dendrites. An individual nerve cell may be up to a metre long.

Many nerve fibres bound together by connective tissue make up a nerve. The signal that passes along the nerve fibre is called a nerve impulse. Nerve impulses allow us to move our muscles when we want to. The functional combination of nerve and muscle tissue is fundamental to most animals, enabling them to move rapidly in response to stimuli. This coordination allows for complex actions, thoughts, and responses, making it one of the most vital tissues in the animal kingdom.

Important Questions and Answers

1. What is a tissue?

Answer: A tissue is a group of cells that are similar in structure and origin, and work together to perform a specific function. For example, in animals, muscle tissue is formed from muscle cells that contract and relax to cause movement. In plants, vascular tissue (xylem and phloem) is formed from various cells that work together to transport water and food.

2. Differentiate between parenchyma, collenchyma, and sclerenchyma on the basis of their cell wall.

Answer:

• Parenchyma: The cell walls are thin and made up of cellulose. The cells are living.
• Collenchyma: The cell walls are irregularly thickened at the corners due to the deposition of pectin. The cells are living.
• Sclerenchyma: The cell walls are very thick and evenly lignified (hardened by lignin). The cells are dead.

3. What is the function of stomata?

Answer: Stomata are tiny pores present on the surface of leaves, primarily on the underside. They have two main functions:

• Gaseous Exchange: They allow for the exchange of gases like carbon dioxide (taken in for photosynthesis) and oxygen (released as a byproduct) between the plant and the atmosphere.
• Transpiration: They are the primary sites for the loss of water in the form of water vapour from the plant, a process known as transpiration. This process helps in the absorption and upward movement of water and minerals from the roots to the leaves.

4. Name the tissue responsible for movement in our body.

Answer: The tissue responsible for movement in our body is muscular tissue. It is composed of specialised cells called muscle fibres which contain contractile proteins. When these proteins contract and relax, they produce movement in various parts of the body.

5. What is the specific function of the cardiac muscle?

Answer: The specific function of the cardiac muscle is to contract and relax rhythmically and continuously throughout an individual's lifetime without fatigue. This rhythmic contraction and relaxation of the heart walls pumps blood to all parts of the body, ensuring the circulation of oxygen, nutrients, and hormones and the removal of waste products.

Chapter Summary

Here are the key takeaways from our detailed exploration of Tissues:

• A tissue is a cluster of cells specialised to perform a specific function.
• Plant tissues are broadly divided into Meristematic (dividing) and Permanent (non-dividing) tissues.
• Meristematic tissues (apical, lateral, intercalary) are responsible for plant growth.
• Permanent tissues are of two types: Simple (Parenchyma, Collenchyma, Sclerenchyma) and Complex (Xylem, Phloem).
• Xylem transports water and minerals, while Phloem transports food.
• Animal tissues are classified into four main types: Epithelial, Connective, Muscular, and Nervous tissue.
• Epithelial tissue forms protective coverings and linings throughout the body.
• Connective tissue (like blood, bone, cartilage) connects, supports, and separates other tissues or organs.
• Muscular tissue (striated, smooth, and cardiac) is responsible for all types of movement in the body.
• Nervous tissue, made of neurons, is specialised for the rapid transmission of signals, enabling communication and control within the body.