Comprehensive theory, key formulas, diagrams, and memory aids for Tissues.
In unicellular organisms like Amoeba, a single cell performs all basic functions such as taking in food, respiration, and excretion. But in multicellular organisms, there are millions of cells. Most of these cells are specialized to carry out only a few functions. Each specialized function is taken up by a different group of cells. Since these cells carry out only a particular function, they do it very efficiently.
A group of cells that are similar in structure and/or work together to achieve a particular function forms a tissue.
Are plants and animals made of the same types of tissues? No. Plants are stationary; most of their tissues are supportive, providing structural strength, and many of these tissues are dead. Animals move around in search of food, mates, and shelter; they consume more energy, and most of their tissues are living. The structural organization of organs and organ systems is far more specialized and complex in animals than in plants.
Plant tissues are broadly classified into two main types based on their dividing capacity: Meristematic Tissue and Permanent Tissue.
The growth of plants occurs only in certain specific regions. This is because the dividing tissue, also known as meristematic tissue, is located only at these points. Depending on the region where they are present, they are classified as: * Apical Meristem: Present at the growing tips of stems and roots. It increases the length of the stem and the root. * Lateral Meristem (Cambium): Found beneath the bark and in vascular bundles. It is responsible for increasing the girth (thickness) of the stem or root. * Intercalary Meristem: Seen in some plants, located near the nodes. It increases the length of the internode.
Characteristics: Cells of meristematic tissue are very active, have dense cytoplasm, thin cellulose walls, and prominent nuclei. They lack vacuoles (since they don't need to store food; they are constantly dividing).
When cells formed by meristematic tissue take up a specific role and lose the ability to divide, they form permanent tissue. This process of taking up a permanent shape, size, and a function is called differentiation.
A. Simple Permanent Tissue Made of only one type of cells. * Parenchyma: The most common simple permanent tissue. Consists of relatively unspecialized cells with thin cell walls. They are living cells, loosely arranged with large intercellular spaces. Functions: Storage of food, provides support. (If it contains chlorophyll, it's called chlorenchyma; in aquatic plants, large air cavities make it aerenchyma). * Collenchyma: Provides mechanical support and flexibility to plants. It allows bending of various parts of a plant (like tendrils and stems of climbers) without breaking. Cells are living, elongated, and irregularly thickened at the corners. There is very little intercellular space. * Sclerenchyma: This tissue makes the plant hard and stiff (e.g., the husk of a coconut). The cells are dead. They are long and narrow as the walls are thickened due to lignin. Often there is no internal space inside the cell. It provides strength to plant parts.
B. Complex Permanent Tissue Made of more than one type of cells that work together as a unit. They are conducting tissues and constitute a vascular bundle. * Xylem: Transports water and minerals vertically from roots to stems and leaves. Consists of tracheids, vessels, xylem parenchyma (stores food), and xylem fibres (supportive). Most cells are dead. * Phloem: Transports food from leaves to other parts of the plant. Consists of sieve tubes, companion cells, phloem fibres, and phloem parenchyma. Except for phloem fibres, other phloem cells are living.
Animal tissues are broadly classified into four types based on their functions: Epithelial, Connective, Muscular, and Nervous tissue.
The covering or protective tissues in the animal body. It covers most organs and cavities and 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. * Simple Squamous Epithelium: Extremely thin and flat cells forming a delicate lining (e.g., in alveoli where gas exchange happens). * Stratified Squamous Epithelium: Skin epithelial cells are arranged in many layers to prevent wear and tear. * Columnar Epithelium: Tall, pillar-like cells. Found where absorption and secretion occur (e.g., inner lining of the intestine). In the respiratory tract, they have cilia (hair-like projections) and are called ciliated columnar epithelium. * Cuboidal Epithelium: Cube-shaped cells providing mechanical support (e.g., lining of kidney tubules).
The cells of connective tissue are loosely spaced and embedded in an intercellular matrix. The matrix may be jelly-like, fluid, dense, or rigid. * Blood: Has a fluid matrix called plasma, in which red blood cells (RBCs), white blood cells (WBCs), and platelets are suspended. Transports gases, digested food, and hormones. * Bone: Forms the framework that supports the body. It is a strong and non-flexible tissue. Bone cells are embedded in a hard matrix composed of calcium and phosphorus compounds. * Ligament and Tendon: Two bones can be connected to each other by a ligament (very elastic, considerable strength). Muscles are connected to bones by tendons (fibrous tissue with great strength but limited flexibility). * Cartilage: Has widely spaced cells and a solid matrix of proteins and sugars. It smoothens bone surfaces at joints and is present in the nose, ear, trachea, and larynx. (You can fold the cartilage of the ear, but not bones). * Areolar & Adipose: Areolar tissue fills the space inside organs and supports internal organs. Adipose tissue is a fat-storing tissue found below the skin; its cells are filled with fat globules.
Consists of elongated cells called muscle fibres. This tissue is responsible for movement in our body. Muscles contain special proteins called contractile proteins, which contract and relax to cause movement. * Striated Muscles (Voluntary): We can move these muscles by conscious will. Mostly attached to bones. Under a microscope, they show alternate light and dark bands (striations). Cells are long, cylindrical, unbranched, and multinucleate. * Smooth Muscles (Involuntary): We cannot start or stop their movement consciously (e.g., movement of food in the alimentary canal, blood vessels). They are unstriated. Cells are long with pointed ends (spindle-shaped) and uninucleate. * Cardiac Muscles: Involuntary muscles of the heart. They show rhythmic contraction and relaxation throughout life. Cells are cylindrical, branched, and uninucleate.
Cells of nervous tissue are highly specialized 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 cells of this tissue are called neurons (nerve cells). * A neuron consists of a cell body with a nucleus and cytoplasm, from which long thin hair-like parts arise. Usually, there is a single long part (axon) and many short, branched parts (dendrites).
The evolution of tissues allowed multicellular organisms to reach incredible sizes and complexity. Plants, needing structural support and stationary existence, evolved heavily lignified, often dead tissues like sclerenchyma and xylem. Animals, requiring active movement and rapid response to environments, evolved contractile muscles, communicative nervous tissue, and diverse connective tissues to bind it all together. Understanding these tissues reveals the architectural blueprint of life itself.