TISSUES

•  All living organisms are made up of cells.

• Unicellular organisms consist of a single cell that performs all basic functions.

• In multicellular organisms, there are millions of specialized cells.

• Specialized cells in multicellular organisms carry out specific functions efficiently.

• Different groups of cells are responsible for different specialized functions.

• Muscle cells in humans contract and relax for movement, nerve cells carry messages, and blood transports oxygen, food, hormones, and waste materials.

• In plants, vascular tissues conduct food and water throughout the plant.

• Multi-cellular organisms exhibit division of labor, where specific functions are carried out by clusters of cells in a definite place in the body.

• These clusters of cells are called tissues.

• Tissues are arranged and designed to maximize efficiency in performing their functions.

• Examples of tissues include blood, phloem, and muscle.

• A tissue is formed by a group of cells that are similar in structure and/or work together to achieve a particular function.

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

• Plants and animals have noticeable differences in their structure and functions.

• Plants are stationary or fixed, while animals move around in search of food, mates, and shelter.

• Plants have a large quantity of supportive tissue to maintain their upright position, which generally consists of dead cells.

• Animals consume more energy compared to plants and have a higher proportion of living tissues.

• Plants exhibit limited growth in certain regions, while animals do not have such limitations.

• Some tissues in plants divide throughout their life and are localized in specific regions. These can be classified as growing or meristematic tissue and permanent tissue.

• Cell growth in animals is more uniform, and there is no clear demarcation between dividing and non-dividing regions.

• The structural organization of organs and organ systems is more specialized and localized in complex animals compared to plants.

• Complex animals have different feeding methods and are adapted for active locomotion, while plants are adapted for a sedentary existence.

• The differences in organ system design reflect the different modes of life pursued by plants and animals.

• The concept of tissues is discussed in detail with reference to the complex bodies of animals and plants.

6.2 Plant Tissues 

6.2.1 MERISTEMATIC TISSUE

• The growth of plants occurs in specific regions where meristematic tissue is located.

• Meristematic tissue is dividing tissue responsible for the production of new cells.

• Meristematic tissues are classified as apical, lateral, and intercalary based on their location.

• Apical meristem is found at the growing tips of stems and roots, contributing to the lengthening of the stem and root.

• Lateral meristem, also known as cambium, increases the girth of the stem or root.

• Intercalary meristem is present near the nodes in some plants.

• Cells of meristematic tissue are highly active with dense cytoplasm, thin cellulose walls, and prominent nuclei.

• Meristematic cells lack vacuoles.

• Vacuoles are organelles responsible for various functions in cells, such as storage, maintaining turgidity, and regulating cell size.

• The absence of vacuoles in meristematic cells may be because their primary function is rapid cell division and growth rather than storage or maintenance of turgor pressure.

6.2.2 PERMANENT TISSUE

• Cells formed by meristematic tissue undergo differentiation, taking up specific roles and losing the ability to divide, resulting in the formation of permanent tissue.

• Differentiation leads to the development of various types of permanent tissues.

• Simple permanent tissues are located a few layers beneath the epidermis and include parenchyma, collenchyma, and sclerenchyma.

• Parenchyma consists of relatively unspecialized, living cells with thin cell walls. It can store food and perform photosynthesis when containing chlorophyll.

• Collenchyma provides flexibility and mechanical support to plant parts, such as tendrils and stems of climbers. Its cells are living, elongated, and irregularly thickened at the corners.

• Sclerenchyma makes the plant hard and stiff, with its cells being dead, long, and narrow with thickened walls due to lignin. It provides strength to plant parts.

• The outermost layer of cells in plants is called the epidermis. It serves as a protective covering and can have a thicker epidermis in plants living in dry habitats.

• Epidermal cells may secrete a waxy layer on their outer surface to protect against water loss and mechanical injury.

• Stomata are small pores found in the epidermis that are enclosed by two kidney-shaped guard cells. They are necessary for gas exchange and transpiration.

• The outer layer of a branch of a tree may differ from the outer layer of a young stem as plants grow older.

• As plants grow older, a strip of secondary meristem in the cortex forms layers of cork cells, which are dead and compactly arranged. Cork cells have a substance called suberin in their walls, making them impermeable to gases and water.

• Complex permanent tissues are made up of more than one type of cell and coordinate to perform a common function.

• The xylem and phloem are examples of complex tissues and constitute the vascular bundle.

• Xylem consists of tracheids, vessels, xylem parenchyma, and xylem fibers. Tracheids and vessels transport water and minerals vertically, while xylem parenchyma stores food and xylem fibers provide support.

• Phloem is composed of sieve cells, sieve tubes, companion cells, phloem fibers, and phloem parenchyma. It transports food from leaves to other plant parts, with sieve tubes being tubular cells with perforated walls.

• Except for phloem fibers, other phloem cells are living cells.

6.3 Animal Tissues

• Movement of body parts, such as the chest during breathing, is enabled by specialized muscle cells that contract and relax.

• Oxygen is inhaled during breathing and is absorbed in the lungs.

• The absorbed oxygen is then transported to all body cells through the bloodstream.

• Cells need oxygen for cellular respiration, a process that occurs in the mitochondria and produces energy (ATP) by breaking down glucose.

• Blood flows throughout the body and carries various substances.

• Blood carries oxygen and nutrients from the lungs and digestive system to all cells, providing them with the necessary resources for their functions.

• Blood also collects waste materials from cells and transports them to organs like the liver and kidneys for disposal.

• Blood is classified as a type of connective tissue.

• Muscles, such as skeletal muscles, are examples of muscular tissue responsible for body movement.

• Muscular tissue consists of elongated cells called muscle fibers that contract and relax to produce movement.

6.3.1 EPITHELIAL TISSUE

• Epithelial tissues are the covering or protective tissues in the animal body.

• Epithelium covers most organs and cavities, forming a barrier that separates different body systems.

• Epithelial tissue cells are tightly packed, forming a continuous sheet with minimal intercellular spaces.

• All substances entering or leaving the body must cross at least one layer of epithelium.

• The permeability of epithelial cells plays a crucial role in regulating the exchange of materials between the body and the external environment, as well as between different body parts.

• The epithelium is usually separated from underlying tissue by an extracellular fibrous basement membrane.

• Different types of epithelia have unique structures correlating with their functions.

• Simple squamous epithelium is a flat and thin layer found in cells lining blood vessels, lung alveoli, the esophagus, and the lining of the mouth.

• Stratified squamous epithelium is found in the skin, arranged in layers to prevent wear and tear.

• Columnar epithelium with tall cells facilitates absorption and secretion, such as in the lining of the intestine.

• Ciliated columnar epithelium in the respiratory tract has hair-like projections called cilia, which help move mucus forward to clear it.

• Cuboidal epithelium with cube-shaped cells lines kidney tubules and ducts of salivary glands, providing mechanical support.

• Epithelial cells can specialize as gland cells, which secrete substances at the epithelial surface.

• Glandular epithelium can form multicellular glands when a portion of the epithelial tissue folds inward.

6.3.2 CONNECTIVE TISSUE

• Blood is called "connective" tissue because it connects different parts of the body and transports various substances.

• Connective tissue has loosely spaced cells embedded in an intercellular matrix.

• The matrix of blood is called plasma, which is a fluid (liquid) containing red blood cells (RBCs), white blood cells (WBCs), platelets, proteins, salts, and hormones.

• Blood flows and transports gases, digested food, hormones, and waste materials throughout the body.

• Bone is a strong and nonflexible connective tissue that forms the framework supporting the body, anchors muscles, and supports organs.

• Bone cells are embedded in a hard matrix composed of calcium and phosphorus compounds.

• Ligaments, another type of connective tissue, connect two bones together and provide elasticity and strength.

• Tendons, also a type of connective tissue, connect muscles to bones, offering strength but limited flexibility.

• Cartilage, with widely spaced cells, has a solid matrix composed of proteins and sugars. It smoothens bone surfaces at joints and is found in the nose, ear, trachea, and larynx.

• Adipose tissue, a type of connective tissue, stores fats and is found below the skin and between internal organs. Its cells are filled with fat globules, and it also acts as an insulator.

• Areolar connective tissue is found between the skin and muscles, around blood vessels and nerves, in the bone marrow, and fills the space inside organs. It supports internal organs and aids in tissue repair.

6.3.3 MUSCULAR TISSUE

• Muscular tissue consists of elongated cells known as muscle fibers and is responsible for movement in the body.

• Muscles contain contractile proteins that contract and relax, causing movement.

• Voluntary muscles, also known as skeletal muscles, are under conscious control and attached to bones, enabling body movement.

• Skeletal muscles appear striated under the microscope due to alternate light and dark bands. They are long, cylindrical, unbranched, and multinucleate.

• Involuntary muscles, called smooth muscles, control movements that we cannot consciously start or stop. They are found in the alimentary canal, blood vessels, iris of the eye, ureters, and bronchi of the lungs.

• Smooth muscles have long, spindle-shaped cells with pointed ends and are uninucleate. They are also called unstriated muscles.

• Cardiac muscles are involuntary muscles found in the heart. They exhibit rhythmic contraction and relaxation throughout life.

• Cardiac muscle cells are cylindrical, branched, and uninucleated.

6.3.4 NERVOUS TISSUE

• Nervous tissue is highly specialized for responding to stimuli and transmitting signals rapidly within the body.

• The brain, spinal cord, and nerves are composed of nervous tissue.

• Nervous tissue cells are called neurons and consist of a cell body with a nucleus and cytoplasm.

• Neurons have long, thin hair-like parts called axons and many short, branched parts called dendrites.

• Neurons can be very long, with individual nerve cells reaching up to a meter in length.

• Nerve fibers, which are bundles of many nerve cells, bound together by connective tissue, make up a nerve.

• The signal that passes along a nerve fiber is called a nerve impulse.

• Nerve impulses allow us to move our muscles in response to stimuli.

• The combination of nerve and muscle tissue is essential for rapid movement in animals.