Structural Organization in Animals PDF - Arjuna NEET 2.0 2025

Arjuna NEET 2.0 2025 100 Zoology BIOLOGY CHAPTER 7 STRUCTURAL ORGANISATION IN...

Arjuna NEET 2.0 2025 100 Zoology BIOLOGY CHAPTER 7 STRUCTURAL ORGANISATION IN ANIMALS 7.1 Animal Tissues In the preceding chapters you came across a large variety of organisms, both unicellular and multicellular, of the animal kingdom. In unicellular 7.2 Organ and Organ organisms, all functions like digestion, respiration and reproduction System are performed by a single cell. In the complex body of multicellular 7.3 Earthworm animals the same basic functions are carried out by different groups of 7.4 Cockroach cells in a well organised manner. The body of a simple organism like Hydra is made of different types of cells and the number of cells in each 7.5 Frogs type can be in thousands. The human body is composed of billions of cells to perform various functions. How do these cells in the body work together? In multicellular animals, a group of similar cells alongwith intercellular substances perform a specific function. Such an organisation is called tissue. You may be surprised to know that all complex animals consist of only four basic types of tissues. These tissues are organised in specific proportion and pattern to form an organ like stomach, lung, heart and kidney. When two or more organs perform a common function by their physical and/or chemical interaction, they together form organ system, e.g., digestive system, respiratory system, etc. Cells, tissues, organs and organ systems split up the work in a way that exhibits division of labour and contribute to the survival of the body as a whole. 7.1 ANIMAL TISSUES The structure of the cells vary according to their function. Therefore, the tissues are different and are broadly classified into four types : (i) Epithelial, (ii) Connective, (iii) Muscular and (iv) Neural. 2015-16(19/01/2015)  STRUCTURAL ORGANISATION IN ANIMALS 101 7.1.1 Epithelial Tissue We commonly refer to an epithelial tissue as epithelium (pl.: epithelia). This tissue has a free surface, which faces either a body fluid or the outside environment and thus provides a covering or a lining for some part of the body. The cells are compactly packed with little intercellular matrix. There are two types of epithelial tissues namely simple epithelium and compound epithelium. Simple epithelium is composed of a single layer of cells and functions as a lining for body cavities, ducts, and tubes. The compound epithelium consists of two or more cell layers and has protective function as it does in our skin. On the basis of structural modification of the cells, simple epithelium is further divided into three types. These are (i) Squamous, (ii) Cuboidal, (iii) Columnar (Figure 7.1). Flattened cell (a) Cube-like cell (b) Tall cell (d) (c) Figure 7.1 Simple epithelium: (a) Squamous (b) Cuboidal (c) Columnar (d) Columnar cells bearing cilia The squamous epithelium is made of a single thin layer of flattened cells with irregular boundaries. They are found in the walls of blood vessels and air sacs of lungs and are involved in functions like forming a diffusion boundary. The cuboidal epithelium is composed of a single layer of cube-like cells. This is commonly found in ducts of glands and tubular parts of nephrons in kidneys and its main functions are secretion and absorption. The epithelium of proximal convoluted tubule (PCT) of nephron in the kidney has microvilli. The columnar epithelium is composed of a single layer of tall and slender cells. Their nuclei are located at the base. Free surface may have microvilli. They are found in the lining of stomach and intestine and help in secretion and absorption. If the columnar or cuboidal cells bear cilia on their free surface they are called ciliated epithelium (Figure 7.1d). Their function is to move particles or mucus in a specific direction over the epithelium. They are mainly present in the inner surface of hollow organs like bronchioles and fallopian tubes. 2015-16(19/01/2015)  102 BIOLOGY Some of the columnar or cuboidal cells get specialised for secretion and are called glandular epithelium (Figure 7.2). They unicellular are mainly of two types: unicellular, gland consisting of isolated glandular cells (goblet cells of the alimentary canal), and multicellular, consisting of cluster of cells Multicelluar gland (salivary gland). On the basis of the mode of (a) (b) pouring of their secretions, glands are divided into two categories namely Figure 7.2 Glandular epithelium : (a) Unicellular (b) Multicellular exocrine and endocrine glands. Exocrine glands secrete mucus, saliva, earwax, oil, milk, digestive enzymes and other cell products. These products are released through ducts or tubes. In contrast, endocrine glands do not have ducts. Their products called hormones are secreted directly into the fluid bathing the gland. Compound epithelium is made of more Multi- layered than one layer (multi-layered) of cells and thus cells has a limited role in secretion and absorption (Figure 7.3). Their main function is to provide protection against chemical and mechanical stresses. They cover the dry surface of the skin, Figure 7.3 Compound epithelium the moist surface of buccal cavity, pharynx, inner lining of ducts of salivary glands and of pancreatic ducts. All cells in epithelium are held together with little intercellular material. In nearly all animal tissues, specialised junctions provide both structural and functional links between its individual cells. Three types of cell junctions are found in the epithelium and other tissues. These are called as tight, adhering and gap junctions. Tight junctions help to stop substances from leaking across a tissue. Adhering junctions perform cementing to keep neighbouring cells together. Gap junctions facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells, for rapid transfer of ions, small molecules and sometimes big molecules. 7.1.2 Connective Tissue Connective tissues are most abundant and widely distributed in the body of complex animals. They are named connective tissues because of their special function of linking and supporting other tissues/organs of the body. They range from soft connective tissues to specialised types, which 2015-16(19/01/2015)  STRUCTURAL ORGANISATION IN ANIMALS 103 Macro- phage Fat storage Fibroblast area Collagen Nucleus fibers fibres Plasma Membrane (b) (a) Mast cell Figure 7.4 Loose connective tissue : (a) Areolar tissue (b) Adipose tissue include cartilage, bone, adipose, and blood. In all connective tissues except blood, the cells secrete fibres of structural proteins called collagen or elastin. The fibres provide strength, elasticity and flexibility to the tissue. These cells also secrete modified polysaccharides, which accumulate between cells and fibres and act as matrix (ground substance). Connective tissues are classified into three types: (i) Loose connective tissue, (ii) Dense connective tissue and (iii) Specialised connective tissue. Loose connective tissue has cells and fibres loosely arranged in a semi-fluid ground substance, for example, areolar tissue present beneath the skin (Figure 7.4). Often it serves as a support framework for epithelium. It (a) contains fibroblasts (cells that produce and secrete fibres), Collagen fibre macrophages and mast cells. Adipose tissue is another type of loose connective tissue located mainly beneath the skin. The cells of this tissue are specialised to store fats. The excess of nutrients which are not used immediately are converted into fats and are stored in this tissue. Fibres and fibroblasts are compactly packed in the dense connective tissues. Orientation of fibres show a regular or irregular pattern and are called dense regular and dense irregular tissues. In the dense regular connective tissues, the collagen fibres are present in rows between many parallel bundles of fibres. Tendons, which attach skeletal muscles to bones and ligaments which attach one bone to another are examples of this tissue. (b) Dense irregular connective tissue has fibroblasts and Figure 7.5 Dense connective tissue: many fibres (mostly collagen) that are oriented differently (a) Dense regular (Figure 7.5). This tissue is present in the skin. Cartilage, (b) Dense irregular 2015-16(19/01/2015)  104 BIOLOGY bones and blood are various types of specialised connective tissues. The intercellular material of cartilage is solid and pliable and resists compression. Cells of this tissue (chondrocytes) are enclosed in small cavities within the matrix secreted by them (Figure 7.6a). Most of the cartilages in vertebrate embryos are replaced by bones in adults. Cartilage is (a) present in the tip of nose, outer ear joints, between adjacent bones of the vertebral column, limbs and hands in adults. Bones have a hard and non-pliable ground substance rich in calcium salts and collagen fibres which give bone its strength (Figure 7.6b). It is the main tissue that provides structural frame to the body. Bones support and protect softer tissues and organs. The bone cells (osteocytes) are present in the spaces called lacunae. Limb bones, such as (b) the long bones of the legs, serve weight-bearing functions. They also interact with skeletal muscles attached to them to bring about movements. The bone marrow in some bones RBC is the site of production of blood cells. Blood is a fluid connective tissue containing plasma, Platelets red blood cells (RBC), white blood cells (WBC) and platelets (Figure 7.6c). It is the main circulating fluid that helps in WBC the transport of various substances. You will learn more (c) about blood in Chapters 17 and 18. Figure 7.6 Specialised connective tissues : (a) Cartilage 7.1.3 Muscle Tissue (b) Bone (c) Blood Each muscle is made of many long, cylindrical fibres arranged in parallel arrays. These fibres are composed of numerous fine fibrils, called myofibrils. Muscle fibres contract (shorten) in response to stimulation, then relax (lengthen) and return to their uncontracted state in a coordinated fashion. Their action moves the body to adjust to the changes in the environment and to maintain the positions of the various parts of the body. In general, muscles play an active role in all the movements of the body. Muscles are of three types, skeletal, smooth, and cardiac. Skeletal muscle tissue is closely attached to skeletal bones. In a typical muscle such as the biceps, striated (striped) skeletal muscle fibres are bundled together in a parallel fashion (Figure 7.7a). A sheath of tough connective tissue encloses several bundles of muscle fibres (You will learn more about this in Chapter 20). 2015-16(19/01/2015)  STRUCTURAL ORGANISATION IN ANIMALS 105 Smooth Striations Striations muscle fibers Nucleus Junction between Nucleus adjacent cells (a) (b) (c) Figure 7.7 Muscle tissue : (a) Skeletal (striated) muscle tissue (b) Smooth muscle tissue (c) Cardiac muscle tissue The smooth muscle fibres taper at both ends (fusiform) and do not show striations (Figure 7.7b). Cell junctions hold them together and they are bundled together in a connective tissue sheath. The wall of internal organs such as the blood vessels, stomach and intestine contains this type of muscle tissue. Smooth muscles are ‘involuntary’ as their functioning cannot be directly controlled. We usually are not able to make it contract merely by thinking about it as we can do with skeletal muscles. Cardiac muscle tissue is a contractile tissue present only in the heart. Cell junctions fuse the plasma membranes of cardiac muscle cells and make them stick together (Figure 7.7c). Communication junctions (intercalated discs) at some fusion points allow the cells to contract as a unit, i.e., when one cell receives a signal to contract, its neighbours are also stimulated to contract. 7.1.4 Neural Tissue Axon Neural tissue exerts the greatest control over the body’s responsiveness to changing conditions. Neurons, the unit of neural Cell system are excitable cells (Figure 7.8). The body neuroglial cell which constitute the rest of with nucleus the neural system protect and support Dendrite neurons. Neuroglia make up more than one- half the volume of neural tissue in our body. When a neuron is suitably stimulated, Neuroglea an electrical disturbance is generated which swiftly travels along its plasma Figure 7.8 Neural tissue (Neuron with neuroglea) 2015-16(19/01/2015) 106 BIOLOGY membrane. Arrival of the disturbance at the neuron’s endings, or output zone, triggers events that may cause stimulation or inhibition of adjacent neurons and other cells (You will study the details in Chapter 21). 7.2 ORGAN AND ORGAN SYSTEM The basic tissues mentioned above organise to form organs which in turn associate to form organ systems in the multicellular organisms. Such an organisation is essential for more efficient and better coordinated activities of millions of cells constituting an organism. Each organ in our body is made of one or more type of tissues. For example, our heart consists of all the four types of tissues, i.e., epithelial, connective, muscular and neural. We also notice, after some careful study that the complexity in organ and organ systems displays certain discernable trend. This discernable trend is called evolutionary trend (You will study the details in class XII). You are being introduced to morphology and anatomy of three organisms at different evolutionary levels to show their organisation and functioning. Morphology refers to study of form or externally visible features. In the case of plants or microbes, the term morphology precisely means only this. In case of animals this refers to the external appearance of the organs or parts of the body. The word anatomy conventionally is used for the study of morphology of internal organs in the animals. You will learn the morphology and anatomy of earthworm, cockroach and frog representing invertebrates and vertebrates. 7.3 EARTHWORM Earthworm is a reddish brown terrestrial invertebrate that inhabits the upper layer of the moist soil. During day time, they live in burrows made by boring and swallowing the soil. In the gardens, they can be traced by their faecal deposits known as worm castings. The common Indian earthworms are Pheretima and Lumbricus. 7.3.1 Morphology Earthworms have long cylindrical body. The body is divided into more than hundred short segments which are similar (metameres about 100-120 in number). The dorsal surface of the body is marked by a dark median mid dorsal line (dorsal blood vessel) along the longitudinal axis of the body. The ventral surface is distinguished by the presence of genital openings (pores). Anterior end consists of the mouth and the prostomium, a lobe which serves as a covering for the mouth and as a wedge to force open cracks in the soil into which the earthworm may crawl. The prostomium is sensory in function. The first body segment is called the peristomium (buccal segment) which contains the mouth. In a mature worm, segments 2015-16(19/01/2015)