TNPSC BIOLOGY (ENGLISH)
WHAT IS THE ORGANISATION IN ANIMALS?
A group of cellswhich is formed to carry out specific task is called a tissue. Formation of tissue is the first step towards division of labour in multicellular animals. Subsequently, a group of tissues form an organ and a group of organs forms the organ system.
Tissues which provide covering to the inner and outer linings of various organs are called epithelial tissue. The cells in the epithelial tissue are compactly packed. There is little intercellular matrix in the epithelial tissue.
The epithelial tissues are further divided into two main types, viz., simple epithelium and compound epithelium.
The simple epithelium is composed of a single layer of cells. It is present in the lining of body cavities, ducts and tubes.
The compound epithelium is composed of more than one layer of cells. The compound epithelium serves the protective function. Compound epithelium is present in skin, in the lining of buccal cavity, pharynx, ducts of salivary glands and pancreatic ducts.
The simple epithelium can be further divided into three types, viz., Squamous, Cuboidal and Coloumnar.
The connective tissues are most abundant and widely distributed in the body of complex animals. The connective tissues support and link other tissues or organs of the body. The cells of the connective tissue secrete fibers of structural proteins called collagen or elastin. But blood is an exception: in which no structural fiber is secreted. The structural fibers provide strength, elasticity and flexibility to the tissue.
(i) Loose Connective Tissue;
(ii) Dense Connective Tissue;
(iii) Specialised Connective Tissues.
Loose Connective Tissue
The cells and fibers are loosely arranged in a semi-fluid matrix in loose connective tissue. Areolar tissue is an example of loose connective tissue. It is present beneath the skin. Adipose tissue is another example of loose connective tissue. It is located beneath the skin. The cells of the adipose tissue are specialized to store fat.
Dense Connective Tissue
The cells and fibers are compactly packed in dense connective tissue. The orientation of fibers can be regular or irregular. On this basis, the dense connective tissue is called dense regular or dense irregular connective tissue.
Specialized Connective Tissue
cartilage, bones, and blood are the specialized connective tissues.
A muscle is made of many long, cylindrical fibres which are arranged in parallel rays. The muscle fibres are composed of very fine fibrils; called myofibril. The muscle fibres can contract and relax in a coordinated fashion. Thus, muscles play an important role in movement and locomotion.
✦ Skeletal Muscles
✦ Smooth Muscles and
✦ Cardiac Muscles.
It is closely attached to skeletal bones. Skeletal muscle fibers are bundled together in a parallel fashion. Several bundles of muscle fibres are covered by a sheath of tough connective tissue. Striations are present in Skeletal muscle and hence they are called striated muscles.
The smooth muscle fibers are spindle- shaped. Striations are absent on smooth muscle. Various smooth muscle fibers are held together by cell junctions. They are bundled together in a sheath of connective tissue.
The muscles in the heart are called cardiac muscles. Cardiac muscles are multi nucleated and have striations. The muscle fibers are held together by cell junctions. Communication junctions are present at some fusion points. Cardiac muscles keep on contracting and relaxing continuously throughout the life.
It is made of neurons. Neurons are excitable cells. The neurological cell protects and supports neurons. Neuroglia makes up more than half of the volume of the neural tissue in human body. When a neuron is stimulated, an electrical disturbance is generated. The electrical disturbance travels along its plasma membrane. When a disturbance arrives at a neuron’s endings, it triggers events which may cause stimulation of inhibition of adjacent neurons and other cells. Thus, neural signals are transmitted to different parts of the body.
The eyes (Photoreceptor)
The sense organ eye in concerned with vision. The eye which is sphericalin shape is kept in the orbit of the skull.
1. Sclerotic coat
The outer sclerotic coat is white in colour except in the front where it forms the transparent cornea
2. The middle choroids coat
It is highly vascularised and deeply pigmented. In front of the eye the choroid coat forms he iris and lens
3. The inner coat (Retina)
✦ Retina is the sensitive part of an eye. It contains two types of receptor cell the rods and cones
✦ Rods are sensitive to different shades of light but not colour.Cones are sensitive to colour
✦ The lens is transparent, elastic and biconvex in shape.It is attached by ligaments to the ring shaped cillary muscles
✦ The Aqueous humors is a clear, watery liquid between the cornea and the lens
✦ Vitreous humour helps in image formation and also maintain the spherical shape of the eye