Revision: Structural Organisation in Animals and Plants >> Structural Organisation in Animals Biology (Theory) ISC (Science) ISC Class 11 CISCE

The epithelial cells are connected to each other laterally as well as to the basement membrane by junctional complexes called cell junctions.

The tissue which connects various tissues together in any organ is called connective tissue.

An organ is a group of tissues that work together to perform a specific function or set of functions in the body.

The heart, lungs, liver, and kidneys are examples, each playing a vital role in maintaining the body's overall health and function.

• A tissue is a group of similar cells along with intercellular substances that perform a specific function together.
• Complex animals are composed of only four basic types of tissues, which are organised in specific proportions and patterns to form organs.
• When two or more organs work together for a common function, they form an organ system (e.g. digestive system, respiratory system).
• The body's survival depends on the division of labour among cells, tissues, organs and organ systems.

• Epithelial tissue has a free surface that faces either body fluids or the external environment, and its cells are tightly packed with very little intercellular space.
• Simple epithelium consists of a single layer of cells and mainly performs functions like lining, secretion, and absorption.
• Squamous epithelium is made of thin, flat cells and helps in diffusion, while cuboidal epithelium has cube-shaped cells and is involved in secretion and absorption.
• Columnar epithelium consists of tall cells that help in secretion and absorption, and ciliated epithelium contains cilia that move substances like mucus in a specific direction.
• Glandular epithelium is specialised for secretion and may be unicellular (goblet cells) or multicellular (such as salivary glands).
• Exocrine glands release their products through ducts, whereas endocrine glands secrete hormones directly into body fluids.
• Compound epithelium has multiple layers of cells and mainly provides protection, while cell junctions (tight, adhering, and gap junctions) help in preventing leakage, holding cells together, and communication between cells.

• Connective tissue is the most abundant tissue in the animal body, linking and supporting other tissues. Its cells (except in blood) secrete collagen or elastin fibres for strength, elasticity and flexibility.
• Loose connective tissue includes areolar tissue (supports epithelium; has fibroblasts, macrophages and mast cells) and adipose tissue (stores fat); both are found beneath the skin.
• Dense regular tissue has parallel collagen fibres; tendons connect muscle to bone, and ligaments connect bone to bone.
• Dense irregular tissue has collagen fibres oriented differently and is present in the skin.
• Specialised connective tissue includes cartilage (chondrocytes + collagen fibres) and bone (calcium salts + collagen fibres with osteocytes in lacunae).

• Muscle fibres are composed of fine fibrils called myofibrils and bring about movement and locomotion.
• Skeletal muscles are long, cylindrical, multinucleated, striated fibres closely attached to skeletal bones (voluntary).
• Smooth muscles are spindle-shaped, uninucleated, non-striated fibres found in the walls of internal organs like blood vessels, stomach and intestine (involuntary).
• Cardiac muscles are short, cylindrical, uninucleated fibres found only in the heart wall, with intercalated discs for communication between cells.