Definitions [3]
Answer the following question.
What is cell junction?
The epithelial cells are connected to each other laterally as well as to the basement membrane by junctional complexes called cell junctions.
Define the term of connective tissue
The tissue which connects various tissues together in any organ is called connective tissue.
Define the following term:
Organ
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.
Key Points
- 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.
- Neurons are the functional unit and are excitable cells.
- Neuroglia cells make up more than half the volume of neural tissue.
- They protect and support neurons.
| Feature | Description |
|---|---|
| Skin texture | Smooth and slippery due to the presence of mucus |
| Skin condition | Always moist |
| Dorsal side colour | Olive green with dark, irregular spots |
| Ventral side colour | Uniformly pale yellow |
| Water absorption | Frogs absorb water through the skin and do not drink it |
| Body division | Divisible into head and trunk; neck and tail are absent |
| Nostrils | Present above the mouth |
| Eyes | Bulged and covered by a nictitating membrane for protection in water |
| Tympanum (Ears) | Membranous structures on either side of the eyes that receive sound signals |
| Limbs | Forelimbs and hind limbs aid in swimming, walking, leaping, and burrowing |
| Hind limbs | End in five digits; larger and more muscular than forelimbs |
| Forelimbs | End in four digits |
| Webbed digits | Present on the feet, aiding in swimming |
| Sexual dimorphism | Male frogs have vocal sacs and copulatory pads on forelimbs; absent in females |
Digestive System
- The alimentary canal runs from the mouth through the buccal cavity, pharynx, oesophagus, stomach, intestine, rectum and exits via the cloaca. It is short due to a carnivorous diet.
- Digestive glands include the liver (produces bile), the pancreas (secretes digestive enzymes) and the stomach glands (secrete HCl and gastric juices).
Respiratory System
-
Frogs use three modes of respiration: cutaneous (through skin in water), buccal (on land) and pulmonary (through lungs). During aestivation and hibernation, gas exchange occurs only through the skin.
Circulatory System
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Frogs have a closed blood vascular system with a three-chambered heart (two atria + one ventricle). RBCs are nucleated and contain haemoglobin.
Excretory System
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Frogs are ureotelic (excrete urea). Kidneys are dark red, bean-shaped organs composed of nephrons. In males, ureters act as urinogenital ducts; in females, ureters and oviducts open separately into the cloaca.
Nervous System
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The brain is divided into the forebrain (olfactory lobes, cerebral hemispheres, diencephalon), midbrain (optic lobes) and hindbrain (cerebellum, medulla oblongata). Ten pairs of cranial nerves arise from the brain.
Reproductive System
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Fertilisation is external and occurs in water. Males have testes connected to the kidneys via 10-12 vasa efferentia. Females can lay 2500-3000 ova at once. Development includes the tadpole stage followed by metamorphosis.
- Basic tissues organise to form organs, and organs associate to form organ systems for efficient coordination in multicellular organisms.
- The heart contains all four basic tissue types: epithelial, connective, muscular and neural.
- Morphology is the study of externally visible features of an organism, while anatomy is the study of internal organs in animals.
- Structural organisation in the complexity of organs and organ systems shows a clear evolutionary trend in animals.
