In a multicellular animal, similar cells work together as tissues. Tissues form organs, and organs work together in organ systems. This organisation allows different parts of the body to carry out different jobs efficiently. In Class 11 Biology Chapter 7, the frog is used to study how an animal’s external features and internal systems support its life on land and in water.
Table of Contents
Levels of Organisation in Animals
Cells → Tissues → Organs → Organ systems → Organism
A tissue is a group of cells, along with the material between them, that performs a function. Different tissues combine to form an organ, such as the heart. Organs then work together in an organ system, such as the circulatory system.
Morphology of Frog
Morphology describes externally visible features. A frog is an amphibian that can live on land and in freshwater.
- Its skin is smooth, moist and covered with mucus. The moist skin also helps in breathing.
- Its body has a head and trunk, but no distinct neck or tail in the adult.
- The eyes have a protective nictitating membrane.
- A tympanum is visible behind each eye and receives sound.
- The hind limbs are larger and more muscular than the forelimbs. Webbed hind feet help in swimming.
- Male frogs have vocal sacs and a copulatory pad on the first digit of each forelimb; females lack these features.
A frog’s body temperature varies with its surroundings. It may shelter during extreme summer heat (aestivation) or winter cold (hibernation).
Digestive System of Frog
The digestive system consists of an alimentary canal and digestive glands. The frog catches prey with its tongue.
Path of food: Mouth → Buccal cavity → Pharynx → Oesophagus → Stomach → Intestine → Rectum → Cloaca
The stomach begins digestion with gastric secretions. The liver produces bile, which is stored in the gall bladder. The pancreas produces pancreatic juice containing digestive enzymes. Digestion continues in the intestine, where nutrients are absorbed. Undigested material leaves through the cloaca.
Remember: The cloaca is a common chamber that opens to the outside.
Respiratory System of Frog
A frog uses different surfaces for gas exchange:
| Situation | Main respiratory surface |
|---|---|
| In water | Moist skin |
| On land | Lungs, skin and buccal cavity |
| During aestivation or hibernation | Skin |
Gas exchange through the skin is called cutaneous respiration. Breathing with lungs is called pulmonary respiration. Keeping the skin moist is therefore important for a frog’s survival.
Circulatory System of Frog
A frog has a closed circulatory system: blood flows through vessels. Its heart has three chambers—two atria and one ventricle. Arteries carry blood away from the heart, while veins return blood towards it.
Frog blood contains plasma, red blood cells, white blood cells and platelets. Unlike human red blood cells, frog red blood cells have nuclei. The frog also has a lymphatic system.
Excretory System of Frog
The excretory system includes two kidneys, ureters, a urinary bladder and the cloaca. The kidneys remove nitrogenous waste from the blood. The main nitrogenous waste excreted by the frog is urea, so the frog is described as ureotelic.
Nervous System and Sense Organs of Frog
The frog’s nervous system coordinates its activities. It includes:
- Central nervous system: Brain and spinal cord.
- Peripheral nervous system: Nerves arising from the brain and spinal cord.
- Autonomic nervous system: Controls many involuntary activities.
The brain has forebrain, midbrain and hindbrain regions. The frog has organs for vision, hearing, smell, taste and touch. Its visible tympanum helps receive sound, while the internal ear also helps with balance.
Reproductive System of Frog
Male frogs have a pair of testes, and female frogs have a pair of ovaries. During breeding, eggs and sperm are released into water. Fertilisation is external.
The fertilised egg develops into a tadpole. The tadpole later undergoes metamorphosis to become an adult frog.
Quick Revision Table
| System | Point to remember |
|---|---|
| Digestive | Food passes from mouth to cloaca; liver and pancreas are digestive glands |
| Respiratory | Skin helps in water; lungs help on land |
| Circulatory | Closed system; heart has two atria and one ventricle |
| Excretory | Kidneys remove waste; frog excretes urea |
| Nervous | Brain, spinal cord and nerves coordinate the body |
| Reproductive | Fertilisation occurs in water; tadpole develops into adult |
Check Your Understanding
- What are the four levels between a cell and a complete organism?
- Name two external features that distinguish a male frog from a female frog.
- How does a frog breathe in water?
- How many chambers does a frog’s heart have?
- What is metamorphosis in a frog?
Answers: (1) Tissue, organ and organ system are the intermediate levels: cell → tissue → organ → organ system → organism. (2) Vocal sacs and copulatory pads. (3) Through its moist skin. (4) Three. (5) The transformation of a tadpole into an adult frog.
For all chapters, visit the CBSE Class 11 Biology – Complete Study Guide.
Frequently Asked Questions
Which animal is studied in detail in this chapter?
The current NCERT chapter studies the morphology, anatomy and organ systems of a frog.
Why is a frog called an amphibian?
It can live both in water and on land during its life cycle.
What type of skin does a frog have?
Frog skin is smooth, moist and glandular, helping with protection and gas exchange.
What is sexual dimorphism in frogs?
Male and female frogs show visible differences; males may have vocal sacs and copulatory pads.
What is the frog’s digestive tract called?
It is the alimentary canal, running from the mouth through the stomach and intestine to the cloaca.
Where does digestion begin in a frog?
Food is swallowed through the mouth, while substantial chemical digestion occurs in the stomach and small intestine.
How does a frog breathe on land?
An adult frog mainly uses lungs and also exchanges gases through its moist skin and buccal cavity.
How does a frog breathe in water?
It exchanges gases mainly through its moist skin; tadpoles use gills.
How many chambers does a frog’s heart have?
It has two atria and one ventricle, making three chambers.
What is the function of a frog’s lymphatic system?
It returns tissue fluid to circulation and participates in immune defence.
What is the chief nitrogenous waste of an adult frog?
An adult frog mainly excretes urea.
What organs make up a frog’s excretory system?
Kidneys, ureters, urinary bladder and cloaca are its main parts.
What is the role of the frog’s tympanum?
The tympanum is an external membrane involved in hearing.
Is fertilisation in frogs internal or external?
Fertilisation is usually external and occurs in water.
What is metamorphosis in a frog?
It is the development of an aquatic tadpole into an adult frog.