Class 7 Science


Various activities always go on inside the body of a living being. These activities are collectively called metabolism. Many harmful substances are created during metabolic activities. These substances can prove lethal if not removed from the body in time. Removal of waste from the body is called excretion.

Excretory System

The human excretory system is composed of a pair of kidneys, two tubes: called ureter and a urinary bladder.

human excretory system


Kidneys are bean-shaped. They work like filters. Blood, laden with waste materials enters the kidney. The waste is filtered from the blood and the purified blood is sent to the normal circulation. The waste, along with water, is transferred to the urinary bladder through the ureters. The content of the ureter is called urine. Urine contains 95% water, 2.5% urea and 2.5% other wastes. Urine is expelled out from time to time.

Type of waste and mode of excretion

Protein is made up of nitrogen. Metabolism of protein creates nitrogenous wastes in the body. The nitrogenous waste is the main waste in animals. The nitrogenous waste takes different forms in different animals. These are, ammonia, urea and uric acid. Based on the type of nitrogenous waste, animals can be divided into following categories:


Ammonia is the main nitrogenous waste in these animals. Lot of water is required for removal of ammonia. Ammonotelism is present in aquatic animals, e.g. fish, frogs, etc.


Urea is the main nitrogenous waste in these animals. Less water is required for removal of urea. Ureotelism is present in mammals.


Uric acid is the main nitrogenous waste in these animals. Removal of uric acid requires negligible amount of water. Uricotelism is present in reptiles and aves.

Note: Some of the wastes are removed along with sweat. Carbon dioxide is an important waste which is removed through the lungs.

Review Questions

Which is the main excretory organ in humans?

Answer: Kidneys

Which is the main excretory product in humans?

Answer: Urea

Transportation in Plants

ascent of sap

For transportation in plants, there are two main tissues, viz. xylem and phloem. These are composed of narrow tube-like structures. Xylem is responsible for transport of water, while phloem is responsible for transport of food.

Transport of Water

Plants take water from soil. Following are the main steps in transport of water in plants.

In Roots

From soil, the water enters the root hairs because of osmosis. From root hairs water enters further because of root pressure.

In Stem

Various factors are at play during transportation of water through xylem in stems. The transport of water through xylem is also called Ascent of Sap.

Note: When water molecules stick to surface of another sbstance, it is called adhesion. Water molecules stick to each other as well, and this is called cohesion.

Transport of Food

Food is prepared in leaves and needs to be transported to different plant parts for use and for storage. The transport of food takes place through phloem. Some biological force is used in transport of food in plants.

Review Questions

Transport of water and minerals in plants happens through which tissues?

Answer: Xylem

Transport of food in plants happens through which tissues?

Answer: Phloem