EXCRETION AND EXCRETORY SYSTEM

Excretion is the process by which metabolic waste products are removed from the body of all living things. Excretion is different from egestion which is the removal of solid waste (undigested food substances i.e.faeces) through the anus. Excretion is necessary for the following reasons:

To avoid or prevent any harm that would be caused by any excretory product.
Some excretory products are poisonous to the body and should be removed.
To maintain water balance in the body (homeostasis).
To avoid interference of waste products with normal metabolic activities in the body.

EXCRETORY STRUCTURES AND WASTE IN ORGANISMS

Organism	Excretory Structure	Excretory Waste Product
Protozoa	Contractile vacuole	C0₂, ammonia and water
Flatworm	Flame cells	C0₂, ammonia and water
Earthworm	Nephridia	C0₂, ammonia and water
Crustacean	Green glands	C0₂, ammonia and water
Insect	Malpighian tubules	C0₂, ammonia and uric acid
Fish	Kidney	C0₂, ammonia and water
Amphibian(toad)	Kidney	H₂0 and salt
Reptiles	Kidney	H₂0 and salt
Birds	Kidney and lungs	C0₂ and water vapour
Mammals	Kidney, lungs, skin and liver	C0₂, water, urea
Flowering plants	Stomata, lenticels and leaves	H₂0, C0₂, 0₂. tannins, gum, alkaloids, oil and latex

EVALUATION

List five excretory structures adapted to aquatic habitats.
Mention four excretory wastes in plants.

TYPES OF EXCRETORY SYSTEMS

CONTRACTILE VACUOLE IN PROTOZOA

The contractile vacuole is a simple structure found in the cell of freshwater protozoa. Water constantly enters the cell of a protozoan through the selectively permeable membrane because the cell is hypertonic to its environment. As water enters the cell, a contractile vacuole is formed which collects the water and expands, when it reaches the maximum size, it contracts and discharges the water through a temporary break in the cell membrane at intervals. The excretion of carbon dioxide and ammonia is by diffusion through the cell membrane.

FLAME CELLS IN FLATWORMS

The excretory system consists of two longitudinal canals with branched tubules which end in flame cells. The flame cell has a large hollow called the cell lumen with a bunch of flagella hung on it. The waste product from the surrounding cells enters the flame cells. The flagella help to propel the fluid into the tubules. The fluid passes into the exterior through a narrow tube called a duct.

NEPHRIDIUM IN EARTHWORM

A pair of nephridia are found on each segment of the earthworm except the three and the last. Each nephridium consists of a ciliated funnel, nephrostome which leads into a long coiled tube (narrow and middle ciliated tubes, wide non-ciliated tubes and muscular tubes). The tube opens to the exterior as a nephriodiophore (excretory pore). Waste product mainly urea is absorbed from blood capillaries surrounding the nephridia. The fluid containing the waste through the long tube of the nephridia, salt and other useful substances are reabsorbed through the wall of the tube. The unabsorbed substances and water gather in the muscular tube and discharge to the exterior through the excretory pore.

EVALUATION

How are excretory products removed in amoeba and earthworms?
Describe the excretory structure in flatworms.

MALPIGHIAN TUBULE IN INSECT

Malpighian tubules are found between the midgut (small intestine) and the hindgut (large intestine). One end opens into the gut while the other end closed freely and floats in the haemocoel. Nitrogenous waste and water in the haemocoel are absorbed in the distal close end into the tubule. The waste is converted into uric acid as it passes along the malpighian tubule towards the gut. A lot of water is also reabsorbed so that by the time the uric acid reaches the proximal end, it is changed to solid crystals. More water is reabsorbed in the rectum therefore concentrated urine leaves the body as almost dried solid.

GENERAL EVALUATION

Define excretion.
Explain the necessity for an excretory system in animals.
Describe the mechanism of excretion in insects.
List five excretory products in plants.
Differentiate between excretion and egestion.

ASSIGNMENT

SECTION A

The organelle which eliminates water from the body of protozoa is A. plasma membrane B. contractile vacuole C. cell wall D. nucleus.
In insects, the structure that performs the same function as the kidney in man is the A. nephridium B. flame cell C. malpighian tubule D. trachea
The excretory structure in earthworms is the A. malpighian tubule B. nephridium C. kidney D. flame cell
Flame cells are the A. excretory systems of worms B. excretory and respiratory systems of flatworms C. excretory systems of flatworms D. secretory systems of flatworms
Which of these is a waste product of an insect? A. alkaloids B. uric acid C. sweat D. mucilage

SECTION B

State the similarities and differences between the excretory organs of a mammal and an earthworm
State three factors that lead to the opening and closing of stomata