Previous Topic : : Respiratory System
Last class we discussed about Respiratory system, I believe you understand, today’s topic is excretion.
First what is excretion.
Excretion : is the process by which an organism gets rid of the waste products of metabolism in its cells.
Note : The undigested food remaining in the food vacuole of the protozoan is not excretory waste and the process of getting rid of it, is not excretory but rather, egestion. And also undigested food egested by higher animals e.g man, is referred to as faeces. top↑
So henceforth, you should know that, whenever you visit the toilet, you didn’t just excrete but egested.
In Organisms, no matter how primitive or simple the organism is, it must get rid of its metabolic wastes, otherwise these will accumulate and interface with normal body metabolism. These wastes are actually harmful to the body, in protists(unicellular organisms), excretion occurs mainly by Cell And Its Environment (Diffusion).
In unicellular organism, the waste materials pass out of the cell by diffusion through the entire cell membrane. While some unicellular organism have a contractile vacuole, in addition as a specialized organelle of excretion.
Experiment To Observe Excretion By Means Of Contractile Vacuole
Materials needed (specimen): Fresh water sample or culture rich in paramecium microscope, cover slips, boiled starch solution, glass slide and a dropping pipette.
a] Observed under the microscope, protozoans tend to be very mobile. You can slow them down so that, they can be more carefully observed. To do this, Put a few drops of boiled starch solution on the glass slide and place a drop of your culture rich paramecium sample.top↑
b] Locate some protozoa, and observed each, first under low power magnification, then under higher power magnification. Locate a paramecium and bring its contractile vacuoles into focus. Patiently and carefully observe both of the samples.
Conclusions And Observations
The contractile vacuole is basically a structure for osomoregulation. It removes extra water that enters with food. As the contractile vacuole removes the extra water, some soluble metabolic wastes may also be collected by it and passed out, in this way, the contractile vacuole functions also as an excretory organelle.
Most of the excretion in paramecium occurs by Cell And Its Environment (Diffusion) through the cell membrane. It is mostly found that, among most animals, the excretory organ is generally the organ of osomoregulation
It is found that, excretion in many organisms can be very complex, for example in pisces (fishes). In marine fishes, excess water salt is excreted through the gills, even against the direction of osmotic pressure.
By osmosis, water flows into the bodies of the freshwater fishes because there is a concentration gradient between the fishes body fluid and the surrounding water. To control the water content of the body, freshwater fishes drink little or no water, but excrete a large quantity of urine, thereby getting rid of much water out of its system.
In mammals, have a very complicated excretory system which involves various organs such as lungs for gaseous excretion of carbon dioxide, the skin for excreting water and salts in form of sweat and kidneys for getting rid of urine.
Forms Of Excretion
Carbon dioxide, produced by tissue respiration is one of the major excretory products which is eliminated by Cell And Its Environment (Diffusion) through the whole body surface or through specialized respiratory organs such as gills or lungs.top↑
Another major group of excretory substances are Nitrogenous waste which arises from the metabolism of nitrogenous compounds such as the break down of protoplasm, the use of amino acids during tissue respiration or the conversion of amino acids to carbohydrates for storage which is refer to as deamination.top↑
Nitrogenous waste substances are excreted in varous ways or forms which have some relationship with the environment of the organisms. In aquatic organisms, almost all, excrete nitrogenous waste in form of ammonia which is readily excreted by aquatic organisms.
brief : Ammonia is a poisonous gas and readily soluble in water, that is why it is use in fountain experiment (because of its high solubility). You might consider joining the chemistry class.
In terrestrial organisms, which have limited access to water, and indeed need to conserve water excrete nitrogenous waste substances in other forms.
Other organisms, such as reptiles, insects and birds excrete nitrogenous waste as uric acid, which is almost insoluble in water and leave the body surface as solid crystals. Uric acid can be stored for a long time without being absorbed into the body to cause harm as of case of urea in mammals.
Mammals excrete nitrogenous waste in form of urea, which is soluble in water.
Let stop here for today, till next class where we be talking about Growthtop↑
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