All organisms carry out hundreds of reactions in their bodies which altogether are known as metabolism. The metabolism can be generally divided into two types: the catabolism and the anabolism.
The catabolism is the breaking down of complex organic molecules into simpler compounds in order to obtain energy which is utilized for the performance of various activities of life. In catabolic reaction carbohydrates, proteins and lipids are broken down or (oxidized) through enzymatic reactions step by step to generate chemical energy in the form of ATP molecules.
Poisonous
By looking at the above reactions you can know that carbon dioxide, water and ammonia are formed in these reactions. They are regarded as waste and surplus products because they are not utilized in the body of organisms (with the exception of plants which can use CO2 for photosynthesis).
These waste products must be removed from the body as soon as possible because these are toxic (poisonous) for the cells if they accumulate in the bodies of the organisms. The CO2 if not removed quickly can lead to lowering of pH which can be dangerous for life.
Ammonia is even more toxic than CO2, it should be quickly removed from the body or converted into less toxic material but even then the less toxic materials like urea, uric acid have to be eliminated from the bodies of organisms.
The concentration of water in the tissue fluid of mammals is maintained within the range 97-99% or a mean of 98%. Similarly the amount of water in the blood is regulated at about 92%. If the water exceeds this level then it collects in the tissues causing dropsy.
If the concentration of H2O falls below the normal line, it can cause dehydration of body. These waste products which are actually metabolic by-products have therefore to be eliminated from the bodies of living organisms. The process of removing the waste products from the body is known as excretion.
Excretion In Plants
The living organisms differ in their methods of excretion. Generally there are no specialized organs or systems in plants for excretion. The unwanted metabolites may be secreted into intra or inter-cellular spaces. The aerating system of plants makes possible the ready diffusion of respiratory CO2 into the surrounding water or atmosphere. In some plants the waste metabolites may be deposited in leaves, seeds and fruits which when shed can easily carry with them these accumulated un-necessary substances. It can be noted that such elimination is purely secondary to the true purposes of leaf fall or seed and fruit dispersal.
Removal of carbon dioxide and oxygen
CO2 is produced as a result of respiration. As the process of respiration takes place continuously in all living cells, CO2 is produced continuously and discharged through the stomata, or lenticels. On contrary, oxygen (O2) is released only during the day-time, because it is a by-product of the process of photosynthesis. The CO2 thus produced during daytime is consumed by the mesophyll cells, in photosynthesis. Only a small quantity of oxygen produced during photosynthesis is used for respiration. The excess oxygen is released from the leaves through stomata.
“Green plants give off oxygen as waste in the day time and CO2 at night.”
At night the photosynthesis stops, the atmospheric O2 is used for respiration in plants and CO2 is evolved as a by-product.
Excretion of extra water
Water is one of the major basic needs of the plants the water in plants is used for photosynthesis, for transpiration and carry out many other functions like transportation of different materials etc. the plants obtain water largely from soil. Whereas a small percentage of it is provided as a result of respiration. The excess water is removed in the form of water vapours by the process of transpiration. Sometimes, the removal of this excess water takes place by guttaion. For example in cabbage, potato and strawberry etc., the droplets of excess water (similar to the dew drops) often appear at the tips and margins of leaves which enlarge and drop-off later on. In this way such plants and few grasses get rid of the extra water (Fig. 13.1)
“Transpiration: it is the process by which the water gets evaporated from the plant cells, through stomata etc.”
Latex, Resins and gums
In some plants the products like latex, resins and gum are secreted in special canals or ducts, present in the plant body. Such products are not ordinarily excreted from the plant, but they trickle out (leak slowly) of the plant when some part of it is damaged. (Due to this unique behavior, some scientists are also reluctant to regard them as waste products of plants.) The common examples of latex producing plants are rubber plant and the pines. Resins and gums are secreted by Acacia Arabica (keekar) etc and mucilage is secreted by Hibiscus (ladyfinger).
Excretion in Animals
The principal metabolic waste products in most animals are water, carbon dioxide, and nitrogenous wastes. Carbon dioxide and some water re eliminated from the bodies mainly by respiratory structures, whereas most of the water and nitrogenous wastes are excreted by special excretory organs.
A nitrogenous waste includes ammonia, uric acid and urea, recall that amino acids and nucleic acids contain nitrogen. During the breakdown of amino acids, the nitrogen-containing amino group is removed by a process called deamination and converted to ammonia. However, ammonia is highly toxic. Some aquatic simple animals excrete it is dissolved form into the surrounding water through the process of diffusion before it can build up to toxic levels in their tissues. But in complex terrestrial organisms, including humans, ammonia being very toxic, is converted to some less toxic nitrogenous wastes such as urea or uric acid.
Uric acid is produced both from ammonia and by the breakdown of nucleotides from nucleic acids. Uric acid is the least toxic and can be excreted in a crystalline form with a little use of water. This is an important adaptation for conserving water in many terrestrial animals such as insects, reptiles and birds.
In birds, the frequent excretion of uric acid in the faeces and the absence of urinary bladder keeps the body weight light which is very important for flight.
Urea is the principal nitrogenous waste product of amphibians and mammals. It is produced mainly in the liver. Urea is far less toxic than ammonia and so can accumulate to some extant in the body without causing tissue damage. Because urea is highly soluble, it is excreted dissolved in water. For its excretion, urea needs less water than ammonia but need more water than uric acid.
Excretion in amoeba
In amoeba and other unicellular freshwater animals contractile vacuoles are present. These contractile vacuoles are osmoregulatory in function as they are involved in the removal of excess water from the body. This water however also carries with it some of the ammonia and CO2 which are produced in metabolism. However, most of the ammonia and CO2 diffuse out of the body through cell membrane in the dissolved form (fig 13.2)
Excretion in planaria
Excretion in Animals – In planaria a clearly defined excretory system exists. It consists of two or more highly branched longitudinal tubules, some side branches of the tubules end in bulb-like cells into which long flagella project. These flagella, beating constantly assume the appearance of a flame and give the system its name- flame cell system. These flagella create continuous currents within the flame cell system and the currents carry
A tuft of flagella present in the flame cell moves in a way similar to that of a flickering candle’s flame, thus given the name “flame”
Fluid and waste materials out of the body through the excretory pores present on the general body surface (fig 13.3). In planaria, ammonia and extra water are eliminated through this system. CO2 is removed through the general body surface by diffusion.
Excretion in earthworm
In the earthworm the excretory system is more advanced and complex than that of planaria. Excretion is carried out by nephredia which are closely associated with its closed circulatory system. Each segment of the earthworm’s body contains a pair or in some species a cluster of nephredia. The body fluids from the body cavity enter a nephridium through the nephridium.
The nephridium then continues into a coiled tubule which is closely associated with blood capillaries. This allows a selective reabsorption of materials. The nephridium ends into a large bladder, which opens to the exterior by means of a nephridiopore. (Fig. 13.4). Water, urea and CO2 are the waste products in earthworm. Most of the CO2 diffuses out in dissolved state through general body surface.
Excretion in cockroach
The nitrogenous wastes, inorganic salts and CO2 are the major excretory products in insects like cockroach, grasshopper etc. these unwanted and poisonous substances are removed through different organs e.g. Malpighian tubules and tracheal tubules. The CO2 is eliminated from the body by means of tracheal system. Water, salts and nitrogenous wastes, chiefly in the form of salts of uric acid, are extracted by Malpighian tubules from the haeemolymph and pass them in the lumen of the tubules. From here the extracted materials move in the solution form called urine towards the ileum.
Water and certain inorganic salts are absorbed from the urine into the proximal part of the tubules. This action of reabsorption leads to the precipitation of the uric acid in the lumen of the tubules. As the urine moves on to the hinder part of the digestive tract, more water is absorbed from the precipitated mass so that solid uric acid is finally thrown out with the faeces.
The ability to conserve water in the excretory process in insects is highly adaptive and is one of the reasons for these insects for successfully invading dryland.
Malpighian tubules are very fine yellowish thread-like processes which open into the anterior end of the ileum. They are arranged in six groups each having about fifteen blind ended tubules which float freely in the haemolymph. Some uric acid is stored in special cells of the fat body and remain their throughout the life of the cockroach.
This is excretion by storage.
Excretion in Vertebrates
Carbon dioxide, mineral salts, urea, creatinin, uric acid, bile pigments and excess water are the waste products in vertebrates.
The kidney is an organ unique to the vertebrates. It is the chief excretory unit in higher vertebrates but its major function in the lower vertebrates (fish) is osmoregulation. In fishes the nitrogenous waste are mainly eliminated through the gills.
In mammals. Skin has sweat glands which produce sweat for cooling the body in hot weather.
In some marine reptiles, external devices have evolved to deal with the excretion of excess salts. These salts excreting glands are similar to those of birds.
This sweat has some quantities of certain waste products like urea, salts, etc. so skin also acts for excretion to some extent. The lungs as you know are involved in elimination of CO2 and water. Some excretion also takes place through intestine as in the amphibians. Main excretory role in reptiles, birds and mammals is however played by a pair of kidneys and their ducts.
Human Urinary System
The human urinary system consists of a pair of kidneys with attached ureters. The ureters from both sides open into the bladder which opens into the urethra. Each kidney is a bean shaped organ enclosed in a fatty membrane. They are found attached to the dorsal body wall within the abdominal cavity, one on each side of the vertebral column, just above the waist-line. The right kidney lies slightly anterior to the left (fig. 13.5). The concave side of the kidney faces the vertebral column. The depression in the center of this surface is called hilus. It provides a place for the renal artery, renal vein and nerves to enter and leave the kidney.
The longitudinal section of a human kidney shows that it mainly consists of two regions. The outer dark red region is called cortex. The inner pale-red region is called medulla. The medulla consists of several cone shaped areas called pyramids.
The pyramids project into a funnel-like space present in center of concave surface of kidney called renal pelvis.
A tube, called ureter, originates from the renal pelvis (of each kidney). The ureters from both the kidneys open in a bladder, called as urinary bladder.
The urinary bladder opens to outside through a duct known as urethra.
Each kidney is composed of large number of tiny tubes – the urinary tubules or nephrons. Each nephron further consists of a cluster of blood capillaries called glomerulus and a long tubule called renal tubule. The renal tubule is closed at one end, which enlarges into a cup-shaped structure called Bowman ’s capsule. The Bowman’s capsule is surrounding the glomerulus and both are collectively called renal corpuscle. F 13.6 &13.7.
Excretion in Vertebrates – The renal tubule of nephron is much longer and it originates from the bowman’s capsule and its wall is single cell in thickness. It is surrounded by a network of capillaries. Its first portion is twisted upon itself in a very complex way and the next portion is quite straight but u-shaped called loop of henle – the name after the scientist who discovered it. The last portion of the tubule is again convoluted and like the last portions of several other nephrons, it ultimately opens into larger duct called collecting duct.
Practical work: examination of a longitudinal section of a mammalian kidney or model of kidney:
- Teacher will make a sheep’s / goat’s kidney available in laboratory / classroom.
- Cut it longitudinally into two equal halves. Keenly observe with the help of hand lense and draw the cut surface.
- Use the similar colours (to the original) in order to differentiate between cortex, medulaa, pyramids and renal pelvis.
- Label the diagram.
- Describe briefly the function of ureter and urinary bladder
Function of Kidney:
The main function of the kidney is the urine formation which takes place in three steps:
In the first step, the process of pressure filtration takes place in the renal corpuscle. Tue to the blood pressure in the glomerulus, the part of blood (without R.B. Cs and plasma proteins) is filtered into the Bowman’s capsule and is called Bowman’s filtrate. It mainly consists of salts, glucose, urea and uric acid dissolved in water. From glomerulus it trickles down in the renal tubule.
In the second step, the process of selective reabsorption takes place by the network of blood capillaries which surrounds the tubule. Firstly, all the glucose with much of water is reabsorbed. Then some of the salts are sent back into the blood. In the third step the unnecessary salts together with urea and uric acid (nitrogenous waste products) with excess water (now called urine) travel down to pelvis of the kidney, from where it moves to bladder through the ureter.
Osmoregulatory Function of Kidney
The osmoregulation is the control of water and other ingredients (like salts and acids etc.) in the blood to maintain constant or nearly constant water potential in the body. In addition to the excretion, the kidneys also play an important function of osmoregulation. If the blood contains too much water, a small amount of water is reabsorbed from the renal tubules. While more water is left to enter into the bladder. In this manner a large volume of dilute urine is produced, and the human body gets rid of extra water and unnecessary salts and uric acid.
If the blood is much concentrated, more water is absorbed back into the blood from kidney tubules, and small quantity of concentrated urine is produced. The message for drinking more water is sent to hypothalamus, the thirst center in the brain, to restore the blood consistency to its normal concentration.
The above mentioned regulatory processes are collectively called osmoregulation, because the regulate the osmotic strength of the blood. The kidneys also regulate the acid-base balance of the body.