Excretion in plants

Plants excrete less poisonous waste products like CO2 through the stomata and acids through dropping leaves and fruits. Plants do not require specialized excretory organs due to;
i) Plants can store excess proteins unlike in mammals.
ii) They accumulate less metabolic wastes due to their low metabolic rate.
iii) Plants synthesize their organic food substances according to their requirements. This ensures that no excess is made.
iv) Plants do not produce nitrogenous waste products. They produce non-nitrogenous wastes, which are less toxic to their bodies.
v) Some wastes accumulate in particular parts of the plant and they are eliminated when this part of the plant falls off.
vi) Some of the wastes are useful in other processes within the plants body. For example Carbon dioxide produced from respiration can be used in photosynthesis.
vii) They do not locomote and they are less metabolically active than animals.

This is the maintenance of a constant internal environment of the body. The internal environment of the body is composed of tissue fluids, which surround cells. Homeostasis involves controlling the blood sugar level, salt level, water level, temperature and Carbon dioxide concentration

It is the maintenance of blood concentration constant.
This is the control of the amount of water in the body.
The water level is kept neither high nor low but within a limit according to the demands of the body.
The level is maintained by loss of excess and gain if more is required.
Water is lost from the body through urine, sweat, expiration, and feaces during egestion and it can be gained through; drinking eating and water from metabolism.
The loss and gain of water brings about changes in blood concentration.

These changes are detected in the brain by the hypothalamus. If the blood passing through the brain is too concentrated, the hypothalamus stimulates the anterior lobe of the pituitary gland to secrete a hormone called antidiuretic hormone (ADH) into the blood stream. When the hormone reaches the kidneys, it causes the walls of the nephrones (distal convoluted tubules and collecting ducts) to become permeable to water and water is reabsorbed from the glomerular filtrate back into the blood. The urine that is secreted becomes more concentrated and yellowish in colour. This reduces the loss of water in urine.
If blood passing through the hypothalamus is too dilute, the production of ADH from the pituitary gland stops and the nephrones become less permeable to water. Less water is therefore reabsorbed from the glomerular filtrate resulting into production of colourless urine in big volumes. This mostly happens during cold conditions where water loss through sweating is minimal.
When conditions are hot, sweating increases, lowering the water level in blood. This causes more re-absorption of water in the nephrones resulting in production of concentrated pale yellow urine.
Because of the high concentration, when urine is poured on grass or any plant, they get scotched because the cells lose water to the surrounding concentrated urine and the plant cells become flaccid. This brings about wilting and drying of the plant.
When the level of water in blood is too low the hormone causes a feeling of thirst, which makes one to drink water in order to bring back the normal water level in blood.
Failure of organisms to secrete ADH leads to constant urination of large amounts of dilute urine thus increases the blood concentration, a condition known as diabetes inspidus.


Amoeba excretes excess water by use of a contractile vacuole. The contractile vacuole is a small sac-like structure lying inside the cytoplasm.
The cell membrane surrounding amoeba is semi-permeable and since the concentration of the cytoplasm is higher than that in the environment surrounding amoeba, water molecules move by osmosis from out into the cytoplasm of amoeba. The organism uses some of the water and excess is secreted into the contractile vacuole, which is formed in the process. As the
vacuole enlarges, it moves towards the cell membrane and finally fuses with it. It then bursts to release the excess water out. A new vacuole is formed when the organism is excreting more water.


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Excretion in insects is carried out by structures called Malpighian tubules, which are found between the mid gut and the rectum of the insect’s alimentally canal.
Insect tissues produce nitrogenous wastes in form of potassium urate, which is liberated into the blood stream and taken to the malpighian tubules. In the tubules, urate reacts with Carbondioxide and water to form uric acid, which is released out of the body along with feaces.

Diagram showing the position of the malpighian tubules

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If the salt levels are high in blood, the blood concentration increases, this is detected by the hypothalamus as blood flows through it. It then instructs the pituitary gland which then instructs the adrenal gland to stop the production of aldosterone thus little or no salts get reabsorbed back into the blood with in the nephrons.
If the salt levels are low in blood, the pituitary gland instructs the adrenal gland to release aldosterone hormone which increases salt re absorption during urine formation leading to dilute urine.

Blood sugar is called glucose. Its concentration is controlled by a section of the pancreas called islets of Langerhans. This gland regulates responding organs mainly the liver and muscles through its secretions.

Importance of blood sugar regulation

  1. It prevents cells running short of glucose in case its level drops. Blood sugar (glucose) is the main source of energy.
  2. Any slight increase in glucose level alters the concentration of blood’s osmotic pressure, which results in alteration of the rate at which water moves in and out of the body cells by osmosis.
    Blood glucose concentration is controlled by the pancreas. The pancreas has glucose receptor cells which monitor the concentration of glucose in the blood, and it also has endocrine cells (called the islets of Langerhans), which secrete two hormones. The alpha cells (α cells) secrete a hormone called glucagon, while the beta cells (β cells) secrete a hormone called insulin. These two hormones are antagonistic, and have opposite effects on blood glucose.

Mechanism of blood sugar regulation
After a meal of carbohydrates, glucose is absorbed from the gut into the hepatic portal vein, increasing the blood glucose concentration. This is detected by the pancreas, which secretes insulin from its beta cells in response.

Insulin causes glucose;

  • To be taken up by the liver and converted to glycogen and stored there.
  • To be converted into fats. Fats are stored in adipose tissue.
  • To be broken down to release energy at higher rate. This energy is stored in a form of high energy compound called ATP. This reduces blood glucose in excess.

Once the concentration of blood glucose is lowered to a normal level, the pancreas stops secreting insulin.
If the glucose level falls too far for example during starvation of fasting, the pancreas detects this and releases glucagon from its alpha cells.

Glucagon causes;

  • Liver cells to convert stored glycogen into glucose.
  • Fats in adipose tissue to be converted to glucose
  • The rate of oxidation of glucose to slow down.
  • This raises the blood glucose concentration to approximately normal level. Once this happens, the pancreas stops producing glucagon.
  • Failure to produce insulin causes the presence of much glucose in urine a condition known as diabetes mellitus.