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Biology
Excretion and Osmoregulation
A waste product during respiration is carbon dioxide, which is made in cells and is called an excretory product. The
removal of excretory products is called excretion.
Some of the food animals eat cannot be digested. The cellulose in our food goes straight through the alimentary canal
and out of the anus as part of faeces. This cellulose is not an excretory product because it has never been involved in any
metabolic reaction inside a cell. It simply passes unchanged through the digestive system and this is called egestion.
Excretion in Plants
Plants are not as metabolically active as animals and do not produce a great quantity of waste products. There are 2
main ways plants get rid of these products. Some literally leave the plant and others are stored in a safe place:
Carbon dioxide from respiration and oxygen from photosynthesis are removed trough the leaves via the stomata.
Leaves sometimes change from green to brilliant colours (yellow, red, etc) before the fall from the tree. These colours
are due to waste pigment such as calcium oxalate (crystals) in the leaves that the plants remove.
A build up of waste products can become poisonous, any which remain must be stored in a safe chemical form. Some
are changed into insoluble substances like calcium oxalate, oils and alkaloids and kept in living cells. Some oils are used
such as citrus oils and pimento, etc.
Excretion in Animals
Animals produce nitrogenous waste which is formed from excess proteins and amino acids. The three forms are
ammonia, urea (mammals), and uric acid (birds and reptiles) which is the least toxic.
When we digest amino acids, its taken directly to the liver. The liver allows some of the amino acids to circulate in the
blood for parts of the body which may need it. It would be very wasteful to excrete the extra amino acids just as they
are. They contain energy which can be stored for later.
Some enzymes in the liver split up each amino acid molecule. The part containing the energy is kept, turned into
carbohydrate and stored. The rest, which is the part that contains nitrogen, is turned into urea. This process is called
deamination. The urea is sent to the kidneys to be excreted.
The kidney is made up of three parts: the cortex, medulla and pelvis. Leading from the pelvis is a tube called the ureter.
The ureter carries urine from the kidney to the bladder. Kidneys are made of thousands of tiny tubules or nephrons.
The Kidney: Filtration
Each Bowman’s capsule is shaped like a cup and it has tangled blood capillaries called a glomerulus in the middle. The
blood vessel bringing blood to the glomerulus is quite wide, but the one taking blood away is narrow. This mechanism
creates a high pressure squeezing the blood in the glomerulus against the capillary walls. The capillary walls and the
Bowman’s capsule have small holes in them to allow only small molecules (water, salt, glucose and urea) to pass through
and enter into the tubule. Most proteins and blood cells are too big and remain in the blood.
The Kidney: Reabsoption
The fluid in the Bowman’s capsule is mainly a solution of glucose, salt and urea dissolved in water. Wrapped around
each kidney tubule are blood capillaries. They reabsorb these useful substances from the tubules in the loop of Henle.
The remaining fluid continues on its way to the collecting duct where it is mostly water, urea and some salt and this fluid
is called urine. The urine is collected in the bladder and excreted from the body via the urethra.
Osmoregulation
Organisms must have ways of keeping the fluids inside their bodies at about the same concentration all the time.
Osmoregulation is the balance of the amount of water and salts in the body.
ADH (Anti-diuretic hormone)