Excretion in living organisms
When nutrients are broken down in cells, waste substances are formed as well as energy. If they build up for long, the body poisons itself, so they are removed. Removing the waste products of metabolism from the body is called excretion. In humans and animals metabolism produces poisonous substances such as ammonia, urea and uric acid. They leave, along with excess water and mineral salts, through the kidneys as urine and through the skin as sweat; carbon dioxide leaves through the lungs. The kidneys filter the blood; the excretory system consists of the kidneys, ureters, bladder and urethra. Plants give out carbon dioxide as a waste product of respiration and oxygen as a by-product of photosynthesis, and release excess water as vapour through stomata. In autumn a plant gets rid of harmful substances stored in leaves by shedding them (leaf fall).
Observation: collect 5 kinds of fallen leaves from trees and shrubs in autumn and compare them in class (colour, shape, veins). Draw the separating layer at the place where the leaf stalk joins the twig. Extra: put two different twigs in water-filled jars, pour a drop of vegetable oil on the water (so it cannot evaporate), mark the level and measure which twig gives off more water in a day. Safety: do not eat unknown plants or fruit, wash hands, and do not touch poisonous (milky-sap) plants.