It is impossible to discuss living things without mentioning excretion. Living organisms have numerous chemical reactions occurring within them at all times. Inevitably, these reactions will produce substances that the organism doesn't want, or that may be harmful to it. So, excretion is the process by which living organisms eliminate unwanted waste products produced by metabolic processes. This may sound similar to egestion, which is the process by which undigested material is evacuated as a solid via the anus, however, they are two completely different processes. Egestion doesn't get rid of any metabolic wastes, only solid undigested material from digestion. Therefore, it cannot be classified as excretion.
Why is excretion important?
Well, without excretion to get rid of waste, the waste would build up within the organism. Most waste products produced are harmful or toxic to the organism, so if they build up within a cell, they will end up killing the cell. For example, ammonia is removed from the body by the synthesis of urea by the liver. Ammonia is very toxic and if not rapidly and efficiently removed from the circulation, will result in central nervous system diseases. Humans and every other organism excrete carbon dioxide as a waste product of respiration. Humans do so through exhalation.
Different organisms have different mechanisms by which they excrete metabolic waste. CSEC requires that you are familiar with those mechanisms in plants and humans.
Excretion in Plants
Plants, unlike humans and higher animals, lack a specialized/well developed excretory system. However, plants do need to excrete certain products, such as:
the oxygen produced as a waste product in photosynthesis is excreted in the daytime when photosynthesis occurs faster than respiration.
the carbon dioxide produced as a waste product in respiration is excreted at night when photosynthesis has stopped occurring or is occurring at a slower rate than respiration. (we discussed this in a previous post on gaseous exchange in plants)
water, produced in respiration and excreted as water vapour at night when photosynthesis is not occurring.
organic waste products, including tannins, alkaloids, anthocyanins and the salts of organic acids like calcium oxalate.
Oxygen, carbon dioxide and water diffuse out through the stomata of leaves and the lenticels of bark covered stems and roots. Organic waste products, however, are usually stored in dead tissue like heart wood. Some plants convert them to insoluble substances (so that they cannot affect metabolic processes and osmotic pressure) like oils or crystals (such as calcium ions combining with the organic acid oxalic acid to form calcium oxalate crystals). These crystals and insoluble substances are stored in the cells of leaves, bark, petals, fruits and seeds, which are removed once the plant sheds those structures.
Excretion in Animals
Animals tend to have well developed excretory systems. They produce several waste products during metabolism, including:
carbon dioxide, produced in respiration
water, produced in respiration
nitrogenous compounds, produced by the deamination of amino acids by the liver for storage, such as urea, ammonia and uric acid
bile pigments (like bilirubin) produced from breaking down haemoglobin from red blood cells by the liver
heat, produced from metabolism in general
These waste products are evacuated by specific organs in humans:
The skin excretes water, salts, some urea and heat.
The kidneys get rid of water, nitrogenous wastes and salts as urine.
The lungs excrete carbon dioxide (and some water vapour) during exhalation/expiration.
The liver excretes bile pigments and makes nitrogenous waste