While the life processes are running inside a body, some substances are made that the body cannot use. If those stay inside they build up into harmful material and do damage. Some other substances are made in larger amounts than the body needs, and the extra has to go. Throwing these unnecessary substances out of the body is called excretion. Both plants and animals excrete, in different amounts and in different forms.
Remember what was said about excretion in the first topic of this unit. It is the odd one out among the life processes, because when it fails nothing is missing at all. The body has food, air and water, and it is poisoned anyway, by something it made itself. That is what makes waste dangerous and it is why every living thing has to deal with it.
Those drops on the grass are not dew
The textbook opens by asking where the water drops on the edges of leaves come from in the morning, and most people answer dew. Sometimes it is dew, but look carefully at the picture. Real dew settles on everything, evenly, wherever air touches a cold surface. These drops are sitting in neat regular positions along the edges and at the tips of the blades, which is not how dew behaves.
They came out of the plant. Leaves have small openings near their tips called hydathodes, and excess water is pushed out through them as liquid drops. This process is called guttation. It usually happens at night or in the early morning, and the reason is worth understanding, because it is the one time a plant really does push.
In the last topic you learned that water is pulled up a plant, never pushed, because it evaporates from the leaves and drags the column behind it. At night the stomata close and the air is damp, so there is almost no evaporation and therefore almost no pull. But the roots keep taking in water anyway, and with nothing leaving at the top the pressure builds. That gentle push from below forces water out of the hydathodes as drops. So guttation is not a contradiction of what you learned. It is what happens on the one occasion when the pull stops.
What a plant throws out, and how
| What is excreted | How it leaves the plant |
|---|---|
| Oxygen | Made during photosynthesis. The plant uses some of it for its own respiration and releases the rest into the atmosphere. |
| Carbon dioxide | Made during respiration. The plant does not need it, so it is released through the stomata. |
| Excess water, as vapour | Pushed out through the stomata as vapour. This is transpiration, and it is the same process that pulls water up the plant. |
| Excess water, as drops | Forced out as liquid drops through small openings near the leaf tips called hydathodes. This is guttation. |
| Resin, latex and oily substances | Some plants excrete these. They are stored inside the plant and released slowly, or when the plant is cut. |
Notice one line in the textbook that is easy to read past: plants do not have special excretory organs like the ones animals have. A plant has no kidney and no bladder. The obvious explanation is that a plant is simpler than an animal, and that explanation is wrong. The real reason is far more interesting.
Why a plant needs no kidney
A plant has three ways of dealing with waste that are simply not available to an animal, and once you see them the missing kidney stops being a puzzle.
| What a plant can do | Why an animal cannot |
|---|---|
| Store waste in a part it is going to throw away anyway. Substances are collected in old leaves and in bark, and then the leaves fall and the bark peels, taking the waste with them. | An animal cannot throw away a leg every autumn. Every part of an animal body is in use all the time, so there is nowhere to put anything permanently. |
| Turn waste into something harmless and simply leave it there. Resin, latex and gum are locked away inside the plant where they do no damage. | An animal is solid, living tissue throughout, and blood reaches every part of it, so nothing can be walled off and forgotten. |
| Make far less waste in the first place, because it does not move. A plant burns food slowly, so it produces waste slowly. | An animal moves, and moving burns food fast. Burning food fast makes waste fast, and fast waste needs a proper organ to remove it. |
So a plant has no kidney because it does not need one, and not because it is a lesser kind of living thing. The absence of an organ is often the most interesting fact about an organism, because it tells you what problem that organism never had.
Excretion in animals, from the simplest upwards
Animals have an excretory system, but not every animal has the same kind of organs. The pattern is the same ladder you met in the last topic, and it is driven by the same thing: distance.
| Animal | How excretion happens |
|---|---|
| Unicellular animals | Through the cell membrane. Useless substances pass out through the membrane by diffusion, with no organ of any kind. |
| Insects such as a grasshopper | Through a tube called the malpighian tubule, which collects waste and passes it out of the body. |
| Humans and other developed animals | Through several organs at once: the skin, the lungs and the kidneys, each removing a different kind of waste. |
Three organs, three different wastes
In a human being the work is shared out, and it is worth noticing that each organ deals with a different kind of waste rather than all of them doing the same job.
| Organ | What it excretes | Worth knowing |
|---|---|---|
| Skin | Sweat, which contains excess water, salt and urea, excreted by the sweat glands. | Sweating does two jobs at once. It removes waste and it also cools you, because evaporating water takes heat with it. |
| Lungs | Carbon dioxide, produced during respiration. | Every breath out is an act of excretion, which surprises people who think excretion only means urine. |
| Kidneys | Urine, which carries away excess water, salts and harmful chemicals such as urea and uric acid. | The kidney is the main organ of the urinary system, and it works by filtering the blood rather than by collecting waste from the gut. |
The urinary system
In the human body the urinary system plays the main role in excretion. It is made of two kidneys, two ureters, one urinary bladder and one urethra. The kidney filters excess water, salts and harmful chemicals such as urea and uric acid out of the blood in the form of urine. The urine is then carried by the ureters to the urinary bladder, and the urine collected there passes out of the body through the urethra.
The word to hold on to in that paragraph is blood. A kidney is not a bin that collects rubbish from the gut. It is a filter placed in the bloodstream, and all the blood in your body passes through it again and again, all day, being cleaned a little each time. That is why a kidney has to be connected to the blood supply and why kidney failure poisons the whole body rather than just one part of it.
This is the mistake that costs the most marks in this topic. Undigested food leaving the body is not excretion. That material was never absorbed into the body at all. It passed through the gut, which is really a tube running through you, and it was never part of the chemistry of any cell. Excretion means getting rid of substances the body itself made while its life processes were running, such as carbon dioxide from respiration and urea from breaking down proteins. If a substance was never inside a cell, throwing it out is not excretion.
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Excretion means throwing out of the body the unnecessary substances made while the life processes are running.
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