The characteristics of living things
In the school yard there is a mango tree, an earthworm under a rock, and a mushroom on a fallen log. They look nothing alike on the outside, but all three are living things. What do an earthworm and a mango tree have in common that a rock does not?
Made of cells and the same elements
If you look at a drop of pond water, a piece of onion, and a scraping from the lining of your cheek under a microscope, in all three cases you will see tiny little compartments. What unit do such different living things share?
Besides cells, all living things share the same bioelements: the chemical elements that make up living matter.
The human body is roughly 60% water, and a head of lettuce is more than 90% water.
List the four most abundant bioelements in living things and explain what it means that all living things share a common chemical composition.
List the four most abundant bioelements in living things and explain what it means that all living things share a common chemical composition.
An earthworm, an oak tree, and a mushroom have very different sizes and shapes. State at least three shared characteristics that prove all three are living things.
An earthworm, an oak tree, and a mushroom have very different sizes and shapes. State at least three shared characteristics that prove all three are living things.
Water is the most abundant substance in most living things. Explain what this statement means and give two examples that show it (for example, what approximate…
Water is the most abundant substance in most living things. Explain what this statement means and give two examples that show it (for example, what approximate percentage of the human body is water, or what happens to a lettuce when it dehydrates).
The three life functions
Let's look at each one in more detail:
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Nutrition. All living things need matter and energy. Plants make their own food using sunlight (photosynthesis). Animals eat other living things. Thanks to that energy you can run, study, or play baseball.
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Interaction. Sensing a stimulus (a change in the surroundings) and giving a response. Animals use the nervous system; plants also respond, but without one.
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Reproduction. Producing new individuals. It can be asexual (a bacterium divides in two) or sexual (a dog has puppies after mating). In both cases, hereditary information is passed on to the offspring.
Two processes are also linked to nutrition. Growth is an increase in size and in the number of cells. Development is the change in form and structure over a lifetime, like the tadpole that loses its tail and develops legs and lungs until it becomes a frog.
Look at the diagram: a living thing carries out three life processes, labeled with letters A, B, and C. Name the life process represented by each letter, briefly…
Look at the diagram: a living thing carries out three life processes, labeled with letters A, B, and C. Name the life process represented by each letter, briefly describe what it involves, and give an example of an animal or plant carrying it out.
State which life process each of these belongs to: a plant makes its food using light, a rabbit flees when it hears a fox, a pair of storks raises their chicks, and a…
State which life process each of these belongs to: a plant makes its food using light, a rabbit flees when it hears a fox, a pair of storks raises their chicks, and a sunflower turns its flower towards the Sun.
A cat that hears a noise turns its ears towards it and freezes. Identify the stimulus and the response, state which life process it is carrying out, and explain why this…
A cat that hears a noise turns its ears towards it and freezes. Identify the stimulus and the response, state which life process it is carrying out, and explain why this is useful to the cat.
Is it a living thing or does it just look like one?
A flame consumes firewood to "grow," sways in the wind, and throws off sparks that can start another fire. Why doesn't science consider it a living thing if it seems to feed, react, and reproduce?
To decide, check the criteria one by one using a table:
| Candidate | Cells? | Bioelements and molecules of life? | Real life functions? | Alive? |
|---|---|---|---|---|
| Mangrove | Yes | Yes | Yes | Yes |
| Fire | No | No | No (it only burns fuel) | No |
| Salt crystal | No | No | No | No |
| Robot vacuum | No | No | No (it uses a battery, it does not feed) | No |
| Virus | No | Yes (nucleic acid and proteins) | It only multiplies inside a host cell | Borderline case: not considered a living thing |
Fire "grows", moves, "feeds" on firewood, and can "reproduce" in sparks that light another fire. Explain why, even so, nobody considers it a living thing.
Fire "grows", moves, "feeds" on firewood, and can "reproduce" in sparks that light another fire. Explain why, even so, nobody considers it a living thing.
A salt crystal "grows" if more salt is added to its solution. Explain why this growth does not make the crystal a living thing, pointing out at least two differences…
A salt crystal "grows" if more salt is added to its solution. Explain why this growth does not make the crystal a living thing, pointing out at least two differences compared to the growth of a plant.
A robot vacuum cleaner moves around the house, avoids obstacles, and returns to its dock by itself to recharge its battery. Check each characteristic of living things…
A robot vacuum cleaner moves around the house, avoids obstacles, and returns to its dock by itself to recharge its battery. Check each characteristic of living things one by one and conclude, providing arguments, whether the robot could be considered alive.
Back to the beginning: The earthworm and the mango tree both are made of cells, have a composition based on C, H, O, N and a lot of water, and carry out nutrition, interaction, and reproduction. The rock has no cells or life functions.