The Characteristics of Living Things
In a milpa (a traditional cornfield) live a corn plant, an earthworm in the soil, and a fungus growing on an ear of corn. They look nothing alike on the outside, and yet all three are living things. What do they have in common that a rock from the same field 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 inside of your cheek under a microscope, in all three cases you discover tiny independent compartments. What unit do such different living things share?
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.
Life processes
These processes are not a whim: each one solves a problem for survival. Without nutrition there is no matter or energy to function; without interaction with the environment, an organism cannot flee danger or find food; without reproduction the species disappears. That is why living things have specialized structures for each one, and these structures are adaptive traits: characteristics that help the species survive.
| Process | Problem it solves | Structures and adaptive traits (examples) |
|---|---|---|
| Nutrition | Getting matter and energy | Leaves with chlorophyll that capture light; mouth, teeth, and stomach; roots |
| Interaction with the environment | Detecting dangers and resources | Ears, eyes, antennae; leaves that close when touched (the sensitive plant) |
| Reproduction | Leaving descendants | Flowers, seeds, eggs, offspring |
In the diagram, each process has a letter (A, B, and C) as well as its name, so you can refer to it.
Two ways of getting nutrients
All living things need matter and energy, but they do not get them in the same way.
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Autotrophic nutrition (plants and algae): they make their own food. Using sunlight as the energy source, they take water and mineral salts from the soil through the roots and carbon dioxide ($\text{CO}_2$) from the air through the leaves, and with them they build their matter. This process is photosynthesis and it releases oxygen. Fertilizer and soil only provide water and mineral salts; they are not the plant's "food."
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Heterotrophic nutrition (animals, fungi, and many bacteria): they cannot make their own food, so they consume matter from other living things or their remains. A deer eats branches and leaves; from them it gets matter to grow and energy to move.
That is why a milpa depends on sunlight, and a deer depends on plants.
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.
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.
A classmate makes the following statement: "Plants like the pine tree feed on soil and fertilizer exactly the same way a deer feeds by eating branches; the only…
A classmate makes the following statement: "Plants like the pine tree feed on soil and fertilizer exactly the same way a deer feeds by eating branches; the only difference is that the deer moves around to look for food and the pine tree does not."
- a) Refute their statement by explaining where each of these two living organisms actually obtains matter and energy.
- b) Predict what would happen to the mass of the deer and the biomass of the pine tree if they are enclosed together in a large transparent dome where no matter other than water enters from the outside, but sunlight does. What if we cover the dome with an opaque tarp that completely blocks light? Justify the ecological role of each in both cases.
Is it a living thing or does it just look like one?
A flame consumes wood to grow, sways in the wind, and throws sparks that can ignite dry leaves. Why doesn't science consider it a living thing if it seems to feed, react, and multiply?
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.
Doubtful cases: viruses and machines
Viruses are a borderline case. They are not made of cells and on their own they do not get nutrients or respond to the environment; they only multiply when they enter a foreign cell, using its machinery. That is why many scientists do not consider them complete living things.
A robot vacuum, on the other hand, moves, avoids obstacles, and "feeds" on electricity, but it has no cells, makes no matter of its own, and does not reproduce: it imitates some traits of life without meeting them all.
To decide any doubtful case, check the traits one by one: cells, chemical makeup, and the three life processes.
Is a virus a living thing? Justify your answer considering which life processes it can carry out and which it cannot (viruses can only replicate when inside a host cell).
Is a virus a living thing? Justify your answer considering which life processes it can carry out and which it cannot (viruses can only replicate when inside a host cell).
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.
An archaeology team has found wheat grains in a sealed jar dating back 3000 years. A member of the team states: "They are completely dry, they neither move nor grow, so…
An archaeology team has found wheat grains in a sealed jar dating back 3000 years. A member of the team states: "They are completely dry, they neither move nor grow, so they are inert matter, exactly like grains of sand."
- a) Design an experimental test to find out whether these seeds are still alive without having to wait weeks to see if a complete plant develops.
- b) Several seeds are placed in an airtight jar with sensors at 20 °C, and it is found that the oxygen level drops from 20.9 % to 20.4 % in 48 hours, while carbon dioxide increases by 0.5 %. Interpret this result: what life process is being detected at the cellular level and what does it prove about the dormancy state of the seeds?
Back to the beginning: the corn, the earthworm, and the fungus in the milpa share being made of cells, having a chemical makeup based on C, H, O, N, and water, and carrying out nutrition, interaction with the environment, and reproduction; the rock lacks cells and life processes.