Raven pins a list to the cupboard beside the Windowsill. "Scientists use eight characteristics to define life," she says.
Rocket reads them aloud, fast. "Order, response, reproduction, adaptation, growth, regulation, energy processing, evolution. Done."
"Slow down," Raven says. "What do you notice? Yesterday we used four. Today there are eight."
Nova dims her light so the list is easier to read. "Some of the eight are big ideas," she says. "Others you can test at a windowsill."
Rocket taps the word response. "A plant bending toward the window. I have seen that!"
"Then you have already seen a living thing respond to a stimulus," Raven says. Nova hums. "Now find the others."
Concepts of Biology says all living things share eight key characteristics. Taken together, these eight define life.
Order means a living thing is highly organized and made of one or more cells. Even a single cell is remarkably complex inside.
Response means living things react to a stimulus. Plants can bend toward light or respond to touch. Some bacteria move toward or away from chemicals.
Reproduction passes DNA, the genetic material, to offspring. That is why offspring belong to the same species and resemble their parents.
Growth and development follow instructions coded in genes. Energy processing means every organism uses a source of energy.
Some organisms capture energy from the Sun and store it in food. Others use chemical energy from molecules they take in, which means eating.
| Characteristic | In plain words | A windowsill example |
|---|---|---|
| Order | Organized, made of one or more cells | A leaf has a pattern of veins and cells |
| Response to stimuli | Reacts to light, touch or chemicals | A plant bends toward the window |
| Reproduction | Makes offspring, passing on DNA | A plant makes seeds |
| Growth and development | Gets bigger and changes by its genes' instructions | A sprout grows taller each day |
| Energy processing | Uses a source of energy | A plant uses sunlight; a person eats lunch |
| Regulation | Keeps steady conditions inside | A body stays warm on a cold day |
| Adaptation | Fits its environment | A cactus has spines and a thick stem to save water |
| Evolution | Populations change over many generations | Too slow to watch at a windowsill |
In 1665 a scientist named Robert Hooke looked at a thin slice of cork through a lens. He saw rows of tiny box-like spaces.
He called them cells, from the Latin word cella, which means small room. The name stuck.
In the 1670s a Dutch lens maker, Antony van Leeuwenhoek, saw bacteria and other single-celled living things for the first time.
By the late 1830s two scientists, Matthias Schleiden and Theodor Schwann, proposed the cell theory. It has three parts.
All living things are composed of one or more cells. The cell is the basic unit of life. New cells arise from existing cells.
| Statement | True or false? |
|---|---|
| Scientists use eight characteristics to define life. | ? |
| Hooke saw cells in a slice of cork. | ? |
| Only animals respond to stimuli. | ? |
| Some organisms get energy from the Sun and others from molecules they take in. | ? |
Strong thinking. Tomorrow is Windowsill Lab: you plant the seeds this course will measure for twelve weeks.