Science Buddies: "Ask an Expert"

We were having a lively discussion in our biology class about the defining characteristics of living things, and the question was posed, "Are there any things that are considered living that are incapable of movement?" For instance are there cellular organisms, or cells from tissues, that if isolated from outside sources of movement, would remain stationary? Is movement limited to exterior movement or does the internal movement within the cell constitute movement? We could not find any official definition of what constitutes movement in scientific terms or if there were cells or organisms that are classified as living that cannot move.

Hi,

Interesting subject. My definition of movement would have to include specificity--movement in response to a perceived stimulus. Most plants don't walk around but they do move in response to light and gravity.

Most of the cells that make up the tissues in our bodies do not move from one location to another, but the cell membrane is capable of extruding parts of itself, which I suppose you could call a form of movement.

What about molecular movement? Ion channels in the membranes of cells open and close in response to specific substances. Would you call that movement? The stomatal cells in plant leaves open and close in response to changes in light and temperature.

Many bacteria like E coli and Salmonella have flagellae that they use to move, but there are some that do not like Staphylococcus, and these are classed as nonmotile. They are alive, but incapable of purposeful movement--as far as we know.

I hope this adds something to your discussion. Keep us posted on your exploration of this interesting subject.

Sybee

Thanks! That was actually very helpful.

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