Expert: Dana Krempels, Ph.D. - 9/12/2007

Question
I can't believe I have to ask this. My daughter Leah has a homework question and I am not 100% sure that our answer is correct. We researched and can't really come up with anything else. Here is the question:

Although a sponge may look like a plant, what CAN'T it do that a plant can?

Our answer is that a sponge can move, but a plant cannot move on its own.

Is this correct?

Thanks!

Answer
Dear Jennifer,

That's not an easy question, actually!

As you know, a sponge is more closely related to animals (some include it in Kingdom Animalia, but there's argument about this) than to plants.  So it would share many more characteristics with animals than plants.

Sponges do not move except on a cellular level, but plants do that, too.  Plants actually do move their bodies in response to sunlight, water, and other stimuli.  So the answer about movement is a good guess, but not really accurate.

However, sponges are heterotrophs, which means they ingest other living organisms in order to obtain energy.  They have specialized cells (choanocytes, also called "collar cells") lining their internal cavities that set up water currents to pull water into their tiny canals, and food particles that enter with the water are filtered from the water, brought to the surface of the sponge, and ingested by specialized amoeboid cells that feed the rest of the cell types in the sponge.

Plants (with rare exceptions) do not eat other organisms to survive.  So my answer would be that sponges ingest other (microscopic) organisms to survive, whereas plants obtain their food via photosynthesis.

When you think about it, plants can do a lot more things than sponges can.  :)

Unlike true animals, however, sponges do not have true tissues, nor do they have a nervous system or muscular system.  This is why they were once classified as plants, before we could look more closely at their cells and find out that wasn't correct.

Hope this helps!

I'm curious to find out what your daughter's teacher thinks is the correct answer.  Please let me know, if you have time.  I'm just curious.  :)

Take care,

Dana