Food and Energy

A Multi-Part Investigation


Part I: who eats whom?


Y'know, I thought I saw a fox this morning.

What do foxes eat around here?

Hey, I have an idea:

Get together in small groups and write down everything you can think of that foxes might eat in southern Indiana.

Now, let's collect all of our data: write the names of these animals on the board.

That's a lot.

Hmm...what else might eat these things?  Let's write these animals down, too.

This gives us many more animals.

Hmm...what might the animals we thought of as "fox-food" eat?

Etc etc--we can keep expanding this food web using completely realistic information, tying into students' existing knowledge. 


-  Note that we eventually get to plants as the base of the food web. 

-  Note also that this is not a food chain--it's too complicated, with too many interactions.  This is the way it really is in nature.  If students develop their own understanding and recognition of this kind of complexity, then they will be better able to recognize the simplifications when we present them.

-  Do we dare draw a diagram of this?  If so, do we connect the animals' names with arrows or just with lines?  If we use arrows, what do the arrows mean?  The tradition is to have the arrows point from prey to predator, indicating the flow of energy from prey to predator--but our students don't have the concept of "food as energy" yet!  Maybe it's best not to use arrows yet.


So, out there, all this stuff is going on at once?  Yikes!

            But, we can summarize:

Animals eat plants


Animals eat animals that eat plants


Animals eat animals that eat animals that eat plants


What does this tell us?

All of life depends on plants!

- if humans disappeared from the planet tomorrow, nothing would happen

- if plants disappeared, everything else would die

            apparently, plants are much more important than we are.



Part II: Plants--The Base of the Food Web


What do plants eat?

I mean, what's "food" to a plant?

We could do an experiment to find out!

For now, let's pretend we did it.

The Experiment:

Germinate seeds ("wheat berries" would be good) in transparent plastic cups, with seeds pressed against side with moist paper towels.

This shows seeds carry their food with them.

Before we can go on, we have to allow the seedlings to exhaust the food that was packaged into the seed.  Otherwise, we can't be sure that we're actually testing the variables we're varying.

Now test variables: water them with or without "plant food" soil fertilizer, or dont water them, or put them in total darkness.  To test "air," we could fill the cup completely with water (ie, drown the seedlings).

            This shows they need minerals, light, and air.


We know from our experiment:

Plants need water, minerals, and the nitrate, phosphate, and such that are in the plant food.  (This explains why they call it plant "food.")

Air?  Something in air seems to be necessary as well, although we can't tell exactly what from this experiment.

Light?  Apparently light is needed as well--but our experiment will probably have shown us that darkness for a short time changes the way the plants grow.  They will have become long and spindly, as if trying to search for light.  It may take rather a long time for them to die outright.

[how else do we know plants care about light?  The phototropism expt.]


These things we've discovered to be various forms of plant "food" are  really different from animals' food.  Animals eat plants; plants eat small molecules.


What does this tell us?

Well, from our prior discussion, plants turn this stuff into animal food! (i.e. into plant)


            ** chemicals we can't eat -----> different chemicals we can eat

                        small molecules                       large molecules


[suggests complex chemistry...we won't go into any details, but it is clear that we have to refer to molecules and basic chemical concepts.]


what's with the light?

Right now, based on the data we have, we don't know.  Rather than just tell our students, it would be much more fun to figure it out.  We can do this later, or we can start now by thinking about it and considering different ideas.  What might they be?  What can we come up  with?  This is a good place for small-group discussions.

Since we know the answer, we can eventually lead our students to the term "energy" ... but energy is a difficult concept when considered as chemistry.


- Note that our students may key on other aspects of light, and may imagine that plants  need light to be warm, or that plants sleep when it's dark so they can't grow...


In the end, we'll have to think of another experiment to figure this out.  This is parts IV and V, which we could do next, or later.


In the meantime, these two Parts of this group leave us with a couple of questions:

1.  What do plants do with light?

2.  What do animals do with food?


Part III:  What do Animals Do With Food?


What does everyone already know about food?

We eat it

Some is pooped out

If we don't get enough we get hungry

            "          "          ...we lose weight

            "          "          ...we get tired

            "          "          ...we could die

If we get too much we get fat


OK, we have some information already.  That is, students' previous knowledge is valid information.  We should use this information--these observations--to develop understanding. 


What does this information tell us?

Food becomes part of our own bodies.  It is ...

            ... needed for young people to grow

            ... needed for adults to maintain their bodies

            ... and it provides energy

It looks like we actually know a lot already.


But...what's this energy bit?  I wonder how this works...


Some classrooms have animals and facilities that allow them to do the complete experiment to find out what happens to food.  Some do not.  Fortunately, the experiment has been performed and published:


In 2000, Knopper and Boily measured lots of stuff for hamsters:

They measured:

Food eaten

Material excreted

Change in body mass


They found:

22% of food eaten is excreted

78% of food eaten is actually used

0.7% of food eaten (and not excreted) appears as increase in body mass

what happens to the remaining 99.3%?

It seems to disappear.


To understand where it goes, we need to think about the chemistry a little bit.  It's not hard.  Let's summarize what we just learned from the data, and then just add a tiny bit to it (i.e., the words "converted to CO2 and water") to indicate the chemistry.



What does this tell us?

Look back at our list of things we know already.

Growing, maintaining adult weight, etc fall under "body mass" part of this diagram.

The other important function of food, providing energy, must be related to the food that does not become body mass.  Therefore, we have to conclude that:


            Animals destroy most of their food to extract energy from it

            And, it seems that destroying food is a chemical process that converts it to CO2 and water.


The reference:

Knopper and Boily, (2000). The Energy Budget of Captive Siberian Hamsters, Phodopus sungorus, Exposed to Photoperiod Changes: Mass Loss Is Caused by a Voluntary Decrease in Food Intake.  Physiol. Biochem.  Zool. 73: 517-522.

4.58 g / day food eaten

excreta = 1.01 g

mass gain = 0.024 g

1.01 / 4.58 = 0.22

22% of food eaten is excreted

0.024 / 4.58 = 0.0052

0.5% of food eaten produces increase in body mass

We can account for 22.5% of food eaten.  The rest is gone.

Let's subtract excreta, and compare food assimilated vs body mass:

4.58 -1.01 = 3.57    0.024 / 3.57 = 0.0067  0.7% body mass

99.3% of food eaten seems to disappear.  Where does it go?



            Uhhh...this leaves us with another question!  how does energy get into food?





Part IV: Plants and Energy


What we've done so far left us with 2 questions:

How do plants use light?

How does energy get into food?


Consider the following:

In about 1600, Jean Baptista van Helmont did an experiment.


Small willow tree (5 pounds)


Grow for 5 years in 200 pounds of soil


Large willow tree (169 pounds)

Used 2 ounces of soil

164 pounds came from water and air

(current measurements indicate that willow trees are typically somewhere between 35% and 61% water.  Let's use 50% as an estimate to think about van Helmont's data.)


            We estimate that van Helmont's willow tree grew 82 pounds of new wood.


What does this tell us?

Apparently, trees (and plants in general) acquire mass from something that is not water, and not soil.  It must be air.  We can summarize it this way:


Chemical in air + water  -------> assembled into plant material, such as wood


We now know that the chemical in air is carbon dioxide, CO2, so:


Water + CO2 ---------> wood and other plant material


Let's think about one more thing we already know:

Wood burns.  Burning wood releases heat, which is energy.  The chemistry is this:


Wood + O2 -------> CO2 + water + heat (energy)



If destroying  wood (converting it to CO2 and water) releases energy, then:

Building wood from CO2 and water must require energy.


Let's think back about plant food

What did our experiment indicate to us that plants need?  Air, minerals, some other small molecules, water, and light.  The only one of these that we can call "energy" is light.

Therefore, light must be the energy source for building plant material


Part V: What is Energy?

The concept of "energy" is a serious "learning bottleneck" for most students at most levels, which suggests that it is a tricky subject.  It is easy to memorize definitions for the test, but it's hard to understand the concept, especially when we refer to "chemical energy" or the "flow of energy in a food chain" or the "energy stored in food."

Perhaps a discussion of energy in general could be helpful.  Here is one way:


Uhhh...what's energy?  How about writing out a list of as many Examples of Energy as we can think of. 

We might expect students to come up with:

Electricity, gasoline, coal, wind

Solar energy

Someone running



What does this tell us?
all of these are examples of the same thing: energy.

- note that this much seems trivially obvious, since we simply wrote down different forms of energy.  The hard part--and the learning opportunity--is to figure out the common aspects of these different things.  In the end, we'll see that "energy" can pretty much be equated with "movement."


Let's look at each one of these in more detail...

Example of Energy

What it is

What it can do


electrons moving in a wire (or in any conductor)

Motor converts it to movement

Heater converts it to heat

Light bulb converts it to light


a chemical

Burn it -------> heat

With engine, this gives movement


Chemicals from old plants

Burn it -------> heat

With turbine, produces electricity


Moving air molecules

With wind turbine, produces electricity

Someone running




Individual molecules moving

Heat other things (heat air, it expands, can drive turbine)

Solar energy


Light absorption converts light to heat

Photovoltaic cells convert light absorption to electricity


Reaction with oxygen

Gives off heat


Do we see any common features that would let us define energy simply?         

energy = movement

Molecule movement = heat

Electron movement = electricity

Photon movement = light


Energy can be stored

Water towers, dams--lift stuff up, so it can fall down

coal, wood, other chemicals--things that burn--store energy in their particular types of chemical bonds.  FOOD is an example!


The biology of energy and food:

Plants use light energy to assemble small molecules into complex organic molecules.

Animals eat plants, and use some of the plants' molecules to assemble into their own bodies.  But they break down most of the food molecules to release the energy stored in those molecules.

Energy enters most food webs as sunlight.  Plants capture the energy and use it to build plants.  Animals eat the plants, or eat each other.  When plants and animals die, bacteria and fungi eat what's left.  So, energy enters food webs, and "flows" from plants to herbivores to carnivores, and to decomposers.  Although the atoms follow a kind of cycle (e.g. the carbon cycle, the water cycle, etc) and can be re-used, energy flows one way.