J. José Bonner

July 2007

Photosynthesis

 

The Learning Bottleneck

The Strategy

The Rationale Behind the Strategy

Assessment and Reinforcement

Extensions: Fuel

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The Learning Bottleneck

 

At a very superficial level, students can learn what we tell them when we say that plants capture light energy and use it to make plant material.  But it's not clear that this really sinks in, or that it even makes sense.  The fundamental reason, of course, is that it's "just magic" if we don't know how it works.  Yet, the details of how it works are unbelievably complicated.  For most students--even after graduating from college--photosynthesis remains little more than something they memorized for Science Class.

 

As with many of the fundamental Learning Bottlenecks in biology, I suspect that a goodly percentage of students' confusion results from being taught that something happens, but without a context to show that such a thing must happen to account for what we see.  The bottleneck, then, would seem to be that students don't really understand why anyone would be so goofy as to imagine that something like photosynthesis could possibly exist.

 

Why not give the students some of the basic information they need to see that photosynthesis must occur?  In a very real sense, our students have a level of background knowledge that is similar to that of scientists in the 1500's.  Perhaps it should come as no surprise that they don't quite get it when we immediately summarize what it took scientists roughly five centuries to figure out.

 

As always, students ask "how do we know that?" -- even if they ask it at a subconscious, intuitive level without articulating these precise words.

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The Strategy

 

Let’s start by asking, “what do plants need to grow?”

Students can kick around ideas, preferably in small groups, followed by a class discussion.  This will provide insight into the pre-conceptions they bring to class.  It will be good to keep track of these.  For example, it is commonly thought by students (and most adults) that most of the mass of plants comes from the soil, by way of the roots.

Great.  We have some ideas.  Some of these ideas may be right, and some may not.  The best thing to do is to find out experimentally.

[It is often tempting to use “the Scientific Method” in experiments in elementary school.  I recommend against it.  See the discussion of the Scientific Method for explanations.]

 

There are various methods for asking this question experimentally.  There are many lesson plans available.  Before choosing one, let’s think about what we need to do. 

1. We need to manipulate variables, perhaps omitting things we think plants require.

2. To determine if a plant needs something to grow, we need to omit that something from its environment--then see if it stops growing.  A very good way to determine that it has stopped growing is to see that it has died.  This tells us that we have to work with whole plants, not just a leaf or two from a larger plant.

3. If we are going use whole plants, we probably don’t want to use oak trees.  Small, inexpensive plants would be best.  We can do this with a package of seeds from the store.  But, seeds are baby plants that have been supplied with food by the mother plant.  We can’t tell what happens if we omit something if the plant already has it stored in the seed.  This tells us that we need to plant seeds, let them germinate and use up the food stored in the seed, and then start the experiment.

4.  What variables should we test?  The obvious things are water, light, air, soil, and “soluble nutrients.”  Let’s design some tests:

 

For all tests, germinate seeds, and let them grow until they’ve used up the food stored in the seeds.  For most of these, some soil in a paper or plastic cup (with holes for drainage) will work.  For some tests, however, we’ll need something else, as described below.  Once the seeds have begun to grow on their own (dicots are growing their first true leaves, or monocots are growing their second leaf):

Test light: put some plants in dark, some in light.  [We expect that that the dark ones don’t grow, and eventually die.  Conclusion: light is necessary for growth.]

Test water: stop watering some of the seedlings.  [We expect that these will die.  Conclusion: water is necessary for growth.]

Test soil and soluble nutrients: this requires planning ahead.  Plant seeds in unflavored gelatin, rather than in soil.  Take care, however: test your seeds ahead of time.  Some, such as wheat (sold as wheat berries in some grocery stores) release enzymes that digest the gelatin, turning it into liquid.  [If someone finds good seeds for this, send us an email and we’ll update this file!]  Make the gelatin as described on the package of “unflavored gelatin,” but at about 1/2 to 3/4 the recommended concentration--enough to gel, but not so “thick” that roots cannot penetrate it.

                                    If you use gelatin and water, you automatically omit both soil and soluble nutrients.  The gelatin will provide the support that soil does, but the soluble nutrients that are in soil.  You will need to add some “plant food” to provide these soluble nutrients.  It may be easiest to do this ahead of time, and then explain to your students that some of the test cups have just water and gelatin, while others have water, gelatin, and the soluble nutrients plants need.

                                    We expect that plants will grow just fine without soil, provided that they have the soluble nutrients that are provided by the plant food (and are in good, fertile soil).  Without the soluble nutrients, the seedlings are likely to turn yellow slowly, stop growing, and eventually die.

 

To test air, we cannot so easily design an experiment.  Fortunately, the critical experiment has been done, by Jean Baptista van Helmont, in 1600.  Here is what he did:

 

Plant a small willow tree (5 pounds) in 200 pounds of soil.

Wait 5 years, watering the tree as needed.

At the end of 5 years, remove the tree from the soil, and weigh the tree and the soil.

 

We now have a large willow tree, weighing 169 pounds.

The soil is almost unchanged, weighing only 2 ounces less.  (2 ounces = 0.125 pounds, not much)

Where did the 194 pounds of new tree come from?

Apparently, not the soil.  We conclude that the 164 pounds came from water and air

 

(Current measurements indicate that willow trees are about 50% water.  If we were to evaporate the water and just measure the wood itself, we would find 82 pounds of dry wood.)

 

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

 

Where did the wood come from?

                  -- not from the soil, since that only contributed two ounces.

                  --the only other things available are water and air, but we evaporated the water, and still had  82 pounds of wood!

Apparently, trees (and plants in general) acquire mass from something that is not in soil.  It must be air. 

 

What can we conclude from this?

 

Some chemical from the air must be assembled into plant material, such as wood

 

Note: it would be really fun if we could simulate this the way TV chefs simulate cooking: set up the initial arrangement (plant a small tree in a pot), then say "now 5 years have passed" and wheel in a much larger tree in a similar pot (on a wheeled cart, of course).  Now we have visual cues that make the experiment much more "immediate."

 

From van Helmont’s experiment, we seem to be forced to conclude that plants take some chemical from air and assemble it into wood and other plant material.  We can write this as a chemical equation:

 

                  Chemical from air ------------> wood and other plant material

 

Not only is air required for plant growth, it provides one of the essential chemicals for growth. 

[This gives us an important clue for understanding our other experiments: if something is required for growth, then it must be used in some way to make growth happen.]

 

We have now performed several experiments ourselves, and evaluated one that was performed by another researcher.  Can we put all of the information together to understand plants?

 

First, we have already considered air.  This led us to write a chemical equation:

 

Chemical from air ------------> wood and other plant material

 

Second, let’s think about water:

Plants wilt before they die, so our first conclusion must be that water is important for plants to maintain their shape and “strength.”  (The same is true of humans.)  There is more to it, however, which we now know based on additional experiments.  Based on these additional experiments, we can modify our chemical equation to include water:

 

chemical from air  + water (which is a chemical) ----------------> wood

 

Third, let’s think about light:

We found that light is essential for plant growth.  But...what is light?  We know things get hot when sunlight shines on them, so light is a form of energy.  Energy is not a “thing” or a chemical; it is more like a force that can make things happen.  One of the things energy can do is enable the assembly of large things from small building blocks.  Let’s add energy to our chemical equation:

 

Chemical from air + water --------------------->  wood

                  and... the energy to do this is provided by light

 

[wood is a "large thing," while the molecules (tiny pieces) of air and water are "small things" that are used as building blocks.]

 

This chemical reaction is “photosynthesis” (which means light-dependent building of stuff).  As a chemical reaction, it seems un-interesting and remote--but let’s let this sink in for a while.  Plants do something remarkable.  They take water from soil, a chemical from air, and use energy from light to assemble these things into plant material. 

                  Then, animals eat the plants.  So do we.  [Even animals that eat other animals depend on plants, because the food-animals eat plants.]  In short, all of life depends upon plants using light energy to build the material upon which everything depends.  This is pretty amazing--and tremendously important.  [Consider: if all humans disappeared from Earth suddenly, little would happen to other species.  But if all plants disappeared, all animals would die.]

 

Now, let’s update this just a bit:

 

We now know the chemical from air is carbon dioxide, CO2.  We can write our chemical equation with this chemical name:

 

Carbon dioxide + water --------------------->  wood

                  and... the energy to do this is provided by light

 

So far, so good...what about the soluble nutrients in plant food, and in soil?  As it turns out, these are all chemicals, such as nitrate, phosphate, magnesium, calcium, etc.  These are necessary parts of the materials that make up plants, but they are needed in much smaller amounts than carbon dioxide.  Remember van Helmont’s experiment: 82 pounds of wood was built from air, water, and only 2 ounces of soil--the soluble nutrients.  These nutrients are important, but the major source of material for building the mass of plants is carbon dioxide from the air.

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The Rationale Behind the Strategy

 

As described above in the "bottleneck" section, the fundamental difficulty is recognizing that some kind of light-dependent process drives the growth of plants.  It is safe to say, based on the general understanding of photosynthesis among the public, that the typical methods of telling students about photosynthesis are fairly ineffective.  Just telling doesn't work very well.

 

Instead, students need evidence--observations that they can understand only if they construct an understanding of the basics of photosynthesis.  They must recognize that the mass of the plant comes primarily from air (that is, from some chemical in air, which we now know is carbon dioxide).  The experiment done by van Helmont makes this conclusion almost inescapable.  When faced with the evidence--the facts--students may (and should) struggle to explain the observations in many different ways.  But the only one that actually fits the data is that air is the primary source of the mass of plants.

 

Similarly, students need evidence that plants build mass only when they have light.  Germinating seeds in the dark doesn't tell us this; rather, it tells us about the developmental programs that plants use during germination in the dark.  (It makes sense that seedlings that germinate under the leaf litter of a forest will grow long and thin, eventually making their way out of the leaves and into the light--and it makes sense that they should do this while they can still use the food stored in the seed.)  Nor does it tell us this to cover a leaf with foil, and see that it turns yellow.  This tells us that the production of chlorophyll is triggered by light, but making chlorophyll is not growth.  To see that growth depends on light, students must compare young plants grown in light to young plants grown for the same length of time in dark (until near death).  It must be obvious to the unaided eye that light-grown plants grow, while plants that are moved to the dark stop growing, and eventually die.

 

The key to developing an understanding of what photosynthesis is about (even if we don't know how it works) is to link these two sets of observations together.  Plants use air to grow.  Plants require light to grow.  Light is energy, not matter.  The only way to put these observations together is to conclude that plants use the energy of light to assemble chemicals from air into the chemicals that make up the plant.

 

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Assessment and Reinforcement

 

The big question here is "what do our students understand about the relationships of light, water, air (CO2), soil, and soluble nutrients that are in soil?"  To a large extent, this is less about photosynthesis than it is about plant biology in general.  But, plant biology cannot be separated from photosynthesis as if they are unrelated topics.  It is all one big picture.

 

Perhaps a way to obtain a "window" into the students' thinking is to ask them to build a concept map, or some other type of diagram that links the different entities (light, water, air, etc) to energy and to each other.  The important question is how students link these things.  If students draw a concept map, with terms in boxes and lines connecting the boxes, it is critical that the lines are labeled to explain the nature of the relationship.

 

An alternate assessment is to ask students to write about how plants grow.  Here, we would want students to articulate not only the relationships of the terms, but also to describe how we know that this might be how things work.  That is, what do you think happens, and what is the evidence that leads you to think this.

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Extensions: fuel

 

Plants take carbon dioxide from the air and build the mass of the plant from it.  That carbon is stored in the plant material as wood.  In the case of fossil fuels (coal and oil) it is still stored in the chemicals of these fuels.  When we burn wood, coal, or oil, we reverse what photosynthesis accomplished.  We convert the stored carbon into carbon dioxide in the air.  Current scientific understanding is that carbon dioxide in the air contributes to climate change, raising the temperature of the earth.  Only one biological process can reduce the amount of carbon dioxide in the air: photosynthesis.  This is one of the reasons that forests are so important. 

 

The only way to stop the buildup of carbon dioxide in the air is to stop burning fossil fuels.  This is why renewable energy is so important.