Chapter 1. Photosynthesis Lab

1.1 Background

Some things are non-negotiable requirements for human life: water, food, and oxygen immediately come to mind. We can live for days without food and water, but if our oxygen supply is cut off, we only have a few minutes. Access to air limits where we can go: for example, astronauts can only go into space if they bring their own air supply, and we can only explore the deep sea if we are inside a submarine or submersible.

What if we could change that? Science fiction writers often describe a process that they call “terraforming.” This means transforming a planet so it resembles earth, including an oxygen atmosphere. Could this ever be possible? Only time will tell.

But let’s start a little smaller. In this lab, your goal is to develop a device that can create breathable air in space or the deep sea. Humans exhale carbon dioxide, and if the levels get too high, heart rate, breathing and blood pressure go up. A person could go into a coma and even die. So your device needs to remove carbon dioxide from air. And while you’re at it, could it create other useful substances, like food?

To be able to do these things, you will need to understand the most important process on earth. Photosynthesis is our source for both oxygen and food. The word photosynthesis defines itself. “Photo” means light, and “synthesis” means building or creating something. The process of photosynthesis uses the energy of sunlight to create sugars. Let’s look at how it works.

(Ins Fig 1. This figure will show a very simplified diagram of photosynthesis. Emphasis is on the inputs of light energy, carbon dioxide and water and the outputs of glucose and oxygen.)

Photosynthesis is a very complex process involving many chemical reactions. The big picture is often summarized in one equation:

Or, in words:

Carbon dioxide + water –(light)→ oxygen + sugar

What does this equation tell us? Photosynthetic organisms take in water (H₂O) and carbon dioxide (CO₂). They also absorb light energy from the sun (or other artificial light sources, for example grow lights). Inside the cells of these organisms, complex chemical reactions convert the water and carbon dioxide into carbohydrates (C₆H₁₂O₆, glucose) and oxygen gas (O₂). The oxygen gas is released out of the cell, but the carbohydrates can be used to make a variety of molecules that a plant needs, such as sucrose, starch, and cellulose. The plant can also use glucose as an energy source, just as our cells do.

There are two main stages of photosynthesis.

• The light reactions are the stage in which chlorophyll absorbs sunlight energy. Some of the energy that is harvested is stored in high-energy electrons, while some of it is stored in adenosine triphosphate (ATP). During this stage, the water molecules will be split apart to provide a continuous supply of electrons. When water is split apart, oxygen gas is formed.
• The Calvin cycle is the stage in which carbohydrates are formed. All of the energy stored in the high-energy electrons and ATP is used to convert CO₂ into larger carbohydrates. Eventually, these molecules will be used by the plant to make sugars (e.g. glucose) and other important molecules.

Which organisms can photosynthesize?

Organisms that can photosynthesize are often called producers because they are producing their own food—they don’t need to eat other organisms. This is in contrast to consumers, who must eat other organisms for energy. Bacteria were the original producers, and they evolved the process of photosynthesis. One group of bacteria, the cyanobacteria, appears to be the ancestors of the chloroplast. The chloroplast is the subcellular structure where photosynthesis takes place in algae and land plants.

There are a wide variety of algae. They vary in size, from single cells to giant kelps. And they vary in color, from green to red to golden to brown. They all contain chloroplasts and chlorophyll. However, some algae contain pigments in addition to chlorophyll that they use for photosynthesis. These pigments give the red, golden, and brown algae their color. In this lab, we are going to focus on green algae and land plants, because their method of photosynthesis uses only chlorophyll. Therefore, for our purposes today, producers must be green.

(Ins. Fig 2. This figure should be a triptych. One panel shows cyanobacteria, one shows green algae, and one shows a land plant. Algae should include single-celled and multicellular ones (seaweeds), while the land plant should be something big to emphasize the difference between them and the algae)

How does photosynthesis work?

First, you have to be able to capture sunlight energy. This is the job of chlorophyll. Chlorophylls are pigments, colored molecules that absorb light. There are several different types of chlorophyll, but they all are involved in absorbing sunlight. The wavelengths of light that are used for photosynthesis are the ones that are visible to us: the colors of the rainbow. Plants are green because chlorophyll is green. And chlorophyll is green because it does not absorb green light. Instead it transmits or reflects it. Chlorophyll is best at absorbing violet, blue, and red light. These wavelengths of light provide most of the energy used in photosynthesis.

(Ins Fig 3. This figure will show the absorption spectrum of chlorophyll, demonstrating that it absorbs violet, blue and red wavelengths the best.

Second, you need carbon dioxide. This is not hard to find. There is always carbon dioxide in the air for plants that live on land. And for plants and algae that live in water, there is carbon dioxide dissolved in water.

Third, you need water. Again, this is not usually hard to find. In desert environments, where water is scarce, plants have adaptations that allow them to conserve as much water as possible.

Photosynthesis is a long process involving many chemical reactions. But at the end of the process, the carbon dioxide molecules have been combined into a larger molecule, glucose. The water molecules have been split apart. When the hydrogen atoms are removed from a water molecule, what is left is oxygen.

So to summarize, photosynthesis starts with: sunlight, chlorophyll, water, and carbon dioxide. It ends with: glucose and oxygen.

What is the point of photosynthesis?

Why do producers spend time and effort on photosynthesis? Well, they need the products. Remember, they are called producers because they are producing their own food (sugar) and oxygen. So they are pretty much self-sufficient.

But that’s not what makes photosynthesis the most important process on earth. Consumers also get their food and oxygen from photosynthesis. The oxygen in the air we breathe comes from photosynthesis. And when a consumer eats a producer, it can use some of the energy stored in the producer’s cells for its own energy needs.

This is how a food web develops. A food web is a description of who eats what in a particular ecosystem. The producers are not eating anybody, because they make their own food. They are always at the base of a food web, supplying food for everybody else. Next, the primary consumers are the herbivores, animals that eat producers. The secondary consumers are the carnivores, animals that eat herbivores. So even though the carnivores don’t eat producers, the energy they get from their food ultimately came from a producer. As your final activity in today’s lab, you will look at how energy is transferred from one organism to another, and you will see that it takes a lot of producers to feed the world!

(Ins. Fig 4 This figure will be a very simple food web, with producers and consumers labeled. Since we are using algae, an aquatic food web will be best because it will include them. First one is really nice! See document for example)

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1.2 Pre-Lab Quiz

Before we can being to design the filter, you’ll have to understand the background material covering photosynthesis—what types of organisms can do it, what it requires, what it produces, etc. Make sure you read the background material thoroughly, and then answer the following questions.

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1.3 Experiment Notebook

Developers: sections 1.3.1 - 1.3.4 should be smaller tabs beneath the main "Experiments" tab.

Section 1.3.1 Photosynthetic Organisms

First, we’ll need to find the ideal organism to use in our bio-filter. I’ve taken us to a wetlands environment containing a variety of photosynthetic producers and non-photosynthetic consumers. Your job is to collect a variety of different living organisms for analysis in the lab.

Collect six different samples by clicking living organisms you find in this environment. Once you find all six, we’ll travel back to the lab so you can analyze them.

Now that you've collected your six samples, we’ll use a carbon dioxide (CO₂) chamber in the lab to measure the amount of photosynthesis occurring in each of the samples you’ve collected. However, before you can test your samples, you will have to run some controls to make sure the equipment in functioning correctly.

We will use two controls for this experiment: an empty chamber and a jar of sterilized water. The sterilized water has been heated to kill off any microorganisms; there is nothing living in the water.

To run the controlled experiments in the lab, you will either click the “test empty chamber” button (for the empty chamber control) or click and drag the jar of sterilized water to the CO₂ chamber. The chamber will automatically fill with 0.5% CO₂ gas.

Now that you know the machine is functioning correctly, we can use it to determine if the specimens we collected are photosynthetic.

To run the next experiment, you will return to the lab (in just a moment) and click and drag any of the specimens to the CO₂ chamber. The chamber will automatically fill with 0.5% CO₂ gas.

But first, think about the gases used and produced by photosynthesis.

Now, let’s go back to the lab and test whether or not your predictions are correct.

Now that you have examined all six of the samples you collected from the wetlands (and all four types of microorganisms in the pond water), answer the questions below.

Here’s what you hypothesized the results would be at the very beginning of the experiment:

Recorded response from earlier in Section 2 displayed here.

Below are several other living organisms that can be found in a variety of environments:

So far, all of the photosynthetic organisms we’ve studied in this lab have been green because of their light-absorbing chlorophyll. However, not all green organisms are photosynthetic! Consider the lizard below:

Now that we have determined the organism with the highest rate of photosynthesis to use for our biological air filter—green algae—we can proceed to the next part of our experiment. In the next section of the lab, we will determine the perfect habitat to make our algae bio-air filter function at peak efficiency!

Section 1.3.2 Requirements for Photosynthesis

We’ll be testing a variety of different environmental conditions to determine what is best for our biological air filters. The five conditions we’ll be testing are: the levels of CO₂, oxygen (O₂), humidity, amount of sugars supplied to the algae, and different colors of light.

Use what you answered above to predict the effects of each of the environmental conditions we are about to test.

Before we begin, we must make our algae biological air filters. The algae in this pond scum will work wonderfully, but we can’t just throw pond scum everywhere. Instead, we’ll grow pure algae on vertical membranes. This will be cleaner and much more portable; we can bring these algae air filters wherever they are needed, whether that’s to the moon or down to the depths of the ocean.

To make a pure algae air filter in the lab, you'll use a pipette to transfer a couple of drops of pond scum onto each of the filter membranes provided. First, click on the pipettes to pick one up, then click on the pond scum sample to collect some of the algae. Next, click EACH of the filters to add just a couple of drops of algae cells to the membrane. Just a few drops will be enough— we’ll wait a week to allow the algae to grow and cover the entire membrane. Once you’ve added the algae to each membrane, click the “Wait for Algae to Grow” button to allow time for the algae to grow over the entire surface of the membrane.

Once you have some bio-filters prepared, we can determine what the ideal conditions are to get them to photosynthesize as much as possible. For each of the conditions, we will test three different levels. First, you'll select one of the five environmental conditions to test. We’ll examine each of these independently to make sure the results are not influencing each other. Once you’ve selected an environment to test, you will click-and-drag an algae bio-filter to each of the three CO₂ chambers. You’ll use the pond scum you’ve collected to keep making more of these, so don’t worry about running out. Lastly, you'll use the controls on top of the CO₂ chamber to set the three different levels we’ll test for this condition. Make sure each chamber is set up differently! Once all three steps are done, you are ready to run the experiment and begin collecting data!

In the previous experiment, we were trying to determine if photosynthesis occurred or not, so it was enough to simply see if the CO₂ levels in the chamber increased or decreased. Now we know photosynthesis is occurring, but we want to determine when it is occurring the fastest, so we’ll have to get a little more detailed in our experiment. We will run each experiment for five hours. After each hour, you will record the level of carbon dioxide gas in each of the CO₂ chambers. This will give us some data we can graph and compare to determine the best environment for maximum photosynthesis efficiency.

The table below shows the hypotheses you made before you started this experiment. In the far right column, select the actual results we saw in our experiments.

Excellent, we now know the best environment to get the highest amount of functionality out of our biological air filter. I’m sure NASA will be excited to hear your results. Meanwhile, there are just a couple more experiments you need to do to make sure you understand and can explain how this biological air filter works. Next we will examine how it performs at the cellular levels.

Section 1.3.3 Photosynthesis Inside the Cell

The algae filter is almost ready to go into production, but the sales staff want some visual aids so they can explain to the NASA astronauts exactly how it works. You’ve been recruited as a science advisor to help the animators create a visual animation that explains the process of photosynthesis—what chemical reactions occur and exactly how it filters out the carbon dioxide.

In the lab, the artists are going to ask you a series of questions about the steps and stages of photosynthesis. As you answer the questions, they will use that information to create animations of the process. If your answers are correct, the entire process should connect together in one sequence.

In a moment, you will go to the lab and answer all of the questions. Sometimes a question may require you to click and drag labels or reactions to the appropriate area inside the cellular organelle. Notice that not all the labels or reactions will be used: the animators may have made a few extra just for fun, so don’t let that trip you up. If at any point you think you have made a mistake, you can click the “reset animation” button to begin again from the very beginning.

Once your animation is correct, we’ll review it for accuracy. If there are any problems, they will be pointed out to you, but you’ll have to reset the animation to correct them. Keep trying until you have correctly constructed an animation that accurately depicts the process of photosynthesis. Then, return here to answer some follow up questions, demonstrating your knowledge of photosynthesis.

Now that you’ve successfully completed your animation, answer the following questions about photosynthesis.

Your animation should really help our clients understand how these air filters work. Your results demonstrate that this type of algae can remove carbon dioxide from the environment very effectively. Soon it will be ready to carry out these same chemical reactions in an enclosed environment.

Section 1.3.4 Photosynthesis for Food

NASA has made a special request for you to test. They love your results so far—it looks like these bio-filters will be a good way to help remove excess CO₂ from the international space station. However, they are a little cramped for space up there, and if these filters could do two jobs that would be even better.

The algae are producers: they can use photosynthesis to produce enough food to be self-sufficient. We, on the other hand, are consumers: we get our energy from eating producers. NASA wants to know if this algae can produce enough food to feed some of the astronauts as it filters the CO₂ out of the air. It’s possible, but we’ll have to run some experiments to see if the idea will actually work. Luckily, the type of algae we’re growing on our bio-filters is actually eaten in a few countries. The scientific name is Cladophora, but some diners know it as “River weed” or “Mekong weed.”

Photosynthesis produces high energy sugar in part because of the First Law of Thermodynamics, which says:

Energy can be changed from one form to another, but it cannot be created or destroyed.

Almost ALL of the energy on the earth comes from the sun! It is captured by producers and converted into forms that other organisms (such as us consumers) can use. So, life on Earth is made possible by the first law of thermodynamics. Even the electrical energy that powers your home probably came from the sun—plants millions of years ago absorbed it through photosynthesis. When these plants died, they decayed and were compacted over LONG periods of time, forming coal or oil. When these fossil fuels are burned, the energy originally stored in the plants is converted into electrical energy.

When we eat food, our bodies also obey the first law of thermodynamics: they convert the chemical energy in the food into the kinetic energy of moving, thinking, living, etc.

However, this transfer of energy is not perfect. For example, plants only successfully store about 1% of the light energy they absorb. Not all of this stored energy is available to consumers, either. Some of the energy will be used by the plant itself, and some of it will be lost as heat energy. This is because of the Second Law of Thermodynamics, which states that:

The disorder or randomness of the universe is always increasing.

In this case, the heat energy given off by the transfer increases the randomness of the whole universe.

On average, only about 10% of energy that the producers transform via photosynthesis is stored. It is only this stored energy that can be passed on to any consumers that eat the plant. The energy in food is measured in “Calories.” The technical definition of a Calorie is the energy required to raise the temperature of 1 Kg of water 1^oC. The average human being uses 1800 – 2500 Calories every day (depending on their age, weight, gender, and activity level).

Let’s look at a more concrete example:

This means the question really is: Can we provide enough light energy to our algae to make it possible for the humans who eat it to get enough energy to live? Obviously, we can’t test this by starving real people, so you’ll run a simulation on the computer in the laboratory. In this simulation, you’ll be able to test the outcomes of providing different light levels to the biological air filters. Examine the outcomes of all the different possible light levels. Make sure to notice how much energy is transferred to the algae in the bio-filter, and how much energy could potentially be transferred to the humans who eat the algae.

To access the simulation, click the computer sitting on the lab bench, then Click the “Run Energy Conversion Simulation” button.

Congratulations, that’s the last experiment to test this new type of bio-filter. These results will be passed on to NASA and they’ll make the final determination whether this algae-based system will be a useful system for their spacecraft. Who knows—your invention here may someday help us walk on Mars, or beyond!

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1.4 Lab Report

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1.5 Vocabulary List

Chlorophylls: the pigment molecules in plants that absorb sunlight energy.

Chloroplast: subcellular structure where photosynthesis takes place in algae and plants.

Carnivore: meat-eating animal.

Consumer: an organism that must eat other organisms for energy.

Cyanobacteria: group of photosynthetic bacteria that gave rise to the chloroplasts in algae and the plants.

Green algae: aquatic organisms found in both salt and fresh water. They may be single-celled or multicellular; they are photosynthetic.

Herbivore: animal that eats plants.

Photosynthesis: the process by which plants use sunlight as an energy source for creating sugars. Oxygen is a byproduct of this process.

Pigment: a colored molecule that can absorb certain wavelengths of light.

Plant: photosynthetic organism that is multicellular, may live in water or on land, and is more complex than algae.

Primary consumer: an animal that eats plants; a herbivore.

Producer: an organism that can make its own food by photosynthesis.

Secondary consumer: an animal that eats animals; a carnivore.