Energy Content of Food


Hello. Thank you for joining me today. My name is Cynthia Sargent, and I’m the biology
curriculum developer and teacher trainer here at PASCO. The lab that I’ll be showing you today is
titled “Energy Content of Food.” It was one of my favorite labs to do with
students when I was teaching. It helped engage students and get them excited
in topics that otherwise were traditionally a little boring to students. Those were the topics of macromolecules, understanding
fats, proteins, and carbohydrates and the structures of those molecules, and being able to relate the structures to
the functions that those molecules have within organisms. I’ve also used this lab to help start my unit
on cellular respiration, getting students engaged in the idea that
food contains energy and learning about the chemical process that
releases energy from that food in a form that’s usable, and helping them understand the overall equation
that we use to represent cellular respiration. I’m going to show you the procedures for the
lab and then talk a little bit more about those connections later on. It’s always good to give students a real-life
scenario to help engage them in the topic as well, and get them thinking and get them discussing
what their prior knowledge is. In this scenario, they’re given the information that a mountain climber needs about 8000 calories per day. You can ask them to predict how that compares
to the number of calories that an average person requires each day. Of course, for a person climbing up to the
top of a mountain, they want to limit the mass that they have in their backpack. If they need a large number of calories per
day and they’re limited in the food that they bring along with them, then they have to make some smart choices
about what they pack in their backpack. You can ask them to think of what kind of
foods they would pack and why. You can have students investigate a nutrition
label and look at the components by mass. They should be able to discover that fats,
proteins, and carbohydrates are the main components of foods. You can ask them to then predict which one
of those they think has the greatest amount of energy. It’s important to connect to students the
idea of calories as a unit of energy. In this case, we’re using calories as a unit
of heat, seeing how much energy a food contains by
calculating the heat released when that food burns. The two foods that we’re going to burn, in
order to compare their energy content, are peanuts and marshmallows. You can just use standard marshmallows and
peanuts from the grocery store. It’s good to keep the packaging that they
come in so that students can then compare the results
of their lab to the nutrition labels that are on those packages. But don’t let the students see those nutrition
labels to start with. You want them to make a prediction — the marshmallow or peanut, which one has the
most energy content, the greatest number of calories. You’ll likely see that students have differing
opinions. Some think the marshmallows, some think the peanuts. It’s important to let them discuss their prediction
and why they have that prediction, but to not tell them ahead of time which one
is the correct answer. You want them to come back and use evidence
from the lab to justify a conclusion. The experiment for measuring calories involves
water being placed into a can. We’re going to place a food item directly
below that can, and allow that food item to burn, transferring its energy to water within
the can. Water is a good substance to heat. It makes
the calculations really easy. A calorie is a standard unit of heat. It takes
one calorie as the amount of energy to raise one gram of water by one degree Celsius. If we put a known amount of water, in this
case 50 mL — you’ll have students measure in a graduated
cylinder 50 mL of water each time — which will be 50 grams of water. They can take that known amount of water,
50 grams, measure the amount of temperature change that occurs within the water, and multiply those values to get the number
of total calories that were gained by the water. In their discussions of their predictions,
students might bring up the idea about the size of the peanut compared to the size of the marshmallow, and whether or not that makes a difference. That can lead into a discussion with students
that really a fair comparison for foods is if we gathered data on calories per gram. This lab will involve measuring a temperature
change, but also a mass change. It’s important to get the initial mass of
the food before burning it. I’m using a simple food holder made from a piece of cardboard wrapped in foil, a paperclip to be the holder. I’m going to put the marshmallow onto my food
holder. For the sake of the video, and saving us a
little bit of time, I went ahead and collected data on the peanut
earlier, so that we can do the comparison. When students get the initial mass of the
food, it’s important that they put the food and
the holder all together on the balance. That way, during burning anything that may
fall off the food is caught on the holder, and can be counted for in mass. My initial mass of the marshmallow and the
holder is 14.44 grams. I’m going to type that into my data table. I’ll come back to this data table after I
burn the food to get the final mass of the food. I’m going to place the food holder with the
food item directly below the can. We want to maximize the amount energy that
the water in the can gains from the food. There’s going to be some energy transfer to
the environment, but we want to minimize that as much as possible. Before I light the food on fire, I’m going to want to see what the temperature is before the burning. I’ve got a Fast Response Temperature probe
that I taped to a coffee stirrer. It’s best if the temperature probe sits submerged
in the water, in the middle of the can, not touching the sides, so the coffee stirrer helps keep the probe in place. And you want to keep the wire of the probe
out of harm’s way from the flame. I’ve got my can. I’m going to add 50 mL of
water to it. I’m going to start recording temperature of
the water in the can by pressing Play. You’ll see on the graph, I already have data
for the peanut. I’m going to hide that for the moment so that
you’re seeing only the data for the marshmallow. You can see the temperature is remaining the
same. The temperature probe is just sitting in the water right now. Now I’m going to light the food. Use a match
to light a long wooden splint. This will help you conserve matches. When students try to light the food with a
match, they often burn their fingers or drop the match. So if you light a wooden splint, the students
can keep their hands well away from the flame. As soon as the food is burning on its own,
you want to remove the splint. You can always adjust the placement of the
food a little bit by moving the holder. You want to keep it as directly underneath
the can as possible. If you’re demonstrating this for students, you can be talking about the energy in the form of heat and light that comes from the food is coming from the energy that was stored within the molecules of that food. Once the food stops burning, you can stop
data collection on temperature. Place the food and holder, again, all together
on the balance. I’m going to return to my data table for collecting
data on mass and type in the final mass here, 13.68 grams. And I’m using a data table in which the third
column, change in mass, automatically calculates the difference between
the initial and final masses. SPARKvue software has a calculator option, and I used that to create an equation that will find the difference. The change in mass is a good opportunity to
bring up the equation for combustion, that we’ve got the sugars in the marshmallow
reacting with oxygen in the air, producing carbon dioxide and water as products
that escape as gases and cause the mass of the food to decrease. Students will notice that a large amount of
the mass of the food is not burned, and that’s OK. In our calculations, we’re going to be getting
calories per gram, so we don’t need 100 percent of the food to be burned. It’s common that a gram or less of the food
will burn. We’ve collected information on the mass before
and after. Now we’re going to look at the change in temperature
of the water, based on the energy that that food released. Our statistics tools of minimum and maximum will be an easy way to find the difference in temperature, and that difference in temperature will be
entered into a data table on the next page. Here students would be able to compare the
peanut result to the marshmallow result, although, at this point, just having a change
in temperature doesn’t give us enough information to compare the calories. It’s important if students are doing this
lab on their own and they’re burning more than one food item, that you remind them the can is going to be
quite warm — as warm as the maximum temperature that
the water reached within that can. Once we have the data on mass and temperature,
here’s how we calculate to calories. On the next data table, I have preset calculations
that will do the math for us today, but these are the three equations that you
would discuss with your students. One is just finding the calories, the heat
gained by the water. We have our 50 grams of water, multiplied by the change in temperature that occurred within that water. Multiplying those values will give us the
quantity of energy that is gained by the water from the food. But if students are then going to compare
their results to a nutrition label, they actually need to do a conversion of calories
to kilocalories. On a nutrition label, this is very subtle. It’s just represented as a capital C versus lowercase C. We’ll need to divide by 1000 to do that conversion. Finally, again, for a fair comparison, we
need Calories per gram. We’ll take our Calorie calculation and divide
it by the grams burned, and that will allow us to get that fair comparison
of the two foods. If you look at the last column of the data
table, the Calories per gram for the peanut was 2.09, and for the marshmallow it’s 1.36. Students can see that there’s a definite difference between a food that is mostly sugar, like
the marshmallow, to a food that has a lot of fat content, like the peanut. Then you would ask students to revisit their
hypothesis, the prediction that they made at the beginning
for marshmallows compared to peanuts, and explain whether or not their prediction
was correct. And then you want to lead students to the
big idea of this lab. Students sometimes get bogged down in the
calculations and the math, but you want to keep students focused on that
final answer of Calories per gram, and then to make the connection to what kinds
of molecules these foods have and the structures of those molecules. You can give them representations of simple
sugars, like a monosaccharide, which is molecule A. This would be a monosaccharide like glucose
or fructose. And then you can ask them to identify molecule
B. Hopefully they would recognize that this is
a fat, or at least part of a fat, like a fatty-acid chain to a triglyceride. You can ask them to think about which of these
two molecules would contain more energy within their structures. And the answer here is molecule B. The large number of carbon-hydrogen bonds
within the fat stores a large amount of energy, much more than the amount present in a carbohydrate
or the amino acids of proteins. You can make the connection, also, to the
fossil fuels that we use as the remains of long-dead plants, and the carbon-hydrogen bonds within those
hydrocarbons of the fuels. You can also make the connection to ecosystems
and cellular respiration. You can show them a picture, like this, of
a predator and its prey, and you can ask them to explain how the predator
eating the prey relates to the lab that they saw. There’s mass contained within the prey’s body.
The cheetah’s going to consume that mass. It’s going to be composed of fats, proteins,
and carbohydrates. And just like the energy was released from
those molecules when we burned the food in the lab, the energy would be released from those molecules
in the cheetah’s body by cellular respiration. You can show students that the equation for
combustion is essentially the same as that for cellular respiration. And you can explain to students that the energy
that we saw released from the marshmallow by burning it is the energy that’s also released in their
bodies when cellular respiration changes that food into carbon dioxide and water. You can also talk about the efficiency of
energy transfer. It’s not 100 percent in the lab. Neither is it 100 percent in an ecosystem. Only 10 percent of the energy, roughly, goes from one trophic level to the next. You can have students calculate the percent
error. If you’ve saved the nutrition label from the
packaging of the foods you use, they can get, from the serving size in grams
and from the Calorie content per serving, a ratio of Calories per gram that they can
compare to their own results. This is a good opportunity to then let students
explore how they can maximize the procedures or change the procedures to reduce their percent
error. You can even make it a competition among students
for which group of students can get the closest results to what’s on the nutrition label. I’ve burned many different foods with students,
not just the peanut and the marshmallow. You can use potato chips. You can use cereal.
You can use crackers. It’s a good chance for students to investigate
energy and how it relates to biology, practice some math skills along the way, and give them an engaging experience that
you can continue to connect content to throughout the year. Thank you for watching today, and I look forward
to bringing you more videos in the future.

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