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Dutchess_III's avatar

Does respiration always involve the release of CO2 as a waste?

Asked by Dutchess_III (25566 points ) May 3rd, 2012

I got lost in a 5th grade science lesson regarding photosynthesis. I started spinning about concerning the fact that photosynthesis releases O2 as a waste, but “respiration” in a plant releases CO2 as a waste. It seemed to me that “respiration” would apply to any gas that is taken in and converted to another gas that is expelled as a waste, but perhaps not. I did a lot of researching on the question this morning including checking out Khun Acadamy, among other sites.

Here is the lesson that started it all:

Green plants use energy through a process called photosynthesis. This process converts solar energy into chemical energy the plant can use for growth and maintenance. It needs chlorophyll to occur and takes place in the leaves of green plants. The three ingredients are carbon dioxide, water, and light. The products of photosynthesis are food and oxygen.

What happens during photosynthesis? A plant takes carbon dioxide in through cells in its leaves. Water comes in through the roots and moves up to the leaves. Light from the sun is absorbed by chlorophyll in the plant’s cells. When carbon dioxide, water, and light are brought together, the plant manufactures sugar and starches, or food. These starch and sugar foods store the energy from the sun in the cells. The food produced by this process that is not used up is stored in a structure inside the cell called a vacuole. Oxygen is released into the air as a waste product of photosynthesis.

Photosynthesis combines carbon dioxide, water, and light to produce food for the plant. But how does the plant get energy from this food? Respiration is the process plants use to change food into energy. This respiration does not mean that plants breathe like animals with lungs. They use oxygen to burn up starches and sugars to release energy. The food chemicals combine with oxygen, and energy is then made available for each cell to grow and reproduce.

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19 Answers

CWOTUS's avatar

I believe that what it means is that for each molecule of CO2 that the plant takes in, it breaks the C and O2 bond to use the carbon in its food-production process. The leftover O2 molecule is “waste” for the photosynthesis process, but that doesn’t mean that all of the O2 that the plant doesn’t utilize immediately is “released” as most animals release most CO2 when they breathe. (In fact, I don’t think you can exhale “all” of the waste CO2 when you breathe. Some is always left in the lungs. I expect that something similar occurs with plants, except for the “lungs” thing, obviously.)

thorninmud's avatar

Plant respiration is a separate process from photosynthesis. In photosynthesis, CO2 is used to create complex carbohydrates for later use as food. What botanists call respiration is the process by which those carbs are later broken down for their energy value. CO2 is liberated as a waste product by the respiration process.

Dutchess_III's avatar

Yes, I got that. Where I’m getting stumped is on the word “respiration.” I understand that a plant doesn’t “respire” the way humans do, but why wouldn’t it be called “respiration” when the plant takes in C02, breaks it down to create the carbohydrates, when releases the O2? Why isn’t that called “respiration”?

XOIIO's avatar

@Dutchess_III It is called respiration, actually.

Dutchess_III's avatar

Well, the question that started it was

During respiration the plant releases
A. Oxygen
B. Nitrogen
C. Carbon Dioxide
D. Argon

The student chose “A”, Oxygen. The lesson marked it wrong. Said the right answer was CO2. That’s what started the search. So @XOIIO…was the student wrong or not? Or is respiration an extremely specific process involving only O2 in and CO2 out that I hadn’t realized before now?

thorninmud's avatar

@Dutchess_III Because they’re using the word to describe a chemical process, not a mechanical one (the moving of gasses). We animals use our oxygen for the same purpose, to oxidize our food, liberating energy and CO2. In plants, the word is used to refer to that same oxidation reaction, so plant respiration is chemically analogous to our respiration, though not mechanically analogous.

XOIIO's avatar

@Dutchess_III It must have been an error because plants don’t release CO2. Respiration basically means the exchange of molecules, be it a human breathing, a plant exchanging CO2 for O2 or cellular respiration, where the cell is exchanging things like piruvic acid to make ADP

Dutchess_III's avatar

During respiration plants DO release O2 @XOIIO. It’s a much smaller process that is separate from photosynthesis, the end result of which is the release of CO2.

@thorninmud So how in the hell am I supposed to explain the difference? Do you think the lesson did a good job of explaining the difference? Should they come up with a different word? Should we call the President about this?

Adirondackwannabe's avatar

@Dutchess_III During respiration the plant is oxygenating it’s tissues and it does release some CO2. During photosynthesis it produces a lot more O2 than the CO2 it releases during respiration. The net is more O2 than CO2.

thorninmud's avatar

@Dutchess_III Your first two sentences above got it backwards. Photosynthesis releases O2, respiration releases CO2

Yeah, confusing. The text you quoted from the lesson is accurate, but it needs a little more detail.

Dutchess_III's avatar

Thanks @thorninmud. You’re right. Correction: During respiration plants DO utilize O2 and release CO2 @XOIIO. It’s a much smaller process that is separate from photosynthesis, the end result of which is utilizing CO2 which results in the release of O2.

@Adirondackwannabe I do understand that, but both processes could be considered forms of respiration, right?

I guess I’m having a problem with semantics. Every person knows from 3rd grade on what respiration is, but it gets switched up when you’re talking about respiration in plants. They need to come up with a different term, I think.

Adirondackwannabe's avatar

@Dutchess_III Photosynthesis is more in the line of digestion and metabolism than respiration. The O2 is just a waste product of photosynthesis.

Dutchess_III's avatar

Ok…that’s making sense @Adirondackwannabe. Kind of. But… in each case you’re talking about the exchange of gases, right? That’s what’s throwing me. Couldn’t the definition of “respiration” be “The exchange of gases”?

So what is the definitive answer to the literal question I asked?

CWOTUS's avatar

I hadn’t known that plants respire. I have known about transpiration, though.

Dutchess_III's avatar

Well, @CWOTUS That would be akin to the plant “breathing in” the CO2 it needs. ‘Ventually it expells the O2. Which sounds like respiration to me. It just gets more convoluted the more it goes on.

Guys…what percentage of a plants manufacturing of sugars would respiration account for? 1%, 5%, 10%, 20%? What do you think?

thorninmud's avatar

@Dutchess_III “what percentage of a plants manufacturing of sugars would respiration account for?”

None. The sugars come entirely from photosynthesis. The respiration breaks down the sugars.

Dutchess_III's avatar

From what I understand, photosynthesis and respiration do the same thing, breaking down the sugars, but come at it from two different directions.

CWOTUS's avatar

Not so. Photosynthesis makes sugars and starches. It’s how they get produced in the first place; they certainly aren’t in the ground for the roots to pull up, and they aren’t in the air.

Rarebear's avatar

Explain it like this. Plants create carbon and store energy through photosynthesis. But they also have to use energy as well. When they use energy, they break down carbon like animals do, and release CO2. That’s what respiration is.

Think of respiration as the opposite of photosynthesis in this context.

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