Illuminating Photosynthesis

  • By Rick Groleau
  • Posted 12.22.16
  • NOVA

Photosynthesis in plants and a few bacteria is responsible for feeding nearly all life on Earth. It allows energy from the sun to be converted into a storable form, usually glucose, which plants use to grow and thrive. Photosynthesis also generates the oxygen that animals need to survive. But here we animals repay the favor. We exhale the carbon dioxide that plants need for photosynthesis. Here, take a closer look at the oxygen/carbon dioxide cycle and the process of photosynthesis.

This story reveals the secret of life,
     or at least it reveals a small bit.
It shows how a plant uses light from the sun
     and from this how we all benefit.

It shows you how plants create glucose.
     It shows you the cycle of gases.
It asks you some relevant questions
     that aren't asked in life science classes.

The Cycle

This kid and her plant use each other,
     and this screen will show you just why.
When the kid and her plant interact,
     all the molecules fly.

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A girl and her plant, together in a room Enlarge Photo credit: © WGBH Educational Foundation

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Here, the girl exhales carbon dioxide and the plant absorbs it, while the plant emits oxygen and the girl inhales it. The plant circulates water. Enlarge Photo credit: © WGBH Educational Foundation

 

Atomic Shuffle

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Photosynthesis is needed for life to subsist. Enlarge Photo credit: © WGBH Educational Foundation

Photosynthesis, the trapper of sunlight;
     it's needed for life to subsist.
All plants use the process to make food;
     without it most life would desist.

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H2O (water) molecules Enlarge Photo credit: © WGBH Educational Foundation

The process begins with plain water
     but not from the tap does it flow.
Some water is made within leaf cells
     and some is sucked up from below.

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H (hydrogen) molecules and O2 (oxygen) molecules Enlarge Photo credit: © WGBH Educational Foundation

With energy gained from sunlight,
     the Hs are stripped from each O.
The oxygen atoms form twosomes,
     and out of the leaf they all go.

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H (hydrogen) molecules and CO2 (carbon dioxide) molecules Enlarge Photo credit: © WGBH Educational Foundation

Meanwhile CO2 has just entered
     through holes in the leaf called stomata.
The gas is exhaled by Earth's creatures,
     including all invertebrata.

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H (hydrogen) molecules, CO (carbon monoxide) molecules, and H2O (water) molecules Enlarge Photo credit: © WGBH Educational Foundation

CO2 taken in goes through changes;
     its atoms get pulled to and fro.
It loses an O, which sticks to two Hs; 
     together they form H2O.

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Glucose molecules Enlarge Photo credit: © WGBH Educational Foundation

But the plant has a goal that is bigger in scope;
     it's out to make food it can keep.
It builds a big structure of Cs, Hs, and Os;
     it's glucose, and boy is it sweet.

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A note about this equation: Sometimes the above formula is shown with six H2Os on the left side and no H2Os on the right side (6H2O + 6CO2 * C6H12O6 + 6O2). This is a simpler way of looking at the process—one that doesn't take into consideration, as this feature does, that the oxygen released through photosynthesis comes only from water molecules. Enlarge Photo credit: © WGBH Educational Foundation

Some folks get a kick from equations,
     which explain things by some kind of law.
If by chance you are one of these people,
     then this explains what you just saw.


12H20 + 6CO2 * C6H12O6 + 6H2O + 6O2

 

Three Puzzlers

PUZZLER 1

 

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A person sits under a tree. Enlarge Photo credit: © WGBH Educational Foundation

 

A tree inhales carbon dioxide;
     it needs to if it's to survive.
But can it release enough O2
     to keep just one person alive?    

 

YES or NO 

 

Puzzler 1: Yes

Well, a bonsai tree couldn't produce enough oxygen to keep someone alive, but an average-sized tree certainly could. In fact, an average-sized tree could produce enough oxygen to keep from two to four people alive.

Puzzler 1: No

Well, if you imagined that the "tree" in the question referred to a bonsai tree, then you made the right selection—it certainly couldn't produce enough oxygen to keep a person alive. An average-sized tree, however, could produce enough oxygen to keep from two to four people alive.

 

Puzzler 2

 

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A potted plant at night Enlarge Photo credit: © WGBH Educational Foundation

 

A plant needs its own food to function.
     It can't make its own food at night.
Beyond a few hours of darkness,
     can a plant stay alive without light?      

 

YES or NO 

 

Puzzler 2: Yes

This is really a trick question. "No" should be correct—a plant can't stay alive without a regular dose of light—but it isn't entirely correct. Yes, plants do need food to survive, and yes, they generally get this food through photosynthesis. However, experiments have shown that it is theoretically possible to keep a non-photosynthesizing plant alive, at least for a while.

John S. Boyer and his coworkers at the University of Delaware have kept the developing kernels of corn plants alive and growing in a low-light environment by feeding them sugar water. The sugar water, which was fed to the plants intravenously, sustained the growing kernels for five days. Without the sugar and light, the kernels would have died within a day.

Puzzler 2: No

This is really a trick question. "No" should be correct—a plant can't stay alive without a regular dose of light—but it isn't entirely correct. Yes, plants do need food to survive, and yes, they generally get this food through photosynthesis. However, experiments have shown that it is theoretically possible to keep a non-photosynthesizing plant alive, at least for a while.

John S. Boyer and his coworkers at the University of Delaware have kept the developing kernels of corn plants alive and growing in a low-light environment by feeding them sugar water. The sugar water, which was fed to the plants intravenously, sustained the growing kernels for five days. Without the sugar and light, the kernels would have died within a day.

 

Puzzler 3

 

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A plant inside a jar and O2 (oxygen) molecules Enlarge Photo credit: © WGBH Educational Foundation

 

A plant that receives the nutrition it needs
     will continue to live, we now know.
But what if we took all its O2 away?
     Do you think that the plant could still grow?   

  

YES or NO 

 

Puzzler 3: Yes

The answer is actually "no, a plant could not grow without oxygen." Perhaps you're thinking that a plant needs to take in carbon dioxide in order to survive and that it expels oxygen as the waste product of photosynthesis. This is certainly true. But a plant doesn't only store the food it produces—it uses some to feed itself. And when a plant feeds on its own food, that food is broken down in the same way that it's broken down in an animal's body (including yours): with oxygen. The oxygen is needed to break down the carbohydrate molecules and release the energy stored in those molecules.

While a plant is photosynthesizing, it's producing more than enough oxygen to break down its own food. But if you were to take away the oxygen surrounding the plant as well as the light it needs for photosynthesis, the plant would in effect starve.

Puzzler 3: No

That's right. The plant would not grow. It is true that a plant needs to take in carbon dioxide in order to survive and that it expels oxygen as the waste product of photosynthesis. But a plant doesn't only store the food it produces—it uses some to feed itself. And when a plant feeds on its own food, that food is broken down in the same way that it's broken down in an animal's body (including yours): with oxygen. The oxygen is needed to break down the carbohydrate molecules and release the energy stored in those molecules.

While a plant is photosynthesizing, it's producing more than enough oxygen to break down its own food. But if you were to take away the oxygen surrounding the plant as well as the light it needs for photosynthesis, the plant would in effect starve.

This feature originally appeared on the site for the NOVA program Methuselah Tree.