Do Plants Do Photosynthesis Even When They Are Not Growing?

3 years ago

I have some plant science questions around photosynthesis:

* The resources linked below are making the claim that plants only do photosynthesis when they are growing. Their analytical method is to look at how much carbon is produced by photosynthesis, as CO2 is split to produce carbon and oxygen. They are inferring that all of that carbon goes to building new tissue in the plant. Therefore their claim is that all photosynthesis goes to plant growth:

http://www.madsci.org/posts/archives/1999-02/917906305.Bt.r.html

https://www.gardenmyths.com/houseplants-increase-oxygen-levels/

Is that strictly true? Can't plants use carbon to simply repair existing tissues, not adding any new net weight to the plant? At some intuitive level, I want to believe that plants produce oxygen whenever they get light and have CO2 and water available, even when the plants are not "growing".

* Plant respiration takes in oxygen and releases CO2. The amounts of CO2 used in photosynthesis greatly outnumbers the CO2 released by plant respiration. But the surprise for me was that plants in the dark only produce CO2, and they never do photosynthesis to release oxygen. So at night plants release more CO2 than they consume.

I connected this to the stories you occasionally read about people dying in root cellars. Most people blame these deaths on some poisonous gas released by rotting potatoes roots. But in a very dark root cellar, is it possible that the plants are just respiring for weeks at a time, and CO2 - which is heavier than oxygen - might just be collecting in the root cellar? That would explain why sometimes whole families get wiped out, with one person after another collapsing in the root cellar, and then rescuers follow them in and also collapse. This suggests to me that a root cellar should always be given some light, so that the plants can do some photosynthesis to use up the CO2 from respiration.

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Comments (5)

westes Zone 9b California SF Bay
Original Author
3 years ago
@tapla (mid-Michigan, USDA z5b-6a) Here are the questions I referenced in another post
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tapla (mid-Michigan, USDA z5b-6a)
3 years ago
True growth is measured by the increase in a plant's dry mass. A plant can be extending its branches, growing taller, or putting on new branches w/o actually growing in the true sense of the word.

Plants grow because they need storage space for the food/energy created during photosynthesis; but in order to 'grow' they need to be creating more food/ energy during photosynthesis than they consume during respiratitive phase. The point at which a plant is using exactly as much food/energy as it's consuming is called the LCP (light compensation point). We know with great certainty that a plant is fully capable of short term survival when it is creating less food/ energy than it takes to keep its systems orderly AND grow, and can actually lose weight during this period (shedding leaves/ roots/ branches); so, YES, plants carry on photosynthesis even while growth is stalled and they are losing dry mass/ shedding parts.

Al
westes Zone 9b California SF Bay thanked tapla (mid-Michigan, USDA z5b-6a)
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westes Zone 9b California SF Bay
Original Author
3 years ago
last modified: 3 years ago
@tapla (mid-Michigan, USDA z5b-6a) Is the carbon from photosynthesis used to construct glucose to serve all of the plant's energy needs? I wasn't sure if the carbon was just being used for the plant's structure.
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tapla (mid-Michigan, USDA z5b-6a)
3 years ago
last modified: 3 years ago
During photosynthesis, light energy is used to drive the reduction (oxidation/ removal of electrons) of 6 molecules of CO2 and 6 molecules of water and reorganizes them to make 1 molecule of sugar (glucose), with O2, hydrogen ions and electrons being a byproduct. After the sugar is synthesized it can be stored or broken down and pressed into immediate use for growth or keeping systems orderly. The formula for photosynthesis looks like:

6CO2 + 6H2O + light energy > C6H12O6 (sugar) + 6O2

The carbon used in photosynthesis comes from the air/ atmosphere. Glucose is a hydrocarbon chain the plant utilizes as the energy source to create other hydrocarbon chains from which the plant is made. Cellulose is the most abundant organic molecule on earth, and it accounts for about 1/3 of the average plant's mass. It's formula is:

(C6H10O5)n .........not very different from glucose, yes?

Al
westes Zone 9b California SF Bay thanked tapla (mid-Michigan, USDA z5b-6a)
westes Zone 9b California SF Bay
Original Author
3 years ago
@tapla That's amazing how close cellulose is to glucose, and it makes you realize you can do amazing things with some simple chemistry.