Where does the plant mass come from?

[keith]

This may turn out to be a silly question, but...

Not being able to wait for Spring and driven by a bad case of cabin fever, I recently made some containers to experiment with hydroponics. It's going surprisingly well I think. The spinach is not happy, but the lettuce and tomatoes are looking good after 3 weeks.

I have some understanding of how a plant works, but what I'm wondering is where does the actual mass (tissue, cells) come from? Does a plant literally create some amount of mass out of "thin air" through photosynthesis? Does carbon from carbon dioxide make up a significant part of the mass? Hypothetically, if you grew a plant in a closed system and measured the amount of water and nutrients that were consumed, would that exactly equal the weight of the plant, or would the plant weight more. I'm thinking it would weigh more, and if that's so, what's the breakdown in percentages of mass from carbon, nutrients and water?

High school biology is a long way gone. :-)

-Keith

[keith]

Answering my own question here...

It appears that, in an average tree at least, some 90% of the bio mass is derived from oxygen and carbon in the air. I would guess that if you weighed a healthy plant, around 90% of that weight would be water, but after removing the water, the remainder - the actual structural parts - would be primarily derived from elements in the air.

I had no idea it was that much. That's fascinating! I'm going to start fermenting my beer next to my plants. See what a little more CO2 does. :-)

--------------------------------

Let's think about that from the perspective of the underlying chemistry. When we talk about water we describe a molecule of two hydrogen atoms and one oxygen atom. But these words refer to atoms, not the energy which bonds the atoms together. Wood is mainly a carbohydrate called cellulose (chemical formula, C6 H10 O5). Compare the mass of these atoms: carbon is about 12 mass units, oxygen about 16, and hydrogen about 1. Thus carbon and oxygen contribute 72 (6 x 12 for the carbon) plus 80 (5 x 16 for the oxygen) mass units while hydrogen contributes only 10 (10 x 1) units. Thus carbon and oxygen make up 152/162 of the cellulose molecule by mass. And experiments using tagged markers have shown that this oxygen comes from the carbon dioxide molecule not from the water molecule. The carbon is not entering the plant in minerals, or in the water, but rather from the air.

Carbon dioxide enters plant leaves as a gas. It is combined with hydrogen from water to produce carbohydrates. In this process of photosynthesis, oxygen gas is produced as a by-product of the reaction. If wood is mostly carbon and oxygen, then wood comes mainly from air! That rather unexpected point should get students attention: the mightiest redwood, or rainforest giant, is made mostly of elements derived from air. These first two Activities use something readily available, a pencil, and something less easy to obtain--dry ice--to show students how carbon behaves as it changes from a solid to a gas.

[Darren Garrison]

interesting:

[DavidPT40]

IIRC, cellulose is the isomer form of glucose. C6-H12-O6. So plants use photsynthesis to make tissues and such.

[Jim Elbrecht]

-snip-

In some plants it promotes blossoming.

Jim

[David Hill]

"..........I'm going to start fermenting my beer next to my plants. See what a little more CO2 does. ........."

Extra CO2 should give you an increase in leaf density and a higher petal count in flowers, which is why you will often find professional growers using Gas burners to add extra CO2

[David J Bockman]

In monocots and dicots, the energy starts in the seed's cotyledons, which are highly modified leaf structures consisting mostly of starch. It's complex mechanism that triggers germination, much more than I know about, but once germination is triggered the growth is fed from the cotyledons.

Secondary growth arises from zones of maturation, in which clusters of 'totipotent' cells called Parenchyma can become any type of cell the plants need.

An excellent book is 'Botany for Gardeners', Capon.

Dave

[Brian]

Until relatively recent times it was believed that plants took nothing from the soil. Experimenters weighed the soil in containers before and after cropping and found no difference. This being due to their inability to weigh such minuscule amounts. To determine the relative quantities a plant should be weighed fresh and then when totally dry. This shows the water content. Burning the dry plant fiercely will give the remaining two figures. The final weight will be that of minerals taken from the soil [very little] and the difference will be that having come from the atmosphere. Best Wishes~~ Brian

.pdf