For example in hydroponics, the plant root has only water… where does it get the extra matter it needs to make more plant cells so it can grow? How does water turn to plant?
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It’s actually weirder. Plants are mostly made out of air. They take carbon dioxide (CO2) and turn it into oxygen (O2). This why planting trees is a popular activity for offsetting carbon emissions. This process generates leftover carbon, and that carbon is used to build the physical structures of the plant. Those physical structures are still packed full of water, which gives the plants most of their mass, but the reason why you see a leaf or stem and not a puddle is because of carbon plucked from the air.
It is not just water but also carbon dioxide. Plants absorb light using chlorophyll, this is why they are green. The energy they get from this is used to take the carbon atoms from carbon dioxide and the hydrogen atoms from the water to make sugars and fats. The remaining oxygen atoms are combined to make oxygen molecules which they release into the atmosphere. They also get nitrates made by nitrogen fixating bacteria in the ground. These nitrates are combined with the sugars to make amino acids which is the building blocks of proteins. So a plant is able to get everything it needs from the soil, the air and the sun.
Plants, through the wonder of chlorophyll, have the amazing ability to take water and atmospheric carbon dioxide, and, harnessing the energy from the sun, combine H2O with CO2 and produce cellulose (plant matter) and oxygen.
You are right to question this – because your premise is a bit off!
The vast majority of “plant flesh” is made up of three elements – Hydrogen, Carbon, and Oxygen. Plants get these elements from water and carbon dioxide gas in the air. They use light to breakdown the molecules and reconfigure them as “plant”. By weight, this is the majority of the plant’s flesh.
But plants need tons of other elements as well, nutrients and minerals like nitrogen and phosphorus and tons of others.
Usually plants get these from the soil, or from other things that get them from the soil, but in the case of hydroponics there is no soil. So for hydroponics the minerals need to be dissolved into the water first, like salt is dissolved in the ocean, and then the plants can absorb the minerals via their roots in the water.
Hydroponics isn’t just water but also nutrients, essentially the plant version of Flintstone vitamins.
If you picture a balloon arch. Plants pull CO2 from the atmosphere to make the balloons, and then fill them with water. They use those vitamins to perform those activities.
There is a process called photosynthesis that converts carbon dioxide + water into oxygen + sugar. Photosynthesis uses sunlight as its driving energy. Sugar can take many forms depending on the type of plant. Sugar is mainly a way of storing energy, but it is also a raw material that the plant can use to make other chemicals needed for life and growth.
Carbon dioxide, water, and sugar are made up of the elements carbon, oxygen, and hydrogen, so photosynthesis just rearranges the atoms into different compounds.
Air.
This is the cycle that happens with grass, leaves, wood, coal, oil, etc. Burn plant matter to make CO2 and heat and then plants and algae pull the CO2 out the air to get carbon to grow in size.
Everyone else has pointed out that a plant growing in water gets its carbon (the backbone of most of its molecular mass) from the air.
I just want to point out one more little thing: the above is true of plants growing in soil as well.
Plants get most of their “plant” from the air, not from water! All their carbon comes from CO2 in the air, not from the ground. That’s why they can be grown hydroponically and how thing like [“air plant” species](https://m.media-amazon.com/images/I/71wm8RbPzcL._AC_UF1000,1000_QL80_.jpg) can exist that don’t need soil or even liquid water around roots!
Through photosynthesis, plants “inhale” carbon dioxide (CO2) and “exhale” oxygen (O2). See the difference – the carbon gets removed and becomes part of the plant.
I’ll go a bit deeper. There are 2 parts of photosynthesis, the part that needs light and the part that doesn’t…cleverly named the light and dark reactions. In the light reaction, light energy is used to break apart water and excite some electrons. The electrons go through a number of proteins and turn NADP+ to NADPH. The hydrogen we got from water goes through ATP synthase to turn ADP to ATP. Thats how we get oxygen as waste, the plant only wanted the hydrogen ion for the process. In the dark reaction, that NADPH and ATP is used to fix CO2 and turn it into a sugar intermediate, G3P. For every 3 CO2s we get 1 G3P, and 2 G3P combine to make 1 glucose, which is either used in cellular respiration, stored as starch, or turned into cellulose for growth.
So the water is just used for the hydrogen ions. But they also need that light and CO2.
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