Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Plants getting food under water

Learn more physics!

Browse our 6758 answers by or search term

Q:
Do underwater plants produce food the same way as plants above water? ... and do underwater plants need carbondioxide? How do they get it if its underwater?
- Nicholas (age 10)
ST Augustines, Kendall Park,NJ
A:
First of all, what is photosynthesis? Well, it's how plants get their food. When you get hungry you probably just sit down and have something to eat. Plants, however, don't have it so easy, they have to make their own food. Plants use water (H2O), carbon dioxide (CO2), and sunlight to make glucose (C6H12O6), also known as sugar. The process follows the chemical equation: 6CO2 + 6H2O+light -> C6H12O6. The light has a special role. Just like your car needs gas in order for it to run, photosynthesis needs some kind of energy source for the reaction to occur, because the product has more energy than the starting materials. That fuel is sunlight. Plants have special little chemical machines based on the light-absorbing molecule chlorophyll to conduct the photosynthesis. Most plants and many bacteria get their food by photosynthesis.

So chemically, the photosynthetic process that land and aquatic plants use to produce food is identical. Both types of plants require carbon dioxide, water, and energy to produce glucose (their food). The only difference between photosynthesis in aquatic and land plants is where in their environments they get these nutrients. Land plants get water from the ground through their extensive root system, carbon dioxide from the air through their stomata (tiny holes in a plant's leaves), and energy from the sun. Aquatic plants get water and carbon dioxide from their aquatic environment and, like the land plants, light energy from the sun. Even though the plant is underwater, it still gets its energy from the sun because sunlight can pass through water.

To understand where aquatic plants find this CO2 think about your favorite kind of pop. Those bubbles that give the pop its fizzy nature come from CO2 that's dissolved in the liquid.


If you would like more information on sunlight passing through water and how CO2 is dissolved in water and taken up by plants, try the following site: http://www.thekrib.com/Plants/phisio.html

-KH (w small changes by Mike W,)

(republished on 07/18/06)

Follow-Up #1: underwater CO2

Q:
As you mentioned that underwater plant get the CO2 from animals ( fish etc). But as we know that in evolution journey, underwater plants came first. So during that era how underwater plants use to get the CO2 ( Since no animal around who is creating the CO2)
- vinay Shrivastava (age 32)
Delhi, Delhi, India
A:
The previous answer was misleading, because underwater CO2 doesn't have to come from underwater- some can dissolve in from the atmosphere. That point is now fixed.
At any rate, photosynthesis began among bacteria, before multi-celled plants and animals became prominent.  However, non-photosynthetic bacteria had been around for billions of years first. We can trace when photosynthesis became common because it was the source of most of the atmosphere's O2, so iron deposits from before the photosynthetic period are not oxidized.
So when modern photosynthesis started, CO2 was already around.

Mike W.

Photosynthetic plants also respire -- they create CO2 when they use their energy-storing compounds during metabolic processes.

Tom

(published on 03/14/07)

Follow-up on this answer.