Botany-How Plants Work

(Unedited version)
Originally appeared in the February 2007 issue of
Tropical Fish Hobbyist Magazine

In the last 2 months we’ve looked at the history of botany and briefly looked at some of the structure of and terminology used for plants. This month we’ll look at how the plants actually work.

Bacteria to Plants

The first organisms to photosynthesis were bacteria. Very recent studies reveal that non-oxygen producing purple and green bacteria were the first to photosynthesis, followed by heliobacteria. Later developing bacteria produced oxygen in photosynthesis, as does the commonly known cyanobacteria which appeared even later, followed by algae and plants.

The Discovery of Photosynthesis

The discovery of photosynthesis began as the question of, “Where do plants get their mass?” Early experiments were carried out in the 1600’s to try to find out. The first was by a Flemish scientist, Jan Baptista van Helmont. He grew a willow tree in a pot and noted that though the plant had grown in mass the soil hadn’t changed much at all, so he hypothesized that the water had contributed to the increased mass of the plant. Later a Cambridge professor, John Woodward, conducted some experiments to test van Helmont’s idea found that it was not the water that was contributing to the increased mass of the plants. But still no one knew what the plants were using to increase their mass.

The next person to contribute to our knowledge of plant photosynthesis and gases in general was an amazing man, Jason Priestly. Priestly, an Englishman, studied to become a minister. He met Benjamin Franklin in 1766 and a year later began an amazing series of discoveries. His accomplishments ranged from grammar to electricity to discovering gasses and he was instrumental in our understanding of the composition of water. In 1772 he presented a paper called “On different kinds of air.” At one point in his life he lived next to a brewery and noticed the gas that came out of the fermenting floated near the ground indicating they were heavier than air. This was carbon dioxide; he found a way to make it at home, mixed it with water and discovered carbonated water. He also conducted experiments with sealed containers, plants and candles, and later mice that showed plants somehow changed the air. He believed plants restored the air. His theory as to how and what happened weren’t entirely correct, but led to a later more clear understanding. Priestly had very liberal ideas for his day and eventually found the need to move to the United States where he became friends with Thomas Jefferson and spent the last 10 years of his life. He contributed a great deal more knowledge to the world and is a very interesting man to study.

Jan Ingenhousz was the next person to make a large contribution to our knowledge of plants. He also used candles and those poor mice and discovered that plants needed light to “purify” the air. He also tested aquatic plants in light and darkness and noted the gas bubbles that form when the plant was in bright light. Finally Julius Robert Mayer, a physicist from Germany, realized that the energy from light was transformed in plants to produce new chemicals.

Details of Photosynthesis

6H2O + 6CO2 à C6H1206 + 6O2

These means that 6 molecules of water and 6 of carbon dioxide, in plants, with energy from sunlight, will produce one molecule of sugar and 6 of oxygen. But this is a simple statement for a very complex process.

In a typical terrestrial plant leaves take in oxygen and carbon dioxide, while the roots supply water through the plants vascular system. The leaves must protect themselves from drying out and still be able to exchange gases so they have special holes called stomata for gases to enter or be released, they still do loose a lot of water but it usually allows them to retain enough to not dry out.

Photosynthesis takes place in chloroplasts. The chloroplast is an organelle, or part of the plants cells. The chloroplasts contain pigments in little stacks of structures called thylakoids. There are many thylakoids in the chloroplast, arranged in pancake like stacks called granum. These are all surrounded by a liquid called stroma, all of which play a part in photosynthesis.

Pigments give living things their colors and are what makes plants green. The pigments absorb some colors of light and reflect others. The colors reflected are what we see. In plants there are several basic types of pigments; chlorophylls, carotenoids, phycobilins and xanthophylls. The most common chlorophylls are simply labeled chlorophyll a, b, and c. Chlorophyll a is the most important and the one that gives plants their green color by reflecting green while absorbing mostly violet, blue red and orange colors. The other pigments in plants are called accessory pigments. They absorb energy from other colors and can help a plant utilize the most energy in its specific surroundings, such as under water where not all colors of light may penetrate.

Photosynthesis takes 2 different steps. These steps are called light reactions and dark reactions. In the first step, the light reaction phase, energy from the sun causes the chlorophyll to loose an electron. It’s going to want to get that electron back so it takes it from the water. This is when the oxygen is released as a waste product. The electron that was originally lost from the chlorophyll goes through a series if proteins called the electron transport chain. This process creates chemical energy called ATP and NADPH which are used in the second step. In the second step the hydrogen left over after the oxygen was released from water are added to carbon dioxide with the energy created earlier to make sugar in a process called the Calvin Cycle. It’s these sugars that everything else in the world needs to feed on, either directly or indirectly through eating the plants.

Photosynthesis, light, aquatic plants and CO2

In the air there’s usually a lot more available light and CO2 for plants to utilize, and the levels are more stable. Aquatic plants have developed some different adaptations to deal with this situation.

Submersed leaves are different from leaves of plants grown emersed, even in the same plant. The leaves of aquatic plants are usually very thin compared to terrestrials. Many aquatic plants have highly dissected leaves. Frilly leaved plants like hornwort is a classic example. They do this to create a higher ratio of surface area to the volume of the plant. This allows them to get more CO2 from the water.

Other strategies include floating leaves, like water lilies and sending up emersed leaves, which quite a few of the stemmed plants will do in your tanks if you let them. These leaves can utilize the full sun and CO2 in the air, just like the fully terrestrial plants do.

The internal structures of submerged plant leaves are different from terrestrial plants also. The stomata I mentioned above that regulate the passage of gases in terrestrial plant leaves are absent in submerged plants. And while they stomata are on the lower part of terrestrial plant leaves they are on the upper part of floating plants.

Other nutrients

In addition to water and carbon dioxide plants need other nutrients to live also. Nitrogen, Phosphorus and Potassium are the other major plant nutrients needed. They also need Calcium, Magnesium, Sulfur, and even smaller amounts of iron, boron, copper, chloride, manganese, molybdenum and zinc. Some of these are usually found in your aquarium water, but others can be used quickly by your plants.

The easiest way to insure your plants have the appropriate nutrients is to do regular water changes and use fertilizers professionally prepared for the aquarium. For those folks who are a little more adventuresome or want more specific control there are a number of DIY recipes available to experiment with. A web pages search is a great way to find some of those recipes.

I had just finished writing this months column on Friday night just before going to San Francisco to spend the weekend at the Aquatic Gardeners Association Convention, which I have to say was a well planned and wonderful event. One of the speakers was Dr. Ole Pedersen from Tropica of Denmark. During his talk he brought up something that I’ve wondered about for quite some time. Though CO2 injection in planted tanks has been popular for a number of years now, I’ve often wondered how the levels people are putting in to their aquariums correspond with what’s actually in natural waters. During Dr. Pedersen’s talk he presented a map with rivers and lakes that showed their natural CO2 levels. Some creeks can have even more CO2 than what’s generally recommended for the planted aquarium. Lake levels on the other hand can be quite low.

The differences in other types of nutrients available in natural waters and the aquarium were quite extreme also Dr. Pedersen showed us some charts that showed that the nutrients most available in the typical tap water and our aquariums is just about opposite of what occurs in the tropical waters many of our plants come in.

For those that have read my column for a while you may remember that I haven’t been using CO2 and very seldom other fertilizers in my tanks. After listening to Dr. Pedersen’s talk I’m finally convinced enough to try a slightly higher tech tank myself, with CO2. Knowing that what levels of nutrients that occur in natural waters can help us all to create the best environment for the plants and animals that once lived there, that we bring in to our homes.

Questions or Comments?

If you have questions or Comments about this column, join the Natural Aquariums Forum and post them here.


Welcome to the Jungle | Into The Forest | The Creepy Crawlies | A Clearing in the Thicket | Algae Eaters for the Planted Aquarium
North American Natives | Why things go wrong Pt 1 | Why things go wrong Pt 2 (Algae) | Algae Eating Shrimp | Lo-Tech Tanks
Welcome to the Fish Room | The Stemmed Plants | Mosses | A Livebearer Biotope | Planted Tank Social | The Genus Hygrophila | Cyanobacteria
Easy Plants | What I Did Last Summer | Decorations in the Planted Tank | Botany-An Introduction to Plant Biology | Botany-Anatomy of a plant
Botany-How Plants Work | Easy Rosettes | Going High-Tech | Floating Plants | Dealing with Success | Bringing the Outside In | Vallisneria
Hair Algae | Flowering Aquarium Plants Part 1 | Flowering Aquarium Plants Part 2 | Liverworts in the Aquarium | Elements of Design
Planted Aquarium Maintenance | More Mosses | Invaders | Ferns in the Aquarium | Setting up a Planted Aquarium
Seaweeds of the Pacific Northwest | Proserpinaca | Hardware for the Planted Aquarium | Rotala | Neocaridina Shrimp | Lo-tech Tank Tips


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