Aquarium sciences: Plant biochemistry – Part 1 Photosynthesis

Aquarium sciences: Plant biochemistry – Part 1 Photosynthesis In this three part series we will look at the key processes that power plant growth. By understanding these processes we can better control growth of plants in our aquarium and avoid unwanted algae. The first in this series will look at Photosynthesis, the second will look at Nitrogen, Phosphorus and Potassium and the third will look at Micro nutrients such as Iron, Magnesium and Calcium. How all of these things work together is the key behind a successful high tech planted tank.

To start with we will look at what is one of the most essential processes on the planet the fixing of carbon dioxide into polysaccharide molecules to produce glucose, better known as Photosynthesis. The best way to explain photosynthesis is through a series of diagrams. The first diagram is representative of what is happening inside a leaf, without going into the microbiology and molecular biology of a plant just understand that inside a leaf there are a number of chemical processes that occur on microscopic structures that exist inside leaf cells.

Image 1. Photosynthesis – The light side

So in this diagram what we see is the sun at the top, the energy from the sun is shining on these two blue oval shapes, underneath we have a water molecule (H2O) splitting into Oxygen (O2) and Hydrogen ions (H+). Above we have a number of molecules, Adenosine diphosphate (ADP), Phosphate (P), Adenosine triphosphate (ATP), NADP and NADPH. Now let’s have a look at what this diagram is trying to tell us. –          Let’s start by following the red arrows, the water molecule is split into oxygen and hydrogen ions, this releases an electron, this electron is transferred to the first blue oval. –          This is blue oval is Photosystem 2, the light from the sun excites electrons inside this photosystem giving them more energy.   –          This electron then passes along an electron transport chain (the purple triangle) and whilst doing this transfers some energy to ADP allowing it to bond with P to make ATP.   –          The electron eventually finds its way to the second blue oval (Photosystem 1) where again it is excited by the suns energy.   –          The electron then passes along this second structure (Orange rectangle) providing the energy to bond the Hydrogen ions to NADP thus producing NADPH.

Before we move on let’s just clarify what ADP, ATP, NADP and NADPH are.  These molecules or ones that are similar exist inside all living things, they can be called carrier molecules or transport molecules. The best way to think of ADP is like a dump truck; ADP gets loaded up with energy as this happens it binds a phosphate which locks this energy in changing it to ATP. The full dump truck goes to where it is needed drops off the energy losing the Phosphate and thus changing back to ADP, this once again gets loaded up with energy and the cycle continues. The same can be said for NADP, this gets loaded up with H+ turning it into NADPH, goes to where it’s needed, loses its H+ changing it back to NADP and the cycle continues again. We will look more at ATP when we discuss the role of nitrates and phosphates in parts 2 and 3.

From this diagram we can draw some conclusions the first is that splitting of water provides the electrons for this process to take place, thus establishing plants need water, obviously in the aquarium we don’t need to worry about this. The second thing we can establish is that light provides the extra energy these electrons need to power the reactions, hopefully you can already appreciate that different wavelengths of light have different energy states so hopefully this explains why we need light of a certain spectrum to grow our plants, providing the right amount of light energy for these electrons. From this first stage of photosynthesis we have added a phosphate to ADP, this process is called phosphorylation and because we used light it’s called photophosphorylation producing ATP. This ATP is what all living things use to transport energy think of it as a currency and certain tasks cost x amount of ATP.  

Image 2. Photosynthesis – The dark side

This diagram represents something called the Calvin cycle this is only used to illustrate the process and is not numerically accurate but what we can see is that Carbon dioxide (CO2) is fed in to this cycle, ATP powers the cycle by transferring the energy it captured during the light phase of photosynthesis returning to ADP, NADPH releases its Hydrogen thus converting back to NADP, these then go back to the light stage of photosynthesis to convert back to ATP and NADPH. What we have established during the dark phase of photosynthesis is that Carbon dioxide is fed in, Plants need CO2 , in the aquarium there is a certain amount of CO2 dissolved in the water but often this is the limiting factor in plant growth in the aquarium. The Glucose that is made in the Calvin cycle is used for respiration, a process by which glucose is used to produce massive amounts of energy that can be used for plant growth.

As a side note it is important to mention that glucose can be converted into a number of other compounds and macro molecules that provide a plant with structure such as cellulose. Cellulose could arguably be called the skeleton of the plant.

In short that’s photosynthesis, for a better understanding of some of these processes why not familiarise yourself with the microbiology of a plant or take a look at the carbon cycle in much more detail.   In summary Plants need light, this light must contain a certain amount of energy and so must be the correct spectrum, it feeds energy into this process. Plants need water, this helps power photosynthesis but also is used to help fill the plant much in the same way it helps to fill our bodies. Plants need carbon dioxide; plants use this to make food which they then break down to free up the energy they captured from the sunlight. Getting the right balance of light and CO2 is vital in maintaining a successful planted tank. Plants can’t survive on CO2 and light alone they need essential nutrients that they use to make things like DNA, proteins, fats and a plethora of other essential molecules, have a look at parts 2 and 3 to see how NPK and trace elements complete the story.