Unravelling the Mystery of Photosynthesis

We’ve all seen the chemical reaction of photosynthesis and been totally confused by a teacher trying to explain something so complex. In fact, photosynthesis is known to have at least 50 intermediate action steps that ultimately generates the plant’s food from light. So, you are correct in thinking that photosynthesis is nothing but intricate. It is totally understandable why it confuses just about everyone, including me. But, lets just take a moment to look at that equation again to understand the general idea of photosynthesis.

Photosynthesis Chemical Equation.

Photosynthesis is essentially the plant's form of respiration. Plants perform the counter part to our own respiration process, breathing in oxygen to give off carbon dioxide. The plant respires to generate energy from light to produce sugars that build up the plant's biomass.

This equation tells us that in order to make plant food in the form of glucose (C6H12O6), the plant will need carbon dioxide (CO2) and water (H2O) along with light as an energy source. Everything about this equation explains photosynthesis, however it should not be taken literally. Remember I said that there are at lest 50 intermediate actions that we know of. So, it is important for us to understand how photosynthesis actually works instead of trying to decode this equation that doesn’t really have enough information. Understanding how photosynthesis works and how the plant utilizes light means we can manipulate the light to our advantage in the planted tank. Throttling the light can result, to a point, in different appearances of a plant. 

SO LETS START BY ASKING AN ESSENTIAL QUESTION. WHY DO PLANTS NEED LIGHT?

Now I am sure you’ve never seen plants gorging on cheeseburgers, donuts, salad (would that be considered cannibalism). Instead of eating delicious human food, plants sustain themselves by making their own food. But in order to do that, plants need an energy source to drive their photosynthetic mechanisms. What’s the most abundant energy source on Earth? Light. Plants need light to power their metabolism to create the building blocks they need to grow. Plants are essentially starving if they have no light. 

HOW EXACTLY DOES PHOTOSYNTHESIS USE LIGHT ENERGY TO MAKE FOOD?

Diagram of a Chloroplast

Here is where photosynthesis takes place, in the organelle called Chloroplast. It is on the thylakoids that light energy is converted to the chemical energy that powers the plant and in the stroma where sugar are produced. 

Image Credit: Plant Physiology 4th Ed.

Photosynthesis is broken down into two different stages; the light-dependent stage and carbon reactions. In the first stage, the light is gathered and transformed into energy that is used in the second stage to create sugars, or plant food.

I like to think of plant leaves as solar panels. It is in the first stage of photosynthesis, the light dependent stage, that the 'solar panel' leaves soak up light energy. All of this takes place within the chloroplast of specialized plant cells. Think of the chloroplast as rechargeable batteries. Inside these chloroplast there are stacks of thylakoids where electron transport chain is housed. To charge the ‘battery’, light is collected by photosynthetic pigments, like chlorophyll. The light energizes the pigments to oxidize water into oxygen gas and collects the extra electrons. These electrons aid in the formation of a useable energy source, ATP and NADPH. As the energy is transformed it is released into the space around the thylakoids, the stroma.

It is important to know that none of this can happen without light. Since light is the limited reagent to photosynthesis, he amount of food a plant makes depends on the amount of light it receives. 

BUT IF THE 1ST STAGE OF PHOTOSYNTHESIS IS LIGHT DEPENDENT, WHAT HAPPENS WHEN THE LIGHT IS TURNED OFF? 

Remember all the ATP and NADPH that was released into the stroma? Well its purpose is to supply the Calvin Cycle with energy in the dark. In this carbon fixation process, carbon dioxide is attached to a simple carbohydrates. ATP and NADPH is then used to reduce this carbohydrates into smaller more simpler sugars. A few of the sugars go on to regenerate the simple carbohydrates that are used in the beginning of the cycle. As the cycle continues, more simple sugars are formed and then combined with other sugars to make sucrose and starch, for example, that build and maintain the plant biomass. It really is a sugar building machine!

Although the carbon fixation process can occur at all times, regardless of the light being present, plants do max out on how much oxygen can be produced in the first stage. At some point, light is not being used anymore. Remember how I related the chloroplast to a rechargeable battery? Like a plant as some point the battery cannot take any more charge because it is already full. Therefore, it is best to give the plant a series of light on and light off phases. It uses up energy during the light off phase and recharge during the light on phase. 

Personally, I am a visual and hands-on learner. Since we can't hold a chloroplast and watch the magic as it unfolds, here is a video so you can see everything in action.

 

Photosynthesis in Action

Watch along as all the players in photosynthesis get lost in the action yet contribute to a spectacular showing of nature.

Video Credit: Encyclopédie Médicale

 

WE NOW KNOW THAT PLANTS NEED SCHEDULED LIGHT ON/OFF TIME TO COMPLETE PHOTOSYNTHESIS. SO, HOW MUCH LIGHT, TIME WISE, DOES A PLANT NEED?

Pearling on Ricca fluitans

Oxygen is accumulating on the leaves of Riccca and is a sign that the first stage of photosynthesis is exhausted. Nap time starts now.

Image Credit: infopimp

In order to know how much light a plant needs, we have to measure when the plants ‘recharging system’ is exhausted. Or, at what point does the battery become full and blinks to be disconnected from the plug? The byproduct of the photosynthesis gives us a clue as to when the lights should be turned off. Remember in the light-dependent reactions water is used to extract electrons to power the regeneration of ATP and NADPH. The byproduct of this reaction is oxygen gas. As light is harvested and used, oxygen begins to builds up and eventually exits through the stomata, root cell walls and other places. Since oxygen gas and useable energy go hand in hand, if oxygen become oversaturated so does ATP and NADPH. 

In aquatic plants it is very easy to tell when a plant’s light-dependent reactions are exhausted because water is abundant. Bubbles gathered on leaf surfaces show the signs of excess oxygen that is being released. In the Aquascaping World this is known as pearling and generally happens between 8 to 10 hour periods of light. 

SO WHAT'S THE TAKE AWAY? WHY DO WE NEED TO KNOW ABOUT PHOTOSYNTHESIS?

Don't think of photosynthesis as another confusing part of growing plants. Just knowing the basics gives you a solid ground on understanding how plants utilize light. Cracking the code of our silent partners lets us know exactly what the plants want and need. Light is an essential player in photosynthesis and without it plant life wouldn't be possible. Applying this knowledge to our aquascapes ensures that we are doing the best we can to maintain an ecosystem that is dependent on light. So, here is a list of all the things I learned from writing this article and how I am going to apply them to my aquascapes.

  1. Plant life in an aquarium requires light to survive. I'll need to provide light to my aquascapes to promote plant growth and metabolism.
  2. In order to complete photosynthesis, plants need 'lights off' time. Just like us, plants need to rest and metabolize the sugars using energy transformed during the light-dependent phase. Turn the lights off to simulate day length.
  3. Plants give off excess oxygen gas to show that the photosystem has plateaued. Watch for pearling. Too much light isn't helping either of us. So, turn the light off when pearls starts to appear.

Next time I'll discuss what color temperature of light plants prefer and how to quantify the amount of light we give to our aquatic plants. Moving forward towards understand light so we ca use it to our advantage in our aquascapes. If you like what I'm doing on The Greener Side, please subscribe to stay updated on my lasted articles, blog post on my aquascape, and news in the Aquascaping World.

Resources & Credits

Lincoln Taiz and Eduardo Zeiger. Plant Physiology 4th Edition: PhotosynthesisISBN-10: 0878938567