Bio 1000- Photosynthesis Introduction

Photosynthesis plays huge factor on carrying out life on Planet Earth. Whether it is producing life's essential molecule OXYGEN or creating the green colour in leaves, believe it or not, photosynthesis is part of your day to day lives.

SO WHAT EXACTLY IS PHOTOSYNTHESIS?

Photosynthesis is defined as the conversion of carbon dioxide into organic molecules such as carbohydrates using light energy provided by the Sun.

GENERAL EQUATION:

6CO₂ + 6H₂O + Energy --> C₆H₁₂O₆ + 6O₂

 Now on planet Earth, there are four types of organisms classified within the realm of photosynthesis which includes:

1. Autotrophs
• Autotrophs ("self-feeding) are photosynthetic organisms that make all their required molecules from carbon dioxide
2. Chemoautotrophs
• Chemoautotrophs use inorganic compounds as source of energy. Ideally, these organisms undergo oxidation reactions of electron donors
3. Photoautotrophs
• Photoautotrophs are photosynthetic organisms that use light energy to synthesize organic molecules
• Note that these organisms are usually primary producers because not only do they produce organic molecules but also represent the source of organic molecules for consumers
4. Heterotrophs
• Heterotrophs are consumers and decomposers that require an already synthesized source of organic molecules
• This would include humans, animals .etc.

As mentioned earlier on another page, organisms are classified into 3 domains: Archaea, Bacteria and Eukarya. Believe it or not, photosynthesis only occurs in Bacteria and Eukarya and isn't present in Archaea. WAIT, DOES THAT MEAN ARCHAEA DON'T UTILIZE LIGHT ENERGY? No, in fact Archaea do harvest light energy to convert into chemical energy almost all the time. The reason, however, why they aren't classified under photosynthesis is that they don't use light energy to convert CO2 into carbs. Instead they undergo a process called phototropy which is a generalization of any process that converts light energy into chemical energy.

THE 2 CYCLES THAT MAKE UP PHOTOSYNTHESIS

The conversion of carbon dioxide into carbohydrates require two processes:

1. Light Reactions
2. Calvin Cycle

THE FIRST PART TO PHOTOSYNTHESIS: LIGHT REACTIONS

The first process that initiates photosynthesis is light reactions. Light reactions can be summed up into two stages:

1. The capture of light energy by pigment molecules
2. The utilization of that energy to synthesize NADPH and ATP

THE SECOND PART TO PHOTOSYNTHESIS: THE CALVIN CYCLE

The second process immediately begins after the completion of light reactions. Now, the Calvin cycle can be said to be a much smoother cycle than light reactions itself and can be summed up through a single stage:

The electrons and protons carried by NADPH and the energy from ATP is used to convert carbon dioxide into an organic molecule. This conversion is referred to as "carbon dioxide fixation"

Carbon Dioxide Fixation

Carbon dioxide fixation is defined as a reduction reaction with electrons and protons being added to carbon dioxide. Often, this conversion results in the formation of carbohydrates in the general formula of :

(CH2O)n where n signifies the number of carbons

 

SO OKAY, I GOT WHAT CO2 FIXATION IS, BUT WHERE EXACTLY DO THESE ELECTRONS COME FROM?

Well, good question. I have no idea. I'm joking. In fact for many organisms, the source of electrons comes from Earth's essential element: WATER! The stripping of electrons from water is referred to as oxygenic photosynthesis and can be summed up as:

2 H2O + light energy------ 4 H+ + 4 e- + O2

 

That is, 2 water molecules with the addition of light energy will produce the stripping of the water molecules into 4 protons or H+, 4 electrons and one oxygen molecule. We will discuss this phenomena further on another page. 

The major direct product of Calvin cycle:

3- carbon sugar which can easily combine to form a 6-carbon monosaccharide

This is now the end of Photosynthesis Introduction. To get further in depth details, make sure to click on the following pages listed as Photosynthesis to learn more.