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Understanding Photosynthesis as a Biologically Closed Process
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# Objective

• To understand the mechanism of photosynthesis using remote experimentation.

# Theory

Photosynthesis is a physico-chemical process by which plants, algae and some types of bacteria convert light energy into chemical energy. Green plants capture light energy and then convert water, carbon dioxide and other minerals into oxygen and other organic compounds.  The energy that drives photosynthesis is the solar energy that originates from the center of the sun. Here the mass gets converted to heat as a result of fusion of hydrogen. As the heat energy reaches the sun’s surface, a small proportion of it is converted to light energy by the process of black body radiation. The light energy is then absorbed by the plants on the earth’s surface. After that, photosynthetic organisms transform light energy to chemical energy through a series of energy transducing reactions.  If the photosynthesis process is ceased, then there would not be sufficient food or other organic matter on Earth and the atmosphere become nearly devoid of gaseous oxygen. In such a situation, the only organisms that can exist is chemosynthetic bacteria, which utilizes chemical energy derived from certain inorganic compounds and thus independent of light energy.

## Overview of photosynthesis reaction

Photosynthesis is generally a light energy mediated oxidation-reduction process. [Oxidation refers to the removal of electrons from a molecule where as reduction refers to the gain of electrons by a molecule]. In green plants, light energy is used for oxidation of water molecule (H2O) resulting in the production of Oxygen gas (O2), Hydrogen ions (H+) and free electrons.  Oxygen (O2) is a byproduct of photosynthesis and is released into the atmosphere. The removed free electrons and hydrogen ions are ultimately transferred to Carbon dioxide (CO2) which is then reduced to form organic end-products.

$6 CO_{2} + 6 H_{2}O\rightarrow C_{6}H_{12}O_{6} + 6 O_{2}$

The remaining free electrons and H+ ions reduce nitrate and sulfate to amino and sulfhydryl groups of amino acids.

The processes of photosynthesis occur in two stages such as Light reaction (Light dependant) and Dark reaction (Light independent). Light reaction results in a series of electron transfers resulting in the production of ATP and Nicotine Adenine Dinucleotide Phosphate (NADPH). In dark reaction, the ATP and NADPH produced in the light reactions are used to reduce Carbon dioxide and other compounds to organic carbon compounds (Glucose).

Sulfur bacteria use Hydrogen sulfide as a source for hydrogen atoms and produce sulfur as a byproduct instead of oxygen during the photosynthesis process.

$CO_{2} + 2H_{2}S\rightarrow CH_{2}O + H_{2}O + S_{2}$

The functional and structural protein complexes involved in the photosynthesis are known as Photosystems. Their specifc function is to carry out primary phytochemical reactions of photosynthesis, such as absorbtion of light and the transfer of energy and electrons. The photosystem families are classified into two groups, Phostosystem I (PSI) that occur in chloroplasts and in green sulfur bacteria, and Photosystem II ( PSI)that occur in chloroplast and in non-sulfur purple bacteria.

## Membranes associated with photosynthetic reactions

The photosynthesis mechanism happens mainly in the chloroplast, where the photosynthetic pigment chlorophyll is present. They are specialized organelles or subunits present in plant and algal cells (Figure 1). [For understanding the detailed structure of chloroplast, refer Isolation of Chloroplast, in Cell biology Virtual Lab 1, http://vlab.amrita.edu/?sub=3&brch=187&sim=878&cnt=1].

Figure 1. Structure of chloroplast

## Thylakoids

Thylakoids are membrane bound compartments located in the innermost part of the chloroplasts and cyanobacteria. Thylakoids are made up of more than hundred proteins which together with pigments, chlorophyll and other membrane lipids functions in the electron transport chain and ATP synthesis. Apart from this, they gather light with the help of photosynthetic pigments in an ordered manner.

It also consists of a thylakoid membrane, in which the pigments for photosynthesis are embedded directly. Hence it is the center for light-dependant reaction of photosynthesis process. The thylakoid membrane is surrounded by a thylakoid lumen, which is a continuous aqueous phase, which also carries a pivotal role in photosynthesis. During the light reaction the protons are pumped across the thylakoid membrane into the lumen, making it acidic (pH =4).

Like the inner mitochondrial membrane the thylakoid membrane and the inner membrane, are impermeable to most molecules and ions. On the other hand, the outer membrane of a chloroplast is like that of a mitochondrion is highly permeable to small molecules and ions. The thylakoid membrane gets folded into many disks called grana. They are connected by intergranal thylakoids that connects the graum stacks together and thus functions as a single compartment.

## Stroma

After the process in thylakoids, the photosynthesis phenomenon moves out to the stroma. It is the location of enzymatic actions, where the different soluble enzymes functions to produces carbohydrate molecules (sugar) by combing carbon, hydrogen and oxygen.  It is the site of of “light independent" photosynthesis reactions or "dark reactions". Here, the sugar molecules are produced enzymatically by utilizing NADPH and ATP synthesized in the thylakoids.

## Study of photosynthesis using Electrical circuit

The electrical phenomenon in plants has been attracted by researchers in the last few years.  Biologically closed electrical circuits were shown to operate over larger distance, especially related to biological tissues. The cells have the ability to produce electrical potentials which results in the flow of electric currents. They are several types of biologically closed electrical circuits in plants. Photophosphorylation electrical circuit and plasma membrane electrical circuits responsible for action and resting potentials are some of the examples of biologically closed electrical circuits. Some of the challenging issues related to the measurement of electrical activity of plants and evoked potentials include type and position of electrodes, measurement methods, reference electrodes, and other synchronizations with external events.

In the light dependent phase of photosynthesis, protons (H+ ions) are usually pumped across the thylakoid membrane into the thylakoid lumen. The addition of more electrons in the lumen makes it acidic in nature. PSI functions to utilize the light energy to reduce NADP+ to NADPH. This is active in non- cyclic and cyclic electron transport process. On the other hand, PSII utilizes light energy for oxidizing water molecules resulting in the formation of electrons, protons and molecular oxygen. The protons can be then used for the synthesis of ATP using the enzyme ATP synthase. A quinone molecule, Plastoquinone, in the electron transport chain in the light-dependent reaction of photosynthesis is reduced (2 protons) from the stroma of the chloroplast; it is then coupled to two electrons from PS II. This results in the formation of plastoquinol, which transports the protons to the thylakoid lumen. The electrons transfer continue using electron transport chain into the cytochrome b6f protein complex.

Figure 1. The equivalent electrical circuit of thylakoid membrane

Cm - Membrane capacitance; Rm - membrane resistance;

cF1 - coupling factor; cF0 - proton channel;
PQ pool - plastoquinone pool.

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