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The process of Adenosine Triphosphate (ATP) formation in chloroplast by photosynthesis is known as photophosphorylation. There are two types of phosphorylation such as cyclic and non-cyclic. The most common light-dependent reaction within photosynthesis is known as non-cyclic phosphorylation (Youtube.com, 2010). Conversely, cyclic photophosphorylation occurs commonly within plants as compared to non-cyclic, and it mostly happens when there are very small amounts of NADP+ (Nicotinamide adenine dinucleotide phosphate) available. It involves PS-I (Photo system-I) and generates adenosine triphosphate (ATP). However, non-cyclic photophosphorylation involves PS-I (light absorption at 700 nm) and II (light absorption at 680 nm) and then generates NADPH and ATP. (Photosynthesis. Ppt and photosynthesis (mssaberi))

Electron transport chain

In the photosynthesis process, electron transport defines a method of light-induced electron transport to produce chemical energy. The electron transport chain provides energy for ATP synthesis by chemiosmosis. The electron transport chain is found in thylakoids (Youtube.com, 2016). As per Chapter 4, during chemiosmosis, ATP is generated by PS-II (Photo system-II) electron transport chain and chemiosmosis. Noncyclical phosphorylation, the chloroplast of thylakoid excites its electron to a high energy level and then it enters the electron transport chain that gives energy for ATP synthesis and then the electron molecule enters the Photo system-I (PS-I).

Proton gradient

The proton gradient is the product of an electron transport chain. In photosynthesis, two photosystems are present such as Photo system-II and Photo system-I. The Photo system-II releases protons into thylakoid that again takes them into the stroma. When excited electrons absorb light, these electrons are again passed on to electron carrier chains that are present in the thylakoid membrane. Again, the light energy is used for pumping protons through the thylakoid membrane to thylakoid space or lumen that creates a proton gradient.

Light-dependent reactions

The light-dependent reaction is the photochemical reaction that occurs in the thylakoid of chloroplasts. It involves both PS-II, and I where PS-I contains P700 (chlorophyll a) and PS-II contain P680 (chlorophyll a and b). In light-dependent reactions, the antenna pigments that excite them to move from ground state to executed state (Youtube.com, 2014; Youtube.com, 2012) absorb light photons. This excitation is passed along with the chlorophyll molecules until this reaches the reaction centre. As per Chapter 4, in the reaction centre, chlorophyll absorbs energy, and at a high-energy state, this molecule causes the emission of two electrons. The primary electron acceptor takes electrons through chlorophyll a. This entire process is called photoexcitation. A common example of this reaction is non-cyclic photophosphorylation. 

Light-independent reactions

This reaction involves the conversion of CO2 to organic compounds within a method called carbon assimilation. Intermediate glyceraldehyde-3-phosphate is a type of intermediate reaction that starts from the formation of various substrates such as fats, protein, and carbohydrates. This reaction occurs within the stroma, where there is no requirement of light (Youtube.com, 2014). As per chapter 4, in this reaction, ATP synthesis is done by consumption of NADP+ of electrons and H+ on one side of the membrane, and on the other side, H+ has been produced by water. Since this reaction does not require sunlight, no electron flow has been found to drive ATP production. Thus, ATP gives the energy, and plants produce glucose with the help of NADP+ and carbon dioxide.


The plant cell consists of chloroplast, and the overall photosynthesis reactions happen in chloroplasts. Chloroplasts consist of some parts, such as the outer membrane, inner membrane, thylakoid, and granum (Youtube.com, 2012; Youtube.com, 2014). As per Chapter 4, thylakoid is the key part where photosynthesis occurs; however, another part is also responsible for photosynthesis reaction, such as stroma. As per Chapter 4, thylakoid contains chlorophyll, the green pigment found in plants. Thylakoid is the site of light-dependent reactions, and it consists of a thylakoid membrane that surrounds the thylakoid lumen or space.


Chlorophyll is the green pigment that is found in plants, and it is present in the chloroplast in thylakoid. There are varieties of pigments; however, chlorophyll is the primary photosynthesis pigment. There are two types of chlorophyll such as chlorophyll a and chlorophyll b. According to information, chlorophyll is a chemical that can absorb specific wavelengths of light, and when this particular pigment absorbs light, its electrons enter the excited state (Youtube.com, 2012; Youtube.com, 2014). As per Chapter 4, regarding light absorption, both chlorophylls a and b absorb mostly the blue and red regions of the spectrum. Additionally, both chlorophylls reflected the colour green, which is why most plants seem green colour.


Stroma is again a major part of chloroplast, and this region does light-independent reactions. According to information, the stroma is a colourless fluid that is present outside of the thylakoids. Stroma encircles the thylakoids and grana and gives support to pigment thylakoids (Youtube.com, 2010). Since stroma is responsible for the light-dependent reaction, the Calvin Cycle happens in the stroma, and carbon dioxide, ATP and NADP+ produce glucose and oxygen.

Photosystem and its types

The photosystem is the functional unit in relation to photosynthesis. Information has suggested that the core group of protein and chlorophyll in the centre is called a photosystem. If chlorophyll molecules are removed from the photosystem, they can absorb one wavelength of light; however, they can absorb various wavelengths of light (Youtube.com, 2014). As per Chapter 4, there are two types of photosystems such as Photosystem I (PS-I) and photosystem II (PS-II). These systems help to bring light-dependent reactions and then yield NADPH and ATP for being used within light-independent reactions such as Calvin Cycle.

Carbon fixation

Carbon fixation occurs in light-independent reactions, for example, in Calvin Cycle. In carbon fixation, carbon dioxide from the atmosphere is combined with the pre-existing ribulose-bis-phosphate that forms unstable compounds, which again break down to form a stable compound of the cycle, known as 3-phosphoglycerate; therefore, this cycle is called the C3 cycle. As per Chapter 4, this specific reaction eases by a particular enzyme called ribulose bis-phosphate carboxylase, also known as ribulose and the most abundant protein on earth. (Photosynthesis. Ppt and photosynthesis (mssaberi))

ATP synthase

In cyclic photophosphorylation, electrons are transported down the electron transport chain, and during this time, some of the energy released is utilized for pumping protons throughout the thylakoid membrane through the stroma of chloroplast to the interior space of thylakoid by producing a proton motive force or proton gradient. When accumulating proteins within the interior space of thylakoid pass back across thylakoid membrane to stroma through ATP synthase complexes, the energy is used for generating ATP from Pi and ADP (adenosine di-phosphate). (Photosynthesis. Ppt and photosynthesis (mssaberi))


Pigments are produced by the living structure that possesses a definite colour and absorbs the materials into a coloured form to the plants (Youtube.com, 2014). The most common pigment in plants is chlorophyll a and b; however, some pigments are used for absorbing over wavelengths, and these are Carotenoids, Phycobilins, Xanthophylls, beta-carotene, Fucoxanthin, and phycocyanobilin. As per Chapter 4, regarding light absorption, Xanthophylls and carotenoids get light from other regions as it absorbs green and blue and that is why they are yellow, red, and orange. According to information, beta-carotenes provide orange colour in plants, for example, carrots. Additionally, this pigment can be converted into vitamin A, which again converts into a retinal, a visual pigment of the human eye.


RUBISCO is an enzyme, easing the combination of CO2 and RUBP (Ribulose 1,5-bisphosphate), and it has a special property of utilizing O2 and CO2 as substrate competes for combining with RUBP. If RUBP combines with CO2, it leads to the process of carbon fixation, and when it combines with oxygen, the process is known as photophosphorylation. In Photorespiration, the product produced will be two carbon compounds that are called phospho-glycolate and 3-phosphoglycerate. At high temperatures, there is more chance to combine oxygen with RUBP. (Photosynthesis. Ppt and photosynthesis (mssaberi))

Calvin Cycle

Calvin Cycle consists of three phases as Carbon fixation, Reduction stage, and Regenerating RuBP. In the first phase, CO2 combines with ribulose-bisphosphate and forms an unstable compound, and after immediate breakdown, it forms a stable compound called 3-phosphoglycerate (PGA) (Youtube.com, 2012). As per Chapter 4, low-energy PGA converted into a high-energy state in the second stage by activating ATP and reducing NADPH producing two molecules of G3P (glyceraldehyde-3-phosphate). In the third stage majority of G3P was used for regenerating RuBP. This cycle occurs 6-times to making one glucose molecule.

C3 Plants

During carbon fixation, carbon dioxide combines with ribulose-bisphosphate and forms an unstable compound, and after the immediate breakdown of that compound, it generates another stable compound named 3-phosphoglycerate. As per Chapter 4, the plants that combine carbon dioxide with ribulose-bisphosphate and form 3-phosphoglycerate are the C3 plants. Generally, C4 plants are C3 plants because, after the formation of 3-phosphoglycerate, the next two steps of the Calvin Cycle occur (Youtube.com, 2012). However, this cycle undergoes different organic compounds in various ways because of the different anatomy of leaves.

C4 plants

The C4 pathway is designed for efficiently fixing carbon dioxide at low concentrations, and those plants that use these pathways are called C4 plants. As per Chapter 4, these plants use carbon dioxide in terms of a 4-carbon compound (C4) called oxaloacetate, which occurs in mesophyll cells. At first, CO2 fixed with 3C called PEP (phosphoenolpyruvate) for producing oxaloacetate and PEP carboxylase catalyzes this reaction (Youtube.com, 2012). Then oxaloacetate is converted into another 4C compound called malate for reducing NAPH power.


  • Youtube.com, (2010). Photosynthesis. Available at <https://www.youtube.com/watch?v=pXSVEKqvyIc> (Accessed 28th September 2021)
  • Youtube.com, (2012). Photosynthesis Part 5: C4 and CAM. Available at <https://www.youtube.com/watch?v=Dq38MpYOb8w> (Accessed 28th September 2021)
  • Youtube.com, (2014). (OLD VIDEO) Photosynthesis and the Teeny Tiny Pigment Pancakes. Available at https://www.youtube.com/watch?v=uixA8ZXx0KU (Accessed 28th September 2021)
  • Youtube.com, (2016). Flow of energy and matter through ecosystem | Ecology | Khan Academy. Available at https://www.youtube.com/watch?v=TitrRpMUt0I (Accessed 28th September 2021

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