formation of ATP and NADPH

Photosynthesis Reactions: Light Dependent Reactions, Chemiosmosis

Reactions of photosynthesis
Carbon dioxide – A reactant

Sugar is formed during light-independent reactions of photosynthesis by the reduction of CO2, utilizing ATP and NADH, the products of light-dependent reactions. Undoubtedly photosynthesis does not happen in the lack of CO2. About 10 percent of total photosynthesis is carried out by terrestrial plants, the rest occurs in oceans, lakes, and ponds.

Aquatic photosynthetic organisms use dissolved CO2, bicarbonates, and soluble carbonates that are present in water as carbon sources. Air includes about 0.03 – 0.04 percent CO2. Photosynthesis taking place on land utilizes this atmospheric CO2.


Carbon dioxide gets in the leaves through stomata and gets dissolved in the water absorbed by the cell walls of mesophyll cells. Stomata are found in a great number in a leaf; their number being proportional to the amount of gas diffusing into the leaf. Stomata cover only 1 – 2 percent of the leaf surface however they allow proportionately a lot more gas to diffuse.

The entry of CO2 into the leaves relies on the opening of stomata. The guard cells guarding the stoma, because of their peculiar structure and changes in their shape, regulate the opening and closing of stomata. Stomata are adjustable pores that are typically open throughout the day when CO2 is needed for photosynthesis and partly closed at night when photosynthesis stops.

Light – The driving energy of Photosynthesis

Light is a kind of energy called electromagnetic energy or radiations or packets of photons. Light acts as waves along with sort of particles called photons. The radiations most important to life are the visible light that ranges from about 380 to 750 nm in wavelength. It is the light energy that is taken in by chlorophyll, converted into chemical energy, and drives the photosynthetic process. Not all the light falling on the leaves is taken in. Just about one percent of the light falling on the leaf surface is absorbed, the rest is reflected back or transmitted.

Absorption spectrum for chlorophylls suggests that absorption is maximum in blue and red parts of the spectrum, two absorption peaks being at around 430 nm and 670 nm respectively. Absorption peaks of carotenoids are different from those of chlorophylls. Different wavelengths are not only differently absorbed by photosynthetic pigments however are likewise differently effective in photosynthesis.

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Reactions of Photosynthesis

These reactions of photosynthesis include two parts:

  1. The light-dependent reactions (light reactions) which use light directly and
  2. The light-independent reactions (dark reactions) do not utilize light directly.
Light-dependent Reactions

(Energy-conversion phase; formation of ATP and NADPH)

formation of ATP and NADPH

Light-dependent reactions make up that phase of photosynthesis during which light energy is absorbed by chlorophyll and other photosynthetic pigment molecules and converted into chemical energy. As a result of this energy conversion, reducing and assimilating power in the form of NADPH (NADPH + H+) and ATP, are formed, both momentarily storing energy to be brought along with H+ to the light-independent reactions.

The sunlight energy which is taken in by photosynthetic pigments drives the procedure of photosynthesis. Photosynthetic pigments are organized into clusters, called photosystems, for efficient absorption and utilization of solar energy in thylakoid membranes.

Each photosystem includes a light-gathering ‘antenna complex’ and a ‘reaction center’. The antenna complex has lots of molecules of chlorophyll a, chlorophyll b, and carotenoids, the majority of them funneling the energy to the reaction center. The reaction center has several molecules of chlorophyll an along with a primary electron acceptor, and associated electron carriers of ‘electron transport chain’(ETC).

Chlorophyll a molecules of reaction center and associated proteins are closely connected to the close-by electron transport system. The electron transport chain plays role in the generation of ATP by chemiosmosis (which is the second phase of light dependent reactions). Light energy absorbed by the pigment particles of the antenna complex is moved ultimately to the reaction center. There the light energy is converted into chemical energy.


There are 2 photosystems, photosystem I (PS I) and photosystem II (PS II). These are called so in order of their discovery. Photosystem I have chlorophyll a molecule which takes in optimum light of 700 nm and is called P700, whereas the reaction center of photosystem II has P680, the form of chlorophyll a which takes in best the light of 680 nm.

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A specialized molecule called; primary electron acceptor is likewise associated nearby each reaction center. This acceptor traps the high energy electrons from the reaction center and then passes them on to the series of electron carriers. During this energy is utilized to create ATP by chemiosmosis.

In the predominant kind of electron transportation called non-cyclic electron flow, the electrons travel through the two photosystems. In a less typical kind of path called cyclic electron flow just photosystem I is involved. The development of ATP during non-cyclic electron flow is called non-cyclic phosphorylation while that throughout cyclic electron flow is called cyclic phosphorylation.

Non-cyclic Phosphorylation or Z scheme:

  1. When photosystem II absorbs light, an electron excited to a greater energy level in the reaction center chlorophyll P680 is captured by the primary electron acceptor of PS II. The oxidized chlorophyll is now an extremely strong oxidizing agent; its electron “hole” needs to be filled.
  2. This hole is filled by the electrons which are drawn out, by an enzyme, from water. This reaction splits a water molecule into 2 hydrogen ions and an oxygen atom, which right away combines with another oxygen atom to form O2. This water-splitting reaction of photosynthesis that releases oxygen is called photolysis. The oxygen produced during photolysis is the primary source of the replenishment of environmental oxygen.
  3. Each photoexcited electron passes from the primary electron acceptor of photosystem II to photosystem I through an electron transport chain. This chain includes an electron carrier called plastoquinone (Pq), a complex of 2 cytochrome, and copper including a protein called plastocyanin (Pc).


  1. As electrons move down the chain, their energy goes on reducing and is utilized by the thylakoid membrane to produce ATP. This ATP synthesis is called photophosphorylation because it is driven by light energy. Specifically, ATP synthesis during non-cyclic electron low is called noncyclic photophosphorylation. This ATP created by the light reactions will supply chemical energy for the synthesis of sugar during the Calvin cycle, the second significant phase of photosynthesis.
  2. The electron reaches the “bottom” of the electron transportation chain and fills an electron “hole” in P700, the chlorophyll ‘a’ molecules in the reaction center of photosystem I. This hole is created when light energy is taken in by particles of P700 and drives an electron from P700 to the primary acceptor of photosystem I.
  3. The primary electron acceptor of photosystem I passes the photoexcited electrons to a 2nd electron transport chain, which transmits them to ferredoxin (Fd), an iron containing protein. An enzyme called NADP reductase then moves the electrons from Fd to NADP. This is the redox reaction that stores the high-energy electrons in NADPH. The NADPH molecule will supply reducing power for the synthesis of sugar in the Calvin cycle.
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The path of electrons through the two photosystems throughout non-cyclic photophosphorylation is referred to as Z-scheme from its shape.


In both cyclic and non-cyclic photophosphorylation, the mechanism for ATP synthesis is chemiosmosis, the process that utilizes membranes to couple redox reactions for ATP production. Electron transport chain pumps protons

(H+) across the membrane of thylakoids in the case of photosynthesis into the thylakoids space. The energy utilized for this pumping comes from the electrons moving through the electron transport chain. This energy is transformed into prospective energy kept in the form of H+ gradient throughout the membrane. Next, the hydrogen ions move down their gradient through special complexes called ATP synthase which is integrated into the thylakoid membrane. Throughout this diffusion of H+, the energy of electrons is used to make ATP.


Frequently Asked Questions

Q1: Difference between

Cyclic phosphorylation Non-cyclic phosphorylation
Only involves PSI Involves both PSI and PS II
Happens when chloroplast lacks ATP All the time in presence of light
Produce ATP Produce ATP, NADPH2, O2


Q2: What is chlorophyll a and b?

Chlorophyll a: C55H72O5N4Mg, blue-green in color

Chlorophyll b: C55H70O6N4Mg, yellow-green in color

Q3: What is the order of transfer of energy in photosynthetic pigments?

Carotenoids –> chlorophyll b –> chlorophyll a

Q4: Chlorophyll exists in how many forms?

Ans: chlorophyll exists in several forms.

Q5: Where the proton gradient develops in chemiosmosis?

Ans: Proton gradient develops across the thylakoid membrane and across inner mitochondrial membrane.