Question

how does photosynthesis occur indrakshi​

Answer 1

Photosynthesis takes place inside plant cells in small objects called chloroplasts . Chloroplasts contain a green substance called chlorophyll . This absorbs the light energy needed to make photosynthesis happen. ... Plants get carbon dioxide from the air through their leaves, and water from the ground through their roots.

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Answer 2

Photosynthesis takes place inside plantcells in small things called chloroplasts. Chloroplasts contain a green substance called chlorophyll. This absorbs the light energy needed to makephotosynthesishappen. Plantscan only photosynthesise in the light.

In depth,,

Process of Photosynthesis

Photosynthesis in plants occurs in two stages. These stages are known as the light-dependent reactions and the Calvin Cycle.

Light-dependent Reactions

The first stage of photosynthesis is the light dependent reactions. These reactions take place on the thylakoid membrane inside the chloroplast. During this stage light energy is converted to ATP (chemical energy) and NADPH (reducing power).



Light-dependent Reactions

Light is absorbed by two Photosystems called Photosystem I (PSI) and Photosystem II (PSII). These protein complexes contain light harvesting chlorophyll molecules and accessory pigments called antenna complexes. The photosystems are also equipped with reactions centers (RC). These are complexes of proteins and pigments which are responsible for energy conversion. The chlorophyll molecules of PSI absorb light with a peak wavelength of 700nm and are called P700 molecules. The chlorophyll molecules of PSII absorb light with a peak wavelength of 68Onm and are called P68O molecules.

The light dependent reactions begin in PSII.

A photon of light is absorbed by a P680 chlorophyll molecule in the light harvesting complex of PSII.

The energy that is generated from the light is passed from one P680 chlorophyll molecule to another until it reaches the reaction center (RC) of PSII.

At the RC is a pair of P680 chlorophyll molecules. An electron in the chlorophyll molecules becomes excited as a result of a higher level of energy. The excited electron becomes unstable and is released. Another electron is released following the capture of another photon of light by the light harvesting complex and the transfer of energy to the reaction center.

The electrons are transported in a chain of protein complexes and mobile carriers called an electron transport chain (ETC). Plastoquinone is the mobile carrier that transports the electrons from the reaction center of PSII to the Cytochrome b6f Complex as shown in the diagram above.

The electrons lost from PSII are replaced by splitting water with light in a process called Photolysis. Water is used as the electron donor in oxygenic photosynthesis and is split into electrons (e-), hydrogen ions (H+, protons) and oxygen (O2). The hydrogen ions and oxygen are released into the thylakoid lumen. Oxygen is later released into the atmosphere as a by-product of photosynthesis.

While the electrons pass through the ETC via Plastoquinone, hydrogen ions (protons) from the stroma are also tranferred and released into the thylakoid lumen. This results in a higher concentration of hydrogen ions (proton gradient) in the lumen.

As a result of the proton gradient in the lumen, hydrogen ions are transferred to ATP synthase and provide the energy needed for combining ADP and Pi to produce ATP.

Cytochrome b6f transfers the electrons to Plastocyanin which then transports them to Photosystem I.

The electrons have now arrived at PSI.

They again receive energy, but this time from light absorbed by P700 chlorophyll molecules.

The electrons are transferred to mobile carrier, ferredoxin.

They are then transported to ferredixin NADP reductase (FNR), which is the final electron acceptor. At this point the electrons and a hydrogen ion are combined with NADP+ to produce NADPH.

The lost electrons from PSI are replaced by electrons from PSII via the electron transport chain

If you look at the actual equation for photosynthesis (not the one you see in Mickey Mouse textbooks) you will learn that water is both a substrate and a product of photosynthesis. It is a substrate only for the light reactions and both a substrate & product of the Calvin Cycle. That is not easy to see because most textbooks leave out the water in the Calvin Cycle and only show the carbon. For example, the RUBISCO step of carbon fixation uses CO2 and H2O as substrates as well as RuBP but most textbook version do not show that.

12H2O + 6CO2 => C6H12O6 + 6O2 + 6H2O