Question

What are thylakoids? Write in detailed points!!

Please urgent

Answer 1

Answer:

Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. ... Chloroplast thylakoids frequently form stacks of disks referred to as grana (singular: granum).

Explanation:

The thylakoids are the site of the light-dependent reactions of photosynthesis. These include light-driven water oxidation and oxygen evolution, the pumping of protons across the thylakoid membranes coupled with the electron transport chain of the photosystems and cytochrome complex, and ATP synthesis by the ATP synthase utilizing the generated proton gradient.

Water photolysis Edit

Main article: photolysis

The first step in photosynthesis is the light-driven reduction (splitting) of water to provide the electrons for the photosynthetic electron transport chains as well as protons for the establishment of a proton gradient. The water-splitting reaction occurs on the lumenal side of the thylakoid membrane and is driven by the light energy captured by the photosystems. This oxidation of water conveniently produces the waste product O2 that is vital for cellular respiration. The molecular oxygen formed by the reaction is released into the atmosphere.

Electron transport chains Edit

Main article: electron transport chain

Two different variations of electron transport are used during photosynthesis:

Noncyclic electron transport or Non-cyclic photophosphorylation produces NADPH + H+ and ATP.

Cyclic electron transport or Cyclic photophosphorylation produces only ATP.

The noncyclic variety involves the participation of both photosystems, while the cyclic electron flow is dependent on only photosystem I.

Photosystem I uses light energy to reduce NADP+ to NADPH + H+, and is active in both noncyclic and cyclic electron transport. In cyclic mode, the energized electron is passed down a chain that ultimately returns it (in its base state) to the chlorophyll that energized it.

Photosystem II uses light energy to oxidize water molecules, producing electrons (e−), protons (H+), and molecular oxygen (O2), and is only active in noncyclic transport. Electrons in this system are not conserved, but are rather continually entering from oxidized 2H2O (O2 + 4 H+ + 4 e−) and exiting with NADP+ when it is finally reduced to NADPH.

Chemiosmosis Edit

Main article: chemiosmosis

A major function of the thylakoid membrane and its integral photosystems is the establishment of chemiosmotic potential. The carriers in the electron transport chain use some of the electron's energy to actively transport protons from the stroma to the lumen. During photosynthesis, the lumen becomes acidic, as low as pH 4, compared to pH 8 in the stroma.[30] This represents a 10,000 fold concentration gradient for protons across the thylakoid membrane.

Source of proton gradient Edit

The protons in the lumen come from three primary sources.

Photolysis by photosystem II oxidises water to oxygen, protons and electrons in the lumen.

The transfer of electrons from photosystem II to plastoquinone during non-cyclic electron transport consumes two protons from the stroma. These are released in the lumen when the reduced plastoquinol is oxidized by the cytochrome b6f protein complex on the lumen side of the thylakoid membrane. From the plastoquinone pool, electrons pass through the cytochrome b6f complex. This integral membrane assembly resembles cytochrome bc1.

The reduction of plastoquinone by ferredoxin during cyclic electron transport also transfers two protons from the stroma to the lumen.

The proton gradient is also caused by the consumption of protons in the stroma to make NADPH from NADP+ at the NADP reductase.

ATP generation Edit

The molecular mechanism of ATP (Adenosine triphosphate) generation in chloroplasts is similar to that in mitochondria and takes the required energy from the proton motive force (PMF).[citation needed] However, chloroplasts rely more on the chemical potential of the PMF to generate the potential energy required for ATP synthesis. The PMF is the sum of a proton chemical potential (given by the proton concentration gradient) and a transmembrane electrical potential (given by charge separation across the membrane). Compared to the inner membranes of mitochondria, which have a significantly higher membrane potential due to charge separation, thylakoid membranes lack a charge gradient.[citation needed] To compensate for this, the 10,000 fold proton concentration gradient across the thylakoid membrane is much higher compared to a 10 fold gradient across the inner membrane of mitochondria. The resulting chemiosmotic potential between the lumen and stroma is high enough to drive ATP synthesis using the ATP synthase. As the protons travel back down the gradient through channels in ATP synthase, ADP + Pi are combined into ATP. In this manner, the light-dependent reactions are coupled to the synthesis of ATP via the proton gradient.

Answer 2

Thylakoids:

• In the stroma the flatted membrenous sacs are called thylakoids.

• The membrane of thylakoids enclose a space.

• The space enclosed by its membrane is called Lumen.

• Chlorophyll is present in thylakoid membrane.

• The photosynthetic functional unit which is located on thylakoid membrane is called Quantasome.

• Quantasome consists if 250- 400 molecules of various pigments.

• Thylakoids are arranged in stacks.

• The stacks are called grana or intragranal thylakoids.

• Each chloroplasts has 40-60 granum.

• Stroma lamellae or Frets channel or stroma thylakoids are flat membrenous tubular connecting the thylakoids of different granum.

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Knowledge Cell:

• The chloroplasts are double membrane bound structures.

• The inner membrane of chloroplast is less permeable.

• The space limited by inner membrane is called stroma.

• Thylakoids are a part of the stroma.

• The outer membrane of chloroplasts contains porins.