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the stroma would have a higher pH

Photosynthesis in a leaf: Chloroplasts, Grana, Stroma, and Thylakoids, the starting point for energy's travels through life

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Chemistry for Biologists: Photosynthesis

If you could measure the pH of the interior of thylakoids and the surrounding stroma during active photosynthesis what would be the relative pH of each?

a separate internal thylakoid membrane system for photosynthesis

The light-independant reactions of photosynthesis occur in the stroma of the chloroplast and involve the conversion of carbon dioxide and other compounds into glucose. The light-independent reactions can be split into three stages, these are carbon fixation, the reduction reactions and finally the regeneration of ribulose bisphosphate. Collectively these stages are known as the Calvin Cycle.

Thylakoid membrane - WikiVisually

During carbon fixation, carbon dioxide in the stroma (which enters the chloroplast by diffusion) reacts with a five-carbon sugar called ribulose bisphosphate (RuBP) to form a six-carbon compound. This reaction is catalysed by an enzyme called ribulose bisphosphate carboxylase (large amounts present within the stroma), otherwise known as rubisco. As soon as the six-carbon compound is formed, it splits to form two molecules of glycerate 3-phosphate. Glycerate 3-phosphate is then used in the reduction reactions.

Photophosphorylation is the production of ATP using the energy of sunlight. Photophosphorylation is made possible as a result of chemiosmosis. Chemiosmosis is the movement of ions across a selectively permeable membrane, down their concentration gradient. During photosynthesis, light is absorbed by chlorophyll molecules. Electrons within these molecules are then raised to a higher energy state. These electrons then travel through Photosystem II, a chain of electron carriers and Photosystem I. As the electrons travel through the chain of electron carriers, they release energy. This energy is used to pump hydrogen ions across the thylakoid membrane and into the space within the thylakoid. A concentration gradient of hydrogen ions forms within this space. These then move back across the thylakoid membrane, down their concentration gradient through ATP synthase. ATP synthase uses the energy released from the movement of hydrogen ions down their concentration gradient to synthesise ATP from ADP and inorganic phosphate.

Structure and function of the photosynthetic membrane

Keep in mind that the flow of actively transported protons from the stroma to the lumen is against their concentration gradient. There wouldn't be a net flow of protons this way without active transport.

NAD+ is also needed for respiration, but is not produced by photosynthesis.PARP (Poly-ADP-Ribose-Polymerase) and PARG (Poly-ADP-Ribose-Glycohydrolase) are important if the plant is exposed to excessive stress factors (such as extreme temperatures).

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Photosynthesis occurs in the thylakoid membrane

The electrons from the chain of electron carriers are then accepted by Photosystem I. These electrons replace electrons previously lost from Photosystem I. Photosystem I then absorbs light and becomes photoactivated. The electrons become excited again as they are raised to a higher energy state. These excited electrons then pass along a short chain of electron carriers and are eventually used to reduce NADP+ in the stroma. NADP+ accepts two excited electrons from the chain of carriers and one H+ ion from the stroma to form NADPH.

Chow Lab - Thylakoid structure and function | RSB

If the light intensity is not a limiting factor, there will usually be a shortage of NADP+ as NADPH accumulates within the stroma (see light independent reaction). NADP+ is needed for the normal flow of electrons in the thylakoid membranes as it is the final electron acceptor. If NADP+ is not available then the normal flow of electrons is inhibited. However, there is an alternative pathway for ATP production in this case and it is called cyclic photophosphorylation. It begins with Photosystem I absorbing light and becoming photoactivated. The excited electrons from Photosystem I are then passed on to a chain of electron carriers between Photosystem I and II. These electrons travel along the chain of carriers back to Photosystem I and as they do so they cause the pumping of protons across the thylakoid membrane and therefore create a proton gradient. As explained previously, the protons move back across the thylakoid membrane through ATP synthase and as they do so, ATP is produced. Therefore, ATP can be produced even when there is a shortage of NADP+.

Chow Lab - Thylakoid structure and function

Energy from an electron transport chain is used to actively transport protons from the stroma to the thylakoid lumen. Thus the lumen has a relatively higher proton concentration and therefore a lower pH than the stroma.

Photosynthesis | Essays in Biochemistry

I understand that proton gradient is from stroma to thylakoid space, but that does not mean stroma would have less protons than thylakoid space, if stroma would have a higher pH, then the proton will never flow from stroma to thylakoid space, right?

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