Chemiosmotic Synthesis of ATP
• In the Electron Transport Chain the process starts with low-energy
and finishes with high-energy
The role of
• Thus to pull electrons along the chain the, the low- energy electrons in the H2O are given potential energy by
photons of light
Formation of NADPH:
of a photoautotrophic cell’s chloroplasts (a cluster of light-absorbing pigments embedded in the thylakoid membrane)
and this energy is transferred to a molecule known as
This process is also known as
• A protein gradient is establish across the thylakoid membrane by three mechanisms:
Protons enter into the lumen (liquid portion of the thylakoid) by being reduced and oxidized by
when the shuttle moves from
is split in the lumen, the concentration of protons is increased (adding
The concentration of protons in the
stroma is reduced
is added to make
(potential energy) is used to synthesize
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+.
So we can summarize by saying that the photosynthetic plantstrap solar energy to form ATP and NADPH (Light Phase) and thenuse these as the energy source to make carbohydrates and otherbiomolecules from carbon dioxide and water (Dark Phase),simultaneously releasing oxygen in to the atmosphere. Thechemoheterotrophic animals reverse this process by using theoxygen to degrade the energy-rich organic products ofphotosynthesis to CO2 and water in order to generate ATP fortheir own synthesis of biomolecules.
Plant photosynthesis, both the Light Phase and Dark phasereactions, takes place in chloroplasts, which may be regarded asthe "power plants" of the green leaf cells. At night,when there is no sunlight energy, ATP continues to be generatedfor the plant's needs by respiration, i.e., oxidation of(photosynthetically produced) carbohydrate in mitochondria(similar to animals).
Photosynthesis converts these energy- depleted compounds (ADPand NADP+) back to the high energy forms (ATP and NADPH) and theenergy thus produced in this chemical form is utilized to drivethe chemical reactions necessary for synthesis of sugars andother carbon containing compounds (e.g., proteins, fats). Theproduction of high energy ATP and NADPH in plants occurs in whatis known as Light Phase Reactions (Z Scheme) (requiressunlight). The energy releasing reactions which converts themback to energy-depleted ADP and NADP is known as Dark PhaseReactions (Calvin Cycle) (does not require light) in whichthe synthesis of glucose and other carbohydrates occurs.
The oxidative chemical reactions of respiration releaseenergy, some of which is heat and some of it is captured in theform of high energy compunds such as Adenosine triphosphate (ATP)and Nicotinamide adenide dinucleotide phosphate (NADPH). Thesecompounds have a high energy (unstable) terminal phosphate bondand that terminal phosphate is easily detached with the transferof the energy to drive chemical reactions in the synthesis ofother biomolecules. In this case, the ATP loses one phosphate tobecome the energy-depleted ADP (Adenosine diphosphate)and the NADPH loses one electron to become energy-depleted NADP+.
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.
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.
In a broad chemical sense, the opposite of photosynthesis isrespiration. Most of life on this planet (all except in the deepsea vents) depends on the reciprocal photosynthesis-drivenproduction of carbon containing compounds by a series of reducing(adding electrons) chemical reactions carried out by plants andthen the opposite process of oxidative (removing electrons)chemical reactions by animals (and plants, which are capable ofboth photosynthesis and respiration) in which these carboncompounds are broken down to carbon dioxide and water.
Glycerate 3-phosphate is reduced during the reduction reactions to a three-carbon sugar called triose phosphate. Energy and hydrogen is needed for the reduction and these are supplied by ATP and NADPH + H+ (both produced during light-dependent reactions) respectively. Two triose phosphate molecules can then react together to form glucose phosphate. The condensation of many molecules of glucose phosphate forms starch which is the form of carbohydrate stored in plants. However, out of six triose phosphates produced during the reduction reactions, only one will be used to synthesise glucose phosphate. The five remaining triose phosphates will be used to regenerate RuBP.