b) What side products might be detected if this mechanism operated?

Having already generated the proton gradient used for ATP generation, what is there left to do?

b) What side products might be detected if this mechanism operated?

It all starts with Photosystem II. Photosystem I comes later; the two complexes are named that way because Photosystem I was discovered before Photosystem II.

Figure PS4.1. X-ray crystallographic structure of photosystem II, showing only the ligands.1

photosystem 1 and 2 Flashcards | Quizlet

2. X-ray crystal structures: Amunts, A., Drory, O., Nelson, N. The structure of a plant photosystem I supercomplex at 3.4 A resolution. Nature 2007, 447, 58-63. Images obtained via RCSB Protein Data Bank (2o01).

Figure PS6.4. Results from an X-ray crystal structure of photosystem I in spinach.2

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+.

c) What is the evolutionary advantage that drives the algae to make this chromophore?


Photosystem I is a trimeric complex that forms a large disk.

In oxygenic photosynthesis, the two protein–pigment complexes photosystem I and photosystem II are involved in the light‐driven charge separation across the thylakoid membrane, which results in the oxidation of water to O2 and in the reduction of NADP+ to NADPH. In addition to a photosynthetic reaction centre, each of these photosystems contains a core antenna system. The crystal structure of photosystem I reveals the spatial organization of protein subunits and cofactors.

8.2 Part 1: The Photosystem - YouTube

Photosynthesis occurs inside chloroplasts. Chloroplasts contain chlorophyll, a green pigment found inside the thylakoid membranes. These chlorophyll molecules are arranged in groups called photosystems. There are two types of photosystems, Photosystem II and Photosystem I. When a chlorophyll molecule absorbs light, the energy from this light raises an electron within the chlorophyll molecule to a higher energy state. The chlorophyll molecule is then said to be photoactivated. Excited electron anywhere within the photosystem are then passed on from one chlorophyll molecule to the next until they reach a special chlorophyll molecule at the reaction centre of the photosystem. This special chlorophyll molecule then passes on the excited electron to a chain of electron carriers.

05/11/2012 · 8.2 Part 1: The Photosystem Ron Kinser

The light-dependent reactions starts within Photosystem II. When the excited electron reaches the special chlorophyll molecule at the reaction centre of Photosystem II it is passed on to the chain of electron carriers. This chain of electron carriers is found within the thylakoid membrane. As this excited electron passes from one carrier to the next it releases energy. This energy is used to pump protons (hydrogen ions) across the thylakoid membrane and into the space within the thylakoids. This forms a proton gradient. The protons can travel back across the membrane, down the concentration gradient, however to do so they must pass through ATP synthase. ATP synthase is located in the thylakoid membrane and it uses the energy released from the movement of protons down their concentration gradient to synthesise ATP from ADP and inorganic phosphate. The synthesis of ATP in this manner is called non-cyclic photophosphorylation (uses the energy of excited electrons from photosystem II) .

10Photosynthesis Concept Outline 10.1 What is photosynthesis

The picture below is a "cartoon rendering" of an X-ray crystal structure of photosystem II. So, researchers first took a solution of protein and somehow coaxed the protein molecules out of solution in the form of a crystal. In a crystal, the molecules are oriented in an organised array, so they are all lined up in a regular way. The researchers took the crystal and placed it in a beam of X-rays, then measured how the X-rays scattered when they hit the crystal. That scattering pattern was used to construct a three-dimensional map of the atomic positions in the protein. Because that is an awful lot of atoms, biochemists often present the map in different ways that are less complicated.