Photosynthesis Details! The Light Reactions and the Calvin Cycle

How it all fits together…

Why do we see green? Different pigments absorb photons of different wavelengths. A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.

Absorption Spectra light reactions: perform work with wavelengths of light that are absorbed. In the thylakoid are several pigments that differ in their absorption spectrum. Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green.

Photon Absorption When a molecule absorbs a photon, one of that molecule’s electrons is elevated to an orbital with more potential energy. Photons are absorbed by clusters of pigment molecules in the thylakoid membranes.

Excited Electrons! Excited electrons are unstable… they drop to their ground state in a billionth of a second, releasing heat energy. Car in the sun gets hot! Some photons release light too…

Photosystems Photosystems: the “light-harvesting units”, made of chlorophyll, proteins and other organic molecules consists of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules Energy is transmitted from pigment molecule to pigment molecule until it reaches a particular chlorophyll a: the reaction center Like a satellite dish! Primary electron acceptor: captures the excited electron

Two Types of Photosystems Photosystem I: has a reaction center chlorophyll, the P700 center, that has an absorption peak at 700nm Photosystem II: has a reaction center with a peak at 680nm differences due to the proteins associated with each reaction center These two photosystems work together to use light energy to generate ATP and NADPH.

Noncyclic Electron Flow… the predominant route: produces both ATP and NADPH

The steps… Photosystem II absorbs light, 2 excited electrons are passed to P680 (chlorophyll a). Then the electrons are captured by the primary electron acceptor. Water is split to replace the lost electrons splits into 2 H+ and an oxygen atom which combines with another to form O2 3. Excited electrons “fall” down the electron transport chain to Photosystem I

Energy of “falling” electrons is used to make ATP using chemiosmosis across the thylakoid membrane: noncyclic photophosphorylation ATP is used by the Calvin Cycle 5. The falling electrons fill a “hole” in P700 (chlorophyll a) in Photosystem I. This hole is created when photons excite electrons on the photosystem I complex. The light energy sends 2 electrons to another primary electron acceptor Electrons “fall” down a second electron transport chain. Electrons are picked up by NADP+ to form NADPH NADPH will go to the Calvin Cycle

Summary The light reactions use the solar power of photons absorbed by both photosystem I and photosystem II to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH

Calvin says, “I need more ATP!” Problem: Noncyclic electron flow produces about the same amount of ATP and NADPH, but the Calvin Cycle uses MORE ATP Cyclic Electron Flow makes up the difference. Cyclic Electron Flow: uses only photosystem I, but electrons are sent down the first electron transport chain to make ATP generate ATP by cyclic photophosphorylation

Cyclic Electron Flow

Chemiosmosis (again?!) Yup! …electron transport chain pumps protons across a membrane as electrons are passed along a series of more electronegative carriers. This builds the H+ gradient across the membrane. ATP synthase molecules generate ATP as H+ diffuses back across the membrane.

The Calvin Cycle Carbon enters the cycle in the form of CO2 and leaves as sugar The actual sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P) G3P is the starting material for making other organic compounds, including glucose and other carbohydrates. spends the energy of ATP and the reducing power of electrons carried by NADPH to make the sugar 1 G3P = 9 ATP and 6 NAPDH The cycle must take place 3 times, fixing 3 molecules of carbon dioxide 3 Phases…

Phase 1: Carbon fixation each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP). This enzyme rubisco catalyzes the first step The 6-carbon intermediate splits in half to form two molecules of 3-phosphoglycerate per CO2.

Phase 2: Reduction each 3-phospho-glycerate receives another phosphate group from ATP NADPH donates a pair of electrons Six molecules of G3P are produced, but only 1 exits the cycle (others are reused) G3P can be used to make glucose and other organic compounds

Phase 3: Regeneration of RuBP five G3P molecules are rearranged to form 3 RuBP molecules. Requires 3 ATP (one per RuBP) to complete the cycle and prepare for the next.

Now I need a nap! 