4.3 Photosynthesis in Detail KEY CONCEPT Photosynthesis requires a series of chemical reactions.

4.3 Photosynthesis in Detail The first stage of photosynthesis captures and transfers energy – The light-dependent reactions. Prefix photo means? The light-dependent reactions are the photo part of photosynthesis Sugars are not made here The light-dependent reactions include groups of molecules called photosystems. When energy is captured from the sun, it is transferred in the thylakoid membranes by two groups of molecules called photosystems. –Photosystem I –Photosystem II

4.3 Photosynthesis in Detail Let’s talk about Atoms for a sec What are the three subatomic particles in an atom? Of these three, which ones move in or out of the atom? While we learn about photosynthesis details, keep in mind that energy is associated with the transfer of electrons in the making or breaking of bonds. In photosynthesis, electrons have a special role or job in getting energy into a living system!

4.3 Photosynthesis in Detail Photosystem II captures and transfers energy to electrons Step 1 –chlorophyll absorbs energy from sunlight –Energy is transferred to electrons –Electrons leave chlorophyll and enter the electron transport chain (a series of proteins in the membrane of the thylakoid)

4.3 Photosynthesis in Detail Step 2 –water molecules are split (enzymes break them down) –Oxygen, hydrogen ions (H+) and electrons are separated from each other –oxygen is released as waste –hydrogen ions (H+) are transported across thylakoid membrane –Electrons from water replace those that left and went into the ETC

4.3 Photosynthesis in Detail Step 3 Hydrogen ions (H+) are transported across thylakoid membrane Electrons move from protein to protein in the ETC Their energy used to pump Hydrogen ions (H+) from outside to inside thylakoid against concentration gradient Hydrogen ions (H+) buildup inside thylakoid Electrons move on to Photosystem I

4.3 Photosynthesis in Detail Photosystem I captures energy and produces energy- carrying molecules. Step 4 –Just like PII, chlorophyll and light absorbing molecules in thylakoid absorb energy from sunlight –Energy is added to electrons (some from PII) and they leave the molecules

4.3 Photosynthesis in Detail Step 5 –energized electrons are added to molecule NADP+ (functions like ADP) – the combination makes NADPH –NADPH functions like ATP in photosynthesis –Molecules of NADPH are transferred to light- independent reactions

4.3 Photosynthesis in Detail Step 6 Hydrogen ion (H+) diffusion: hydrogen ions (H+) flow through protein channel in thylakoid membrane Hydrogen ion (H+) concentration is higher inside the thylakoid than outside it Difference between the two concentrations is called chemiosmotic gradient (stores potential energy) Ions flow by diffusion …………..

4.3 Photosynthesis in Detail Step 7 ADP changed to ATP when Hydrogen ions (H+) flow through ATP synthase The protein channel in step 6 is part of a complex enzyme called ATP synthase As ions flow through the channel, ATP synthase makes ATP by adding phosphate groups to ADP.

4.3 Photosynthesis in Detail Summary of the Light-Dependent Reactions Energy is captured from sunlight by light-absorbing molecules. The energy is transferred to electrons that enter an electron transport chain. Water molecules are broken down into H+ ions, electrons, and oxygen molecules. The water molecules provide the H+ ions and electrons that are used in the light-dependent reactions. Energized electrons have two functions. –They provide energy for H+ ion transport –They are added to NADP+ to form NADPH. The flow of H+ ions through ATP synthase makes ATP. The products are oxygen, NADPH, and ATP. Oxygen is given off as a waste product. Energy from ATP and NADPH is used later to make sugars.

4.3 Photosynthesis in Detail The light-dependent reactions produce ATP. –The synthesis part of photosynthesis (synthesis = to build) What is built here in light-independent? –hydrogen ions (H+) flow through a channel in the thylakoid membrane –ATP synthase attached to the channel makes ATP FIGURE 4.8 Scientists have made detailed computer models of ATP synthase (left). Scientists are still working on viewing the actual molecule (right).

4.3 Photosynthesis in Detail Light-independent reactions occur in the stroma and use CO 2 molecules. Can take place anytime energy is available energy sources for the light-independent reactions are the molecules of ATP and NADPH formed during the light- dependent reactions. The second stage of photosynthesis uses energy from the first stage to make sugars.

4.3 Photosynthesis in Detail Energy is needed series of chemical reactions called the Calvin cycle Cycle named for the scientist who discovered the process A molecule of glucose is formed as it stores some of the energy captured from sunlight. Can NOT take place without the ATP from light-dependent reactions. Also use CO2 from atmosphere and ADP and NADPH energy to make simple sugars! Steps to Calvin Cycle:

4.3 Photosynthesis in Detail Label the Steps to the Calvin Cycle!