light dependent reaction lecture note-130411061851-phpapp01.ppt
- 1. The light dependent reaction Photosynthesis
- 2. One can think of the light dependent reactions as a way to increase the free energy of the system and the light independent reactions as a way to convert that new free energy into the bonds of glucose.
- 3. Photosystems Photosystems • The light-absorbing pigments of thylakoid membranes and their associated electron carriers are arranged in functional sets or clusters. • These clusters are called photosystems. • These photosystems can absorb light over the entire visible spectrum but especially well between 400 to 500 nm and 600 to 700 nm.
- 4. When a chlorophyll molecule in the thylakoid membrane is excited by light, the energy level of an electron in its structure is boosted The packet of excitation energy now migrated rapidly through the light harvesting pigment molecules to the reaction centre of the photosystem where it causes an electron to acquire the large amount of energy.
- 5. Photosystem I and II • The thylakoid membranes of plant chloroplasts have two different kinds of photosystems each with its own set of light harvesting chlorophyll and carotenoid molecules and the photochemical reaction centre. • Photosystem I - is maximally excited by light at longer wavelengths. (P700) • Photosystems II - is maximally excited by shorter wavelengths. (Less the 680)
- 6. The light reaction These increase the free energy made available to the system. This free energy can be used in three ways: 1) To build the chemiosmotic or proton gradient. 2) Generate ATP. 3) Reduce NADP+ to NADPH. • There are two ways to generate ATP • 1) Non cyclic photophosphoraltion. • 2) Cyclic photophosphoralation. • These two systems differ in the route taken by the "light activated" electrons and in some of the products formed.
- 7. Photosystem II • Light energy is trapped in photosystem II and boosts electron to a higher level. • The electrons are received by an electron acceptor. Thus leaving the chlorophyll molecule positive. • It then splits water by taking 2e- from it. • The electrons are passed from one acceptor (Plastoquinone and Cyctochrome) to the next to photosystem I. • Along the way the energy lost reduces ADP-> ATP
- 9. Photosystem I • The light energy in Photosystem I boosts the electrons to an even higher level. • The electrons are received by another acceptor and these combine with the protons from the split water to reduce NADP called non cyclic photophosphorylation. • Any left over electrons will return to Photosystem I (via the electron transport chain) to produce more ATP. This is called cyclic photophosphorylation. (This is the common pathway in prokaryotes)
- 11. The ‘z’ scheme The products so far…. -O2 -H+ ions -ATP -NADPH
Editor's Notes
- #3: For example: in spinach chloroplasts these photosystems contain about 200 chlorophyll molecules and about 50 carotinoids, arranged in what are called light harvesting antenna.