Photosynthesis!!!!

Photosynthesis is divided into two parts The overall reaction in photosynthesis: Light energy 6CO2 + 12 H2O + C6H12O6 + 6O2 + 6 H2O Two components: Light-dependent reactions Light-independent reactions Chemical energy (ATP, NADPH) Light energy Chemical energy (ATP, NADPH) Chemical energy (C6 H12O6) H2O O2 CO2 Energy Harvest Synthesis

Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 3)

Photosynthesis starts in the thylakoid membrane These aggregations of pigment molecules are called photosystems

Figure 10.11 How a photosystem harvests light

The energy from light is captured and converted in chloroplasts Energy from light excites an electron in chlorophyll. This electron is then passed to acceptor molecules to be used to make ATP and NADPH Stroma Light photon Electron acceptor molecule e- Reaction center (chlorophyll a ) Thylakoid Thylakoid space Chloroplast Photosystem Stroma

There are 2 types of reaction centers in green algae and plants: Photosystem I: activated by light of wavelengths of 700nm Photosystem II: activated by light of wavelengths of 680nm These photosystems are named after the order in which they were discovered, not the order in which they act during photosynthesis Figure: 7.6c Caption: (c) In chloroplasts, excited electrons transmit energy to a reaction center. There the electron is captured by an electron acceptor that becomes reduced. As a result, fluorescence is rare.

Noncyclic electron flow during the light reactions generates ATP and NADPH

The electrons in photosystem II are replaced by electrons from water

The electrons from chlorophyll are passed to an ETC which makes ATP

The electrons reach photosystem I where another photon of light excites them once again

The electrons from photosystem II are passed down a second ETC, in which they are used to make NADPH

Figure 10.13 A mechanical analogy for the light reactions

Figure 10.16 The light reactions and chemiosmosis: the organization of the thylakoid membrane

Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts

Sometimes an alternate pathway happens In cyclic electron transport, which drives cyclic photophosphorylation, photosystem I transfers electrons to plastoquinone (PQ).

Summary of the Light Dependent Reaction of Photosynthesis Light energy is absorbed by chlorophyll molecules, which excites electrons The energy from the excited electrons is converted to chemical energy: Photosystem II: electron transport chain produces ATP The excited electrons are replaced by splitting a water molecule, releasing O2 Photosystem I: The excited electron is donated to NADP+ to produce NADPH

Photosynthesis is a two-step process The light independent reactions (Calvin cycle) use the energy in ATP and NADPH to reduce organic compounds to form glucose.

Figure 10.17 The Calvin cycle (Layer 1)

Figure 10.17 The Calvin cycle (Layer 2)

Figure 10.17 The Calvin cycle (Layer 3)

Summary Light Dependent Reaction Light + chlorophyll --> ATP + NADPH + (O2 as waste) Light Independent Reaction (Calvin Cycle) CO2 + ATP + NADPH --> glucose

What happens to the glucose produced by photosynthesis? Sucrose CH2OH O O H H HOCH2 H H OH H O H HO Figure: 7.17a Caption: (a) In plants, sugars are usually transported in the form of sucrose.  CH2OH HO H OH HO H Fructose subunit Glucose subunit

Starch Glucose subunit Glucose subunit Glucose subunit CH2OH CH2OH Up to 1000 or more monomers H H H O OH H O OH H O OH H O H OH H OH H OH Figure: 7.17b Caption: (b) Plants store sugars in the form of starch, which consists of long chains of glucose molecules. Glucose subunit Glucose subunit Glucose subunit