Photosynthesis Process

Photosynthesis

• Photosynthesis is the ability of plants, algae, and some bacteria to produce their own food in the presence of sunlight.
• All heterotrophs (consumers) are dependent on autotrophs (producers) for their food.

Structures Involved in Photosynthesis

• Chloroplasts are the organelles within plant cells where photosynthesis occurs.
• Thylakoids are membranous compartments within chloroplasts that contain chlorophyll.
• Grana are stacks of thylakoids within chloroplasts.
• Stroma is the fluid-filled space surrounding the grana within the chloroplast.

Light-Dependent Reactions

• Light-dependent reactions occur in the thylakoid membranes of chloroplasts.
• Light energy is absorbed by reaction-center chlorophyll, typically chlorophyll a.
• Electrons become excited and escape, moving to a nearby electron-acceptor molecule.
• There are two electron pathways: cyclic and noncyclic.

Cyclic Electron Pathway

• Electrons flow in a circle within Photosystem I.
• Only produces ATP.
• No oxygen is released.

Noncyclic Electron Pathway

• Electrons flow in a straight line from water to NADP+ via both Photosystem II and Photosystem I.
• Produces both ATP and NADPH.
• Oxygen is released when water splits.

ATP Synthesis

• The flow of hydrogen ions down the electrochemical gradient through a channel in the thylakoid membrane provides the energy for ATP production.
• ATP synthase complex facilitates the synthesis of ATP.
• ATP is produced in the stroma of the chloroplast.

Light-Independent Reactions (Calvin Cycle)

• Carbon dioxide is reduced to form a carbohydrate.
• The reduction of carbon dioxide occurs in the stroma.
• RuBP carboxylase (Rubisco) combines RuBP with carbon dioxide during carbon fixation.
• 5 carbons are in RuBP.
• 3-PGA is the first detectable molecule in the Calvin cycle after carbon dioxide fixation.
• PGA, BPGA, and G3P are intermediate molecules in the Calvin cycle.
• Five molecules of G3P are used to re-form 3 molecules of RuBP.
• G3P represents the end product of the Calvin cycle.
• ATP and NADPH are converted to ADP + Pi and NADP+ during the light-independent reaction.
• Glucose is the energy-rich organic compound produced as a result of the Calvin cycle.

Electron Transport Chain

• A series of protein complexes that generate a proton gradient across the thylakoid membrane.
• Energy from light is used to pump protons across the membrane, creating a proton gradient.
• The proton gradient is used to drive ATP synthesis.
• Involves the transfer of electrons from high-energy molecules to lower-energy molecules.

Photosystem I

• Absorbs light energy at a wavelength of 700 nm (P700).
• Generates a high-energy electron that is used to reduce NADP+ to NADPH.
• Produces a proton gradient that drives ATP synthesis.
• Located in the unstacked regions of the thylakoid membrane.

Photosystem II

• Absorbs light energy at a wavelength of 680 nm (P680).
• Generates a high-energy electron that is used to reduce plastocyanin.
• Produces a proton gradient that drives ATP synthesis.
• Located in the stacked regions of the thylakoid membrane (grana).