Photosynthesis Overview

Photosynthesis

• Chlorophyll absorbs red and blue light, reflecting green light.
• Beta-carotene, another pigment found in leaves, absorbs blue and green light and reflects yellow, orange, and red light.
• Photosystems are light-harvesting complexes in chloroplasts composed of a core group of chlorophyll molecules and proteins.
• The antenna complex is composed of pigment molecules that absorb light and transfer the energy to the reaction center (a pair of chlorophyll-a molecules).
• When light energy is absorbed by the reaction center, an electron is excited and passed to an electron acceptor.

Photosystem I and Photosystem II

• Plants and algae have two photosystems: Photosystem I (P700) and Photosystem II (P680).
• Photosystem II comes first in the photosynthetic process, followed by Photosystem I.

Light-Dependent Reactions

• The light-dependent reactions occur in the thylakoid membrane.
• In step 1, light energy excites the P680 molecule, causing it to become positively charged and pull electrons from water molecules by an enzyme in the water splitting complex. This creates oxygen, hydrogen ions (H+), and electrons.
• In step 2, these energized electrons are transferred through an electron transport system (containing various electron-carrying molecules). Energy released in this process is used by the b6f complex to pump H+ from the stroma into the thylakoid space. This pumping creates a proton gradient across the thylakoid membrane.
• In step 3, P700 absorbs light energy and passes its excited electrons to a high-energy electron acceptor. The lost electrons are replaced by electrons from the end of the electron transport chain from P680.
• In step 4, the electrons from P700 are utilized by NADP reductase to reduce NADP+ to form NADPH. This reducing power of NADPH is used in the light-independent reactions.
• The electron transport chain looks like this: PSII (P680) -> plastoquinone (PQ) -> b6f (cytochrome b6f complex) -> plastocyanin (PC) -> PSI (P700) -> ferredoxin (FD) -> FNR (ferredoxin reductase) (NADP reductase).

ATP Production

• The movement of H+ across the thylakoid membrane is facilitated by chemiosmosis and its energy is used to create ATP by ATP synthase. This process is called photophosphorylation.
• Non-cyclic photophosphorylation produces both ATP and NADPH, as the flow of electrons is unidirectional from PSII to NADP reductase. This process generates one NADPH and slightly more than one ATP per electron pair.
• Cyclic photophosphorylation produces only ATP as electrons from PSI are passed to an electron acceptor and then back to the b6f complex and to PSI, continuously driving H+ pumping. This process does not produce NADPH or oxygen.

Cellular Respiration: Glycolysis

• Glycolysis is the breakdown of glucose into pyruvate occurring in the cytosol.
• It yields two pyruvate molecules, two ATP, and two NADH per glucose molecule.
• This process can occur in both prokaryotes (lacking organelles) and eukaryotes (possessing organelles).

Cellular Respiration: Pyruvate Oxidation

• Pyruvate produced by glycolysis is transported into the mitochondrial matrix, where it undergoes pyruvate oxidation (also known as pyruvate decarboxylation) through a series of five steps catalyzed by pyruvate dehydrogenase.
• This process yields carbon dioxide, NADH, and acetyl-CoA.

Cellular Respiration: Krebs Cycle

• The Krebs cycle is a cyclic pathway in the mitochondrial matrix that oxidizes the two-carbon acetyl-CoA molecule.
• It occurs twice for each glucose molecule (yielding two molecules of acetyl-CoA).
• The cycle produces: two ATP, six NADH molecules, and two FADH2 molecules per glucose molecule.

Cellular Respiration: Oxidative Phosphorylation

• Oxidative phosphorylation is the process of ATP synthesis driven by the electron transport chain in the mitochondrial membrane.
• During this process, electrons from NADH and FADH2 are passed along a series of electron carriers, releasing energy during each transfer.
• This energy is used by protein complexes to pump H+ across the mitochondrial membrane, creating a proton gradient.
• The energy stored in the gradient is used by ATP synthase to convert ADP to ATP. This process is called chemiosmosis.

Cellular Respiration: Net Yield

• The net yield of ATP from the complete oxidation of one glucose molecule is estimated to be 36-38 ATP.
• This value is theoretical and can vary due to several factors such as proton leakage and energy expenditure for transporting molecules into and out of the mitochondria.
• Prokaryotes generally produce 38 ATP as they don't need to transport NADH across the mitochondrial membrane.
• Experimental values are closer to 30-32 ATP per glucose molecule.

Anaerobic Respiration

• Anaerobic respiration takes place in environments where oxygen is limited and uses an inorganic molecule other than oxygen as the final electron acceptor in ATP synthesis.
• Many single-celled organisms use only glycolysis for energy, while multicellular organisms utilize it as the first step in aerobic respiration.

Fermentation

• During exercise, oxygen supply may be insufficient, leading to reliance on glycolysis for energy.
• To regenerate NAD+ for glycolysis to continue, the reduced NADH must find another pathway.
• This process is called fermentation and is used for reoxidizing reduced NADH by reducing organic molecules.
• Fermentation is less efficient than aerobic respiration and only produces the amount of ATP generated during glycolysis.

Lactate Fermentation

• During lactate fermentation, NADH reduces pyruvate to lactate.
• Lactate builds up in muscles and needs to be reoxidized to prevent tissue acidification.
• Once oxygen becomes available, lactate can be transported out of cells, converted back into pyruvate, oxidized, or stored as glycogen in muscles.

Ethanol Fermentation

• In ethanol fermentation, yeast and certain bacteria convert pyruvate to acetaldehyde and carbon dioxide, which is used to reoxidize NADH to NAD+.
• This process produces ethanol as a byproduct.
• The byproducts of ethanol fermentation have practical uses in baking (carbon dioxide production) and alcoholic beverages (ethanol production).