Photosynthesis

• The energy from light is used to break the strong hydrogen (O-H) bonds in the water molecules. The hydrogen released is combined with carbon dioxide to form glucose and the oxygen is released into the atmosphere as a waste product.
• Photosynthesis is a two stage process. The light-dependent reactions produce materials that can be used in the light-interdependent reactions. The whole process continues during the hours of daylight, and the light-interdependent reactions can continue when it is dark
• The light dependent stage occurs on the thylakoid membranes of the chloroplasts and has two main functions:

  • to break up water molecules in a photochemical reaction, producing hydrogen ions to reduce carbon dioxide and produce carbohydrates in the light-interdependent stage
  • to produce ATP, which supplies energy to build carbohydrates
• Light is a form of electromagnetic radiation, and the smallest unit of light is a photon.
• When a photon hits a chlorophyll molecule, the energy is transferred to the electrons of the chlorophyll molecule. The electrons are excited and raised to a higher energy levels. If an electron reaches a high enough electron level, it leaves the chlorophyll completely and is collected by a carrier molecule called an electron acceptor. This results in the synthesis of ATP in either cyclic phosphorylation or non-cyclic phosphorylation. These processes occur at the same time
• In both cyclic phosphorylation and non-cyclic phosphorylation, ATP is formed as the excited electron is transferred along an electron transport chain. In non-cyclic phosphorylation, reduced NADP is also formed
• Phosphorylation means adding a phosphate group to ADP
• Cyclic phosphorylation involves only photosystem I (PSI) and drives the production of ATP. When an electron returns to a chlorophyll molecule in PSI, it can be excited again.

  
• In non-cyclic phosphorylation, water molecules are broken down, providing hydrogen ions to reduce NADP. This process involved both photosystem I and II. this
• In non-cyclic phosphorylation, light energy hits photosystem II in the thylakoid membrane, and an electron gains energy and is excited to a higher energy level. This electron is transferred along an energy transport chain to PSI, driving the synthesis of ATP. This allows PSI to receive an electron to replace the one lost in the light-interdependent reactions. However, now the chlorophyll molecule in PSII is missing one electron and is unstable. H+ and OH- ions in the chloroplasts, formed by the photolysis of water are used to replace the lost electrons in the chlorophyll.
• At the same time as non-cyclic phosphorylation, electrons in PSI are also being excited and transferred along an electron transport chain and collected by the electron acceptor NADP. The NADP also collects a H+ from the dissociated water. The reduced NADP and ATP produce the source of reducing power and energy to make glucose. The remaining hydroxide ions react to make water and 4 chlorophyll molecules regain electrons in the production of an oxygen molecule 4OH- - 4e- -> O2 + H2O
• The light-interdependent stage uses the reducing power (reduced NADP) and ATP produced by the light-dependent stage. This stage consists of a series of reactions known as the Calvin cycle and occurs in the stroma of the chloroplast. This results in the reduction of carbon dioxide from the air leading to the synthesis of carbohydrates. The Calvin cycle is controlled by enzymes.
• The Calvin cycle reactions can continue in the dark, and only stop when there is no longer reduced NADP or ATP.
• In the first step of the Calvin cycle, carbon dioxide combines with the 5-carbon compound ribulose biphosphate (RuBP) in the chloroplasts in a process called carbon fixation. This steps involved the enzyme ribulose biphosphate carboxylase/oxygenase (RUBISCO), a rate limiting enzyme in photosynthesis. The result is highly unstable and immediately separates into two molecules: glycerate 3-phosphate (GP, 3-carbon compound) which is then reduced to form gylceraldehyde (GALP). The hydrogen comes from reduced NADP, and the energy required comes from ATP.
• Much of the GALP produced in the Calvin cycle is used to replace the RuBP used in the first step, but some is used to make glucose or transferred directly into the glycolysis pathway for the synthesis of other molecules. The rest is used to make glucose in a process called gluconeogenesis. The GALP that entered cellular respiration is used to provide energy in the form of ATP, and can also combine with phosphates in the soil to produce nucleic acids.
• Photosynthesis is limited by the availability of light and carbon dioxide and also by temperature.
• Light can limit photosynthesis as light intensity affects the amount of chlorophyll which can be excited, and therefore the amount of reduced NADP and ATP produce. Light wavelength also affects photosynthesis
• Carbon dioxide concentration affects photosynthesis as if there is not enough carbon dioxide available for fixing in the Calvin cycle, then the reactions cannot proceed at maximum rate.
• Temperature affects photosynthesis as all of the Calvin cycle reactions and many of the light-dependent reactions are controlled by enzymes, which are proteins and can denature in too high/low temperatures