Photosynthesis

• The role of photosynthesis is to make food for plants. It also produces oxygen which is used to make energy. It removes carbon dioxide form the air.
• Photosynthesis requires carbon dioxide, water, light energy and chlorophyll.
• Photosynthesis produces glucose and waste oxygen.
• Photosynthesis converts light energy into chemical energy.
• In photosynthesis, there are five stages: Light energy is absorbed by chlorophyll, which is converted by plants into glucose. Some of the trapped light energy is used to split water into oxygen, protons and electrons.
• The electrons made from photosynthesis are passed into chlorophyll, the protons are stored in a proton pool for later use, and the oxygen passes into the atmosphere. The electrons become energized by light energy and become high-energy electrons. These along with the protons are combined with CO2 to form glucose.
• Artificial light is usually used in greenhouses to stimulate growth.
• Plants get carbon dioxide internally from their own cellular respiration. They externally source it from the atmosphere.
• Water is absorbed from the soil by the roots of plants.
• Photosynthesis is divided into two stages: the light stage and the dark stage.
• the light stage of photosynthesis is light dependent, while the dark stage is light independent
• Light stage of photosynthesis takes place in the granum (stack of thylakoids)
• The dark stage of photosynthesis takes place in the stroma.
• There are four events in the light stage- light absorption, light energy transfer to electrons, and flow of electrons to pathway one and two.
• In light absorption, there are coloured pigments in the grana to absorb the seven colors of white light. The color green is usually reflected.
• Chloroplast

  A) outer membrane

  B) inner membrane space

  C) inner membrane

  D) stroma

  E) granum

  F) thylakoid

  G) lamella

  H) lumen

  
• Chloroplast pigments are arranged in clusters in chloroplast. ​A cluster consists of: ​

  1. Variety of pigments​
  2. Strategically placed Chlorophyll molecule​
  3. Electron Acceptor 
• • In Stage 2, light energy is transferred to electrons.
  • The function of a cluster is to absorb as much light as possible​.
  • Different pigments transfer the absorbed energy from one to another until it reaches the chlorophyll associated with the electron acceptor.
  • Energy is transferred to electrons and they become energized. 
  • Energized electrons pass from chlorophyll to the electron acceptor​. Then, the Electron passes along either​ Pathway 1 or Pathway 2  ​

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• Adenosine Diphosphate (ADP) is a low energy carrier.
• Adenosine Triphosphate (ATP) is a high energy carrier.
• When energy is given off, ADP forms ATP through phosphorylation.
• In Pathway 1, a high energy electron leaves the chlorophyll molecule and passes from electron acceptors giving off energy to form ATP. The electron returns to the original electron acceptor (cyclic)
• Nicotinamide Adenine Dinucleotide (NADP+) is a low-energy carrier.
• Nicotinamide Adenine Dinucleotide Phosphate (NADPH) is a very high energy carrier.
• In Pathway 2, two electrons leave the chlorophyll molecule. They pass through electron acceptors to form ATP. They combine with NADP+ to form NADP-.

  • The chlorophyll molecule is now short of electrons. ​
  • It gains new electrons from the splitting of water (photolysis)
  • Protons formed from water splitting are stored in a pool of protons in the chloroplast. ​
  • These protons are now attracted to NADP- and combine to REDUCE it to NADPH
  • Because the electrons start at a chlorophyll and finish at NADPH and form ATP on their way
• The products of the light stage are:

  ATP- energy for dark stage

  NADPH- proton and energised electron supply for dark stage

  oxygen- respiration
• The dark stage is controlled by enzymes and therefore can be affected by temperature
• In the dark stage, Carbon dioxide from the air enters the chloroplast where they combine with protons + electrons to form glucose
• In the dark stage,

  • It uses energy from breaking down ​

  ATP into ADP + P​

  • It uses hydrogen ions and electrons obtained from breaking down ​

  NADPH into NADP+ + 2 electrons + H+
• The dark stage is classed as 'anabolic'