plants 3-1

• Photosynthesis


  The production of sugar in plants using carbon dioxide and water in the presence of light. Solar energy is used to produce chemical energy which is used to produce organic molecules

  
• Photosynthesis takes place in plants and trees, algae and kelp, and photosynthetic bacteria

  
• Light absorbing compounds in leaves

  • Need red or blue light, green light will kill
• Chlorophyll


  There are two forms - 'a' and 'b'. Plants only respond to light in the visible spectrum. Carotenoids are other pigments that are orange and yellow and absorb light around 480-500 nm wavelength. Chlorophyll 'a' absorbs light around 440 nm wavelength (blue color) and also at 680-700 nm (red color). Carotenoid pigments show up when chlorophyll is broken down in the Fall season to produce vivid colors of leaves. They also are anti-oxidants that reduce oxidative damage due to sunlight and UV rays.

  
• Chloroplasts


  • Chlorophyll is contained within chloroplasts. Chloroplasts are found in the mesophyll cells of leaves. They are surrounded by a double layer of cell membranes. Inside the chloroplasts are stacks of granum (pancake shaped) that are surrounded by the thylakoid membrane. The space around the granum is called stroma. Light is absorbed by the granum, except for green wavelength which is transmitted. Light energy is packaged into photons which strike the chlorophyll and cause it to emit higher energy level electrons.

  
• Photosynthetic reactions


  During the light reactions, ADP and NADP are combined with P to form ATP and NADPH. This provides energy for the next step in photosynthesis. Oxygen is released from water. The next step is the Calvin cycle which is light-independent in which energy from the light reactions is used to drive the formation of carbon molecules from CO2. The chlorophyll and pigment molecules are arranged in a light-harvesting complex called a Photosystem. Inside is a primary electron acceptor called pheophytin. The Photosystem I and Photosystem II are embedded in the thylakoid membrane.

  
• Photosystems


  • Photosystem I absorbs light in the range of 700 nm (P700). Photosystem II absorbs light in the range of 680 nm (P680). Electrons released from the splitting of water by light photons reach P680 first and the chlorophyll energizes electrons to the primary electron acceptor. The electrons are transferred down an "electron transport chain" to Plastoquinone (Pq), then to Cytochrome complex (Cc) and then to Plastocyanine (Pc). At the point of reaching Cc, energy from the electrons is used to generate ATP.

  
• Electron transport chain


  Light energy then strikes PS I (P700) and electrons are transferred to the primary electron acceptor. The electrons continue down the electron transport chain to the next molecule which is Ferredoxin (Fd). The energy from the electrons creates the formation of NADPH using the enzyme NADP reductase. The end result of electrons being released from water is the production of ATP and NADPH and the production of oxygen.

  
• Noncyclic (linear) electron flow


  Produces NADPH and ATP
• Cyclic electron flow


  Produces more ATP but no NADPH
• Calvin cycle


  Also called the C3 cycle. Named after Melvin Calvin who discovered the cycle in plants. Results in the conversion of carbon dioxide to sugar (sucrose) using energy (ATP, NADPH) from the light reactions. Also called "carbon fixation". About 160 x10^12 kg/yr is fixed by plants. Occurs in the stroma of chloroplasts. Reaction starts with ribulose-1,5-bisphosphate (RuBP) adding one CO2 molecule to form (2X) 3-phosphoglycerate. The enzyme involved here is called Rubisco = ribulose-1,5-bisphosphate carboxylase. This is the most abundant enzyme on Earth. The next step is a requirement for ATP to produce 1,3-bisphosphoglycerate (3C). Then NADPH is required to form glyceraldehyde-3-phosphate. This goes on to form sugar. And RuBP is reformed to continue the cycle. Energy in the form of ATP is required here. Total energy required for producing a 6C sugar molecule is 18 ATP molecules and 12 NADPH.

  
• Plants growing in hot climates

  • Plants growing in deserts will close their stomata during the day to conserve water. This causes CO2 levels in leaf cells to decline, and oxygen builds up. The Calvin cycle slows down as there is less CO2. Plants then undergo "photorespiration" where in the presence of oxygen, the phosphoglycerate molecule is oxidized to release CO2. This can cause up to 50% of the carbon to be lost.
• C4 pathway in plants

  They use a molecule of malate (C4) instead of phosphoglycerate (C3). This is produced in mesophyll cells by adding CO2 to a molecule of phospho-enol-pyruvate (PEP) to form oxaloacetate which is converted to malate. The enzyme involved is PEP carboxylase, which has a higher affinity for CO2 than Rubisco and so can capture lower concentrations of CO2 in hot climates. These plants also have specialized cells called "bundle sheath cells". These cells break down malate to release CO2 and form pyruvate. This CO2 molecule is used in the Calvin cycle to form sugar. This allows plants to grow in hot climates eg. sugarcane.
• CAM plants


  • These are Crassulacean Acid Metabolism plants. They occur in plants such as cactus and pineapple. During the day, stomata are closed. CO2 is taken up at night to produce organic acids eg. crassulacean acid. This is stored in mesophyll cells at night. In the day time, light reactions continue, and ATP and NADPH are produced. Then Crassulacean acid is broken down to release CO2. This is used in the Calvin cycle which can operate while the stomata are closed during the day.
• The product of photosynthesis (sugar) is used in the plant to form cellular structures and tissues. Some of it is stored in roots and tubers as starch. Other uses are for fruits and seeds.

  
• Requirements for photosynthesis


  • light energy
  • chloroplasts
  • water
  • electrons
  • photosystems
  • electron transport
  • Calvin cycle

  
• Carbon fixation is a very important process for planet Earth.