Unit 4: Section 1- Photosynthesis

A summary of photosynthesis notes in unit 4

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  • Created by: R_Hall
  • Created on: 12-09-13 19:45

Photosynthesis and Energy Supply

  • Plants need energy for photosynthesis, active transport, DNA replication, cell division and protein synthesis
  • Animals need energy for muscle contraction, homeostasis, active transport, DNA replication, cell division and protein synthesis
  • Photosynthesis- the process when light energy is used to break strong bonds in H2O molecules, to release H to combine with CO2 to form glucose. O2 is released into the air
  • Energy is stored in the glucose until the plants release by respiration. Animals obtain glucose by eating plants, the use it to release energy through respiration, which powers all the biological processes in a cell
  • Aerobic respiration-with oxygen. Anaerobic respiration- without oxygen
  • A cell cannot get energy directly from glucose. So energy released from glucose is used to make ATP (adenine+ ribose sugar+ 3 phosphate), which carries energy around. 
  • It is synthesised from ADP (by ATP synthase enzyme) and inorganic phosphate using energy from a reaction, with energy stored as chemical energy in the phosphate bond
  • It then diffuses to where needed
  • Here it's broken down to ADP and phosphate (ATPase enzyme)- releasing chemical energy for the cell
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Photosynthesis and Energy Supply 2

  • Phosphorylation- adding phosphate to a molecule
  • Photophosphorylation- adding phosphate to a molecule using light
  • Photolysis- the splitting of a molecule using light energy
  • Hydrolysis- the splitting of a molecule using water
  • Redox reaction- a reaction with involves oxidation and reduction
  • A coenzyme is a molecule which aids the function of an enzyme by transferring a chemical group from one molecule to another. NADP is a coenzyme which transfers H from one molecule to another- it can reduce (give H) or oxidise (take H)
  • Chloroplasts are flattened organelles with a double membrane- the chloroplast envelope
  • Thylakoids are stacked into grana within the chloroplasts. Grana are linked with lamellae (bits of thylakoid)
  • Chloroplasts cont. photosynthetic pigments (chlorophyll a+b, carotene) which absorb light energy. They are found attached to proteins in thylakoids- a photosystem
  • Stroma is a jelly contained in the inner membrane of chloroplast. It contains enzymes, sugars, organic acids and oil droplets
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The Light-Dependent Reaction

  • It requires light energy and takes place in the thylakoid membranes. Light energy is absorbed by photosynthetic pigments and converted into chemical energy. During the process, H20 is oxidised into O2
  • Light energy absorbed is used to make ATP, make reduced NADP and split H20 into protons (H+), e- and O2
  • Non cylic photosphosphorylation- produces ATP, reduced NADP and O2
  • 1. Light energy absorbed by PSII photosystem, which excites electrons in chlorophyll. The electrons move to a higher energy level, and move along the electron transport chain (chain of proteins) to PSI
  • 2. Light energy splits H2O into protons (H+), e- and O2. The new electrons replace the ones lost from PSII
  • 3. The excited electrons lose energy as they move along the electron transport chain. This energy is used to transport protons into the thylakoid, so it has a higher H+ conc than stroma- forms a proton gradient. Protons move down the gradient into stroma, via ADP synthase enzyme. The energy from the movement creates ATP (ADP + P)
  • 4. Light energy is absorbed by PSI, which exictes e- to a higher energy level. The e- are transferred to NADP along with H+ from stroma-> reduced NADP
  • Cyclic phosphorylation- only produces small amounts of ATP- electrons passed back to PSI
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The Light-Dependent Reaction

  • Takes place in stroma of chloroplasts. Called the Calvin Cycle
  • 1. CO2 enters leaf through stomata and diffuses into stroma. Combined with Ribulose biphosphate (5C)-> unstable 6C -> two molecules of Glycerate 4-phosphate (3C). Rubisco (ribulose biphosphate carboxylase) catalyses CO2 and RuBP reaction
  • 2. ATP provides energy to reduced GP to triose phosphate, a different 3-carbon compound. Reduction also requires H+ ions rfom reduced NADP, which is recycled to NADP. Triose phosphate is converted into useful organic compounds (carbohydrates, lipids, amino acids and nucleic acids)
  • 3. 5/6 molecules of TP made in the cycle are used to regenerate RuBP. This uses the rest of the ATP from the light-dependent reaction
  • The calvin cycle needs to turn 6 times to make one glucose molecule
  • The chloroplast envelope keeps reactants for photosynthesis close to reaction sites
  • Thylakoids have a large surface area to maximise light absorption
  • Lots of ATP synthase molecules are present in the thylakoid membrane to produce ATP in the light-dependent
  • The stroma contains enzymes, sugars and organic acids needed in the light-independent
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