Photosynthesis and Respiration

• Why ATP is a good energy source?

  • Hydrolysed quickly in a single reaction providing an immediate energy source
  • Energy can be released in small manageable amounts, so doesn't damage cells
  • It cannot leave the cell
• ATP is a universal energy currency. It is used by allorganisms
• Why organisms need energy from ATP?


  • Active Transport
  • Movement - e.g. muscle contraction
  • Metabolic Processes - including synthesis of proteins, DNA and RNA
  • Secretion - exocytosis
  • Activation of Molecules - ATP can lower the activation energy of some molecules when a phosphate is transferred.
• Energy Sources for ATP Synthesis:

  • oxidative phosphorylation
  • photophosphorylation
  • substrate level phosphorylation
• Adding a phosphate group to another molecule is phosphorylation but adding it to ADP is specifically substrate level phosphorylation
• Coenzymes are not enzymes but assist enzymes. They are reduced and then oxidised in respiration.
• Glycolysis - In the cytoplasm

  Glucose is phosphorylated by 2 ATP into Glucose Phosphate. This is unstable so it breaks down into 2 molecules of Triose Phosphate. Each Triose Phosphate is oxidised into Pyruvate, as NAD is reduced and 2 molecules of ATP are synthesised via substrate level phosphorylation.
• Glycolysis has the net gain of 2 ATP
• If no oxygen is present, Pyruvate is reduced to Ethanol/Lactate as NADH is oxidised back to NAD being regenerated.
• Pyruvate is actively transported into the mitochondrial matrix
• Link Reaction - In the matrix

  Pyruvate is oxidised into Acetate as NAD is reduced and it is decarboxylated. Acetate the binds with Coenzyme A to form Acetyl coenzyme A.
• Krebs Cycle - In the matrix

  Acetyl Coenzyme A combines with a 4C compound to form a 6C compound. Then the 6C is decarboxylated and oxidised to a 5C compound as NAD is reduced. 5C is then decarboxylated again. Reduction of 2 NAD and FAD takes place as well as synthesis of 1 ATP.
• Oxidative Phosphorylation 1

  1. The reduced coenzymes NADH and FADH are oxidised to release hydrogen ions and electrons
  2. The electrons enter the ETC and move between proteins from a high to low energy level - releasing energy
  3. The energy released is used to pump the protons across the inner mitochondrial membrane and into the intermembranal space.
• Oxidative Phosphorylation 2

  1. This forms a chemiosmotic gradient of protons
  2. The protons diffuse through a channel protein embedded in the inner membrane causing ATP synthase to catalyse the reaction
  3. Oxygen is the final electron acceptor of the electron from the ETC and the protons entering the matrix

  2H+ + e- + 1/2O2 --> H2O
• Other Respiratory Substrates - Lipids

  Hydrolysed triglycerides turn into fatty acids and glycerol. Glycerol is converted into triose phosphate on entering glycolysis. Fatty acids are hydrolysed to 2C fragments and are converted into acetyl coenzyme A.
• Other Respiratory Substrates - Proteins

  Hydrolysed into amino acids. Amine group is removed by deamination. R group is split into either 3C (triose phosphate), 4C or 5C (Krebs Cycle) molecules.
• Light travels in packets called photons
• The energy content of a photon is dependent on the wavelength of light. The shorter the wavelength, the higher in photon energy. Blue light has a relatively short wavelength.
• Plants are able to absorb a wide range of wavelengths because they contain a variety of pigments with different strutures and absorption properties
• Light Dependent Stage 1

  1. Light is absorbed by chlorophyll, causing it to become photoionised, as electrons are excited to a higher energy level
  2. At the same time, water undergoes photolysis producing protons, electrons and oxygen.
  3. The excited electrons are replaced by the ones from photolysis, and these former ones enter the ETC.
  4. As electrons move down the ETC, energy is released and used to pump the protons (produced by photolysis) from the stroma into the thylakoids - this establishes a chemiosmotic gradient.
• Light Dependent Stage 2

  1. The protons diffuse back into the stroma through a channel protein and cause ATP Synthase also embedded in the membrane to catalyse the reaction. ADP + Pi --> ATP
  2. The electrons at the end of the ETC and these protons bind with coenzyme NADP to form NADPH2.
• Light Independent Stage

  1. CO2 combines with RuBP - catalysed by rubisco
  2. The 6C product splits into 2 molecules of Glycerate 3 Phosphate.
  3. The Glycerate 3 Phosphate is reduced to Triose Phosphate using the NADPH2 and energy from ATP hydrolysis - LDS products.
  4. Some of the Triose Phosphate is used to make useful organic substances such as glucose and some regenerates the RuBP.
• Limiting Factors of Photosynthesis:

  • Light Intensity
  • CO2 concentration
  • Temperature
• In modern agriculture greenhouses have burners of hydrocarbon fuels which release CO2 whilst raising the temperature.