topic 5

• Photosynthesis

  The process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen

  
• Light-dependent reaction


  1. Chlorophyll absorbs light, leading to photoionisation of chlorophyll

  2. Energy from electrons released during photoionisation is conserved in the production of ATP and reduced NADP

  3. Production of ATP involves electron transfer associated with the transfer of electrons down the electron transfer chain and passage of protons across chloroplast membranes, catalysed by ATP synthase (chemiosomotic theory)

  4. Photolysis of water produces protons, electrons and oxygen
• Light-independent reaction

  1. Carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP), catalysed by rubisco

  2. ATP and reduced NADP from the light-dependent reaction are used to reduce GP to triose phosphate

  3. Some triose phosphate is used to regenerate RuBP in the Calvin cycle

  4. Some triose phosphate is converted to useful organic substances

  
• Environmental factors that limit the rate of photosynthesis

  • Factors that can be identified and evaluated to overcome their effects

  
• ATP (Adenosine Triphosphate)

  A nucleotide derivative formed from a molecule of ribose, a molecule of adenine and three phosphate groups. It is the short-term energy store of the cell and is often called the 'energy currency' of the cell.

  
• Hydrolysis of ATP

  Catalysed by the enzyme ATP hydrolase, it releases energy that can be coupled to energy-requiring reactions within cells. The inorganic phosphate released can be used to phosphorylate other compounds.

  
• Resynthesis of ATP

  The condensation of ADP and inorganic phosphate, catalysed by ATP synthase during photosynthesis or respiration.

  
• Uses of energy (and therefore ATP) in plants and animals

  • Active transport
  • Photosynthesis
  • DNA replication
  • Cell division
  • Protein synthesis
  • Muscle contraction

  
• Energy cannot be made or destroyed, it can only be transferred
• When ATP is broken down

  The energy is used directly by another reaction and not lost as heat
• Advantages of ATP

  • Instant source of energy in the cell
  • Releases energy in small amounts as needed
  • Mobile and transports chemical energy to where it is needed in the cell
  • Universal energy carrier that can be used in many different chemical reactions
• There is a constant cycle of ATP being broken down and remade
• Limiting Factors

  Plants need many things in order to photosynthesise as quickly as possible. If any are in short supply the rate of photosynthesis will be limited. That factor is known as a "limiting factor"
• Keywords: Limiting Factors, Photosynthesis, Chloroplast

  
• How does light intensity change the rate of photosynthesis?

  Light gets brighter so photosynthesis gets faster. Photosynthesis cannot get faster because something is limiting
• How does CO2 change the rate of photosynthesis?

  More CO2 allows photosynthesis to happen faster. Photosynthesis cannot get faster because something is limiting
• How does temperature change the rate of photosynthesis?

  Above 40oC the enzymes needed for photosynthesis are destroyed
• Scientists investigated the effects of temperature and light intensity on the rate of photosynthesis in creeping azalea
• Chloroplast

  Consists of two membranes enclosing a gelatinous matrix called the stroma. Suspended in the stroma are thylakoids. These are disc-like membrane sacs, stacked in a group to form a granum (plural grana). The space inside each thylakoid is connected with the other thylakoids in the stack, forming a continuous fluid-filled compartment called the thylakoid space. The thylakoid membranes contain photosynthetic pigments, including chlorophyll.
• Chlorophyll

  A group of light absorbing pigments. Chlorophyll A is in Photosystem 2 and absorbs best at around 680nm. Chlorophyll B is in photosystem 1 and absorbs best at 700nm. Having multiple pigments allows the plant to harvest light more efficiently.
• Chlorophyll absorbs light energy mostly in the red and blue parts of the spectrum. Green light is mostly reflected. More pigments you have the more photosynthesis you can do in the same time.
• Photosynthesis

  Photosynthesis is made up of two linked sets of reactions: the light dependent reactions and the light independent reactions (Calvin cycle)

  
• Chlorophyll

  A group of light absorbing pigments
• NADP

  A chemical that can accept H+ ions and store them for later use
• Electron transfer chain

  A group of proteins capable of carrying electrons from one place to another
• ATP

  A short term store of energy in the cell
• Light Dependent Reaction

  Occurs on the Thylakoid membranes. Photosystem 2 absorbs light and emits a pair of excited (high energy) electrons. Electrons pass down electron transfer chain and is used to convert ADP to ATP. Photosystem 1 re-excites the electron, one is used to reduce NADP to NADPH, one generates more ATP. Photosystem 2 is now positively charged. It takes 2 electrons from H2O breaking the bond. 2H2O → 4H+ + O2 + 4e-. The H+ ions are used in reduction of NADP.

  
• Photophosphorylation


  Energy used to join ADP and Pi to form ATP

  
• Photolysis


  Photolysis of water produces protons, electrons and oxygen
• NADP reduced by electrons / electrons and protons / hydrogen
• ATP production


  Non-cyclic photophosphorylation - all the time and uses energy from ETC. Cyclic photophosphorylation - occurs when there is plenty of NADPH in the chloroplast (a sunny day) - They go from PS1 to the ETC twice.

  
• Light dependent stages take place in the thylakoid space. Light independent stages take place in the stroma.
• Calvin Cycle


  During the Calvin cycle, carbon dioxide is fixed and reduced to 3-C sugars from which other products can be synthesized; RuBP used for fixation is regenerated.
• 5C / RuBP combines with CO2 using enzyme rubisco to form x2 3C compound / TP / GP. Using energy from ATP being hydrolysed into ADP and Pi and reducing it using NADPH, 2 molecules of 3C compound / TP / GP form hexose e.g. glucose. All RuBP is regenerated. 10 molecules of 3C / TP / GP form 6 molecules of 5C / RuBP.

  
• Photosynthesis


  1. CO2 combines with rubisco

  2. Forms 2 x 3C compound/TP/GP

  3. Using energy from ATP hydrolysis

  4. Reducing with NADPH

  5. 2 molecules of 3C compound form hexose e.g. glucose

  6. All RuBP is regenerated

  7. 10 molecules of 3C/TP/GP form 6 molecules of 5C/RuBP

  
• In an exam...
• Learning Objective
• To explain the action of the chloroplast
• How do we know about the pathways?
• Calvin Cycle WS