cellular respiration

• Light Dependent Inputs

  > 12 H2O (water) molecules

  > 12 NADP+

  > 18 ADP+Pi
• Light Dependent Outputs

  > 6 O2 (oxygen) molecules

  > 12 NADPH

  > 18 ATP
• Light Independent Inputs

  > 6 CO2 (carbon dioxide) molecules

  > 12 NADPH

  > 18 ATP
• Light Independent Outputs

  > Glucose (C6 H12 O6)

  > 6 H2O water molecules

  > 12 NADP+

  > 18 ADP + Pi
• Fermentation


  Involves the conversion of pyruvate into an alternative carbon compound via a reaction that oxidises the hydrogen carrier

  • This restores the stocks of unloaded coenzyme needed for glycolysis, allowing ATP production to continue in the absence of oxygen
  • Fermentation is therefore considered to be a vital component of anaerobic respiration, as glycolysis could not otherwise be sustained
• Anabolism Features


  >

  • Synthesises larger molecules from smaller ones
  • Uses energy to construct new bonds
  • Involves reduction or condensation reactions
  • Photosynthesis overall is an anabolic process.
• Catabolism Features


  >

  • Digests complex molecules to form simpler & smaller ones.
  • Releases energy once bonds are broken
  • Involves oxyidation or hydrolysis reactions
  • Cell respiration is overall a catabolic process
• Photosynthesis versus Cell Respiration

  The general structure of the biochemical pathways in photosynthesis and cell respiration are effectively reversed

  • Photosynthesis uses an electron transport chain to generate chemical energy for a subsequent Calvin cycle which forms glucose
  • Aerobic respiration uses the Krebs cycle to break down glucose in order to generate energy (ATP) via an electron transport chain
• Photosynthesis involves the 


  anabolic production of glucose from inorganic components (using light as an initial energy source)

  • Water is broken down by sunlight (photolysis) into hydrogen and oxygen (a waste product)
  • Chlorophyll absorbs light energy which is used by an electron transport chain to produce ATP (photophosphorylation)
  • The hydrogen is combined with carbon dioxide (and ATP) to form glucose via the Calvin cycle (i.e. anabolism)
• Cell respiration involves the

  catabolic digestion of glucose into inorganic components (releasing chemical energy as ATP)

  • Glucose is broken down into carbon dioxide and hydrogen via glycolysis and the Krebs cycle (i.e. catabolism)
  • The hydrogen is used by an electron transport chain in order to produce ATP (oxidative phosphorylation)
  • Oxygen collects the de-energised hydrogen to form water (a waste product)
• A catalyst is a substance


  that enables a chemical reaction to proceed at a faster rate or under different conditions (e.g. lower temperature)

  • Catalysts lower the amount of energy required for a reaction to proceed (activation energy), allowing a reaction to occur more readily
  • Catalysts are not changed or consumed by the reactions they influence and so occur at relatively low levels and can be re-used
• Enzymes are 


  globular proteins which act as biological catalysts and speed up the rate of a reaction by lowering the activation energy

  • Every enzyme will only react with specific molecule called a substrate, which binds to a region of the enzyme called the active site
  • The active site and substrate will complement each other in terms of both shape and chemical properties (e.g. opposite charges)
• Enzyme-Substrate 


  When a substrate binds to the active site, an enzyme-substrate complex is formed (as per the ‘lock and key’ model)

  • The active site is not completely rigid however and may undergo a conformational change in shape to better fit the substrate
  • This conformational change may stress and destabilise the bonds in the substrate, hence lowering the activation energy
  • When the substrate has been converted into a product, it will dissociate from the enzyme (allowing the enzyme to be re-used)
• Coenzymes


  A coenzyme is a complex organic molecule that is required for an enzyme’s metabolic activity (it assists with the catalysis of a reaction)

  • Coenzymes cycle between two states: a loaded form that can be used and an unloaded form (similar to a charged or expended battery)
  • Examples of biologically significant coenzymes include ATP (transfers energy) and hydrogen carriers (transfers protons and electrons)
• Hydrogen Carriers


  Hydrogen carriers are coenzymes that transport protons and electrons between chemical reactions (functions like a chemical taxi)

  • Hydrogen carriers are loaded by oxidation reactions (become reduced), and unloaded in reduction reactions (become oxidised)
  • The protons and electrons can be used to help synthesise organic macromolecules via anabolic reactions (e.g. photosynthesis)
  • Hydrogen carriers also function as intermediate energy sources (via their energised electrons) and can be used to make ATP
• The rate of enzyme catalysis can be increased by

  improving the frequency of successful collisions via:

  • Increasing the molecular motion of the particles (thermal energy can be introduced to increase kinetic energy)
  • Increasing the concentration of particles (either by increasing the substrate or enzyme concentrations)
  • Maintaining enzyme-substrate specificity (enzyme denaturation will change the conformation of the active site)
• pH


  • Changing the pH will alter the charge of the enzyme, which in turn will alter protein solubility and overall enzyme shape
  • Changing the shape or charge of the active site will diminish its ability to bind the substrate, abrogating enzyme function
  • Enzymes have an optimal pH (may differ between enzymes) and moving outside this range diminishes enzyme activity
• Substrate Concentration


  • Increasing the substrate concentration will increase the activity of a corresponding enzyme
  • More substrates mean there is an increased chance of enzyme and substrate colliding and reacting within a given period
  • After a certain point, the rate of activity will cease to rise regardless of any further increases in substrate levels
  • This is because the environment is saturated with substrate and all enzymes are bound and reacting (Vmax)
• Enzyme Inhibitors Types


  • Irreversible inhibitors form strong covalent bonds with the enzyme in order to form a permanent attachment
  • Reversible inhibitors form weaker, temporary attachments with the enzyme and thus can potentially be dissociated
• Competitive Inhibition


  • Competitive inhibition involves a molecule, other than the substrate, binding to the enzyme’s active site
  • The molecule (inhibitor) is structurally and chemically similar to the substrate (hence able to bind to the active site)
  • The competitive inhibitor blocks the active site and thus prevents substrate binding
  • As the inhibitor is in competition with the substrate, its effects can be reduced by increasing substrate concentration
• Non-competitive Inhibition


  • Non-competitive inhibition involves a molecule binding to a site other than the active site (an allosteric site)
  • The binding of the inhibitor to the allosteric site causes a conformational change to the enzyme’s active site
  • As a result of this change, the active site and substrate no longer share specificity, meaning the substrate cannot bind
  • As the inhibitor is not in direct competition with the substrate, increasing substrate levels cannot mitigate the inhibitor’s effect
• Chloroplasts are an 


  organelle in plant cells that convert light energy into chemical energy via the process of photosynthesis

  • The chemical energy may be transferred to a molecule that is immediately accessible (ATP) or stored as an organic compound (glucose)
  • Only photosynthetic tissues will possess chloroplasts (i.e. they are present within the leaf tissue but are not found in the roots of plants)
• Structure

  The chloroplast is structured to support the two distinct stages of

  photosynthesis (the light dependent and light independent stages)

  • Chloroplasts contain a series of flattened discs called thylakoids, which are arranged into stacks called grana (increases SA:Vol ratio)
  • These membrane discs contain photosynthetic pigments (such as chlorophyll) and are the site of the light dependent reactions
  • The internal fluid is called the stroma and contains carbon-fixating enzymes that are responsible for the light independent reactions
• Chlorophyll


  The main photosynthetic pigment is chlorophyll, which absorbs red and blue light while reflecting green light

  • The absorbed light energy functions to energise chlorophyll electrons, which are transferred to an electron transport chain
  • Accessory pigments may capture additional wavelengths of light in order to maximise light absorbance by the photosystems 
• Light Dependent Reactions

  occur within the membranous discs called thylakoids (which are arranged into stacks called grana)

  • Light is absorbed by chlorophyll pigments (in photosystems), resulting in the release of energised electrons
  • The electrons enter an electron transport chain, which results in the production of ATP (via photophosphorylation)
  • Light is also absorbed by water, which is split (photolysis) to produce oxygen and hydrogen (carried by NADPH)
  • The hydrogen and ATP are used in the light independent reactions, the oxygen is released from stomata as a waste product
• Light Independent Reactions


  Occur within the fluid-filled interior of the chloroplast called the stroma

  • ATP and hydrogen (carried by NADPH) are transferred to the site of the light independent reactions
  • The hydrogen is combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates, amino acids, etc.)
  • The carbon is fixed by the enzyme Rubisco, with ATP providing the chemical energy required to join the molecules together
  • This process is also commonly known as the Calvin cycle
• Which Plants use which enzyme for a specific fixation?

  C3 plants use a RUBISCO enzyme for carbon fixation in order to make carbohydrates (e.g glucose).
• Photorespiration


  Rubisco can alternatively use oxygen (O2) as a substrate to undergo a different series of reactions known as photorespiration

  • Photorespiration creates a product that cannot be used to make sugars and hence reduces the efficiency of the Calvin cycle
  • Photorespiration reduces levels of photosynthesis by up to ~25% in C3 plants, reducing energy yield in these plants.
• Oxygen is a competitive inhibitor to what enzyme?

  Rubisco. Therefore photosynthesis for C3 plants slows down in the presence of oxygen.

  • C3 plants are less efficient in hot and dry regions, as the stomata must remain closed in order to prevent excessive water loss
  • When the stomata are closed, oxygen cannot diffuse out of the leaf, increasing oxygen concentration relative to CO2 levels
• Maximising Photosynthesis


  • C4 and CAM plants use an alternate enzyme called PEP carboxylase to initially fix the carbon and make a 4C compound
  • PEP carboxylase has a higher affinity for carbon dioxide than Rubisco and doesn’t bind to oxygen at all
  • These plants can then store and transfer the 4C compounds to regions with lower oxygen concentrations
• C4 Pathway 


  In the C4 pathway, carbon dioxide is physically separated from oxygen in order to improve CO2 binding to Rubisco 

  • The CO2 is converted to the 4C compound in the mesophyll and then sequestered to a deeper tissue layer where less O2 is present
  • In this deeper tissue layer (the bundle sheath), the CO2 is released and can enter the Calvin cycle without competition from oxygen
• CAM Pathway


  In the CAM pathway, carbon reserves are created at night and then released for use during the day (temporal isolation)

  • CAM plants are suited to hot and arid environments where water loss is high and stomata must therefore remain closed during the day
  • The CO2 is converted into the 4C compound during the night, when stomata are open and the CO2 is able to diffuse into the leaf
  • The stored CO2 is then released for use during the day, when closed stomata would otherwise prevent photosynthesis from proceeding
• Mitochondria Structure 


  • The mitochondrion is a double membrane structure with the inner membrane arranged into folds (cristae) to increase the SA:Vol ratio
  • The inner membrane contains the ETC and ATP synthase (which is used for oxidative phosphorylation)
  • The gap between the membranes (intermembrane space) is very small to maximise the gradient upon proton accumulation
  • The internal fluid is called the matrix and contains the enzymes responsible for the Krebs cycle
• Cell respiration


  is the controlled release of energy from the breakdown of organic compounds (principally glucose) 

  • Glucose can be partially broken down under anaerobic conditions (no oxygen) within the cytosol for a low energy yield
  • The partially digested products can be completely broken down under aerobic conditions (oxygen) in mitochondria for a higher yield
• ATP Production


  When organic compounds are broken down, the energy released is transferred to one of two coenzyme molecules

  • ATP is the primary energy carrier and can be produced directly from ADP (and Pi) via substrate-level phosphorylation
  • Hydrogen carriers act as a transitional energy carrier and can transfer energy to form ATP via oxidative phosphorylation
  • O.P involves an electron transport chain located on the inner membrane (cristae) of the mitochondria.
  • O.P requires oxygen to function, hence only aerobic respiration can produce ATP from hydrogen carriers
• Anaerobic Respiration


  does not require oxygen and involves the partial breakdown of glucose via the process of glycolysis 

  • It occurs in the cytosol and results in a low yield of ATP (net 2 molecules total) via substrate level phosphorylation
• Glycolysis


  • In glycolysis, glucose is broken down within the cytosol into two 3C-compounds called pyruvate (or pyruvic acid)
  • This process uses two molecules of ATP but produces four molecules (for an overall net gain of 2 ATP molecules)
  • Hydrogen atoms are also removed (via oxidation) and transferred to unloaded coenzymes (NAD)
  • This results in a small yield of loaded hydrogen carriers (NADH) which can be used by the mitochondria
• Aerobic Respiration


  Involves the complete breakdown of glucose in the mitochondria for a higher ATP yield (net 30 or 32 ATP)

  • It is preceded by the anaerobic breakdown of glucose into two molecules of pyruvate (via glycolysis in the cytosol) 
  • It occurs across two key stages: the Krebs cycle (in the matrix) and the electron transport chain (on the inner membrane)
• Krebs Cycle


  • The pyruvate produced via glycolysis is transferred to the mito matrix and oxidised to form acetyl coenzyme A
  • Acetyl CoA is then introduced into the Krebs cycle, -> combined with a 4C compound to make a 6C intermediate.
  • This 6C intermediate (citrate) is broken down into the original 4C compound over a series of sequential chemical reactions
  • -> formation of one ATP per pyruvate (2 in total) and a large number of loaded hydrogen carriers
  • The breakdown of pyruvate also results in the formation of carbon dioxide (3 molecules per pyruvate, for 6 molecules in total)
• Electron Transport Chain


  • Hydrogen carriers (NADH and FADH2) donate energised electrons (and protons) to an electron transport chain on the cristae
  • The ETC utilises the energy stored in the donated electrons to make ATP via oxidative phosphorylation
  • A total of 26 or 28 molecules of ATP are produced as a result of the unloading of hydrogen carriers within the mitochondria
  • For the ETC to continue to function, the donated electrons (and protons) must be removed from the chain
  • O2 acts as the final electron acceptor in the transport chain and is complexed with the protons to form water molecules