Cellular Respiration

• Metabolism is the totality of an organism’s chemical reactions
• There are two types of metabolic reactions taking place in the cell:

  • Catabolic – Breaking down which releases energy by breaking down complex molecules into simpler compounds
  • Anabolic – Building up, which consumes energy to build complex molecules from simpler ones
• Energy coupling is the use of an exergonic process to drive/fuel an endergonic one

  Most energy coupling in cells is mediated by ATP

  ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant

  The recipient molecule is now called a phosphorylated intermediate
• Energy transfer molecules:

  • NAD+, NADP+, FAD, and the cytochromes
  • These exist in two forms, oxidized and reduced form
  • These molecules act as energy carriers in a series of oxidation-reduction reactions in metabolism
  • Their reduced forms represent stored energy that is tapped to synthesize ATP
• Nicotinamide adenine dinucleotide (NAD) is a coenzyme (vitamin B3) central to metabolism
• Cellular processes required for cell function, growth, development, and reproduction require energy

  Photosynthesis generates O2 and organic molecules, which are used in cellular respiration
• Stages of Cellular Respiration:

  Glycolysis – breakdown of glucose into two molecules of pyruvate

  Citric acid cycle (Krebs cycle) – completes the breakdown of glucose

  Oxidative phosphorylation - ~90% of the ATP generated by cellular respiration
• Glycolysis breaks down glucose into two molecules of pyruvate, two molecules of ATP, and two NADH molecules

  This occurs in the absence of O2

  Glycolysis occurs in the cytoplasm and has two major phases:

  • Energy investment phase
  • Energy payoff phase
• Lactic acid fermentation:

  In lactic acid fermentation, pyruvate is reduced by NADH, forming lactate as a product, with no release of CO2

  Human muscle cells use lactic acid fermentation to generate ATP when oxygen is scarce
• Photosynthesis is the process that converts solar energy into chemical energy

  Plants are photoautotrophs, meaning they use energy from sunlight to make organic molecules

  Chloroplasts are solar-powered chemical factories

  They are double membraned

  Two stages of photosynthesis:

  Light-dependent reaction

  Light-independent reaction – aka the Calvin Cycle
• Light-dependent stage:

  Occurs in the thylakoids

  Splitting of H2O

  Release O2

  Reduces NADP+ to NADPH

  Generate ATP from ADP by photophosphorylation
• Light-independent stage:

  The Calvin cycle occurs in the stroma to form sugar from CO2, using ATP and NADPH produced from light-dependent reaction

  The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
• The Photosystem:

  A photosystem consists of a reaction-centre complex surrounded by light-harvesting complexes

  The light-harvesting complexes (pigment molecules bound to proteins transfer the energy of photons to the reaction centre

  There are two photosystems in the thylakoid

  Photosystem II (PSII) functions first and is best at absorbing a wavelength of 680nm

  Photosystem I (PSI) is the best at absorbing a wavelength of 700nm. This only produces a little ATP
• Light-independent reaction:

  This stage is dependent on the products of the light-dependent reaction

  Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phosphate (G3P)

  For the net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2

  The Calvin cycle has three stages:

  • Carbon fixation (catalyzed by rubisco)
  • Reduction
  • Regeneration of the CO2 acceptor (RuBP)
• The electron transport chain is the final step in cellular respiration, where electrons are passed from one carrier molecule to another.
• Cellular respiration occurs in three stages: glycolysis, pyruvate oxidation, and the citric acid cycle (Krebs cycle).
• Metabolism is the totality of an organism's chemical reactions
• Two types of metabolic reactions in cells:

  • Catabolic: Breaking down complex molecules into simpler compounds, releasing energy (exergonic)
  • Anabolic: Building complex molecules from simpler ones, consuming energy (endergonic)
• Energy coupling is using an exergonic process to drive an endergonic one

  • Most energy coupling in cells is mediated by ATP
• ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to another molecule, creating a phosphorylated intermediate
• Energy transfer molecules:

  • NAD+, NADP+, FAD, and cytochromes exist in oxidised and reduced forms
  • Act as energy carriers in oxidation-reduction reactions in metabolism
• Cellular respiration is necessary for cell function, growth, development, and reproduction

  • Photosynthesis generates O2 and organic molecules used in cellular respiration
• Stages of Cellular Respiration:

  1. Glycolysis: Breakdown of glucose into pyruvate, ATP, and NADH
  2. Citric acid cycle (Krebs cycle): Completes glucose breakdown
  3. Oxidative phosphorylation: Generates ~90% of ATP
• Glycolysis:

  • Breaks down glucose into pyruvate, ATP, and NADH in the absence of O2
  • Occurs in the cytoplasm with energy investment and payoff phases
• Lactic acid fermentation:

  • Pyruvate is reduced to lactate by NADH, no CO2 released
  • Catalysed by alcohol dehydrogenase
  • Used by human muscle cells to generate ATP when oxygen is scarce
• Alcohol fermentation:

  • Pyruvate converted to ethanol in two steps, releasing CO2 and oxidising NADH
  • Allows NAD to be re-oxidised for glycolysis to continue
• Photosynthesis converts solar energy into chemical energy, an endergonic process

  • Plants are photoautotrophs, using sunlight to make organic molecules
• Photosynthesis occurs in chloroplasts, with two stages:

  • Light-dependent reaction
  • Light-independent reaction (Calvin Cycle)
• Light-dependent stage:

  • Occurs in thylakoids, splitting H2O, releasing O2, reducing NADP+ to NADPH, and generating ATP
• Light-independent stage (Calvin Cycle):

  • Occurs in the stroma, forms sugar from CO2 using ATP and NADPH from the light-dependent reaction
  • Begins with carbon fixation, incorporating CO2 into organic molecules
• Photosystem:

  • Reaction-centre complex surrounded by light-harvesting complexes
  • Light-harvesting complexes transfer photon energy to the reaction centre
  • Two photosystems in the thylakoid: PSII (P680) and PSI (P700)
• Light-independent reaction:

  • Dependent on products of the light-dependent reaction
  • Carbon enters as CO2 and leaves as glyceraldehyde 3-phosphate (G3P)
  • Calvin cycle has three stages: Carbon fixation, reduction, and regeneration of the CO2 acceptor (RuBP)
• Glucose is converted into two molecules of pyruvic acid during glycolysis.
• Glycolysis takes place in the cytoplasm and does not require oxygen.
• Cellular respiration occurs in three stages: glycolysis, pyruvate oxidation (in aerobic conditions), and the citric acid cycle/Krebs cycle.
• In anaerobic conditions, fermentation can occur instead of pyruvate oxidation.