Respiration is the means by which cells extract energy stored in food and transfer that energy to molecules of ATP.

The equation for respiration. Notice that it is nearly the reverse of photosynthesis.

Occurs in the cytoplasm with or without oxygen present, breaks down 1 molecule of glucose into two 3-carbon molecules of pyruvate (also called pyruvic acid).

• releases 4 molecules of ATP but takes 2 ATP to complete process so net gain is 2 ATP

• the end product pyruvate is the raw material for Kreb’s cycle

• the cell regulates ATP production through allosteric inhibition, so the enzyme for the third step, phosphofructokinase (PFK), is an allosteric enzyme that inhibits glycolysis when the cell has enough

2 ATP + 1 Glucose → 2 pyruvate + 4 ATP

Please watch this video to learn more about glycolysis.

Anaerobic Respiration:
  • There are two types of anaerobes: 
    • facultative - can tolerate the presence of oxygen
    • obligate - cannot live in an environment containing oxygen
  • alcoholic fermentation can generate ATP as long as there is an adequate supply of NAD+ to accept electrons during glycolysis
    • cells convert pyruvate from glycolysis into ethyl alcohol and carbon dioxide in the absence of oxygen and oxidize NADH back into NAD+
  • in lactic acid fermentationpyruvate from glycolysis is reduced to form lactic acid or lactate and NADH gets oxidized back into NADP+
    • human skeletal muscles carry out lactic acid fermentation when the blood cannot supply adequate oxygen to muscles

Aerobic Respiration:

Please watch this video to gain an understanding of aerobic respiration.

Aerobic respiration consists of the Kreb’s Cycle and the Electron Transport Chain, processes that use the products of glycolysis
  • Krebs Cycle:

    •  It takes place in matrix of mitochondria
    •  It requires pyruvate-glucose is broken into two pyruvates so the cycle goes twice every one molecule of glucose
    • Pyruvate combines with acetyl co-A and produces 2 molecules of NADH
    • ATP is produced by substrate-level phosphorylation (enzyme kinase transfers a phosphate from a substrate directly to ADP, a small amount is produced from Glycolysis and the Krebs cycle)
  • Electron Transport Chain:
    • NAD+ and FAD are coenzymes that carry protons or electrons from glycolysis and the Krebs cycle to the electron transport chain
    • It takes place in the cristae membrane
    • A proton pump that couples exergonic and endergonic uses energy released from exergonic flow of electrons to pump
      protons against a gradient 
      from the matrix to outer compartment, resulting in a proton gradient; does not make ATP but sets the stage for ATP production during chemiosmosis
    • ETC carries electrons delivered by NAD and FAD from glycolysis and Krebs cycle to oxygen, the final electon acceptor, through a series of redox reactions
    • Oxidative phosphorylation: most of the energy produced during cell respiration occurs during this process
      • phosphorylation of ADP into ATP by the oxidation of the carrier molecules NADH and FADH2
      • chemiosmosis uses potential energy in the form of a proton (H+) gradient to phosphorylate ADP and produce ATP (ADP+P=ATP) (powered by the redox reactions)
      • protons (H+) pumped from matrix to outer compartment by ETC, and proton gradient is created
      • protons flow down the gradient into the matrix through ATP synthase channels, generating energy
      • oxygen is final hydrogen acceptor, combining ½ an oxygen molecule with 2 electrons and 2 protons, forming water
Overall, this is the yield of aerobic respiration:

Using the information above, you should be able to complete your Photosynthesis and Respiration worksheet.