8.1a Aerobic Respiration

29/11/2012 § Leave a comment

Establishing the structure of the power houses of the cell, mitochondria, will be important for this unit, so let’s get started. Membranes are a vital part of the structure of mitochondria: there is an outer smooth membrane that has an inner membrane inside of it. The inner mitochondrial membrane has larger space within it than the narrow space of the outer mitochondrial membrane. It normally has folds or invaginations that create a maze-like matrix within the organelle. This is what you should focus on when identifying organelles under a microscope: the double membranes and the folds will make mitochondria easy to recognize.

So why do we need to know about the mitochondria and all of its invaginations? Great question. Okay, the mitochondria is the power house of the cell, as I’ve been saying multiple times during the year, and that’s because ATP molecules are produced within the inner mitochondrion membrane. Reaching the last bit of aerobic respiration, we are introduced to oxidative phosphorylation which is really exactly what it sounds like. ADP (adenine diphosphate) is phosphorylated (phosphate is added) to produce ATP (the energy currency) using the energy released by oxidation (the loss of electrons). The substances involved are NADH and a proton (H+) and the process involves multiple small steps, therefore we can deduce that oxidative phosphorylation must be another example of a metabolic pathway. What happens during oxidative phosphorylation is chemiosmosis, which, like water in osmosis, is when a chemical substance moves across a membrane down a concentration gradient.

And now is when we’ll probably need a drawing to help us out in the process of chemiosmosis.

  • NADH and its proton start off the process by supplying the first of the electron carriers with pairs of hydrogen atoms (so, two hydrogen atoms). The newly converted NAD+ returns to the mitochondrial matrix.
  • The hydrogen atoms split and release two electrons.
  • The electrons move from carrier to carrier and release energy. Some carriers use the energy to transfer protons from the matrix to the intermembrane space, building a concentration gradient around the membrane. (This stores potential energy).
  • The electrons continue flowing by being transferred to a terminal electron acceptor and become oxygen (O2-) but then combine with existing protons in the matrix to become water.
  • Protons from the intermembrane space transfer back to the matrix through the enzyme ATP synthase. Their movement down the concentration gradient releases energy that helps ATP synthase to phosphorylate ADP.


Essay Questions

  1. Outline the process of glycylosis. (5)
  2. Draw the structure of a mitochondrion as seen in an electron microscope. (5)
  3. Explain how the structure of the mitochondrion allows it to carry out its function efficiently. (8)
  4. Explain the reactions that occur in the matrix of the mitochondrion that are part of aerobic respiration. (8)
  5. Explain the process of aerobic respiration. (8)
  6. Outline the role of oxygen in providing cells with energy. (6)
  7. Explain how chemiosmosis assists in ATP production during oxidative phosphorylation. (9)
  8. Explain the similarities and differences in anaerobic and aerobic cellular respiration. (8)
  9. Describe the central role of acetyl (ethanoyl) CoA in carbohydrate and fat metabolism. (5)



Page 97, oxygen consumption by mitochondria

1. Explain why oxygen consumption by the mitochondria could not begin unless pyruvate had been added.

Pyruvate is like the gateway for further steps into cellular respiration. After being produced by glycolysis from glucose, it can be further broken down into more substances to produce more ATP. In steps such as reduction and oxidation, oxygen is consumed, but all of it can only happen after pyruvate is added since pyruvate starts the rest of aerobic respiration.

2. Deduce what prevented oxygen consumption between points I and II.

Prior to oxygen consumption during oxygen phosphorylation, ADP was not added to the medium until point II, and since ADP is needed for the creation of ATP (and because ATP synthase uses the energy from the transfer of oxygen to make ATP), no oxygen will be consumed.

3. Predict, with reasons, what would have happened if ADP had not been added at point III.

Had ADP not been added at point III, a significant amount of ATP would not have been produced, even with the chemiosmotic movement of the protons across the matrix. ADP needs to be phosphorylated to produce ATP and a lack of ADP will restrict the mitochondria from making this happen.

4. Discuss the possible reasons for oxygen consumption not being resumed after ADP was added at point IV.

In this controlled test, the amount of pyruvate and supply of protons (from the NADH + (H+) molecules) must have been completely consumed and converted into ATP molecules and the concentration gradient of protons balanced. Even if ADP is added at point IV, the lack of NADH + (H+) and pyruvate initiating aerobic respiration would keep oxygen consumption from resuming. 

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