Cellular Respiration Notes

Learning Outcomes

• Understand the differences between autotrophs and heterotrophs.
• Understand the difference between aerobic and anaerobic respiration.
• Understand the four main stages of aerobic respiration and their locations within the cell.
• Understand the production of energy (ATP) in each main stage.

Recap: Oxidation and Reduction

• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.

ATP Cycle

• How cells harvest energy.

Respiration

• Organisms are classified based on how they obtain energy.

Autotrophs (Self-Feeders)

• Able to produce their own organic molecules through photosynthesis.
• Examples: plants, algae, and some bacteria.

Heterotrophs (Fed by Others)

• Live on organic compounds produced by other organisms (autotrophs).
• Examples: animals, fungi, most prokaryotes.
• All organisms use cellular respiration to extract energy from organic molecules.

Cellular Respiration

1. Oxidation of organic compounds (glucose) to extract energy from chemical bonds.
2. Cellular respiration is a series of reactions.
3. Oxidation reactions are often coupled with reduction reactions (redox reaction).
   • Oxidation – loss of electrons.
   • Reduction – gain of electrons.

Electron Acceptors

Aerobic Respiration

• The final electron acceptor is oxygen $(O_2)$.

Anaerobic Respiration

• The final electron acceptor is an inorganic molecule (not $(O_2)$), such as sulfur, nitrate, carbon dioxide, etc.

Fermentation

• The final electron acceptor is an organic molecule, such as an organic acid.

Electron Transport

Electron Carriers

• Many types of carriers are used:
  • Soluble, membrane-bound, moves within the membrane.
  • All carriers can be reversibly oxidised and reduced.
  • Some carry just electrons, and some carry electrons and protons.
  • $NAD^+$ acquires two electrons $(e^-)$ and a proton $(H^+)$ to become NADH.

Oxidation of Glucose (Aerobic Respiration)

• The complete oxidation of glucose proceeds in stages:
  1. Glycolysis
  2. Pyruvate oxidation
  3. Citric acid cycle (TCA cycle/ Krebs Cycle)
  4. Electron transport chain and chemiosmosis

Aerobic Respiration Overview

Stage 1: Glycolysis

1.  Converts 1 glucose (6 carbons) to 2 pyruvate (3 carbons).
2.  10-step biochemical pathway.
3.  Occurs in the cytoplasm.
4.  Net production of 2 ATP molecules by substrate-level phosphorylation.
5.  2 NADH produced by the reduction of $NAD^+$.

Glycolysis Overview:

Glycolytic Pathway:

Fate of Pyruvate

• Depends on oxygen availability:
  • When oxygen is present, pyruvate is oxidised to acetyl coenzyme A (acetyl-CoA) which enters the citric acid cycle.
    • Aerobic respiration.
  • Without oxygen, pyruvate is reduced in order to oxidise NADH back to $NAD^+$.
    • Fermentation / anaerobic respiration.

Regeneration of $NAD^+$

Stage 2: Pyruvate Oxidation (with $O_2$)

• Location: Mitochondria.
• Oxidation through decarboxylation by a multienzyme complex (pyruvate decarboxylase/pyruvate dehydrogenase).

Stage 3: Citric Acid Cycle

• Oxidises the acetyl group (2C) from pyruvate (3C).
• Occurs in the matrix of the mitochondria.
• Biochemical pathway of nine steps in three segments:
  1. Acetyl-CoA (2C) + oxaloacetate (4C)
  2. Citrate rearrangement and decarboxylation
  3. Regeneration of oxaloacetate

Location

• Mitochondrial matrix

Net Energy Production

• 6 NADH
• 2 $FADH_2$
• 2 ATP

Citric Acid Cycle Yield

• Each Acetyl-CoA entering the citric acid cycle:
  • Reduce 3 $NAD^+$ to 3NADH. (Since 2Ace-CoA = 6NADH)
  • Reduce 1 FAD (electron barrier) to $FADH<em>2$. (= 2 $FADH</em>2$)
  • Produces 1 ATP (= 2 ATP)
  • Regenerates oxaloacetate (C4)
• $NAD^+$ - Nicotinamide adenine dinucleotide
• FAD - Flavin adenine dinucleotide
• In general, they are called co-factors.

4. Electron Transport Chain

• Electron transport chain (ETC) is a series of membrane-bound electron carriers
• Embedded in the inner mitochondrial membrane
• Electrons from NADH and $FADH_2$ are transferred to complexes of the ETC

Location

• Inner mitochondrial membrane

Stage 4 - Electron Transport Chain

• Each complex in the chain operates as a proton pump, driving protons in the intermembrane space
• Electrons move from one protein complex to the other

ATP Synthase Structure

• Accumulation of protons in the intermembrane space drives protons into the matrix via diffusion, but this occurs slowly since the membrane is relatively impermeable to ions
• Most protons can only re-enter the matrix through ATP synthase
• Uses energy of electrochemical gradient to make ATP from ADP + P
• Process called chemiosmosis.

Aerobic Respiration in Mitochondria:

Cellular Respiration:

Theoretical Yield of Respiration:

• 32 ATP per glucose for bacteria.
• 30 ATP per glucose for eukaryotes.