C1.2 Cell Respiration Study Notes for First Exams 2025

IB Guiding Questions for Cell Respiration

• Roles of Hydrogen and Oxygen: What are the specific roles of hydrogen and oxygen in the release of energy within cells?

• Energy Distribution: How is energy distributed and subsequently used inside cells?

SL and HL Core Content: C1.2 Cell Respiration

• C1.2.1 ATP Distribution: ATP is the primary molecule that distributes energy within cells.

• C1.2.2 ATP-Supplied Processes: Specific life processes within cells are supplied with energy by ATP.

• C1.2.3 ATP/ADP Interconversions: Energy transfers occur during the interconversions between ATP and ADP.

• C1.2.4 Respiration Systems: Cell respiration is a system for producing ATP within the cell by using energy released from carbon compounds.

• C1.2.5 Respiration Differences: There are distinct differences between anaerobic and aerobic cell respiration in humans.

• C1.2.6 Rate Variables: Various variables affect the rate of cell respiration.

Adenosine Triphosphate (ATP) Structure and Function

• Full Name and Classification: ATP stands for Adenosine Triphosphate. It is classified as a nucleotide.

• Function: ATP provides the activation energy for the majority of chemical reactions occurring within cells. It is considered the "energy currency" of the cell.

• Molecular Components:

  • Adenine: A nitrogenous base.

  • Ribose: A five-carbon sugar.

  • Phosphate Groups: Three phosphate groups linked in a chain.

• High-Energy Bonds:

  • High-energy bonds exist between the three phosphate groups.

  • The bond between the final two phosphates is specifically unstable.

  • When this unstable bond is broken, energy is released. This released energy serves as activation energy for metabolic chemical reactions.

Life Processes Requiring ATP

• Active Transport: The movement of substances across cell membranes against a concentration gradient.

• Anabolism: The synthesis of macromolecules from smaller units (e.g., proteins from amino acids).

• Whole Cell Movement: The physical movement of the entire cell.

• Intracellular Movement: The movement of components inside the cell, such as the transport of chromosomes during the processes of mitosis and meiosis.

The ATP-ADP Cycle and Energy Transfers

• Hydrolysis Reaction:

  • Energy is released through the hydrolysis of ATP into Adenosine Diphosphate (ADP) and an inorganic phosphate ($P_i$).

  • The reaction provides energy sufficient for many cellular tasks, though specific kilojoule values vary.

• Condensation Reaction (Regeneration):

  • Energy is required to synthesize ATP from ADP and phosphate.

  • This is a condensation reaction, also referred to as phosphorylation when a phosphate is added to the ribosome-adenine complex (the ADP).

• Source of Energy for Regeneration: Cellular respiration provides the necessary energy for the regeneration of ATP.

Cell Respiration and Substrates

• Definition: Cell respiration is the controlled release of energy (in the form of ATP) from organic compounds (food) within cells.

• Organic Compounds: These are carbon-containing compounds, excluding oxides (like $CO_2$) or carbonates.

• Principal Substrates:

  • Glucose: Primary carbohydrate substrate.

  • Fatty Acids: Principal lipid substrate.

  • Other Compounds: A wide range of other carbon/organic compounds can be used if necessary.

• Gas Exchange vs. Respiration:

  • Gas Exchange: The physical exchange of carbon dioxide and oxygen between living cells/tissues and the environment (e.g., in the alveoli of human lungs via diffusion).

  • Respiration: The metabolic process occurring within cells to release ATP energy.

Aerobic and Anaerobic Respiration in Humans

• Shared Characteristics:

  • Both occur within cells.

  • Both use glucose as the initial substrate.

  • Both utilize enzymes to catalyze metabolic reaction pathways.

  • Both result in the production of ATP.

• Aerobic Respiration:

  • Oxygen Requirement: Always requires oxygen ($O_2$).

  • Yield: High ATP yield, totaling a net gain of approximately $36\,\text{ATP}$ per glucose molecule.

  • Waste Products: Carbon dioxide ($CO_2$) and water ($H_2O$).

  • Location: Most of the process occurs within the mitochondria.

  • Word Equation: $\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} + \text{ATP}$

• Anaerobic Respiration:

  • Oxygen Requirement: Oxygen is not used.

  • Yield: Low ATP yield, totaling a net gain of exactly $2\,\text{ATP}$ per glucose molecule.

  • Waste Product: Lactate (lactic acid).

  • Location: Occurs entirely in the cytoplasm; does not use the mitochondria.

  • Word Equation: $\text{Glucose} \rightarrow \text{Lactate} + \text{ATP}$

Investigating the Rate of Respiration

• Chemical Indicators of Rate:

  • Decrease in Oxygen: Measured as it is consumed during the reaction.

  • Increase in Carbon Dioxide: Measured as it is produced as a waste product.

• Respirometers: Devices used to determine the rate of respiration. They often utilize a base (such as Potassium Hydroxide, $KOH$) to absorb carbon dioxide so that volume changes can be attributed to oxygen consumption.

• Variables Affecting Insects or Seeds:

  • Temperature (must remain within the organism's normal range).

  • Mass of the respiring organisms.

• Variables Affecting Yeast:

  • Temperature.

  • Mass of yeast.

  • pH of the suspension.

  • Type of substrate (type of food source).

  • Substrate concentration.

• Alternative Measurement Methods:

  • Oxygen probes to measure concentration changes.

  • Carbon dioxide probes to measure concentration changes.

  • Measuring the volume of gas produced by yeast.

• Calculations: The rate is determined by calculating the gradient of a graph (Change / Time) at any specific point.

Additional HL Content: C1.2 Cell Respiration

• C1.2.7 Role of NAD: NAD acts as a hydrogen carrier; oxidation occurs via the removal of hydrogen.

• C1.2.8 Glycolysis: The conversion of glucose to pyruvate through stepwise reactions with a net yield of ATP and reduced NAD.

• C1.2.9 Anaerobic Regeneration of NAD: Pyruvate is converted to lactate to regenerate NAD in humans.

• C1.2.10 Yeast Respiration: Anaerobic respiration in yeast produces ethanol and $CO_2$ for brewing and baking.

• C1.2.11 Link Reaction: Oxidation and decarboxylation of pyruvate to form acetyl groups.

• C1.2.12 Krebs Cycle: Oxidation and decarboxylation of acetyl groups yielding ATP and reduced NAD.

• C1.2.13 Energy Transfer to ETC: Transfer of energy by reduced NAD to the electron transport chain.

• C1.2.14 Proton Gradient: Generation of a proton gradient via the flow of electrons along the ETC.

• C1.2.15 Chemiosmosis: ATP synthesis coupled with the flow of protons through ATP synthase.

• C1.2.16 Terminal Electron Acceptor: Oxygen's role in accepting electrons and protons to form water.

• C1.2.17 Lipid vs. Carbohydrate Substrates: Differences in energy yield and metabolic pathways.

Oxidation, Reduction, and NAD

• Redox Reactions: Respiration pathways involve coupled oxidation and reduction reactions.

• Oxidation:

  • Addition of oxygen.

  • Removal of hydrogen (dehydrogenation).

  • Loss of electrons ($e^-$).

• Reduction:

  • Removal of oxygen.

  • Addition of hydrogen.

  • Gain of electrons ($e^-$).

• NAD (Nicotinamide Adenine Dinucleotide):

  • An electron carrier.

  • When NAD accepts two electrons and hydrogen from a substrate, it becomes Reduced NAD ($NADH$).

  • The substrate that loses the hydrogen/electrons is oxidized.

  • Reduced NAD carries these to the electron transport chain where it is re-oxidized back to NAD.

Glycolysis (HL Only)

• Definition: The breakdown of a 6-carbon glucose molecule into two 3-carbon pyruvate molecules.

• Location: Cytoplasm.

• Step 1: Phosphorylation: Two ATP molecules are hydrolyzed. The resulting phosphates bond to glucose, creating an unstable 6-carbon compound known as Hexose bisphosphate.

• Step 2: Lysis: The unstable Hexose bisphosphate splits into two 3-carbon compounds called Triose phosphates.

• Step 3: Oxidation (Reduction of NAD): Each Triose phosphate is oxidized as it loses electrons and hydrogen to NAD, forming two molecules of Reduced NAD.

• Step 4: ATP Formation: Through the conversion of Triose phosphates to pyruvate, four ATP are produced.

• Net Yield of Glycolysis:

  • $2\,\text{Pyruvate}$

  • $2\,\text{Reduced NAD}$

  • $2\,\text{ATP}$ (net gain, as 4 were produced but 2 were used).

Anaerobic Respiration and NAD Regeneration (HL Only)

• Purpose: Glycolysis requires a constant supply of NAD to continue. Anaerobic respiration exists to regenerate NAD from Reduced NAD when oxygen is absent.

• In Humans:

  • Pyruvate is converted to Lactate.

  • This involves oxidizing Reduced NAD to NAD.

• In Yeast (Fermentation):

  • Pyruvate is converted to Ethanol and Carbon Dioxide ($CO_2$).

  • This also regenerates NAD.

  • Baking: $CO_2$ causes dough to rise.

  • Brewing: Ethanol is used in alcohol production.

The Link Reaction (HL Only)

• Location: Mitochondrial Matrix.

• Process: Pyruvate enters the mitochondrion for the link reaction.

• Decarboxylation: Pyruvate ($3C$) loses a carbon atom in the form of $CO_2$, resulting in a 2-carbon acetyl group.

• Oxidation: The acetyl group is oxidized, and NAD is reduced to Reduced NAD.

• Formation of Acetyl Coenzyme A: The acetyl group combines with Coenzyme A to form Acetyl CoA, which carries the acetyl group into the Krebs cycle.

The Krebs Cycle (HL Only)

• Location: Mitochondrial Matrix.

• Entry: Acetyl CoA transfers the 2-carbon acetyl group to a 4-carbon compound called Oxaloacetate.

• Formation of Citrate: This combination produces a 6-carbon compound called Citrate.

• Decarboxylation: Citrate undergoes two decarboxylations, releasing two molecules of $CO_2$ and returning to a 4-carbon state.

• Oxidation (Dehydrogenation): Four oxidation reactions occur per turn:

  • Three result in the reduction of NAD to Reduced NAD.

  • One results in the reduction of FAD to Reduced FAD ($FADH_2$).

• ATP Production: One ATP is produced per turn via a condensation reaction between ADP and $P_i$.

• Regeneration: Oxaloacetate is regenerated at the end of the cycle to cycle again.

Electron Transport Chain (ETC) and Chemiosmosis (HL Only)

• Location: Inner Mitochondrial Membrane (specifically on the cristae).

• Electron Transfer: Reduced NAD and Reduced FAD donate electrons to the first and second protein complexes of the ETC, respectively, regenerating NAD and FAD.

• Proton Pumping: As electrons pass through the chain via redox reactions, energy is released. This energy is used to actively transport protons ($H^+$) from the matrix into the intermembrane space.

• Proton Gradient: This creates a high concentration of protons in the intermembrane space.

• Chemiosmosis: Protons flow back into the matrix through a special enzyme called ATP Synthase via facilitated diffusion.

• ATP Synthesis: The kinetic energy of the moving protons provides the energy for ATP synthase to phosphorylate ADP into ATP.

• Terminal Electron Acceptor: Oxygen accepts the electrons at the end of the ETC and combines with protons from the matrix to form water ($H_2O$). Without oxygen, the ETC stops functioning.

Respiration of Lipids (HL Only)

• Mechanism: Triglycerides are hydrolyzed into fatty acids and glycerol. Fatty acids are converted directly into multiple Acetyl CoA molecules in the mitochondrion, bypassing glycolysis.

• Yield Comparison: Lipids provide a much higher energy yield per gram than carbohydrates.

  • Lipids contain less oxygen and more oxidizable hydrogen and carbon.

  • More 2-carbon acetyl groups can be formed from a single lipid molecule.

• Limitations: Anaerobic respiration and glycolysis can only occur with carbohydrates, not lipids.

Mitochondrial Adaptations (HL Only)

• Outer Membrane: Contains protein channels allowing pyruvate to enter; impermeable to protons to maintain the gradient.

• Inner Membrane: Contains the ETC proteins and ATP synthase.

• Intermembrane Space: A very small space allowing for the rapid accumulation and concentration of protons.

• Cristae: Folds in the inner membrane that increase the total surface area for the ETC and ATP synthase.

• Matrix: Semi-fluid interior containing DNA (single circular chromosome), $70S$ ribosomes, and enzymes needed for the Link Reaction and Krebs cycle.

• Circular DNA and Ribosomes: Allow the mitochondrion to synthesize its own proteins and enzymes required for respiration.