Cellular Respiration

Energy Processing

Photosynthesis and cellular respiration are two fundamental biological processes that enable organisms to capture, store, and utilize energy. These processes are closely linked in the flow of energy and cycling of matter within ecosystems.

• Photosynthesis allows plants and other autotrophs to capture energy from sunlight (kinetic energy) and convert it into chemical energy stored in sugar molecules (potential energy).

• The general equation for photosynthesis is:

• Plants can store the produced sugars or use them in cellular respiration to release energy.

Cellular Respiration

Cellular respiration is the process by which cells break down glucose and other organic molecules to produce ATP, the main energy currency of the cell. This process is nearly the reverse of photosynthesis.

• ATP (Adenosine Triphosphate) is crucial for coupling endergonic (energy-requiring) and exergonic (energy-releasing) reactions.

• ATP is constantly used and regenerated; each ATP molecule is cycled more than once per minute in a typical cell.

• ATP is a very unstable molecule, which makes it efficient for energy transfer but not for long-term storage.

• The general equation for cellular respiration is:

• Cellular respiration yields approximately 29 ATP molecules per glucose molecule (compared to a theoretical maximum of 38).

• Because water is a smaller molecule than carbon dioxide, cells lose more weight in carbon dioxide during respiration.

Energy vs Atoms

Both photosynthesis and cellular respiration involve the conversion of energy and the rearrangement of atoms, particularly carbon atoms, through the breaking and forming of chemical bonds.

• It is important to track both the flow of energy and the movement of atoms (e.g., carbon) during these processes.

• In photosynthesis, carbon atoms are combined to form glucose; in respiration, glucose is broken down, releasing carbon dioxide.

Different Ways of Making ATP

Cells can produce ATP through several mechanisms, depending on the availability of oxygen and the type of organism.

• Aerobic respiration: Occurs in the presence of oxygen and is the most efficient pathway for ATP production.

• Alternate electron acceptors: Some organisms use molecules other than oxygen for respiration (anaerobic respiration).

• Fermentation: An anaerobic process that is less efficient and occurs when oxygen is absent.

• Key question: Why does aerobic respiration yield ~29 ATP per glucose, rather than the theoretical 38?

Energetic Coupling

Energetic coupling refers to the linking of exergonic and endergonic reactions, allowing cells to efficiently manage energy transfer.

• The overall equation for glucose combustion is the same as cellular respiration:

• Instead of releasing all energy as heat, cells control the release of energy through redox reactions.

• Energy released is used to make ATP and other energy carriers.

• Cellular respiration consists of four interconnected stages:

  • Glycolysis

  • Pyruvate processing

  • Citric acid cycle

  • Electron transport chain + oxidative phosphorylation

• When simplified: ;

First 3 Stages of Cellular Respiration

The initial stages of cellular respiration involve the controlled release of energy from glucose, primarily through electron carriers.

• Electrons are carried by NADH and FADH2.

• These stages also produce carbon dioxide and a few ATP molecules.

• Substrate-level phosphorylation: ATP is produced by the direct transfer of a phosphate group from a phosphorylated substrate to ADP.

• This process is exergonic, except for the reverse reaction.

Last Stage of Cellular Respiration

The final stage involves the electron transport chain, where energy in NADH and FADH2 is transferred to drive ATP synthesis.

• Energy from NADH and FADH2 is transferred to the electron transport chain, coupling multiple redox reactions to the transport of H+ ions across a membrane.

• This creates a proton gradient that powers ATP synthase, resulting in oxidative phosphorylation and the bulk of ATP production.

Summary Table: Comparison of Photosynthesis and Cellular Respiration

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• Photosynthesis: The process by which plants and some other organisms convert light energy into chemical energy.

• Cellular Respiration: The process of breaking down glucose to release energy for cellular activities.

• Redox Reaction: A chemical reaction involving the transfer of electrons between two species.

• Substrate-level Phosphorylation: Direct formation of ATP by transfer of a phosphate group to ADP.

• Oxidative Phosphorylation: ATP formation driven by the electron transport chain and chemiosmosis.

Example: ATP Yield from Glucose

• Complete aerobic respiration of one glucose molecule yields approximately 29 ATP molecules.

• Some energy is lost as heat and due to inefficiencies in the process.