The energy from food is ultimately captured in the molecule adenosine triphosphate (ATP).
Chemical structure of ATP includes:
Three phosphate groups.
Breakage of ATP chemical bonds releases energy, allowing cells to perform work.
Reaction for ATP breakage:
ext{ATP} + ext{H}2 ext{O}
ightarrow ext{ADP} + ext{P}i + 7.3 ext{ kcal/mol}
Importance of ATP
ATP is crucial for cellular processes as it serves as the primary energy currency in biological systems.
Inside cells:
Enzymes break bonds of subunits.
Converts potential chemical energy from food into the molecular bonds that constitute ATP.
Aerobic Respiration
A series of reactions converting stored food energy into ATP.
Occurs in the presence of oxygen.
Primary energy source for nearly all eukaryotic organisms is glucose.
Overall Reaction for Aerobic Respiration
The aerobic respiration of glucose can be described as follows:
ext{Glucose} + ext{O}2
ightarrow ext{CO}2 + ext{H}_2 ext{O} + ext{Energy} (+ ext{Heat})
Stages of Aerobic Respiration
Three main stages:
Glycolysis
Citric Acid Cycle
Electron Transport Chain
1. Glycolysis
First step of aerobic respiration:
Takes place in the cytoplasm.
Breaks down glucose into smaller molecules (e.g., pyruvate).
Converts some energy into a small number of ATP molecules.
2. Citric Acid Cycle
Second step of aerobic respiration:
A series of reactions that extracts energy from food:
High-energy electrons are stripped from bonds between carbon and hydrogen atoms.
Carried by NADH molecules to the inner mitochondrial membrane.
This step releases carbon dioxide (CO₂).
A small amount of ATP is produced during this cycle.
3. Electron Transport Chain (ETC)
Final step of aerobic respiration:
NADH transports high-energy electrons to the inner membranes of the mitochondria.
Passes electrons down a chain of molecules to oxygen.
Oxygen accepts electrons, combining with hydrogen atoms to produce water (H₂O).
This stage produces the majority of the ATP.
Diagram of the ETC
Features:
Multiple transport proteins facilitate movement of electrons and protons (H+).
ATP Synthase: Utilizes the proton gradient established by the electron transport to produce ATP from ADP and inorganic phosphate (P).
Energy Yield of Aerobic Respiration
Glycolysis produces 2 ATP.
Citric Acid Cycle and Electron Transport together can yield about 34 ATP.
Total potential yield from one glucose molecule through aerobic respiration is approximately 36 ATP molecules.
Alternative Fuels for Aerobic Respiration
Cells can utilize other molecules for energy:
Fats and amino acids can also serve as fuel sources.
Fats contain higher electron density, resulting in the production of more ATP per gram compared to carbohydrates.
Anaerobic Conditions and Fermentation
Under conditions where oxygen is scarce:
Oxygen consumption can exceed oxygen intake.
The electron transport chain cannot function properly due to lack of oxygen, so the process shifts to fermentation.
Fermentation Process:
Occurs in the cytoplasm.
Glycolysis is retained, yielding pyruvate, which is then converted into lactic acid or alcohol depending on the organism.
Interrelationship Between Photosynthesis and Respiration
Photosynthesis and aerobic respiration create a continuous cycle:
Outputs of one process serve as inputs for the other.
Photosynthesis: Utilizes CO₂ and H₂O to produce glucose and O₂.
Aerobic Respiration: Uses glucose and O₂ to produce CO₂ and H₂O, releasing energy in the form of ATP.
Summary of Key Concepts
ATP is the energy currency of the cell.
Cellular respiration involves sequential processes that convert glucose and oxygen into usable energy.
The aerobic respiration pathway is crucial for optimal energy yield, while anaerobic processes provide alternatives under oxygen-limited conditions.
Understanding the balance between photosynthesis and respiration is critical for comprehending energy dynamics in biological systems.