Cellular Respiration and Photosynthesis: Key Concepts and Pathways

Metabolism: Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a series of metabolic processes by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It is essential for the production of energy in both eukaryotic and prokaryotic organisms.

• Glycolysis occurs in the cytosol and breaks down glucose into pyruvate, generating a small amount of ATP and NADH.

• Krebs Cycle (Citric Acid Cycle) takes place in the mitochondrial matrix, oxidizing acetyl-CoA to CO2 and producing NADH and FADH2.

• Electron Transport Chain (ETC) and Oxidative Phosphorylation occur in the inner mitochondrial membrane, using electrons from NADH and FADH2 to generate a proton gradient that drives ATP synthesis.

Substrate-Level Phosphorylation

Substrate-level phosphorylation is a process of ATP formation in which a phosphate group is directly transferred from a phosphorylated substrate to ADP, forming ATP. This occurs during glycolysis and the Krebs cycle.

• Definition: Direct enzymatic transfer of a phosphate group to ADP from a phosphorylated intermediate.

• Example: In glycolysis, 1,3-bisphosphoglycerate donates a phosphate to ADP, forming ATP and 3-phosphoglycerate.

Oxidative Phosphorylation

Oxidative phosphorylation is the production of ATP using energy derived from the transfer of electrons through the electron transport chain to oxygen, the final electron acceptor. This process is coupled to the generation of a proton gradient across the inner mitochondrial membrane.

• Definition: ATP synthesis powered by the transfer of electrons from NADH and FADH2 to O2 via the electron transport chain.

• Key Equation: $\text{ADP} + \text{P}_i + \text{NADH} + \text{O}_2 \rightarrow \text{ATP} + \text{NAD}^+ + \text{H}_2\text{O}$

Oxygen Dependence in Cellular Respiration

• Glycolysis: Oxygen independent. Glycolysis does not require oxygen and can occur anaerobically.

• Electron Transport Chain (ETC): Oxygen dependent. Oxygen acts as the terminal electron acceptor, allowing the chain to function.

• Krebs Cycle: Indirectly oxygen dependent. While O2 is not directly used, the cycle relies on NAD+ and FAD, which are regenerated by the ETC. Without O2, the ETC halts, and NADH/FADH2 accumulate, stopping the cycle.

What Happens if Oxygen is Cut Off?

If oxygen is unavailable, the electron transport chain cannot operate, leading to a halt in oxidative phosphorylation. NADH and FADH2 cannot be oxidized back to NAD+ and FAD, causing glycolysis and the Krebs cycle to stop due to lack of electron carriers.

• In animal cells: Metabolism stops, especially in oxygen-dependent tissues like the brain.

• In some cells: Metabolism continues via fermentation, which regenerates NAD+ from NADH.

Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue by regenerating NAD+ from NADH. There are two main types:

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid, regenerating NAD+. Occurs in muscle cells and some bacteria.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2, regenerating NAD+. Occurs in yeast and some bacteria.

Main disadvantage: Fermentation yields much less ATP per glucose molecule compared to aerobic respiration.

Main advantage: Allows ATP production in the absence of oxygen.

Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which light energy is converted into chemical energy in the form of glucose, primarily in plants, algae, and some bacteria. It is the reverse of cellular respiration in terms of reactants and products.

• General Equation:

$6\text{CO}_2 + 12\text{H}_2\text{O} + \text{Light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 + 6\text{H}_2\text{O}$

• Occurs in: Chloroplasts of eukaryotic cells, using chlorophyll in the thylakoid membranes.

• Two main stages: Light reactions and the Calvin cycle.

Light Reactions

The light reactions (the "photo" part) occur in the thylakoid membranes and convert solar energy to chemical energy.

• Split water molecules, releasing O2.

• Transfer electrons and protons to NADP+, forming NADPH.

• Generate ATP from ADP by photophosphorylation.

The Calvin Cycle

The Calvin cycle (the "synthesis" part) occurs in the stroma of the chloroplast and uses ATP and NADPH to convert CO2 into glucose.

• Does not require light directly, but depends on products of the light reactions.

• Key enzyme: Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase).

Chloroplast Structure

• Outer membrane

• Inner membrane

• Thylakoid membrane (contains chlorophyll)

• Stroma (fluid surrounding thylakoids)

• Granum (stack of thylakoids)

Global Importance of Photosynthesis

• Photosynthesis is responsible for the majority of oxygen production on Earth.

• Oceans (phytoplankton, cyanobacteria) contribute 50-80% of global oxygen; terrestrial plants contribute 25-30%.

• Photosynthesis helps regulate atmospheric CO2 levels, as shown by long-term monitoring at Mauna Loa Observatory.

Organisms Capable of Photosynthesis

• Plants

• Multicellular algae

• Unicellular eukaryotes

• Cyanobacteria

• Some sea slugs (kleptoplasty)

Comparison Table: Cellular Respiration vs. Photosynthesis

Process	Location	Reactants	Products	Energy Flow
Cellular Respiration	Mitochondria	Glucose, O2	CO2, H2O, ATP	Releases energy
Photosynthesis	Chloroplasts	CO2, H2O, Light	Glucose, O2	Stores energy

Key Equations

• Cellular Respiration:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Photosynthesis:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Additional info:

• Some slides referenced class activities and environmental careers, but the main academic content is focused on cellular respiration and photosynthesis.