Cellular Respiration and Photosynthesis: Energy Pathways in Cells

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a multi-stage metabolic process by which cells extract energy from glucose and other organic molecules to produce ATP, the universal energy currency. This process occurs in both prokaryotic and eukaryotic cells, though the location of some steps differs.

• Purpose: To convert the chemical energy in glucose into ATP.

• Overall Equation:

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \sim 30-32\ ATP}$

• Stages: Glycolysis, Transition Reaction, Krebs Cycle, Electron Transport Chain & ATP Synthase.

Glycolysis

• Location: Cytosol of the cell.

• Oxygen Requirement: Anaerobic (does not require oxygen).

• Mechanism: Breaks down one 6-carbon glucose molecule into two 3-carbon pyruvate molecules.

• Energy Yield: Net gain of 2 ATP and 2 NADH molecules.

• Key Reaction:

$\mathrm{Glucose\ (6C) \rightarrow 2\ Pyruvate\ (3C) + 2\ ATP + 2\ NADH}$

• Example: Glycolysis is the only energy-yielding pathway in some anaerobic bacteria.

Transition Reaction (Oxidative Decarboxylation)

• Location: From cytosol into the mitochondrial matrix (in eukaryotes).

• Mechanism: Each pyruvate (3C) is converted into an acetyl group (2C) attached to Coenzyme A (Acetyl CoA).

• Byproducts: Releases CO2 and produces NADH.

• Key Reaction:

$\mathrm{2\ Pyruvate + 2\ NAD^+ + 2\ CoA \rightarrow 2\ Acetyl\ CoA + 2\ CO_2 + 2\ NADH}$

Krebs Cycle (Citric Acid Cycle)

• Location: Mitochondrial matrix.

• Mechanism: Each Acetyl CoA is fully oxidized to CO2, transferring electrons to NAD+ and FAD.

• Energy Yield (per glucose): 2 ATP, 6 NADH, 2 FADH2, 4 CO2.

• Key Reaction:

$\mathrm{2\ Acetyl\ CoA + 6\ NAD^+ + 2\ FAD + 2\ ADP + 2\ P_i \rightarrow 4\ CO_2 + 6\ NADH + 2\ FADH_2 + 2\ ATP}$

Electron Transport Chain (ETC) & ATP Synthase

• Location: Inner mitochondrial membrane.

• Mechanism: Electrons from NADH and FADH2 are transferred through a series of protein complexes, releasing energy to pump protons (H+) into the intermembrane space.

• ATP Synthesis: Protons flow back into the matrix through ATP Synthase, driving the phosphorylation of ADP to ATP.

• Final Electron Acceptor: Oxygen (O2), which combines with electrons and protons to form water (H2O).

• Energy Yield: Approximately 25 ATP per glucose molecule.

Summary Table: Stages of Cellular Respiration

Stage	Location	ATP Produced	Key Products
Glycolysis	Cytosol	2	2 NADH, 2 Pyruvate
Transition Reaction	Mitochondrial Matrix	0	2 NADH, 2 Acetyl CoA, 2 CO2
Krebs Cycle	Mitochondrial Matrix	2	6 NADH, 2 FADH2, 4 CO2
ETC & ATP Synthase	Inner Mitochondrial Membrane	~25	H2O

Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert solar energy into chemical energy, producing glucose and oxygen from carbon dioxide and water. This process occurs in the chloroplasts of eukaryotic photoautotrophs.

• Overall Equation:

$\mathrm{6CO_2 + 6H_2O + light\ energy \rightarrow C_6H_{12}O_6 + 6O_2}$

• Two Main Phases: Light Reactions and Calvin Cycle (Light-Independent Reactions).

Light-Dependent Reactions

• Location: Thylakoid membranes of the chloroplast.

• Inputs: Sunlight (captured by Photosystem II and I), Water (H2O).

• Mechanism: Light energy excites electrons, which move through the photosynthetic electron transport chain, generating ATP and NADPH. Water is split to provide electrons, releasing O2 as a byproduct.

• Outputs: ATP, NADPH, and O2.

• Example: The oxygen released during photosynthesis is essential for aerobic life on Earth.

Calvin Cycle (Light-Independent Reactions)

• Location: Stroma of the chloroplast.

• Inputs: CO2, ATP, and NADPH (from light reactions).

• Mechanism: CO2 is fixed into a 5-carbon sugar (RuBP) and converted through a series of reactions into G3P, a 3-carbon sugar. This process uses ATP and NADPH as energy sources.

• Outputs: G3P (used to synthesize glucose, sucrose, starch, amino acids, and fatty acids), ADP, inorganic phosphate, and NADP+ (recycled back to the light reactions).

• Key Reaction:

$\mathrm{3CO_2 + 9ATP + 6NADPH + 6H^+ \rightarrow G3P + 9ADP + 8P_i + 6NADP^+ + 3H_2O}$

Summary Table: Photosynthesis Phases

Phase	Location	Inputs	Outputs
Light Reactions	Thylakoid Membrane	Light, H2O	ATP, NADPH, O2
Calvin Cycle	Stroma	CO2, ATP, NADPH	G3P, ADP, NADP+

Comparison: Cellular Respiration vs. Photosynthesis

• Cellular Respiration: Catabolic process (breaks down glucose to release energy).

• Photosynthesis: Anabolic process (builds glucose using energy from sunlight).

• Energy Flow: Respiration releases energy; photosynthesis stores energy.

• Gas Exchange: Respiration consumes O2 and releases CO2; photosynthesis consumes CO2 and releases O2.

Additional info: In prokaryotes, cellular respiration occurs in the cytoplasm and across the plasma membrane, as they lack mitochondria and chloroplasts.