Photosynthesis and Cellular Respiration: A Comprehensive Analysis

Posted on Dec 1, 2024 in Biology

Photosynthesis and Cellular Respiration

What is Photosynthesis?

Photosynthesis is the process of transforming water (H2O) and carbon dioxide (CO2) into organic matter using light energy.

Chloroplast Structure

  1. Outer membrane
  2. Inner membrane
  3. Grana
  4. Intergrana lamellae
  5. Stroma

Light-Dependent Reactions

Light energy, via ATP and NADPH2, fuels the process. There are two types of photophosphorylation:

Cyclic Photophosphorylation

  • Occurs in the presence of light during photosynthesis.
  • Synthesizes ATP from ADP and a phosphate group (Pi): Pi + ADP + energy —> ATP
  • Electrons from the chlorophyll molecule, after passing through the electron transport chain, return to the chlorophyll molecule (cyclic phase).
  • Produces 2 ATP molecules per electron transferred.
  • Does not require an external electron source.
  • Continuously produces ATP when light energy is available.

Non-Cyclic Photophosphorylation

H+ protons are captured by reduced NADP, which received electrons from ferredoxin, creating NADPH2 used in glucose synthesis. The splitting of the water molecule is called photolysis.

Light-Dependent Phase Products

The light-dependent reactions occur in the thylakoid membranes. Electrons released by photons striking photosystems reduce NADP+ to NADPH. The electron transport chain uses electron energy for ATP synthesis. This phase produces reduced coenzymes (NADPH), ATP via phosphorylation, and molecular oxygen (O2) as a byproduct.

Photosynthesis End Products

The end product of photosynthesis is glucose (C6H12O6), a six-carbon sugar, with molecular oxygen (O2) as a byproduct. The equation is:

6CO2 + 6H2O + light energy —> C6H12O6 + 6O2

Cellular Respiration

Cellular respiration is the process where living cells use oxygen to release chemical energy stored in food.

Stages of Cellular Respiration

  • Glycolysis: Occurs in the cytoplasm, similar to fermentation.
  • Krebs Cycle (Citric Acid Cycle): Carbon dioxide (CO2) is released from glucose in the mitochondria.
  • Electron Transport Chain: Hydrogen atoms from glucose are transferred to O2, forming water (H2O), also in the mitochondria.

Glycolysis Products

In the absence of oxygen, each glucose molecule splits into two pyruvic acid (3C) molecules:

2 pyruvate + 2 ADP + 4 ATP + 2 NADH + 2H+ + 2H2O

Lactic and Alcoholic Fermentation

Lactic fermentation is an anaerobic process where glucose is used for energy, producing lactic acid as a waste product. This process is carried out by lactic acid bacteria, fungi, protozoa, and some animal tissues (e.g., muscle tissue during intense activity when oxygen is insufficient).

Krebs Cycle Products

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle is the final pathway for oxidizing pyruvic acid, fatty acids, and amino acid carbon chains. Acetyl CoA (2 carbons) combines with oxaloacetic acid (4 carbons) to form citric acid (6 carbons). Citric acid undergoes oxidation, reducing NAD+ to NADH and FAD+ to FADH2. Two carboxyl groups are released. The cycle regenerates oxaloacetic acid.

Cellular Respiration End Product

Cellular respiration produces carbon dioxide (CO2), water (H2O), and up to 38 ATP molecules from the breakdown of pyruvic acid.

Photosynthesis and Cellular Respiration: A Comparison

Here’s a comparison of the chemical equations:

Photosynthesis: 6CO2 + 6H2O + light energy —> C6H12O6 + 6O2

Respiration: C6H12O6 + 6O2 —> 6CO2 + 6H2O + Energy (ATP)