Week 7 - Oct 24

Photosynthesis: Transforming the Earth

Photosynthesis is a fundamental biological process that converts light energy into chemical energy, sustaining life on Earth. This process not only produces oxygen but also forms the basis of the food chain by generating organic molecules from carbon dioxide and water.

Introduction to Photosynthesis

• Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose and other organic molecules.

• This process is essential for life, as it provides both food and oxygen for aerobic organisms.

• Photosynthetic organisms are capable of 'ingesting the sun,' transforming solar energy into a usable form.

Light Absorption and Pigments

Why Does Wavelength Absorption Vary?

The absorption of light by photosynthetic pigments depends on their molecular structure and the chemical environment.

• Chlorophyll a and chlorophyll b are the main pigments in plants, each absorbing light at specific wavelengths.

• The porphyrin ring is the light-absorbing 'head' of the molecule, with a magnesium atom at its center.

• The hydrocarbon tail anchors the pigment in the thylakoid membrane.

• Even slight variations in pigment structure or their protein environment (e.g., in photosystem II vs photosystem I) can shift absorption peaks.

Example: The difference in absorption spectra between chlorophyll a and b allows plants to capture a broader range of the solar spectrum.

The Light Reactions

Overview of the Light Reactions

The light reactions occur in the thylakoid membranes of chloroplasts and convert solar energy into chemical energy in the form of ATP and NADPH.

• Light energy excites electrons in chlorophyll, raising them to higher energy levels.

• These high-energy electrons move through an electron transport chain, driving the formation of a proton (H+) gradient across the thylakoid membrane.

• The energy stored in this gradient is used to synthesize ATP (via chemiosmosis).

• NADP+ is reduced to NADPH, a high-energy electron carrier.

Equation:

$2H_2O + 2NADP^+ + 3ADP + 3P_i + light \rightarrow O_2 + 2NADPH + 3ATP + 2H^+$

The Calvin Cycle (Light-Independent Reactions)

Overview of the Calvin Cycle

The Calvin Cycle occurs in the stroma of the chloroplast and uses ATP and NADPH from the light reactions to fix carbon dioxide into organic molecules.

• Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the enzyme that catalyzes the fixation of CO2.

• Rubisco is the most abundant protein on Earth, responsible for converting about 100 billion tons of CO2 into carbohydrates each year.

• The cycle produces glyceraldehyde-3-phosphate (G3P), which is used to synthesize glucose and other organic molecules.

Equation for the Calvin Cycle:

$6CO_2 + 18ATP + 12NADPH + 12H^+ + 12H_2O \rightarrow C_6H_{12}O_6 + 18ADP + 18P_i + 12NADP^+ + 6O_2$

Key Steps:

1. Carbon Fixation: CO2 is attached to ribulose-1,5-bisphosphate (RuBP) by Rubisco, forming 3-phosphoglycerate (3-PGA).

2. Reduction: ATP and NADPH are used to convert 3-PGA into G3P.

3. Regeneration: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.

Stoichiometry: To produce one glucose molecule, the cycle must turn six times, fixing six CO2 molecules and consuming 18 ATP and 12 NADPH.

Summary Table: Calvin Cycle Inputs and Outputs

Input	Output
6 CO2	1 Glucose (C6H12O6)
18 ATP	18 ADP + 18 Pi
12 NADPH	12 NADP+ + 12 H+

Fate of G3P and Types of Photosynthesis

• G3P is a versatile intermediate used to synthesize glucose, sucrose, starch, cellulose, amino acids, fatty acids, and nucleic acids.

• The described process is oxygenic photosynthesis (produces O2), typical of plants, algae, and cyanobacteria.

• Some bacteria perform anoxygenic photosynthesis, which does not produce oxygen.

Comparison: Photosynthesis vs. Cellular Respiration

Similarities and Differences

Photosynthesis and cellular respiration are complementary processes in the flow of energy and matter in living systems.

Feature	Photosynthesis	Cellular Respiration
Location	Chloroplasts (thylakoid & stroma)	Mitochondria (matrix & inner membrane)
Source of Electrons	Water (H2O)	Glucose (C6H12O6)
Final Electron Acceptor	NADP+ (light reactions)	O2
Energy Source	Light	Chemical bonds (glucose)
ATP Production	Light reactions (photophosphorylation)	Oxidative phosphorylation
H+ Gradient	Across thylakoid membrane (into thylakoid space)	Across inner mitochondrial membrane (into intermembrane space)
Role of O2	Product (from splitting H2O)	Reactant (final electron acceptor)
Overall Equation	$6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2$	$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + energy$

Example: The ATP and NADPH produced in the light reactions of photosynthesis are used in the Calvin Cycle, while in cellular respiration, glucose is broken down to produce ATP.

Summary

• Photosynthesis transforms solar energy into chemical energy, supporting nearly all life on Earth.

• It involves two main stages: the light reactions (producing ATP and NADPH) and the Calvin Cycle (fixing CO2 into sugars).

• Photosynthesis and cellular respiration are interconnected processes that cycle energy and matter through ecosystems.