Photosynthesis: Conversion of Light Energy to Chemical Energy

Introduction to Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert solar energy into chemical energy, producing organic molecules from carbon dioxide and water. This process is fundamental to life on Earth, as it provides both food and oxygen for most living organisms.

• Autotrophs: Organisms that produce their own food from inorganic substances. Most plants are photoautotrophs, using sunlight as an energy source.

• Heterotrophs: Organisms that obtain organic material by consuming other organisms. They depend on autotrophs for food and oxygen.

• Chloroplasts: Organelles in plant cells where photosynthesis occurs. Structurally similar to photosynthetic bacteria, they are mainly found in mesophyll cells of leaves.

• Mesophyll: The interior tissue of the leaf, rich in chloroplasts (each cell contains 30–40 chloroplasts).

• Stomata: Microscopic pores on the leaf surface that allow CO2 to enter and O2 to exit.

• Stroma: The dense fluid within the chloroplast, surrounding the thylakoid membranes.

• Thylakoids: Flattened, interconnected sacs within the chloroplast, often stacked into grana. The thylakoid membrane contains chlorophyll, the pigment responsible for capturing light energy.

Overall Equation of Photosynthesis

The overall process of photosynthesis can be summarized by the following equation:

• Oxidation: H2O is oxidized to O2.

• Reduction: CO2 is reduced to glucose.

• Endergonic Process: Requires an input of energy (from light).

Stages of Photosynthesis

Photosynthesis consists of two main stages:

1. Light Reactions ("photo" part): Occur in the thylakoid membranes. Convert solar energy to chemical energy (ATP and NADPH), split water, and release O2.

2. Calvin Cycle ("synthesis" part): Occurs in the stroma. Uses ATP and NADPH to convert CO2 into sugar (G3P).

Key Terms and Molecules

• NADP+/NADPH: NADP+ is an electron carrier reduced to NADPH during the light reactions. NADPH carries high-energy electrons to the Calvin cycle.

• Photophosphorylation: The process of generating ATP from ADP and inorganic phosphate using light energy during the light reactions.

• Carbon Fixation: The incorporation of CO2 into organic molecules, catalyzed by the enzyme rubisco in the Calvin cycle.

Light Reactions

The Nature of Light

Light is a form of electromagnetic energy that travels in waves. The wavelength determines the type of electromagnetic energy. The electromagnetic spectrum includes all wavelengths of electromagnetic radiation, but only a small portion (380–750 nm) is visible light, which drives photosynthesis.

• Photon: A discrete particle of light energy.

• Pigments: Substances that absorb visible light. Different pigments absorb different wavelengths.

Photosynthetic Pigments

• Chlorophyll a: Main pigment, absorbs blue-violet and red light; directly participates in light reactions.

• Chlorophyll b: Accessory pigment, absorbs blue and orange light; transfers energy to chlorophyll a.

• Carotenoids: Accessory pigments, absorb blue-green light; provide photoprotection by dissipating excess energy.

Leaves appear green because chlorophyll reflects and transmits green light. The absorption spectrum shows which wavelengths are absorbed by a pigment, while the action spectrum shows which wavelengths are most effective in driving photosynthesis.

Excitation of Chlorophyll

• When a pigment absorbs a photon, an electron is elevated to an excited state (unstable).

• As the electron returns to the ground state, energy is released as heat or fluorescence.

Photosystems

Photosystems are complexes in the thylakoid membrane that organize chlorophyll and other molecules to capture light energy efficiently.

• Photosystem II (PSII): Functions first; reaction-center chlorophyll a is called P680 (absorbs 680 nm).

• Photosystem I (PSI): Functions second; reaction-center chlorophyll a is called P700 (absorbs 700 nm).

• Each photosystem consists of a reaction-center complex (with special chlorophyll a molecules and a primary electron acceptor) surrounded by light-harvesting complexes (pigment molecules bound to proteins).

Linear Electron Flow (Noncyclic Electron Flow)

The primary pathway for electron flow during the light reactions, producing both ATP and NADPH.

1. A photon excites a pigment in PSII; energy is transferred to P680.

2. P680 transfers an excited electron to the primary electron acceptor (becoming P680+).

3. Water is split; electrons reduce P680+ back to P680, O2 is released.

4. Electrons move down an electron transport chain to PSI, creating a proton gradient.

5. ATP is produced by chemiosmosis (photophosphorylation) as protons diffuse through ATP synthase.

6. Light excites P700 in PSI; an electron is transferred to its primary acceptor.

7. Electrons from PSII replenish P700+.

8. Electrons are transferred to NADP+, forming NADPH.

Cyclic Electron Flow

• Involves only PSI; electrons cycle back to PSI instead of reducing NADP+.

• Produces ATP but not NADPH; no O2 is released.

• May protect cells from light-induced damage.

Comparison: Chemiosmosis in Chloroplasts and Mitochondria

• Both use an electron transport chain to create a proton gradient, driving ATP synthesis via ATP synthase.

• Chloroplasts: Light energy is the source; protons are pumped into the thylakoid space and diffuse back into the stroma.

• Mitochondria: Chemical energy from food is the source; protons are pumped into the intermembrane space and diffuse back into the matrix.

The Calvin Cycle

Overview

The Calvin cycle uses ATP and NADPH from the light reactions to convert CO2 into sugar. It is an anabolic pathway, regenerating its starting material (RuBP) after molecules enter and leave the cycle.

• Location: Stroma of the chloroplast.

• Product: Glyceraldehyde 3-phosphate (G3P), a three-carbon sugar.

• For net synthesis of 1 G3P, the cycle must turn 3 times, fixing 3 CO2 molecules.

Phases of the Calvin Cycle

1. Carbon Fixation: CO2 is attached to ribulose bisphosphate (RuBP) by the enzyme rubisco.

2. Reduction: ATP and NADPH are used to reduce 3-phosphoglycerate to G3P.

3. Regeneration of RuBP: Some G3P is used to regenerate RuBP, enabling the cycle to continue.

• After 3 turns: Net production of 1 G3P (the rest is recycled to regenerate RuBP).

• Plants use G3P to synthesize glucose, starch, cellulose, and other organic molecules.

Importance of Photosynthesis

• Stores solar energy as chemical energy in organic compounds.

• Provides food and oxygen for most living organisms.

• Removes CO2 from the atmosphere, helping regulate global climate.

• Excess sugar is stored as starch in roots, tubers, seeds, and fruits.

Summary Table: Key Components of Photosynthesis

Example: Use of Photosynthetic Products

• Glucose produced can be used for cellular respiration, stored as starch, or used to build cellulose for cell walls.

• Oxygen released is essential for aerobic respiration in most organisms.

Additional info: The Calvin cycle is sometimes called the "dark reactions" because it does not require light directly, but it depends on ATP and NADPH from the light reactions. The enzyme rubisco is the most abundant protein on Earth and is critical for carbon fixation.