Photosynthesis: Conversion of Light Energy to Chemical Energy

Photosynthesis

Introduction to Photosynthesis

Photosynthesis is the process by which plants, algae, and some prokaryotes convert solar energy into chemical energy, sustaining almost all life on Earth. This process occurs primarily in chloroplasts and is essential for the production of organic molecules and oxygen.

• Autotrophs: Organisms that produce their own food from inorganic substances. Most plants are photoautotrophs, using sunlight to synthesize organic compounds.

• Heterotrophs: Organisms that obtain organic molecules by consuming other organisms. This group includes animals, fungi, and many bacteria.

• Producers: Autotrophs that form the base of the food chain by producing organic molecules from CO2 and other inorganic substances.

• Consumers: Heterotrophs that rely on autotrophs or other heterotrophs for food.

Chloroplasts: The Sites of Photosynthesis

Photosynthesis in plants occurs mainly in the leaves, within organelles called chloroplasts. These organelles are structurally adapted to capture light energy and convert it into chemical energy.

• Mesophyll: The interior tissue of the leaf where most chloroplasts are found.

• Stomata: Microscopic pores on the leaf surface that allow gas exchange (CO2 in, O2 out).

• Thylakoids: Flattened sacs within the chloroplast, organized into stacks called grana. The thylakoid membrane contains chlorophyll, the pigment responsible for capturing light energy.

• Stroma: The dense fluid surrounding the thylakoids, where the Calvin cycle takes place.

The Overall Equation of Photosynthesis

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

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

• This equation shows that carbon dioxide and water are converted into glucose and oxygen, using light energy.

• Photosynthesis is the reverse of cellular respiration.

The Splitting of Water

During photosynthesis, water molecules are split to provide electrons and protons, releasing oxygen as a by-product. The source of hydrogen varies among photosynthetic organisms (e.g., some bacteria use H2S instead of H2O).

Photosynthesis as a Redox Process

Photosynthesis involves the transfer of electrons (redox reactions). In this process, water is oxidized and carbon dioxide is reduced. The energy required for these reactions is provided by light, making photosynthesis an endergonic process.

The Two Stages of Photosynthesis

Photosynthesis consists of two main stages: the light reactions and the Calvin cycle.

• Light Reactions (in the thylakoids):

  • Split water, releasing O2

  • Produce ATP and NADPH by photophosphorylation

• Calvin Cycle (in the stroma):

  • Uses ATP and NADPH to convert CO2 into sugar (G3P)

  • Regenerates its starting material (RuBP)

The Light Reactions

The Nature of Sunlight

Light is a form of electromagnetic energy that travels in waves. The electromagnetic spectrum includes all wavelengths of electromagnetic radiation, but only visible light (380–740 nm) is used in photosynthesis.

• Photons: Discrete particles of light, each with a fixed amount of energy inversely related to its wavelength.

Photosynthetic Pigments

Pigments are molecules that absorb specific wavelengths of light. The main pigments in chloroplasts are:

• Chlorophyll a: The primary pigment involved in light reactions.

• Chlorophyll b: An accessory pigment that broadens the spectrum of light used.

• Carotenoids: Accessory pigments that absorb additional wavelengths and provide photoprotection.

Leaves appear green because chlorophyll absorbs violet-blue and red light, reflecting green light.

Excitation of Chlorophyll by Light

When chlorophyll absorbs light, its electrons are excited to a higher energy state. In isolation, these electrons return to the ground state, releasing energy as heat or fluorescence.

Photosystems

A photosystem is a complex of proteins and pigments that captures light energy and initiates electron transfer. Each photosystem consists of a reaction-center complex surrounded by light-harvesting complexes.

• Photosystem II (PSII): Contains P680 chlorophyll a, best at absorbing 680 nm light.

• Photosystem I (PSI): Contains P700 chlorophyll a, best at absorbing 700 nm light.

Linear Electron Flow

Linear electron flow is the primary pathway during the light reactions, involving both photosystems and resulting in the production of ATP and NADPH.

1. Light excites electrons in PSII, which are transferred to the primary electron acceptor.

2. Water is split, providing electrons to PSII and releasing O2.

3. Electrons move through an electron transport chain, generating a proton gradient used to produce ATP.

4. Electrons reach PSI, are re-excited by light, and transferred to NADP+ to form NADPH.

Cyclic Electron Flow

In cyclic electron flow, electrons cycle back from ferredoxin to the cytochrome complex and then to PSI. This process produces ATP but not NADPH or O2. It may provide photoprotection and is considered an evolutionary remnant.

The Calvin Cycle

Overview of the Calvin Cycle

The Calvin cycle uses ATP and NADPH from the light reactions to convert CO2 into sugar. It is an anabolic pathway that regenerates its starting molecule, ribulose bisphosphate (RuBP).

• For each G3P produced, the cycle must turn three times, fixing three CO2 molecules.

• The cycle has three phases: carbon fixation, reduction, and regeneration of RuBP.

Phases of the Calvin Cycle

• Phase 1: Carbon Fixation

  • CO2 is attached to RuBP by the enzyme rubisco, forming two molecules of 3-phosphoglycerate.

• Phase 2: Reduction

  • 3-phosphoglycerate is phosphorylated by ATP and reduced by NADPH to produce G3P.

  • For every three CO2 molecules, six G3P are formed, but only one is a net gain.

• Phase 3: Regeneration of RuBP

  • Five G3P molecules are rearranged to regenerate three RuBP molecules, using additional ATP.

For the net synthesis of one G3P, the Calvin cycle consumes nine ATP and six NADPH molecules. The G3P produced can be used to synthesize glucose, sucrose, and other carbohydrates.