Photosynthesis I

Chapter 10: Photosynthesis Part 1

1. Overview of Photosynthesis

Photosynthesis is the metabolic process through which autotrophic organisms convert sunlight energy into chemical energy stored in carbohydrates. This process occurs mainly in the chloroplasts of plant cells and consists of two main pathways:

• Light-dependent reactions:

  • Occur in the thylakoid membranes of chloroplasts and utilize absorbed light to produce ATP (adenosine triphosphate) and NADPH + H+ (nicotinamide adenine dinucleotide phosphate).

  • Water (H2O) molecules are split (photolysis) to provide electrons, releasing oxygen (O2) as a by-product.

• Light-independent reactions (Calvin cycle):

  • Take place in the stroma of chloroplasts, utilizing ATP, NADPH + H+, and carbon dioxide (CO2) from the atmosphere to produce glucose and other carbohydrates.

  • This cycle involves several enzymatic reactions, with RuBisCO (ribulose bisphosphate carboxylase/oxygenase) being a key enzyme that facilitates carbon fixation.

2. Properties of Light

Light is a form of electromagnetic radiation; its energy inversely correlates with its wavelength. Shorter wavelengths (e.g., ultraviolet) carry more energy compared to longer wavelengths (e.g., infrared).

• Dual Nature of Light:

  • Light behaves both as particles (photons) and waves. Each photon has a defined energy based on its wavelength, which is crucial for the photosynthesis process.

3. Photon Absorption by Molecules

Molecules can absorb photons, leading to electron excitation to higher energy levels, making them more reactive. The interaction with photons can yield several outcomes:

• Bouncing Off

• Passing Through

• Absorption

  • Upon absorption, these molecules enter an excited state, where the energy can be utilized for metabolic processes such as photosynthesis.

4. Pigments

Pigments play a critical role in photosynthesis by absorbing specific light wavelengths in the visible spectrum. Their types include:

• Chlorophylls:

  • These green pigments absorb blue (around 430 nm) and red (around 660 nm) wavelengths effectively. Primarily found in the thylakoid membranes of chloroplasts, they are essential for the light-dependent reactions.

• Carotenoids:

  • Accessory pigments that absorb wavelengths between red and blue, enhancing the light-harvesting efficiency by transferring energy to chlorophyll molecules.

• Phycobilins:

  • A group of accessory pigments found in cyanobacteria and some algae, assisting in capturing light energy.

5. Accessory Pigments

Accessory pigments, including carotenes and xanthophylls (types of carotenoids), play vital roles in broadening the spectrum of light absorption. Additionally, flavonoids can protect chlorophyll from UV radiation damage, thereby preserving the efficiency of photosynthesis.

6. Absorption/Action Spectrum

• Absorption Spectrum:

  • A graphical representation showing the range of wavelengths a substance absorbs.

• Action Spectrum:

  • Illustrates the overall rate of photosynthesis at different wavelengths, indicating the wavelengths most effective for driving the process.

7. Chlorophyll

Predominant Types:

• Chlorophyll a:

  • The primary pigment of photosynthesis, absorbing blue and red light effectively.

• Chlorophyll b:

  • Works in conjunction with chlorophyll a, increasing the range of absorbed wavelengths. Both types consist of a porphyrin ring and a hydrocarbon tail, which embed into thylakoid membranes.

8. Photosystems

Photosystems are complex structures comprising 200-300 pigment molecules grouped within the thylakoid membranes, acting as units in the light-dependent reactions. They consist of:

• Antenna Complex:

  • The light-harvesting mechanism that captures and transfers light energy.

• Reaction Center:

  • A specialized site that converts absorbed light energy into chemical energy through electron transfer processes.

9. Light-Harvesting Antennae

When pigment molecules absorb photons, they enter an excited state temporarily. If this energy isn’t released as fluorescence, it can be transferred to adjacent pigments with lower energy thresholds, facilitating effective energy transfer within the photosystem.

10. Reaction Centre

The excitement energy from absorbed photons is relayed to the reaction center, which is centered around a chlorophyll a molecule. This reaction center is crucial for converting light energy into chemical energy, initiating redox reactions that ultimately culminate in ATP and NADPH production.

11. Overview of the Light Reactions

The light-dependent reactions encompass two major processes:

• Reduction Reactions:

  • In which excited electrons initiate a series of redox reactions in the electron transport chain, leading to the formation of NADPH.

• Photophosphorylation:

  • The process of coupling the electron transport system to ATP production via chemiosmosis, occurring within the thylakoid membrane.

12. Two Electron Transport Systems

Photosynthesis employs two distinct electron transport systems:

• Noncyclic Electron Transport:

  • Produces NADPH by transferring lost excited electrons to NADP+, splitting water to replenish those electrons and release O2 as a by-product.

• Cyclic Electron Transport:

  • This system recycles electrons back to photosystem I, focusing solely on ATP production without reducing NADP+. It is mainly utilized by ancient bacteria for ATP generation through sunlight absorption.

13. Noncyclic Electron Transport

This involves both photosystems:

• Photosystem I (P700):

  • Absorbs light energy primarily at 700 nm and facilitates the synthesis of NADPH.

• Photosystem II (P680):

  • Absorbs light energy at 680 nm, splits water to release electrons, forming oxygen, and contributes crucially to the electron transport chain.

14. Cyclic Electron Transport

This process produces only ATP by recycling electrons back to photosystem I, thereby enhancing ATP availability for the Calvin cycle. It serves as a critical adaptation for certain prokaryotes in different environmental conditions.

15. ADP Phosphorylation

The light energy captured during the light reactions creates a proton motive force within the thylakoids, which drives ATP synthesis through chemiosmosis. This process is facilitated by the activity of ATP synthase, which synthesizes ATP from ADP and inorganic phosphate (Pi) as protons flow back into the stroma.

This detailed overview encompasses the fundamental aspects of photosynthesis, focusing particularly on the mechanisms involved in light capture, energy conversion, and pigment functions, ultimately contributing to the plant's ability to convert solar energy into chemical energy.