Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing organic molecules such as glucose from carbon dioxide and water. This process is essential for life on Earth, as it provides the primary energy source for most living organisms.

• Photosynthesis occurs in chloroplasts, mainly within the mesophyll cells of leaves.

• It transforms light energy into chemical energy stored in sugars and other organic molecules.

• Uses CO2 as a carbon source and light energy as the energy source.

• Directly or indirectly supplies energy to most living organisms.

Structure of the Leaf and Chloroplast

Leaves are the major organs of photosynthesis. Chloroplasts are specialized organelles within plant cells where photosynthesis takes place.

• Chlorophyll is the green pigment responsible for absorbing light energy and giving leaves their color.

• Chloroplasts are primarily found in the mesophyll cells of leaves.

• Stomata are pores on the leaf surface that allow CO2 and H2O to enter and exit.

• Chloroplasts contain thylakoids (flattened membranous sacs) and stroma (fluid-filled space).

• Thylakoids are arranged in stacks called grana.

Light-Dependent Reactions

Introduction to Light-Dependent Reactions

The light-dependent reactions are the first stage of photosynthesis, converting light energy into chemical energy in the form of ATP and NADPH. These reactions occur in the thylakoid membranes of the chloroplast.

• Solar energy is converted to chemical energy.

• Occurs in the thylakoid membranes.

• NADP+ is reduced to NADPH.

• Water (H2O) is split, releasing oxygen (O2) as a byproduct.

Summary equation for the light-dependent reaction:

Role of the Stroma

The stroma is the fluid-filled space outside the thylakoids and inside the inner chloroplast membrane. It is the site of the light-independent reactions.

Light-Independent Reactions (Calvin-Benson Cycle)

Introduction to Light-Independent Reactions

The light-independent reactions, also known as the Calvin-Benson cycle, use the chemical energy stored in ATP and NADPH from the light-dependent reactions to synthesize glucose from CO2 and H2O. These reactions occur in the stroma of the chloroplast and do not require direct light.

• CO2 is fixed into organic molecules.

• ATP and NADPH are consumed to drive the reactions.

Summary equation for the light-independent reaction:

Phases of the Calvin-Benson Cycle

• Carbon fixation: Atmospheric CO2 is incorporated into organic molecules.

• G3P (PGAL) synthesis: ATP and NADPH are used to convert fixed carbon into G3P (glyceraldehyde-3-phosphate).

• Regeneration of ribulose bisphosphate: Sugar carbons are shuffled to regenerate the CO2 acceptor, ribulose bisphosphate.

Overall reaction for the Calvin-Benson cycle:

One glucose molecule consumes 18 ATPs and 12 NADPHs. The ADP and NADP+ return to the light-dependent reactions.

Capturing Light Energy

The Nature of Sunlight

Light is a form of electromagnetic energy that travels in waves. The visible spectrum ranges from 400 nm to 750 nm.

• Electromagnetic energy includes gamma rays, radio waves, and visible light.

• Light energy is carried in discrete particles called photons.

Photosynthetic Pigments

Pigments are substances that absorb visible light. The main pigment in photosynthesis is chlorophyll, which captures light energy in the thylakoid membranes.

• Chlorophyll is the key light-capturing molecule.

• Accessory pigments (such as carotenoids and phycocyanins) absorb different wavelengths of light.

• The pattern of absorption is called the absorption spectrum.

Photosystems and Electron Transport

Photosystems

Photosystems are assemblies of proteins and pigments that convert light energy into chemical energy in the thylakoid membranes. There are two types: Photosystem I (PS-I) and Photosystem II (PS-II).

• Each photosystem contains:

  • Light-harvesting complex

  • Reaction-center chlorophyll

  • Electron transport system

Steps in the Light-Dependent Reaction

1. Light energy (photons) is absorbed by the light-harvesting complex in PS-II.

2. Light excites electrons in PS-II, boosting them from the ground state to an excited state.

3. Excited electrons are transferred to the PS-II electron transport system.

4. Electrons flow down the electron transport system to PS-I.

5. Light energy is absorbed by the light-harvesting complex in PS-I.

6. Light excites electrons in PS-I, boosting them to an excited state.

7. Excited electrons are transferred to the PS-I electron transport system.

8. Electrons are transferred to NADP+ and stored in NADPH.

9. Electrons lost from PS-II are replaced by electrons from the splitting of H2O, generating O2.

Photophosphorylation and Chemiosmosis

As electrons flow down the PS-II electron transport system, ATP is generated indirectly by photophosphorylation. Chemiosmosis is the coupling of electron flow down the electron transport chain to ATP production by creating a proton gradient across the membrane.

• PS-II generates ATP by chemiosmosis.

• PS-I generates NADPH.

• Both ATP and NADPH are sources of stored chemical energy.

Summary Table: Key Components of Photosynthesis

Additional info:

• The Calvin-Benson cycle is also known as the C3 cycle, as the first stable product is a 3-carbon compound.

• Accessory pigments broaden the range of light wavelengths that can be used for photosynthesis.

• Photophosphorylation refers specifically to ATP synthesis driven by light energy.