Which organelles are involved in photosynthesis?

Photosynthesis is the process by which green plants, algae, and certain bacteria convert light energy from the sun into chemical energy in the form of glucose. This process involves the absorption of carbon dioxide (CO₂) from the air and water (H₂O) from the soil, using sunlight, which is captured by the chlorophyll in plant cells. The byproduct of this process is oxygen (O₂), which is released into the atmosphere.

The chemical equation for photosynthesis is:

6CO₂ + 6H₂O + light energy = C₆H₁₂O₆ + 6O₂

The primary organelles involved in photosynthesis are the chloroplasts.

Chloroplasts

Except for vacuoles, chloroplast, the double-mebraned organelle, are the largest and the most characteristic organelles in plant cell. These are specialized organelles found in the cells of plants and algae. Chloroplasts contain the pigment chlorophyll, which captures light energy for photosynthesis.

Like mitochondria, chloroplasts have their own genome and contain 70s ribosomes, DNA and enzymes involve in protein synthesis.

The chloroplast belongs to a group of related organelles in plants called plastids. Plastids differ in structure, carry out diverse functions, and are classified according to the kinds of pigments they contain. Chloroplasts contain chlorophyll while chromoplasts are lycopene-containing plastids and are responsible for yellow, orange and red colors of some flowers and fruits. Leucoplasts are non-pigmented plastids, and amyloplasts are starch-storing plastids of potato tubers. All plastids are believed to develop from a common proplastid precursor.

Structure of Chloroplast

Outer Membrane:

Smooth and permeable, allowing small molecules and ions to pass through easily. Protects the inner contents of the chloroplast.

Inner Membrane:

Inner Membrane also smooth but selectively permeable, regulating the movement of materials into and out of the stroma. Contains transport proteins that facilitate the passage of metabolites.

Intermembrane Space:

The narrow space between the outer and inner membranes. Plays a role in the transport of molecules across the chloroplast envelope.

Stroma:

The fluid-filled matrix inside the inner membrane. Contains enzymes involved in the Calvin cycle (light-independent reactions of photosynthesis). Houses the chloroplast DNA, ribosomes, and various proteins.

Calvin Cycle: Takes place in the stroma, using ATP and NADPH to convert CO₂ into glucose and other carbohydrates.

Lamellae:

The inner membrane give the rise to series of internal parallel in foldings called lamellae. These lamellae are suspended in a granular fluid or Matrix called the stroma. The lamellae are organised to form flattered disc shape sacs called thylakoids, which are arranged in stacks called grana.

Thylakoid:

Flattened, membrane-bound sacs where the light-dependent reactions of photosynthesis occur. It contains important components to capture light energy and convert it into ATP and NADPH.

Thylakoid Membrane: Contains chlorophyll and other pigments that capture light energy, as well as the electron transport chain and ATP synthase for ATP production.
Thylakoid Lumen: The internal space of the thylakoid, where proton accumulation occurs during the light-dependent reactions.

Grana:

Grana (singular = Granum) enhance the chloroplast's ability to capture light by stacking thylakoids. A typical chloroplast comprises of almost 40-60 grana, and each granum may be composed of 2-100 small, flattened thylakoids.

Process of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy, usually from the sun, into chemical energy stored in glucose and other organic molecules. This process occurs in chloroplasts in plant cells and involves two main stages:

The light-dependent reactions
The light-independent reactions (Calvin cycle)

1. Light-Dependent Reactions

Location: Thylakoid membranes of the chloroplast.

Steps:

I. Absorption of Light: Chlorophyll and other pigments in the thylakoid membranes absorb photons (light particles) from sunlight. This energy excites electrons in the chlorophyll molecules, raising them to a higher energy level.

II. Water Splitting (Photolysis): The excited electrons are passed along a series of electron carrier molecules (such as NADP⁺ and ADP), creating a flow of electrons. Water molecules are split into oxygen (released as a byproduct), protons (H⁺ ions), and electrons. This process replenishes the electrons lost from chlorophyll.

III. Production of ATP: As electrons move through the electron transport chain, their energy is used to pump protons (H⁺ ions) from the stroma into the thylakoid lumen, creating a proton gradient. This gradient drives ATP synthase, a protein complex, to produce ATP from ADP and inorganic phosphate (Pi).

IV. Production of NADPH: The electrons and protons generated by water splitting combine with NADP⁺ (Nicotinamide adenine dinucleotide phosphate) to form NADPH, a molecule that temporarily stores the energized electrons.

2. Light-Independent Reactions (Calvin Cycle)

Location: Stroma of the chloroplast.

Steps:

I. Carbon Fixation: Atmospheric CO₂ molecules enter the stroma of the chloroplast through diffusion and combine with a 5-carbon sugar, ribulose-1,5-bisphosphate (RuBP), catalyzed by the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). This forms an unstable 6-carbon compound that immediately breaks down into two molecules of 3-phosphoglycerate (3-PGA).

II. Reduction: ATP and NADPH from the light-dependent reactions provide the energy and electrons needed to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar phosphate. Some G3P molecules are used to regenerate RuBP, while others are used to synthesize glucose and other carbohydrates.

III. Regeneration of RuBP: The remaining G3P molecules are rearranged and phosphorylated using ATP to regenerate RuBP, completing the cycle and ensuring that the Calvin cycle can continue.

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