Photosynthesis: The Process That Powers Life on Earth

Posted on Nov 12, 2024 in Biology

Photosynthesis

Photosynthesis is an anabolic and autotrophic primary process upon which life on Earth depends. It involves the conversion of light energy from the sun into electrical energy and then into chemical energy by photosynthetic organisms. This energy is used to form their own organic matter or biomass (carbohydrates) from inorganic molecules such as water, CO2, and mineral salts. O2 molecules, resulting from the breakdown of water molecules involved in the process, appear as a waste product.

Chloroplasts

Chloroplasts are cytoplasmic organelles whose main function is to carry out photosynthesis. They are surrounded by two membranes with an intermembrane space between them. The outer plastid membrane is smooth, while the inner plastid membrane has invaginations parallel to the longitudinal axis of the chloroplast, resulting in the thylakoid membrane (lamellae or thylakoids). The inner membrane encloses a space called the stroma. The thylakoid membrane is organized to form discoidal vesicles and flattened, overlapping piles called grana.

Phases of Photosynthesis

Photosynthesis takes place in two stages known as the light phase and the dark phase.

Light Phase of Photosynthesis

The light phase of photosynthesis takes place in the presence of light and includes a set of reactions that occur in the thylakoid membranes. In it, the electrons released by the impact of photons on the photosystems are used to reduce NADP+ to NADPH. Through the electron transport chain, the energy lost by electrons is used for ATP synthesis. Therefore, during this phase, two very important processes take place: photolysis of water, obtaining reducing power in the form of reduced coenzymes (NADPH), and ATP production through photophosphorylation. The waste product of this phase is molecular oxygen.

In general, the light reactions take place as follows:

  • Photolysis of water produces H+ that will reduce CO2 to organic matter in the dark phase. H2O → ½ O2 + 2H+ + 2e– This reaction also occurs in the thylakoid membranes, as do all processes that take place during the light phase of photosynthesis.
  • Absorption of sunlight: This is carried out by photosynthetic pigments, including chlorophylls and carotenoids. These pigments, along with specific proteins, are grouped together, forming photosystems, which are located in the thylakoid membranes of chloroplasts. All pigments of a photosystem can absorb light, but only one, called the reaction center, is capable of converting light energy into electrical energy. The reaction center consists of a chlorophyll a molecule and a specific protein. Other pigments are called antenna pigments, and their function is to transfer the photon energy to the reaction center.

Two photosystems are involved in light absorption:

  • Photosystem I (PSI): Able to absorb light of λ < 700 nm.
  • Photosystem II (PSII): Able to absorb light of λ < 680 nm.

If a photon hits an electron in an atom belonging to a photosynthetic pigment molecule, the electron captures the energy of the photon and jumps to more distant orbits from the nucleus. It can be lost, leaving the atom ionized. The molecule containing this atom is then eager to gain electrons, which are supplied by the photolysis of water.

Therefore, the absorption of light by the reaction center chlorophyll causes it to release an electron, which travels along an electron transport chain to reach NADP+. The reaction center then becomes ionized and takes an electron provided by the photolysis of H2O.

  • Photosynthetic electron transport: The electrons released from the reaction center, loaded with photon energy, are transported by a set of transport proteins located in the thylakoid membrane to the coenzyme NADP+, which is reduced to NADPH.
  • Photophosphorylation: ATP formation due to light. The dark phase of photosynthesis requires NADPH and ATP. According to Mitchell’s “chemiosmotic hypothesis”, the energy released by electron transport is used to pump protons against a gradient from the stroma to the thylakoid space. These protons return to the stroma down the gradient through the enzyme complex called ATP synthase, which uses the energy released in the transport to phosphorylate ADP into ATP.
Dark Phase of Photosynthesis

In the dark phase of photosynthesis or biosynthesis stage, ATP and NADPH from the light phase are used to convert inorganic compounds into organic compounds, i.e., to reduce CO2 and synthesize carbohydrates. The Calvin cycle is the metabolic pathway by which most photosynthetic organisms fix CO2, thereby obtaining the carbon necessary for building their organic biomolecules.

The Calvin cycle includes several phases:

  1. Carboxylation phase: CO2 is incorporated into a 5-carbon molecule, ribulose-1,5-bisphosphate, to produce two molecules of 3-phosphoglycerate. This reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (RUBISCO), which is the most abundant enzyme on Earth.
  2. Reduction phase: In this phase, the CO2 incorporated in the carboxylation phase reduces the carboxyl group of 3-phosphoglycerate, using ATP and NADPH produced in the light phase.
  3. Regeneration phase: In this phase, the glyceraldehyde-3-phosphate obtained in the reduction phase is converted into glucose-6-phosphate and partly into ribulose-1,5-bisphosphate to close the cycle again.

The balance of the Calvin cycle is:

6 CO2 + 18 ATP + 12 NADPH + 12 H+ + 12 H2O → glucose + 18 ADP + 18 Pi + 12 NADP+

This high consumption of ATP reflects the fact that CO2 is the most oxidized form of carbon that can be used to build the carbon skeletons of organic molecules.