Photosynthesis and Cellular Metabolism: An In-Depth Look

Posted on Dec 19, 2024 in Biology

Anabolism

The path is constructive metabolism, the synthesis pathway from simple molecules to complex molecules. There are two stages:

  • Heterotrophic Anabolism: Simple organic molecules to complex organic molecules such as starch, fat, or proteins. Its objective is the synthesis of macromolecules (energy reserves).
  • Autotrophic Anabolism: The passage of inorganic molecules (H2O, CO2, NO3–) to simple molecules such as glucose or glycerin. These are beings that make their organized matter from inorganic material. According to the source:
  • Photosynthetic Anabolism: The source of energy is light.
  • Chemosynthetic Anabolism: The source of energy is chemical reactions.

Photosynthesis: Conversion of light energy into chemical energy that is stored in organic molecules. It is possible thanks to pigments, able to: collect light energy and use it to promote electrons to transfer them to other substances. The lost electrons are retrieved by two possible routes:

H2O → 2H+ + 2e– + ½ O2 (oxygenic photosynthesis, e.g., plants, algae, and cyanobacteria)

H2S → 2H+ + 2e– + S (anoxygenic or bacterial photosynthesis, e.g., purple and green sulfur bacteria that live in sulfur water)

Photosynthetic pigments are molecules that have conjugated double bonds (resonance). Resonance is a delocalization that does not know where to stand. There is a molecule capable of giving and picking up thanks to a donor. Three things that light can do to interact with matter:

  1. Shake it thermally.
  2. Promote electrons to a higher level.
  3. Ionize the molecule.

Photosynthetic Apparatus

Photosystems, electron transport chain, and ATP synthase.

Photosystems = antenna (sensor complex) + reaction center.

Antenna: It is a group of pigments (chlorophyll a, b, carotenoids). Reaction center: Target pigments capable of transferring electrons to the first acceptor and receiving them from the first donor. The photosystem pigments are xanthophylls, chlorophylls, and, in a few cases, phycobilins. In chlorophylls, there are two components: fatty alcohol (cytosol) and tetrapyrrole (nucleus).

On the upper floors, they act differently:

  1. Photosystem I (PSI) and chlorophyll P700. Do not break the water molecules (the target molecule can capture wavelengths equal to or less than 700 nm).
  2. Photosystem II (PSII) or chlorophyll P680 (can capture photons with wavelengths equal to or less than 680 nm). Breaks water molecules to replenish the electrons lost by the target pigment.

Photosynthesis of Carbon Compounds

In chloroplasts, the synthesis of compounds is made of C, N, and S.

Formula: 6CO2 + 12H2O + light energy → C6H12O6 + 6O2

Phases: The light-dependent phase requires light for its development, and the dark phase does not need light. But without the light-dependent phase, there is no dark phase. The light-dependent phase includes two types of electron transport: acyclic and cyclic.

Light-Dependent Phase: Acyclic

Process:

  1. Photolysis of H2O: H2O → ½ O2 + 2H+ + 2e–

Start with PSII. Excitation causes the pigment to lose two electrons when photons are absorbed. The electrons are passed to the first acceptor and then to plastoquinone (PQ). Hydrolysis of water molecules occurs (photolysis of H2O), leading to terms on the inner face of the thylakoid membrane.

  1. Photophosphorylation of ADP: ADP + Pi → ATP + H2O

PQ is activated and captures two protons from the stroma. When electrons are transferred to the cytochrome bf complex, two protons enter the thylakoid. It is resolved, according to Mitchell’s chemiosmotic hypothesis, with the output of protons through the ATP synthase enzyme to the stroma (photophosphorylation of ADP).

  1. Photoreduction of NADP+: NADP+ + 2H+ + 2e– → NADPH + H+

The two electrons lost by PSI pass through the first acceptor and then to ferredoxin (Fd). The lost electrons are replenished by plastocyanin (cytochrome bf). The light energy in the acyclic stage is used to reduce NADP+ to NADPH + H+, and two protons will be needed from the photolysis of a water molecule.

Light-Dependent Phase: Cyclic

Only PSI is involved. The purpose of this phase is to solve the deficit of ATP found in the acyclic phase to carry out the subsequent dark phase. The cyclic phase occurs when illuminated with light of wavelength greater than 680 nm. Photons affect PSI, releasing electrons that reach Fd, which will transfer them to cytochrome b6 and then to PQ, which captures two protons. PQ gives the electrons to cytochrome bf, and two protons are inserted into the interior of the thylakoid.

Photosynthetic Pigments and the Absorption of Light

Chlorophyll a absorbs light at 683 nm, and chlorophyll b absorbs it at 660 nm. Carotenoids are isoprenoids and absorb light at 440 nm. They can be carotenes (red) and xanthophylls (yellow). Phycocyanin (blue) and phycoerythrin (red).

Biosynthetic or Dark Phase

In the dark phase, CO2 is fixed with ATP and NADPH obtained in the light phase. This process occurs in the chloroplast stroma thanks to the Calvin cycle and involves four sub-phases: fixation of CO2, reduction of fixed CO2, regeneration of ribulose-1,5-bisphosphate, and synthesis of products.

Photorespiration and the Hatch-Slack Pathway

In hot and dry environments, when the stomata are sealed, the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase acts as an oxidase and destroys ribulose-1,5-bisphosphate (5C), reducing the photosynthetic capacity of the plant by 50%. To avoid this process, many tropical plants use the Hatch-Slack pathway (C4 pathway).

C4 plants have two types of chloroplasts: those of the vascular bundle sheath surrounding the vascular bundles and those of the mesophyll. In the mesophyll, the acceptor molecule of CO2 is phosphoenolpyruvate (PEP), and the enzyme is phosphoenolpyruvate carboxylase.

Chemosynthesis

It is carried out exclusively in bacteria and uses NADPH instead of NADH. It has two phases:

  1. Phase 1: Oxidation of inorganic substances to produce ATP. A portion of the ATP is used to obtain NADH by reverse electron transport.
  2. Phase 2: The metabolic pathways coincide with the dark phase of photosynthesis. CO2 enters through the Calvin cycle.

Factors Influencing Photosynthesis

Temperature, CO2 concentration, O2 concentration, light intensity, and water shortages.