Lichtabhängige Reaktion /Lichtreaktion /Primärreaktion der Fotosynthese [1/2] -[Biologie, Oberstufe]

Photosynthesis is a metabolic process used by higher plants, algae, and some bacteria. It converts carbon dioxide and water into glucose and oxygen using solar energy. Water is mainly absorbed from the soil, and carbon dioxide from the air. Glucose is a simple sugar used for growth, and oxygen is a byproduct vital for humans. The balanced chemical equation for photosynthesis is crucial for understanding the process.

Photosynthesis primarily occurs in chloroplasts within plant cells. Chloroplasts contain flattened membrane systems called thylakoids, where pigments like chlorophyll are embedded. This detailed structure allows for the localization of complex photosynthetic steps. The inner space of the chloroplast is called the stroma, and the thylakoids have an internal space called the lumen.

Photosynthesis consists of two main stages: the light-dependent reaction (primary reaction) and the light-independent reaction (secondary reaction). The light-dependent reaction takes place on the thylakoid membrane, converting light energy into chemical energy stored in ATP and NADPH. The light-independent reaction, also known as the dark reaction, occurs in the stroma and uses ATP and NADPH to synthesize carbohydrates like glucose in a process called the Calvin cycle, which will be discussed in the second video.

Light is a form of energy stored in photons. Plants and other photosynthetic organisms possess pigment molecules, mainly chlorophyll, that absorb light energy. When chlorophyll absorbs a photon's energy, it transitions from a low-energy ground state to a higher-energy excited state. This energy is not lost but stored within the pigment molecule. On an atomic level, this means an electron is moved to a higher, less stable orbit, making it more chemically reactive.

The light-dependent reaction occurs on the thylakoid membrane, where light-absorbing pigments like chlorophyll are integrated into protein complexes called photosystems. Chlorophyll molecules are arranged in antenna complexes to efficiently capture and funnel light energy to a central reaction center. The reaction center contains two chlorophyll a molecules that convert light energy into chemical energy. Upon light absorption, an electron in the chlorophyll molecule is elevated to a higher energy level, becoming loosely bound, and is then transferred to a primary redox acceptor, beginning its journey through the thylakoid membrane.

The chlorophyll molecule loses an electron and becomes oxidized (loses negative charge), specifically Photosystem II (P680), which absorbs light best at 680 nm. This oxidized chlorophyll is unstable and seeks an electron to regain stability. These electrons are provided by water molecules through a process called photolysis, or water splitting, carried out by a water-splitting complex. During photolysis, water molecules are broken down into oxygen atoms (which combine to form O2, released into the atmosphere), positively charged hydrogen ions (protons), and electrons. One of these electrons replenishes the missing electron in Photosystem II, confirming that the oxygen produced in photosynthesis originates from water, not CO2.

The excited electron, after being accepted by the primary acceptor, travels through an electron transport chain embedded in the thylakoid membrane. This chain consists of various enzyme complexes like plastoquinone, cytochrome, and plastocyanin. As the electron moves along this chain, its stored energy is used to pump protons (H+ ions) from the stroma into the thylakoid lumen. This, combined with protons from photolysis, creates a high concentration of protons inside the lumen relative to the stroma, establishing a concentration gradient and an electrical potential across the membrane. The stroma becomes more negatively charged than the lumen. This combined concentration and electrical gradient is known as the electrochemical gradient or proton-motive force.

The proton-motive force drives the transport of protons back into the stroma through a specialized channel protein called ATP synthase, embedded in the thylakoid membrane. ATP synthase couples the movement of protons with the synthesis of ATP from ADP and inorganic phosphate. This process, known as photophosphorylation, is an endergonic reaction, meaning it requires energy input. The energy for ATP formation comes directly from the proton flow (proton-motive force). ATP is an energy-rich molecule, and its formation is crucial for the second stage of photosynthesis, where its stored energy will power the synthesis of carbohydrates like glucose. The coupling mechanism of proton-motive force and ATP synthesis is called chemiosmosis. This proton flow is essential; without it, the proton gradient would perpetually build up, leading to unsustainable charge and concentration differences.

Photosynthesis involves plants taking in CO2, water, and light energy to produce glucose and molecular oxygen. The process is divided into the light-dependent and light-independent reactions. The light-dependent reaction converts light energy into chemical energy in the form of ATP and NADPH (to be discussed later). Photosynthetic organisms use chlorophyll in antenna complexes to absorb light and funnel energy to reaction centers. This excites an electron in chlorophyll, which is then transferred through an electron transport chain in the thylakoid membrane via a series of redox reactions. This transfer releases energy, driving the pumping of H+ ions into the thylakoid lumen, creating a proton gradient. This gradient, in turn, powers ATP synthase to produce ATP through photophosphorylation. The electron, now low in energy, reaches Photosystem I.

In Photosystem I (P700), chlorophyll molecules absorb light at 700 nm, exciting another electron. The low-energy electrons from Photosystem II replace the lost electron in Photosystem I. The excited electron from Photosystem I is transferred to ferredoxin, and then, with the help of NADP+ reductase, it combines with another electron and two H+ ions from the stroma to reduce NADP+ to the energy-rich hydrogen carrier NADPH. Thus, the electron transport chain extracts electrons from water, transfers them through a chain to NADP+ (forming NADPH) using light energy absorbed by Photosystems I and II, and also generates ATP via chemiosmosis.