Lecture 4, full

What is the fundamental biological importance of Fotosynthese?

It is the process that converts Sonnenenergie (solar energy) into biochemische Energie (biochemical energy) stored in sugar, providing both the energy source and the organic carbon building material for all life

What are the two main processes of photosynthesis?

• Light reaction (Lichtreaktion):
  Conversion of light energy (Lichtenergie) into chemical energy (chemische Energie) in the form of ATP and NADPH + H⁺ (NADPH/H⁺, ATP).

• Dark reaction / Calvin cycle (Dunkelreaktion):
  Uses chemical energy (chemische Energie) to reduce carbon (Kohlenstoff) into sugars (Zucker) and starch (Stärke).

What is the structure of chloroplasts (Aufbau der Chloroplasten)?

• All photosynthesis processes (Photosynthese) take place in the chloroplasts (Chloroplasten).

• Chloroplasts are filled with internal membrane structures called thylakoids (Thylakoide).

• Thylakoids divide the chloroplast into:

  • lumen (Thylakoid-Lumen) = inside the thylakoids

  • stroma (Stroma) = space outside the thylakoids

• Photosynthetic pigments (Photosynthese-Pigmente) and the main protein complexes of the light reactions (Lichtreaktion) are embedded in the thylakoid membrane (Thylakoidmembran).

How are pigments arranged in antenna complexes (Antennenkomplexe) around Photosystem II (PSII)?

• Photosynthetic pigments (Photosynthese-Pigmente) are associated with membrane-bound chloroplast proteins (CP-Proteine).

• The main antenna complex is the Light Harvesting Complex II (LHCII).

• Pigments are arranged to allow efficient energy transfer (Energieübertragung) to the reaction center (Reaktionszentrum) of Photosystem I or II.

• Each reaction center is connected to many antenna pigments:

  • Photosystem I (PSI): ~200 pigments

  • Photosystem II (PSII): ~300 pigments

• This organization increases light absorption efficiency (Lichtabsorptionseffizienz) and funnels energy to the reaction center.

How does light energy lead to electron transfer (von Lichtenergie zum Elektronentransfer) in photosynthesis?

• Pigment electrons (Elektronen der Pigmente) absorb light energy (Lichtenergie) and are excited to higher energy levels (höhere Energieniveaus) → the molecule enters an excited state (angeregter Zustand).

• The absorbed energy is transferred between pigments in antenna complexes (Antennenkomplexe) via exciton transfer (Excitonentransfer).

• This energy transfer is directed toward a chlorophyll a (Chla) molecule in the reaction center (Reaktionszentrum) of PSI or PSII.

• The reaction center Chla uses this energy to transfer an electron (Elektron) to a primary electron acceptor (Elektronenakzeptor).

What is photoinhibition (Photoinhibition)?

• Excess light (Lichtüberschuss) can become damaging to the photosynthetic system.

• When the demand for NADPH + H⁺ and ATP is low, pigments are over-excited (übermässig aktiviert) and exciton energy cannot be used (kein Excitonenabfluss).

• This leads to the formation of reactive oxygen species (Sauerstoffradikale), which can damage Photosystem II (PSII).

• As a result, photosynthetic performance (Photosyntheseleistung) decreases.

• Typical conditions include:

  • closed stomata during drought (geschlossene Stomata bei Trockenheit)

  • cold temperatures (Kälte)

  • insufficient light acclimation (mangelnde Akklimatisierung an Licht)

What happens in the dark reaction (Dunkelreaktion / Calvin cycle)?

• In the dark reaction (Dunkelreaktion), carbon dioxide (CO₂ / Kohlenstoff) is reduced to carbohydrates (CH₂O)ₙ.

• Electrons and energy are provided by NADPH + H⁺ (NADPH) and ATP.

• The dark reaction itself does not require light (kein Licht erforderlich), only ATP and NADPH.

• However, it only occurs in the light in nature, because ATP and NADPH are produced by the light reactions (Lichtreaktion).

What happens in the carboxylating phase (carboxylierende Phase) of the Calvin cycle?

• In the carboxylating phase (carboxylierende Phase), the enzyme RuBisCO attaches a CO₂ molecule to ribulose-1,5-bisphosphate (RuBP / Ribulose-1,5-Biphosphat).

• This unstable 6-carbon intermediate immediately splits into two molecules of D-3-phosphoglycerate (3-PGA / D-3-Phosphoglycerat).

What happens in the reduction phase (Reduktionsphase) of the Calvin cycle?

• Under ATP consumption (ATP-Aufwand), 3-phosphoglycerate (3-PGA / D-3-Phosphoglycerat) is converted into 1,3-bisphosphoglycerate (1,3-Biphosphoglycerat).

• This is then reduced by NADPH + H⁺ (NADPH) to form D-3-phosphoglyceraldehyde (G3P / Triosephosphat).

• Out of 12 Triosephosphates per cycle, 2 molecules leave the cycle and represent the net carbon gain (Fixierungsgewinn).

What happens in the regeneration phase (regenerierende Phase) of the Calvin cycle?

• In the regeneration phase (regenerierende Phase), 10 triose phosphates (Triosephosphate) are used.

• Through a complex series of reactions and ATP consumption (ATP-Aufwand), they are converted back into ribulose-1,5-bisphosphate (Ribulose-1,5-Biphosphat / RuBP).

• This regenerates the CO₂ acceptor so the Calvin cycle can continue.

What is the overall balance (Bilanz) of the dark reaction (Dunkelreaktion / Calvin cycle)?

• To reduce 6 molecules of CO₂ (Kohlenstoff) into 1 hexose (Hexose, e.g. glucose), the cycle requires:

  • 24 electrons (24 Elektronen)

  • 18 ATP molecules (18 ATP)

• During this process, carbon is reduced from its highest oxidation state (+4 in CO₂) to its lowest oxidation state (0 in carbohydrates).

How is sucrose synthesis (Synthese der Saccharose) linked to photosynthesis?

• In the cytoplasm (Cytoplasma), triose phosphates (Triosephosphate) are converted into hexose phosphates (Hexosephosphat).

• From these intermediates, the disaccharide sucrose (Saccharose) is synthesized.

• Sucrose consists of one unit of glucose (Glukose) and one unit of fructose (Fructose).

How is starch (Stärke) synthesized in leaves (Synthese der Stärke im Blatt)?

• In the chloroplasts (Chloroplasten), triose phosphates (Triosephosphate) are converted into hexose phosphates (Hexosephosphate) such as F6P, G6P, and G1P.

• G1P (Glucose-1-phosphate / Glukose-1-Phosphat) is converted into ADP-glucose (ADP-Glucose).

• Starch synthases (Stärke-Synthasen) use ADP-glucose to build glucose polymers (Glucosepolymere) → starch (Stärke).

• Starch consists of:

  • Amylose (geradkettig / linear)

  • Amylopectin (verzweigt / branched)

How is starch stored in leaves (Synthese und Speicherung der Stärke im Blatt)?

• Starch (Stärke) is stored in plastids (Plastiden) in the leaf as starch granules (Stärkekörner).

• These starch granules contain amylose (Amylose) and amylopectin (Amylopektin).

• They show a characteristic banding pattern (charakteristische Bänderung).

How is starch remobilized (Remobilisierung der Stärke) in leaves?

• Amylases (Amylasen) break down amylose (Amylose) and amylopectin (Amylopektin) into the disaccharide maltose (Maltose).

• Maltose is exported from the chloroplast (Chloroplast).

• In the cytosol, maltose is converted into glucose-6-phosphate (G6P / Glukose-6-Phosphat).

What is the general chemical equation for Fotosynthese?

6 CO2 + 12 H2O + Energy → C6H12O6 + 6O2 + 6H2O

Into which two main processes is photosynthesis divided?

The Lichtreaktion (light reaction), which converts light into chemical energy (ATP/NADPH), and the Dunkelreaktion (dark reaction), which uses that energy to reduce carbon into sugar and starch

Why is the Dunkelreaktion technically a misnomer?

While it does not directly require light, it only occurs during the day because it depends on the immediate supply of unstable ATP and NADPH produced by the Lichtreaktion

What are the three main compartments of the Chloroplasten (chloroplasts)?

The Thylakoide (membrane structures), the Lumen (space inside the thylakoids), and the Stroma (fluid space outside the thylakoids)

What is the Grünlücke (green gap) in plant light absorption?

It refers to the fact that plant pigments (chlorophylls) primarily absorb red and blue light (400–700 nm), but absorb very little in the 500–600 nm range, causing plants to appear green

What are the two primary types of Photosynthesepigmente?

Chlorophylle (Chlorophyll a and b) and Carotinoide (accessory pigments like carotenes and xanthophylls)

What are the structural components of a Chlorophyll molecule?

A Porphyrinring containing a central Magnesium atom (the "head" that absorbs light) and a long hydrophobic Phytolkette (phytol tail) that anchors it in the membrane

How are pigments arranged to capture light efficiently?

They are organized in Antennenkomplexen (antenna complexes) associated with proteins, such as the Light Harvesting Complex II (LHCII)

How many antenna pigments are assigned to Photosystem I (PSI) and Photosystem II (PSII)?

Approximately 200 pigments for PSI and 300 for PSII

What is an Excitonentransfer (exciton transfer)?

The process where absorbed energy is transferred from pigment to pigment within an antenna complex toward the reaction centre

What happens at the Reaktionszentrum (reaction centre) when energy arrives?

The energy is used to eject an electron from a special pair of Chlorophyll-a molecules to an Elektronenakzeptor (electron acceptor)

What is the function of the Wasserspaltungskomplex (water-splitting complex) at PSII?

It performs the Photolyse of water 2H2O → O2 + 4(H+) + 4e- to provide electrons to regenerate oxidized chlorophyll molecules

Where does the Sauerstoff (oxygen) released by plants actually come from?

It is a "waste product" resulting from the Photolyse of water, not from CO2

What are the mobile electron carriers in the Lichtreaktion?

Plastochinon (PQ) (carries electrons and protons within the membrane), Plastocyanin (PC) (a copper-containing protein in the lumen), and Ferredoxin (Fd) (on the stroma side)

What is the role of the Cytochrom-b6-Komplex?

It facilitates electron transfer from PQ to PC and pumps Protonen (H+) into the thylakoid lumen to create an energy gradient

What is Photophosphorylierung (photophosphorylation)?

The synthesis of ATP from ADP and Pi by ATP-Synthase, driven by the proton gradient (H+ concentration) between the lumen and the stroma

What is the net balance of the Lichtreaktion per O2 produced?

2H2O + 2(NADP+) + approx 3ADP + 3Pi → O2 + 2NADPH + 2(H+) + approx 3ATP

What is Photoinhibition?

Damage to Photosystem II caused by excess light; when energy demand is low, surplus excitons create Sauerstoffradikale (oxygen radicals) that inhibit photosynthesis

What are the three phases of the Calvin Zyklus?

1. Carboxylierende Phase (carbon fixation),

2. Reduzierende Phase (reduction of carbon),

3. Regenerierende Phase (regeneration of RuBP)

What is RuBisCO and why is it special?

• It is the enzyme that fixates CO2 onto Ribulose-1,5-Bisphosphat

• It is the most abundant protein on Earth, making up about 25% of leaf nitrogen.

What is the primary product of the Reduzierende Phase of the Calvin Cycle?

Triosephosphat (specifically D-3-Phosphoglycerinaldehyde), which is the biochemical basis for all organic molecules in the plant

How much energy is required to reduce 6 molecules of CO2 into one Hexose (sugar)?

18 molecules of ATP and 12 molecules of NADPH (carrying 24 electrons)

What are the two primary end-products of the carbon pathway?

Saccharose (sucrose) for transport and Stärke (starch) for storage

Where is Saccharose (sucrose) synthesized?

In the Cytoplasma (cytoplasm) of the cell

What are the two types of molecules found in Stärke (starch)?

Amylose (alpha 1-4 glycosidic bonds, linear, 20-30%) and Amylopektin (alpha 1-6 glycosidic bonds, branched, 70-80%)

What is Transitorische Stärke (transitory starch)?

Starch that is synthesized and stored in the chloroplast during the day and remobilized into sucrose at night for export

What enzyme remobilizes starch and what is the intermediate product?

Amylasen split starch into the disaccharide Maltose (Malzucker/malt sugar), which is then exported from the chloroplast

Why do plants store energy as Stärke instead of sugar?

Starch is not water-soluble and thus osmotisch inaktiv (osmotically inactive), allowing large amounts of energy to be stored without affecting the cell's water potential

What is the Z-Schema in the light reaction?

A diagram showing the energy levels (redox potential) of electrons as they are boosted by light at PSII, drop through the transport chain, and are boosted again at PSI