Lecture 5, full

What are the two main processes of photosynthesis described in the sources?

1. Lichtreaktion (Light reaction): Conversion of light energy into chemical energy (ATP and NADPH), 2. Dunkelreaktion (Dark reaction): Reduction of carbon to sugar/starch using chemical energy.

What determines whether RuBisCO catalyzes carboxylation or oxygenation? How frequent is oxygenation?

• Substratangebot (substrate availability): CO₂ vs O₂

• Key factor: CO₂ concentration in mesophyll (ci)

• High ci (CO₂ hoch) → Carboxylierung (carboxylation)

• Low ci / relatively more O₂ → Oxygenierung (oxygenation)

• Under normal conditions:

  • 20–50% der Reaktionen = Oxygenasereaktion

• Reason: limited specificity of RuBisCO for CO₂

Why does photorespiration increase energy demand?

• Ratio Carboxylase : Oxygenase (Carboxylierung : Oxygenierung) = 2:1 – 4:1

• Presence of Oxygenasereaktion (oxygenation) triggers Photorespiration

• Leads to Mehrbedarf (increased demand) of:

  • ATP

  • NADPH

• Increase ≈ +50% Energieverbrauch

• Reason: recycling of 2-phosphoglycolate requires energy-consuming pathway

What factors influence stomatal conductance (stomatäre Leitfähigkeit)?

• Licht (Licht): ↑ Licht → Stomata öffnen (open)

• CO₂ (CO₂-Konzentration im Mesophyll): ↓ CO₂ → Stomata öffnen

• Wasser (Wasserpotential): ↓ Wasserpotential → Stomata schließen (close)

How does C4 carbon fixation (C4-Kohlenstofffixierung) work in terms of CO₂ transport and fixation?

• CO₂ is fixiert im Mesophyll (mesophyll cells) → formation of Malat (malate)

• Malat wird transportiert (transported) to Bündelscheidenzellen (bundle sheath cells)

• In bundle sheath cells:

  • Malat → Pyruvat + CO₂ (Decarboxylierung)

• Released CO₂ is used for:

  • Calvin-Zyklus (Calvin cycle) Assimilation

• Purpose: CO₂-Konzentrierung → reduces Oxygenierung of RuBisCO

What are the main steps of C4 photosynthesis (C4-Photosynthese)?

• PEP Carboxylase (PEP-Carboxylase):

  • Phosphoenolpyruvat + HCO₃⁻ → Oxalacetat

• In Mesophyllzellen (mesophyll cells):

  • Oxalacetat → Malat (reduction)

• Transport to Bündelscheidenzellen (bundle sheath cells)

• In bundle sheath cells:

  • Malat → Pyruvat + CO₂ (oxidation / decarboxylation)

• Result:

  • CO₂-Anreicherung (CO₂ concentration increase) in bundle sheath cells

What are the differences in CO₂ concentration (CO₂-Konzentration) and enzyme properties in C3 vs C4 photosynthesis?

• PEP Carboxylase (PEP-Carboxylase):

  • no Oxygenase-Funktion

  • very high CO₂-Affinität

  • can fix CO₂ even at low ci (intrazelluläres CO₂)

• In C4 plants (C4-Pflanzen):

  • CO₂ is fixed in mesophyll → Malat formation

  • CO₂ is transported and konzentriert (concentrated) in Bündelscheidenzellen

  • acts as a “CO₂-Pumpe” (CO₂ pump)

• In C3 plants:

  • CO₂ fixation directly by RuBisCO

  • no CO₂ concentration mechanism → lower efficiency under low CO₂ / high O₂

What is the energy balance (Bilanz) of C3 vs C4 photosynthesis and when is each advantageous?

• C3 Photosynthese (C3 plants):

  • 3 ATP + 2 NADPH pro fixiertem CO₂

    • Photorespiration (Photorespiration) present → energy loss

• C4 Photosynthese (C4 plants):

  • 3 ATP + 4 NADPH pro fixiertem CO₂

  • keine Photorespiration (no photorespiration)

• Vorteil abhängig von Umweltbedingungen (environmental conditions):

  • hohe Temperatur + Wasserknappheit (low water) + niedrige CO₂-Konzentration → C4 vorteilhaft

  • kühle Temperaturen + ausreichendes Wasser + hohe CO₂-Verfügbarkeit → C3 vorteilhaft

What is the difference between obligate and facultative CAM plants (CAM-Pflanzen)?

• Obligate CAM plants (obligat):

  • always use CAM-Stoffwechsel

  • fixed diurnaler Säurerhythmus

  • CAM is permanent

• Facultative CAM plants (fakultativ):

  • can switch between C3 and CAM metabolism

  • CAM activated under Stressbedingungen (stress conditions):

    • Trockenheit (drought)

    • Salzstress (salinity)

  • C3 mode used under favorable conditions

• Key idea:

  • most CAM plants are facultative, not obligate

What is the diurnal acid rhythm (diurnaler Säurerhythmus) in CAM plants?

• Night (Nacht):

  • Stomata open (geöffnet)

  • Stärke → PEP (Abbau transitorischer Stärke)

  • PEP-Carboxylase (PEP Carboxylase): PEP + CO₂ → Oxalacetat → Malat

  • Malat stored in Vakuole (vacuole) → Ansäuerung (acidification)

• Day (Tag):

  • Stomata closed (geschlossen)

  • Malat → Pyruvat + CO₂ (Decarboxylierung)

  • CO₂ used by RuBisCO in Calvin-Zyklus (Calvin cycle)

• Regulation:

  • PEP-Carboxylase phosphoryliert (phosphorylated) during day

  • Inhibited by high Malat-Konzentration (malate concentration) in cytosol

What physical law governs the uptake of stomatäre CO2 Aufnahme (stomatal CO2 uptake)?

Fick’sches Diffusionsgesetz (Fick's Law of Diffusion), where uptake is a function of a CO2 gradient and resistance.

In the diffusion equation J{CO2} = DeltaCO2 x 1/r, what does Delta CO2 represent?

The Diffusionsgradient (diffusion gradient) between the CO2 partial pressure of the outside air (Außenluft, C_a) and the air inside the leaf (Blattinneres, C_i).

What is the name for the resistance r_a in the CO2 diffusion pathway?

Grenzflächenwiderstand (Boundary layer resistance), which is a function of leaf morphology and wind speed.

What is the name for the resistance r_s in the CO2 diffusion pathway?

Stomatärer Widerstand (Stomatal resistance).

What is the name for the resistance r_w in the CO2 diffusion pathway?

Zellwandwiderstand (Cell wall resistance).

What is the "Dilemma of Gas Exchange" regarding water loss?

For every 1 mole of CO2 taken up, the plant loses approximately 1000 moles of H_2O (Wasser).

How does the CO2 gradient compare to the H_2O gradient in a leaf?

The CO2 gradient is small (-100 ppm), while the H_2O gradient is massive (+12000 ppm).

Which ion's movement into the Schliesszellen (guard cells) regulates stomatal conductance?

Kalium-Ionen (K+ ions).

How do stomata react to an increase in Licht (light)?

The stomata open (öffnen) to allow CO2 uptake for photosynthesis.

How do stomata react to a decrease in CO2 concentration in the Mesophyll?

The stomata open to take in more CO2.

How do stomata react to a sinking Wasserpotential (water potential)?

The stomata close (schliessen) to prevent further water loss.

What does the acronym RuBisCO stand for?

Ribulose-1,5-Bisphosphat-Carboxylase-Oxygenase.

What are the two competing reactions catalyzed by RuBisCO?

Carboxylasereaktionen (fixing CO2) and Oxygenasereaktionen (fixing O_2).

What are the products of the Oxygenasereaktion catalyzed by RuBisCO?

One C3 molecule and one C2 molecule (2-Phosphoglykolat).

What is the primary goal of Photorespiration?

To recover the "lost" reduced carbon from the C2 molecule (2-phosphoglycolate) produced by RuBisCO's oxygenase activity.

Which three Zellorganellen (cell organelles) are involved in photorespiration?

Chloroplast, Peroxisom, and Mitochondrium.

What percentage of "lost" carbon is recovered during photorespiration?

75% of the reduced Kohlenstoff (carbon).

What is the energy cost of photorespiration per fixed O_2 molecule?

5 ATP and 3 NADPH.

Why does photorespiration increase with Temperatur?

RuBisCO's affinity for CO2 sinks, and CO2 Löslichkeit (solubility) decreases more rapidly than O_2 solubility at higher temperatures.

When RuBisCO evolved, why was the Oxygenasereaktion not a significant problem?

The atmosphere had very low oxygen and very high CO2 levels at that time.

What is a potential positive role of photorespiration during stress?

It can dissipate up to 30% of Excitonen Energie (exciton energy) when CO2 is low and electron acceptors are missing, preventing light damage.

What anatomical feature is characteristic of C4-Pflanzen?

Kranz-Anatomie (Kranz anatomy), featuring specialized bundle sheath cells (Bündelscheidenzellen).

Which enzyme performs the primary CO2 fixation in C4 plants?

PEP-Carboxylase (Phosphoenolpyruvat-Carboxylase).

What is the first stable product of CO2 fixation in C4 plants?

Malat (Malate) or Oxalacetat (Oxaloacetate), both of which are 4-carbon bodies.

How do C4 plants achieve a "CO2 Pump"?

CO2 is fixed in the mesophyll as malate, transported to bundle sheath cells, and released, concentrating CO2 near RuBisCO.

What is a major advantage of the high CO2 affinity of PEP-Carboxylase?

It allows C4 plants to keep their stomata more closed, drastically reducing water loss.

What does the acronym CAM stand for?

Crassulacean Acid Metabolism.

How do CAM-Pflanzen separate photosynthesis processes?

Through zeitliche Trennung (temporal separation): CO2 uptake at night and the Calvin cycle during the day.

Why do CAM plants open their stomata at Nacht (night)?

To take up CO2 when the Dampfdruckdefizit (vapor pressure deficit) is low, minimizing water loss.

Where do CAM plants store the CO2 (as malic acid) overnight?

In the Vakuole (vacuole).

What is the Diurnaler Säurerhythmus (diurnal acid rhythm) in CAM plants?

The acidification of the cell at night due to Äpfelsäure (malic acid) accumulation and its subsequent de-acidification during the day.

What is Sukkulenz (succulence) in the context of CAM?

The degree of CAM activity often depends on the leaf's succulence (water storage capacity).

Define Wassernutzungseffizienz (Water Use Efficiency - WUE).

The amount of Biomasse (biomass) in grams produced per kilogram (or liter) of transpired water.

Rank C3, C4, and CAM plants by their typical WUE from lowest to highest.

1. C3-Pflanzen (lowest)

2. C4-Pflanzen

3. CAM-Pflanzen (highest).

What environmental conditions favor C4-Pflanzen over C3?

Dry (Trocken), high temperatures, and low atmospheric CO2.

True or False: C4 and CAM photosynthesis evolved only once.

False. They evolved multiple times unabhängig (independently) in different plant lineages.

Name three important Agrarpflanzen (agricultural plants) that use C4 photosynthesis.

Mais (Corn), Zuckerrohr (Sugarcane), and Hirse (Millet).

Name three important plants that use CAM-Photosynthese.

Ananas (Pineapple), Agave (for Tequila), and Vanille (Vanilla).