Photosynthesis during light reactions can be measured using a fluorometer by detecting fluorescence in chlorophyll. Chlorophyll fluorescence: Chlorophyll absorbs light energy, mainly in the blu and red regions of the spectrum. However, not all absorbed light energy is used for photochemistry. A small portion of it is re-emitted as fluorrscence. By measuring the fluorescence we can estimate the efficiency of PSII.

For measuring the photosynthesis in dark reactions where the carbon fixiation takes place, gas exchange must be measured. -> uptake of CO2 and consumption of O2

Ideally -> measure both at the same time !

When a plant absorbs light, the chlorophyll molecules in Photosystem II have three different ways to use that absorbed energy.

• Photochemistry: using the nergy for photosynthesis

• Heat: releasing the energy safely as heat

• Fluorescence: re-emitting a small part of the energy as red light

Under normal, healty conditions, most of the absorbed energy is used for the photosynthetis.

Under stress, (such as drought, heat, cold, nutrient deficiency), the photosynthtic machinery becomes less efficient.

• the electron transport chain slows down.

• Less energy is used for CO2 fixiation.

• As a result, ore absorbed energy must be released in other ways.- as heat or fluorescence.

Changes in chlorophyll fluorescence directly indicate disturbances in the photosynthetic process.

Generally fluorescence tells us the maximum efficiency of PSII, non-photochemical quenching (indicates how much energy is dissipated as heat.

Chl a is the main pigment , responsible for absorbing red light (670-700 nm) and blue light (400-450nm) in the spectrum and excite electrons. (Also special pair of chl a molecules in the reaction center of photosystems)

• These excited electrons are transferred through a chain of proteins- this is how light energy is transformed into chemical energy.

• Porphyrin ring with a magnesium ion in the center

• It has a long hydrophobic tail (phytol chain) that anchors it into the thylakoid membrane.

Chl b: helps absorb additional wavelenghts ( mainly blue light around 455 nm) and transfers that energy to chl a.

accessory pigments:

• increase range of wavelenght that can be exploited.

• protection by energy dissipation

• carotenoids (carotenes and xanthophylls)-> 460nm - 550 nm

Main pathway of the light reactions of photosynthesis, where electrons flow from water to NADP+, producing ATP and NADPH- that energy is needed by plants for the calvin cycle (dark reactions)

1. Water splitting (Photolysis) :

   • Light excites PSII

   • Electrons are transferred to plastoquinone (PQ)

   • Water is split into O2, protons (H+) and electrons. (Oxygen in the athmosphere)

     
     

2. Electron transport through the chain:

   • Electrons move from PQ to cytochrome b6f complex -> plastocyanin (PC) -> PSI (oxidation of PQ is the slowest reaction of photosynthetis etwa 5 milisec)

   • Photon gradient is generated across the thylakoid membrane -> drives ATP synthetis

3. Excitation at PSII

   • Light excites electrons in PSI

   • Electrons are transferred to ferredoxin (FD) -> NADP+ reductase-> reduce NADP+ to NADPH

Main role of it is to capture light energy and transfer it efficiently to the reaction center of photosystem.

The photons are not directly absorbed by reaction center. Antenna molecules are arranged around those centers in complex arrays held in place by a protein superstructure.

Think of the antenna complex as a solar panel array that collects sunlight and funnels it to a central battery !

P-E Curve Photosynthetis rate to Irradiance ( Light Intensity)

Alpha : Slope of initial -> at low light , the curve starts almost as a straight line

• it shows how efficiently a plant can do photosynthesis under low light

• steeper (larger) alpha , the better the plant uses light

• if the slope is shallower, the plant is less effcient at low light

  
  

Shade-adapted plant: high alpha

Sun-adapted plant: lower alpha, slow photosynthetis at low light, but very fast in high light

Light Saturation Point: the light intensity at which photosynthesis starts to level off

Maximum Photosynthesis:

Photoinhibition: or photodamage - at very high irradiance , the rate drops due to light-induced damage.

1. Excitation: occurs when a molecule absorbs energy usually from light and an electron is promoted from a lower energy level to a higher energy level.

2. De-excitation: is how the excited molecule returns to its lower energy state. (ground state)

   • through fluorescence - loss of energy be emitting a red photon

   • heat : non specific loss of energy to neighbouring molecules as heat

   • resonance energy transfer : loss of energy by passing it to a neighbouring chlorophyll thereby exciting the other molecule

   • oxidation: for photosynthesis - loss of energy by removing one electron from chlorophyll a that then enters the electron chain

   Quencing: any process that dissipates energy from an excited molecule can be regarded as a quenching process

protective method for plants- thah helps protect plants from the damaging effect of excess light energy by dissipating it as heat rather than using it in photosynthesis. This process helps regulate the flow of energy within the Photosystem 2 of the plant and prevents the formation of harmful reactive oxygen species.

Xanthophyll cycle is a photoprotection mechanism in plants, algae, and some photosyntetic bacteria . It helps plants dissipiate excess light energy as heat, protecting the photosynthetic apparatus from damage.

The cycle involves three pigments

1. Violaxanthin

2. Antheraxanthin

3. Zeaxanthin

   
   

under high light: Enzym Violaxanthin de-epoxidase, VDE converts violaxanthin in to zeaxanthin. Function of ZX: enhances thermal dissipation of excess energy in the antenna of PS2, and acts as antioxidant in the thylakoid membrane.

PSBS: is a special protein located in the PS2 light harvesting complex ( LHC2) It is not a pigment. It is a sensor that activates qE.

• Under high light, protons acumulate in the thylakoid lumen -> lumen becomes acidic.

• PSBS becomes protonated.

• changes its conformation.

• Structure of antenna changes also.

• more effective heat dissipation.

A reaction center is open, when it is ready to accept a new electron. The primary electron acceptor QA is oxidized. The reaction center can perform photochemistry. It can use light to move an electron.

A reaction center is closed when it cannot accept another electron.

Beacuse the electron acceptor is already reduces. The reaction center is busy- it has already absorbed light and sent an electron forward, but the electron hasnt been passed further yet. Fluorescence is at max. Beacuse energy can notbe quenched via reduction of QB. Excitation energy is re-emitted as fluorescence.

Linear: main pathway of photosynthetic electron transport

• Electrons move a line from H2O -> PSII -> PSI -> NADP+

• Produces ATP

• Produces NADPH

• Releases O2

• drives the Calvin cycle

Cyclic Electron Transport: (CET)

• Electrons flow in a cycle around PSI

• PSI -> cyctochrome b6f -> back to PSI

• Produces only ATP

• does NOT produce NADPH

• does NOT release O2

• Plants use it to make extra ATP when th plant needs more ATP than NADPH

• important under stress

• PGR5 - importsnt protin

Pseudocyclic Electron Transport (PET)

• Electrons are transfered from PSI directly to oxygen O2 instead of NADP+

• Electrons from PSI reduce O2 -> becomes superoxide (o2-) -> converted to H2O2 -> than to H2O

• consumes electrons

• Protects PSI from over-reduction.

• Produces ATPProduces reactive oxygen species (ROS) -> detoxified by enzymes (superoxide dismutase, peroxidase)

It is a part of pseudocyclic electron transport.

• PSI becomes highly reduces (too many electrons)

• Instead of giving electrons to NADP+, PSI gives them to O2.

• Oxygen is reduced to superoxide (O2-)

• Superoxide is quickly converted to H₂O₂ (hydrogen peroxide).

• Antioxidant enzymes (SOD, APX, catalase) detoxify H₂O₂ to water (H₂O).

• So the electron flow looks like this:

  PSI → O₂ → O₂⁻ → H₂O₂ → H₂O

• Plants use the Mehler reaction to:

  • Remove excess electrons from PSI

  • Prevent over-reduction of electron carriers

  • Avoid photodamage under strong light

  • Support lumen acidification → helps activate NPQ

High light: ETR becomes over reduced - Too much light energy is absorbed. But the calvin cycle can nıt use all that energy.

Low Temp: dark reaction enzymes are slowed down -> electrons can not be processed fast enough

High temp: Enzymes get damaged or change their catalytic activity.

Water stress: stomates close -> co2 becomes limited -> no electron sink available anymore.

Backlash of electrons in the ETC - end result of these factors -> light energy can not be used adequately -> electrons spill over -> Formation of ROS

Reactive oxygen species (ROS) are highly reactive forms of oxygen that are produced as by-products of metabolism—especially during photosynthesis and respiration.

They are normal, unavoidable, and even useful at low levels. But at high levels they can become toxic and damage cells.

🌬️ Main types of ROS

The most important ROS in plants are:

1. Superoxide (O₂⁻)

A radical formed when PSI transfers electrons to oxygen (e.g., during the Mehler reaction).

2. Hydrogen peroxide (H₂O₂)

Not a radical, but still reactive. It can diffuse across membranes.

3. Hydroxyl radical (•OH)

Extremely reactive and damaging; formed from H₂O₂ in the presence of metals (Fenton reaction).

4. Singlet oxygen (¹O₂)

Produced when excess energy in PSII cannot be dissipated (e.g., low NPQ).