Light as a source of energy

1.1. What is Light? The Nature of Electromagnetic Radiation

Light is a form of radiant energy, a narrow band in the continuous electromagnetic spectrum of radiation emitted by the Sun (Hopkins & Hüner, 2009). But what does this mean from a physics perspective?

Light has a dual nature. It behaves both as a wave and as a stream of particles (Taiz et al., 2023).

Light as a wave is characterized by its wavelength — the distance between adjacent wave crests. Wavelength is denoted by the Greek letter λ (lambda) and measured in nanometers (nm) or micrometers (μm). Frequency (ν) is the number of crests passing a point in space per second. These quantities are related by the fundamental equation:

where c is the speed of light (3 × 10⁸ m·s⁻¹).

Light as a stream of particles consists of photons. Each photon carries a specific amount of energy — a quantum. According to Planck's law, the energy of a photon is directly proportional to its frequency and inversely proportional to its wavelength:

where h is Planck's constant (6.626 × 10⁻³⁴ J·s).

This is a key relationship for our topic. The shorter the wavelength, the higher the photon energy, and vice versa (Hopkins & Hüner, 2009; Taiz et al., 2023).

Let's remember this: violet light (short wavelength) carries more energy per photon than red light (long wavelength).

1.2. The Solar Spectrum and its Usable Portion

The Sun emits electromagnetic waves across a vast range: from hard ultraviolet to radio waves. However, the Earth's atmosphere is not just a transparent balloon. It filters solar radiation (Schopfer & Brennicke, 2016).

• Short-wavelength ultraviolet radiation (λ < 300 nm) is almost entirely absorbed by the ozone layer in the stratosphere. If this radiation reached the surface, it would ionize molecules and destroy biological structures (Hopkins & Hüner, 2009).
• Long-wavelength infrared radiation (λ > 700 nm) is actively absorbed by water vapor and carbon dioxide in the atmosphere. Its primary effect is thermal: it enhances the vibrational movements of molecules.

As a result, only part of the solar spectrum reaches the Earth's surface. And only its narrow segment can be used for photosynthesis — this is Photosynthetically Active Radiation (PAR).

1.3. What is PAR and Why is Only This Part of the Spectrum Used?

PAR (Photosynthetically Active Radiation) is the wavelength range of electromagnetic radiation from 400 to 700 nm (Connor et al., 2011; Hopkins & Hüner, 2009; Lambers & Oliveira, 2019). In this range, the photon energy is sufficient to excite electrons in pigment molecules but insufficient to destroy the molecules.

Why only this narrow range?

The answer lies in the quantum nature of light-matter interaction. For a molecule to absorb light, the photon energy must exactly match the energy difference between electronic levels in that molecule (Taiz et al., 2023).

• Infrared photons have too little energy. They can only enhance the vibrations and rotations of atoms in molecules, manifesting as heat. This energy is insufficient to "boost" an electron to a higher energy level.
• Ultraviolet photons, on the contrary, are too energetic. They can knock an electron out of a molecule, causing ionization, leading to the breaking of chemical bonds and the formation of dangerous free radicals.
• Photons of visible light (400–700 nm) hit the "sweet spot": their energy is enough to transfer an electron to an excited state — to a higher orbital — while maintaining the integrity of the molecule (Hopkins & Hüner, 2009).

Thus, evolution "selected" for photosynthesis precisely those pigments capable of capturing photons with energies in this range — from approximately 180 to 300 kJ per mole of photons.

1.4. Photon Energetics: Why Red and Blue Light are Key

Let's translate photon energy into units we are more familiar with.

Let's take two extreme values of PAR:

Calculation according to the formula E = hc/λ

We can see that a blue-violet photon carries almost twice the energy of a red photon. However, for the photosynthetic apparatus, this difference is not as significant as it might seem. The energy gained from absorbing a blue photon is quickly "dissipated" as heat, and the electron ends up at the same lower excited level as when absorbing a red photon (Taiz et al., 2023; Schopfer & Brennicke, 2016).

An important practical conclusion: for photosynthesis, the energy of each individual photon is less critical than the number of photons that a plant can absorb. Therefore, when assessing the light regime for plants, one uses not energy (watts) but photon flux — the number of photons falling per unit area per second (μmol·m⁻²·s⁻¹ or mol·m⁻²·s⁻¹) (Connor et al., 2011; Hopkins & Hüner, 2009). This quantity is called Photosynthetic Photon Flux Density (PPFD).

A huge flow of energy falls on the Earth's surface daily. However, the efficiency of its use by plants is low. Even under ideal conditions, no more than 5% of the total incident solar energy is stored in chemical energy, and in real agroecosystems, even less, about 1% (Connor et al., 2011).

1.5. PAR and the Energy Threshold of Photosynthesis: Why is This Specific Energy Needed?

Why does initiating photosynthesis require energy in precisely this amount? Let's recall the overall equation for photosynthesis:

This reaction is endergonic, meaning it requires an input of energy. The standard change in free energy for forming one molecule of glucose is about +2870 kJ (Taiz et al., 2023), or +470 kJ per mole of CO₂ (Connor et al., 2011).

The energy of one mole of red photons is about 170 kJ. To "cover" the energy cost of fixing one CO₂, at least 470 kJ is needed. This is the energy of roughly 3 photons with a wavelength of 700 nm (3 × 170 ≈ 510 kJ). But the actual process requires more — about 8 photons per O₂ molecule and 10 photons per CO₂ molecule.

Some energy is inevitably lost:

• Losses during energy transfer in the antenna.
• Losses during relaxation from higher excited states (blue light → heat).
• Energy costs for creating the proton gradient (Taiz et al., 2023).

Nevertheless, the quantum yield of the photochemical reaction can be surprisingly high — nearly 100% under optimal conditions, where every absorbed photon initiates the transfer of one electron (Taiz et al., 2023).

1.6. Units of Measuring Light and Light for Plants

We come to an important practical question. How to measure light correctly?

In science and practice, different approaches are used, and it is important to understand their differences.

1. Energy-based measurement (radiant flux)

• Units: J·m⁻²·s⁻¹ or W·m⁻².
• Principle: Measures the total radiation energy falling on a surface (e.g., using a thermopile).
• Problem: This method includes infrared radiation and other waves not involved in photosynthesis. This is not informative for plant physiology.

2. Photon-based measurement (quantum flux)

• Units: mol photons·m⁻²·s⁻¹ or μmol photons·m⁻²·s⁻¹.
• Principle: Counts not the energy but the number of photons in the PAR range (e.g., using specialized quantum sensors with filters).
• Why this is better for physiology: Photosynthesis primarily depends on the number of absorbed photons, not their total energy. A plant is a "photon counter," not a "joule counter" (Hopkins & Hüner, 2009; Taiz et al., 2023).

Consider this: one photon at 400 nm and one photon at 700 nm trigger the same photochemical act. Their energy is different, but their biological effect is the same.

3. Photometric units (lux, lumens)

• Principle: Measures the brightness of light as perceived by the human eye. The spectral sensitivity of the eye is shifted towards the yellow-green region, where photosynthesis is weakest.
• Why this is bad for plants: Lux meters do not reflect the photosynthetic value of light at all. For the same light source, one can roughly convert lux to PPFD, but for different sources (sun, incandescent lamp, LED), this ratio will differ. Therefore, lux is not used for working with plants — only μmol·m⁻²·s⁻¹.

Summary. Key Ideas of the First Part

Today we laid the foundation for understanding how plants convert light into chemical energy.

1. Light is a form of energy. Plants use photons from the visible part of the spectrum.

2. PAR (400–700 nm) is the "working range" of photosynthesis. The energy of PAR photons is large enough to excite electrons but insufficient to destroy molecules.

3. For plant physiology, the number of photons is critically important, not their total energy. Therefore, we measure photon flux (PPFD) in μmol·m⁻²·s⁻¹.

4. Plant evolution is "tuned" to capturing photons. Even at low light levels, specialized antenna pigments collect light and transfer it to reaction centers.

In the next lecture, we will move on to answering the question: "How does a plant collect light?" We will get acquainted with chlorophylls, carotenoids, and antenna complexes — unique molecular structures that allow the plant to use available light most efficiently.

2.1. The Discovery of the Photosynthetic Unit: The Emerson and Arnold Experiment

In 1932, American researchers Robert Emerson and William Arnold conducted an experiment that forever changed the understanding of photosynthesis (Taiz et al., 2023). They illuminated a suspension of the green alga Chlorella with very short flashes of light — only 10⁻⁵ seconds. Such a flash was so short that each reaction center could receive only one photon and could not complete the cycle before the next flash. The scientists varied the flash intensity and measured the amount of oxygen evolved.

The result was astonishing. At very weak flashes, oxygen was released proportionally to the amount of light absorbed — each photon worked. However, with increasing flash intensity, saturation occurred: further increasing light did not lead to an increase in O₂ evolution. Calculations showed that at the saturation point, one molecule of oxygen was released for approximately every 2500 molecules of chlorophyll in the sample.

What did this mean? Clearly, the release of one O₂ molecule requires not one chlorophyll molecule but the work of a collective of many pigments that collect light and transfer energy to a single center where the photochemical act occurs. This collective was named the photosynthetic unit (Schopfer & Brennicke, 2016).

Modern research has refined this: in higher plants, there are between 200 and 300 chlorophyll molecules per reaction center, and in some algae, up to several thousand (Taiz et al., 2023). Of these, only one or two chlorophyll molecules are photochemically active — it is in them that primary charge separation occurs. All the others work as an antenna, capturing and transmitting energy.

2.2. Pigments — The "Catchers" of Photons

To collect light, a plant needs molecules capable of absorbing photons in the visible region. Such molecules are called pigments. In higher plants, we encounter three main classes of photosynthetic pigments: chlorophylls, carotenoids, and (in some groups) phycobilins. We will focus on the first two.

2.3. Antenna (Light-Harvesting) Complexes: The Architecture of Light Collection

Now that we know the main pigments, we need to understand how they are organized in space to transmit energy efficiently.

In thylakoid membranes, pigments do not float freely. They are bound to proteins, forming pigment-protein complexes. These complexes constitute the antenna (Light-Harvesting Complexes — LHC). This organization is critically important: proteins hold the pigments at a strictly defined distance (2–3 nm) and at the right angle, which is necessary for rapid energy transfer (Schopfer & Brennicke, 2016; Taiz et al., 2023).

Several types of antenna complexes are distinguished in higher plants:

• Inner (core) antennas — directly associated with the reaction centers. In Photosystem II, these are proteins CP43 and CP47, containing about 30 chlorophyll a molecules. In Photosystem I, the inner antenna is built into the reaction center proteins themselves (PsaA/PsaB) and contains about 90 chlorophyll a molecules (Medvedev, 2012).
• Outer (peripheral) antennas — are the most abundant. These are LHCII complexes (for Photosystem II) and LHCI (for Photosystem I). They bind the majority of chlorophyll b and carotenoids.

LHCII is the most common antenna complex in nature. It consists of three identical protein subunits (trimer), each containing 14 chlorophyll molecules (8 chlorophylls a and 7 chlorophylls b) and 4 carotenoids (Taiz et al., 2023). The total number of pigments in such a trimer is 42 chlorophylls and 12 carotenoids. This is a true "light harvester." Notably, LHCII can migrate between photosystems, redistributing energy depending on illumination (more on this in the regulation section).

2.4. How Energy Transfer Occurs: From Photon to Reaction Center

So, a photon is absorbed by a pigment in the antenna. What next?

Excitation energy does not stay in place. Due to the close packing of pigments and the overlap of their absorption and fluorescence spectra, excitation migrates from molecule to molecule. This phenomenon is called inductive resonance energy transfer (or Förster resonance) (Schopfer & Brennicke, 2016; Taiz et al., 2023).

The mechanism is simple: the excited electron creates a changing electric field, which induces electron oscillations in the neighboring acceptor molecule. Transfer occurs without electron transfer — only energy moves. The speed of this process is picoseconds (10⁻¹² s), which is thousands of times faster than fluorescence or thermal dissipation.

Key principle of directionality: energy moves from pigments with shorter-wavelength absorption (higher excitation energy) to pigments with longer-wavelength absorption (lower energy). This creates an energy gradient — a "funnel" down which energy flows to the reaction center (Taiz et al., 2023). In the reaction center are special pairs of chlorophyll a molecules, absorbing at the longest wavelengths — P680 (Photosystem II) and P700 (Photosystem I). They serve as energy "traps."

The efficiency of energy transfer in the antenna is exceptionally high — 95–99% of absorbed photons reach the reaction center (Taiz et al., 2023). It is precisely because of this organization that the photosynthetic unit works as a single "quantum device," where individual photons falling on different pigments combine their energy to initiate a single photochemical event.

2.5. Why Must the Antenna be Flexible?

We have already mentioned that LHCII can move from Photosystem II to Photosystem I. This is not coincidental. The plant faces a problem: the intensity and spectral composition of light constantly change (cloud cover, time of day). If one photosystem complex receives more light than the other, an imbalance occurs. To avoid this, there is a mechanism of state transitions. When there is excess energy on Photosystem II, part of LHCII is phosphorylated, detaches from PS II, and attaches to PS I, balancing the load (Taiz et al., 2023). This is a fine regulation at the antenna level, which we will discuss further in the context of protection.

What We Learned Today About Light Collection?

1. The plant does not rely on individual chlorophyll molecules. They are united into photosynthetic units, where hundreds of pigments work as a collective antenna.

2. Chlorophylls (a and b) and carotenoids together cover the main part of the visible spectrum. Carotenoids also serve as protection against photooxidation.

3. Pigments are organized into antenna complexes (LHC) — rigid pigment-protein structures where the distance and orientation of pigments are optimized for ultrafast energy transfer.

4. Energy migrates down the energy funnel to the reaction center (P680 or P700) with over 95% efficiency. The transfer speed is picoseconds, allowing it to outcompete losses from fluorescence and heat.

5. The antenna can rearrange, redistributing energy between photosystems depending on illumination.

Now that we know how a plant collects light, the next question arises: "Where does this energy go?" How is it transformed into chemical form? This is the subject of the next part of our lecture.

3.1. What is a Reaction Center? The Special Pair of Chlorophylls

In each photosynthetic unit, for every hundreds of antenna molecules, there is only one pair of special chlorophyll a molecules that act as an energy "trap." Unlike all other molecules, they do not fluoresce — they are capable of a photochemical reaction (Schopfer & Brennicke, 2016). This pair of chlorophylls is called the reaction center or trap pigment (Schopfer & Brennicke, 2016).

In higher plants, there are two types of such centers, each with its characteristic absorption spectrum:

• In Photosystem I, the reaction center absorbs at 700 nm — it is called P700 (from Pigment, 700 nm).
• In Photosystem II — at 680 nm — P680 (Taiz et al., 2023; Medvedev, 2012).

These chlorophylls are not just individual molecules but dimers — two chlorophyll a molecules closely associated with each other and with surrounding proteins. This specific structure provides the necessary energy level and the ability for primary charge separation. It is important to understand: the energy transferred from the antenna reaches precisely these molecules, exciting one of their electrons.

3.2. The Primary Photochemical Act: Charge Separation

What happens when the excited energy reaches P680 or P700?

The chlorophyll molecule in the reaction center transitions to an excited state (P). In this state, it becomes a very strong reductant — its electron is at a high energy level and can easily be transferred to another molecule. This transfer occurs within a few picoseconds* (10⁻¹² s) — incredibly fast, much faster than possible energy loss through fluorescence or heat (Taiz et al., 2023).

Here is the sequence of events (Hopkins & Hüner, 2009; Taiz et al., 2023):

1. P* donates an electron to the nearest primary acceptor. In Photosystem II, the primary acceptor is pheophytin — a molecule similar to chlorophyll but without the magnesium atom (replaced by two hydrogen atoms). In Photosystem I, the primary acceptor is a special chlorophyll a molecule (A₀).

2. As a result, the reaction center chlorophyll is oxidized (loses an electron) becoming P⁺ (positively charged cation radical), and the acceptor is reduced (gains an electron) — A⁻.

3. This is primary charge separation: the photon energy is converted into the energy of separated electrical charges. A system with very high energy is formed, which is then stabilized as the electron moves along the chain of carriers, and the positive charge P⁺ is filled by an electron from a donor (Taiz et al., 2023).

Key moment: it is here that light energy first becomes chemical energy — in the form of an electrochemical potential difference. It is analogous to charging a battery.

3.3. Stabilization of Charge Separation: The Chain of Carriers

Separated charges are very unstable. The electron could easily return to P⁺, and all the energy would be lost as heat or fluorescence. To prevent this, evolution created a system of sequential carriers that gradually "move" the electron away from the reaction center, making the reverse process increasingly unlikely (Taiz et al., 2023).

In Photosystem II, the electron transferred to pheophytin then sequentially passes to:

• The first bound plastoquinone (QA) — this carrier accepts only one electron and acts as a relay.
• The second bound plastoquinone (QB) — this one can accept two electrons and two protons, being converted into plastoquinol (QH₂). This is a crucial carrier that leaves the reaction center and transfers electrons in the membrane to the next complex (the cytochrome complex) (Taiz et al., 2023; Medvedev, 2012).

In Photosystem I, the electron passes through the chain: A₀ (chlorophyll) → A₁ (phylloquinone, or vitamin K₁) → iron-sulfur proteins (FX, FA, FB) → ferredoxin — a small water-soluble protein that then transfers electrons to NADP⁺ (Taiz et al., 2023; Medvedev, 2012).

3.4. Two Photosystems — Two Steps of the Energy Staircase

We now come to one of the central discoveries in the history of plant physiology: oxygenic photosynthesis requires two photosystems working in sequence (Taiz et al., 2023; Schopfer & Brennicke, 2016). This was proven in a series of experiments by Robert Emerson (the Emerson enhancement effect), which we have already mentioned.

What is the difference?

• Photosystem II (P680) — acts as a "water-oxidizing" unit. Its oxidized P680⁺ is the strongest biological oxidant (potential around +1.1 V). It can "strip" electrons from water, splitting it into oxygen, protons, and electrons. This is the first stage of raising electrons to a high energy level.
• Photosystem I (P700) — acts as a "reductive" unit. Its excited P700* is a very strong reductant (potential around -0.6 V). It can donate electrons to ferredoxin, which then reduces NADP⁺ to NADPH.

In other words, two photochemical acts act as two pumps, sequentially raising electrons to increasingly higher energy levels. Between them is a chain of intermediate carriers, which also creates a proton gradient for ATP synthesis (discussed in the next lecture).

Schematically:

H₂O → (PS II) → plastoquinol → cytochrome complex → plastocyanin → (PS I) → ferredoxin → NADPH

3.5. Spatial Organization: PS II in Grana, PS I in Stroma

It is important to note that these two complexes are not only functionally distinct — they are physically separated in the thylakoid membrane (Taiz et al., 2023; Medvedev, 2012).

• Photosystem II is predominantly localized in the stacked (granal) regions of the thylakoid membranes — where the membranes are tightly appressed, forming grana.
• Photosystem I and ATP synthase are located in the unstacked (stromal) regions, as well as in the marginal zones of the grana.
• The Cytochrome b₆f complex is more evenly distributed but is often concentrated at the boundary between the regions.

This separation is not accidental: it allows for the optimization of electron and proton flows and creates opportunities for mobile carriers (plastoquinol, plastocyanin) to transfer electrons between distant photosystems. It is also important for regulation — upon changes in illumination, the LHCII antenna complex can move between photosystems, as we mentioned.

3.6. What Does Charge Separation Give the Plant?

So, we have traced the path of energy from photon to charge separation in the reaction center. What does this give the plant in physiological terms?

1. Creation of reducing equivalents — NADPH, which will be used in the Calvin cycle for CO₂ fixation.

2. Creation of a proton gradient (along with electron transfer) — this is energy for ATP synthesis.

3. Oxidation of water — this is the only biological source of molecular oxygen on the planet.

All of this becomes possible only because the photon energy is converted into the energy of separated electrons and holes, which is then used to drive the complex electron transport chain. Photosystems can be compared to miniature solar cells that generate an electric current, but in a biological package, where the "current" is a flow of electrons and the "voltage" is the difference in redox potentials.

Summary of Part 3

1. Energy collected by the antenna is directed to the reaction center — a special pair of chlorophylls (P680 or P700), which is the only photochemically active molecule in the photosynthetic unit.

2. Charge separation occurs: the excited chlorophyll (P*) donates an electron to the primary acceptor (pheophytin or A₀). This results in P⁺ and acceptor⁻ — this is the conversion of light energy into electrical (chemical) energy.

3. Charge separation is stabilized through the sequential transfer of the electron along a chain of carriers (plastoquinones, iron-sulfur proteins), making back-recombination unlikely.

4. There are two photosystems operating in sequence (in the order PS II → PS I). PS II is responsible for water oxidation, PS I for NADP⁺ reduction. This allows two light quanta to raise electrons to different energy levels.

5. Photosystems are physically separated in the membrane: PS II in grana, PS I in stromal thylakoids. This ensures directed electron and proton flow and allows for regulation.

6. Outcome of primary processes: creation of reducing power (NADPH) and a proton gradient — the two main forms of stored energy, later used for the synthesis of organic substances.

Now that we know how light energy is converted into the energy of separated charges and how the two photosystems work in concert, we are ready to move on to the next, culminating stage: how this energy is used to synthesize ATP and NADPH, and what the Z-scheme of photosynthesis is. This will be the concluding part of our introductory lecture.

4.1. The Energy Staircase: What Does the Z-Scheme Show?

You have probably heard of the Z-scheme of photosynthesis. It is not just a pretty picture — it is a fundamental physiological diagram showing how light energy is converted into the energy of chemical bonds.

The Z-scheme is a graph where the horizontal axis shows the stages of electron transfer, and the vertical axis shows the redox potential (in volts) (Taiz et al., 2023). The lower on the graph, the higher the reducing ability (more negative potential). Electrons move from donors (with lower potential) to acceptors (with higher potential), but are twice raised upwards by light energy — as if electrons climb a staircase and then roll down, releasing energy (Schopfer & Brennicke, 2016).

Here are the main stages of the Z-scheme (Taiz et al., 2023; Medvedev, 2012):

1. Photosystem II (P680) receives energy from a photon. Excited P680* becomes a very strong reductant (potential about –0.6 V) and donates an electron to pheophytin. This is the first "lift" of the electron to a high energy level.

2. Oxidized P680⁺ (potential about +1.1 V) is reduced by stripping electrons from water — this is where water photolysis occurs and oxygen is released.

3. The electron from pheophytin is transferred along a chain of carriers: plastoquinones → cytochrome b₆f complex → plastocyanin. In this section, the electron's energy is partially dissipated, and it "rolls down" to a potential of about +0.4 V. However, the released energy is used for pumping protons across the membrane.

4. Photosystem I (P700) receives a second quantum of light. Excited P700* (potential about –0.6 V) donates an electron to ferredoxin, and then to NADP⁺. This is the second "lift" of the electron.

5. The electrons, now "lifted" twice by light, possess sufficient reducing power to reduce NADP⁺ to NADPH.

The Z-shape of the diagram (resembling the letter Z) arises precisely because electrons are lifted upwards twice (by two photons), and between these lifts, they descend, releasing part of their energy (Taiz et al., 2023).

4.2. How is the Proton Gradient Created?

Now let's analyze what happens in the section between the photosystems. Three mechanisms work together to create the proton gradient — the basis for ATP synthesis (Taiz et al., 2023; Medvedev, 2012):

1. Water photolysis (in PS II). Splitting two water molecules releases 4 protons (H⁺) and 4 electrons. Protons are released directly into the lumen (the inner space of the thylakoid). This already creates local acidity.

2. Operation of plastoquinones (Q-cycle). When plastoquinone (PQ) is reduced to plastoquinol (PQH₂) on the stromal side, it picks up two protons from the stroma. Plastoquinol then moves to the cytochrome complex and is oxidized, releasing these two protons into the lumen. Moreover, due to the Q-cycle, for every pair of electrons transferred to Photosystem I, four protons enter the lumen (Taiz et al., 2023).

3. Additional contribution: during electron transfer from plastoquinol to the cytochrome complex, proton transfer across the membrane also occurs (due to the operation of the complex itself).

In total, for each pair of electrons passing from water to NADP⁺, between 6 and 8 protons are pumped into the lumen (Taiz et al., 2023). Outside (in the stroma), protons become fewer, and inside, more. A ΔpH (pH difference) and an electrical potential (the lumen becomes positively charged relative to the stroma) arise. Together, these create a proton motive force (pmf) — approximately –180…–200 mV (Schopfer & Brennicke, 2016). This is the energy that will then be used for ATP synthesis.

4.3. ATP Synthase: How Proton Flow Generates ATP

The proton gradient is like a "waterfall" or a "water tower." But how is this potential energy converted into the chemical energy of a phosphate bond?

This is where ATP synthase (or CF₀-CF₁ complex) comes into play — an astonishing molecular motor embedded in the thylakoid membrane (Taiz et al., 2023; Medvedev, 2012). It consists of two parts:

• CF₀ — a channel portion spanning the membrane. Protons can pass through it from the lumen to the stroma, following the gradient.
• CF₁ — a catalytic "head" protruding into the stroma. This is where ATP is synthesized from ADP and inorganic phosphate (Pi).

The mechanism works as follows: protons rushing from the lumen (where they are abundant) to the stroma (where they are scarce) pass through CF₀. This movement causes rotation of part of CF₀ (a ring of subunits), which is transmitted to the inner "stalk" of CF₁ (the γ-subunit). The rotation changes the conformation of the catalytic centers on the β-subunits of CF₁, forcing them to bind ADP + Pi, synthesize ATP, and then release the completed molecule (Taiz et al., 2023; Medvedev, 2012). The entire process is like a microscopic hydroelectric turbine, where a flow of protons drives a molecular rotor, producing ATP.

How many protons are required to synthesize one ATP molecule? According to current estimates, about 4–5 protons per ATP (Taiz et al., 2023). This means that for one full rotation of the rotor (14 subunits), approximately 3 ATP molecules are synthesized. The efficiency is not absolute but is sufficient for the cell's needs.

Outcome: the energy of the proton gradient is converted into the energy of the ATP macroergic bond. This is a universal energy carrier, which will then be used in the Calvin cycle and in many other processes (protein synthesis, ion transport, etc.).

4.4. NADPH — Reducing Power

In parallel with ATP synthesis, NADPH is formed in Photosystem I. This is another form of stored energy, but not as a phosphate bond, but as reducing potential.

The process is simple: electrons passing through PS I and reaching ferredoxin are transferred to ferredoxin-NADP⁺ reductase — an enzyme located on the outer side of the membrane (in the stroma). This enzyme transfers two electrons and one proton to NADP⁺, converting it to NADPH + H⁺ (Taiz et al., 2023; Medvedev, 2012).

NADPH is an "energetic" reductant that will be used in the Calvin cycle to reduce carbon dioxide to carbohydrates. Without NADPH, CO₂ fixation is impossible: not only ATP for energy expenditure is needed, but also hydrogen to reduce carboxyl groups. Thus, Photosystem I produces reducing equivalents, while Photosystem II and the proton gradient produce ATP.

4.5. ATP and NADPH Ratio: Balancing the Cell's Needs

It is important to understand that the amount of ATP and NADPH produced by the light stage is roughly balanced with the needs of the Calvin cycle and other metabolic pathways. For each CO₂ molecule fixed in the Calvin cycle, approximately 3 ATP and 2 NADPH molecules are required (Connor et al., 2011). In standard linear (non-cyclic) electron transport, for each pair of electrons forming one NADPH molecule, roughly 1.5–2 ATP molecules are synthesized (Taiz et al., 2023). This is close to the requirements of the Calvin cycle.

However, in some cases, the plant needs more ATP than linear transport provides. For example, during active synthesis or under stress. Then, cyclic electron transport around PS I is activated: electrons do not go to NADP⁺ but are returned to plastoquinone, passing through the cytochrome complex and creating an additional proton gradient, but without producing NADPH (Taiz et al., 2023). This allows the plant to additionally produce ATP without changing the amount of NADPH — a flexible mechanism for regulating the energy balance.

4.6. Overall Outcome of the Light Stage

Let's put everything together. The following key events occur during the light stage of photosynthesis (Connor et al., 2011; Taiz et al., 2023):

1. Absorption of light by antenna complexes and transfer of energy to the reaction centers.

2. Primary charge separation in PS II and PS I, creating two "lifts" of electrons to high energy levels (Z-scheme).

3. Water oxidation in PS II with the release of O₂ and protons.

4. Electron transfer along the chain of carriers, coupled with the creation of a proton gradient (proton pumping into the lumen).

5. ATP synthesis due to the backflow of protons through ATP synthase (photophosphorylation).

6. Reduction of NADP⁺ to NADPH by electrons from PS I.

As a result, light energy is converted into two forms of chemical energy: ATP (energy carrier for most synthetic processes) and NADPH (reductant for the synthesis of organic substances). These molecules, along with carbon dioxide, will participate in the dark phase — the Calvin cycle — where carbohydrates will be synthesized from inorganic carbon.

Summary of Part 4

1. The Z-scheme is an energy diagram showing two sequential light-driven "lifts" of electrons (in PS II and PS I) and one intermediate "descent," during which energy is released to create the proton gradient.

2. The proton gradient is created by: a) water photolysis (H⁺ into the lumen), b) plastoquinone operation (transfer of H⁺ into the lumen), c) cytochrome complex operation.

3. ATP synthase is a molecular motor using proton flow to synthesize ATP from ADP and Pi.

4. NADPH is formed from electrons from PS I reducing NADP⁺.

5. The ATP/NADPH ratio is regulated, including through cyclic electron transport, allowing flexible adaptation to the cell's needs.

6. The light stage culminates in the creation of two key forms of energy — ATP and NADPH — which will be used in the dark stage for CO₂ fixation.

We have completed the logical journey from photon to ATP and NADPH. In the next, fifth and final part of the lecture, we will discuss why the photosystem can become overloaded, what photoinhibition is, and how the plant protects itself from excess light — this is a natural conclusion to the whole story, linking physics, chemistry, and physiology into a coherent whole.

5.1. Why Does an Excess of Light Energy Occur?

Photosynthesis is a sequence of reactions working in series. The speed of the whole process is determined by the slowest step. And there are several slow steps: Rubisco activity (carboxylation), regeneration of ribulose bisphosphate, and the rate of assimilate export from the leaf (Connor et al., 2011; Medvedev, 2012).

When light falls on a leaf, the antennas absorb photons at a high rate. However, if downstream enzymatic reactions (Calvin cycle, sucrose transport) cannot keep up with using ATP and NADPH, a "bottleneck" occurs in the electron transport chain — oxidized NADP⁺ becomes scarce, plastoquinones become over-reduced (saturated with electrons), and electron flow slows down (Taiz et al., 2023; Lambers & Oliveira, 2019).

What happens to the excited chlorophyll if it cannot pass its electron further down the chain? It must somehow get rid of the excess energy. If this does not happen, the excitation can transition to a triplet state, which is very dangerous: it can interact with molecular oxygen (mostly triplet, ³O₂) and generate singlet oxygen (¹O₂) — a highly reactive form capable of oxidizing unsaturated fatty acids in membranes, proteins, and chlorophylls (Hopkins & Hüner, 2009; Medvedev, 2012). This is photooxidation — irreversible damage to the photosynthetic apparatus.

Thus, excess light is not dangerous in itself, but in combination with the cell's insufficient capacity to use the products of the light reaction (CO₂ deficiency, low temperature, drought, nutrient deficiency, especially N, Mg, K) (Connor et al., 2011; Medvedev, 2012). Under these conditions, even moderate light becomes "excess."

5.2. Two Types of Photoinhibition: Dynamic and Chronic

The term "photoinhibition" describes the decrease in photosynthetic efficiency under excess light (Lambers & Oliveira, 2019). But this is not always a catastrophe. Two main types are distinguished:

1. Dynamic photoinhibition — a reversible, rapid decrease in quantum yield that occurs under high light but recovers within minutes or hours upon return to normal conditions. This is a protective mechanism, part of normal physiology. For example, the activation of protective mechanisms (dissipation of energy as heat) leads to a temporary decrease in efficiency but prevents damage (Lambers & Oliveira, 2019; Taiz et al., 2023).

2. Chronic photoinhibition — a long-lasting (hours to days) decrease in photosynthetic activity caused by actual damage to reaction centers, especially the D1 protein in Photosystem II. This is no longer a defense but a breakdown requiring repair (Lambers & Oliveira, 2019; Taiz et al., 2023).

The key target of photoinhibition is the D1 protein (a component of the Photosystem II reaction center). It is particularly vulnerable to reactive oxygen species and is damaged most rapidly under excess light (Taiz et al., 2023). However, plants have an effective repair mechanism: the damaged D1 is removed, and a new one is synthesized in its place (chloroplasts have their own ribosomes), and Photosystem II is restored (Taiz et al., 2023). This requires energy expenditure but allows survival under variable light conditions.

5.3. The First Line of Defense: Dissipation of Energy as Heat (Xanthophyll Cycle)

How does the plant avoid damage? The first and fastest line of defense is the conversion of excess excitation energy into heat without the involvement of photochemical reactions. This process is called non-photochemical quenching (NPQ) (Taiz et al., 2023; Medvedev, 2012). It is triggered when protons accumulate inside the thylakoid (in the lumen), and the pH drops (becomes acidic, around 5.0–5.5). This is a signal that the electron transport chain is overloaded and protons are not exiting fast enough through ATP synthase.

The acidic environment in the lumen activates the enzyme violaxanthin de-epoxidase, which initiates the xanthophyll cycle (Taiz et al., 2023; Medvedev, 2012):

Violaxanthin → Antheraxanthin → Zeaxanthin

Under bright light, violaxanthin (containing epoxide groups) is converted to zeaxanthin (without epoxides). Zeaxanthin, together with the PsbS protein (of Photosystem II), induces changes in the LHCII antenna complex that allow excess energy to be dissipated as heat. This reduces the likelihood of singlet oxygen formation. When light subsides, the enzyme zeaxanthin epoxidase converts zeaxanthin back to violaxanthin (Taiz et al., 2023; Medvedev, 2012).

This mechanism operates within minutes, allowing the plant to adapt quickly to changes in illumination. Plants adapted to high light have a larger pool of xanthophylls (violaxanthin + antheraxanthin + zeaxanthin) than shade-tolerant ones (Lambers & Oliveira, 2019). This is one reason why sun-loving species tolerate direct sunlight better.

Important detail: zeaxanthin acts like a "valve" — if light becomes excessive, it switches on and dumps the excess energy as heat. This does not reduce the maximum rate of photosynthesis but decreases its efficiency under high light, preventing damage.

5.4. The Second Line of Defense: Antioxidant Systems

If the first line of defense (thermal dissipation) is insufficient and reactive oxygen species (superoxide anion O₂⁻, hydrogen peroxide H₂O₂, singlet oxygen ¹O₂) are still formed, a powerful antioxidant system comes into play (Medvedev, 2012; Taiz et al., 2023).

A cascade of enzymes operates in the chloroplast:

1. Superoxide dismutase (SOD) converts superoxide anion to hydrogen peroxide.

2. Ascorbate peroxidase reduces hydrogen peroxide to water, using ascorbic acid (vitamin C) as an electron donor.

3. Regeneration of ascorbate occurs via the glutathione cycle (involving glutathione and NADPH) (Medvedev, 2012; Connor et al., 2011).

Furthermore, carotenoids (especially β-carotene) can directly quench singlet oxygen, accepting its energy and dissipating it as heat (Hopkins & Hüner, 2009; Medvedev, 2012). This protects chlorophyll from photooxidation.

This system operates continuously, but under severe stress, its capacity may be insufficient, and then damage (chlorosis, necrosis) begins. This is why mineral nutrient deficiency (especially Mg, K, Zn) or water deficit leads to increased photooxidation — the defense systems are weakened (Connor et al., 2011; Medvedev, 2012).

5.5. The Third Line of Defense: Repair of Photosystem II

If damage does occur (especially to the D1 protein in PS II), the plant does not leave things as they are. It initiates a repair mechanism (Taiz et al., 2023; Lambers & Oliveira, 2019):

1. The damaged D1 is recognized, cleaved from the complex, and degraded by proteases.

2. A new D1 protein is synthesized in the chloroplast (based on plastid DNA information) on chloroplast ribosomes.

3. The new D1 is inserted in place of the old one, and Photosystem II regains its activity.

This process requires ATP and occurs with some delay (from minutes to hours). If damage occurs too frequently (e.g., during prolonged excess light in the cold), the repair system may not keep up, and chronic photoinhibition ensues, leading to reduced productivity (Taiz et al., 2023). This is why cold, bright morning hours are particularly dangerous for heat-loving plants (corn, tomatoes) — the repair rate is reduced by low temperature.

5.6. Other Adaptive Strategies: Morphological and Behavioral

In addition to biochemical mechanisms, plants use other ways to reduce light load:

1. Changing leaf orientation. Many plants (e.g., cotton, soybean) turn their leaves parallel to the sun's rays at noon, reducing the light-absorbing area (Lambers & Oliveira, 2019).

2. Chloroplast movements. Under bright light, chloroplasts move to the side walls of cells, aligning parallel to the surface, which reduces light absorption (Medvedev, 2012; Taiz et al., 2023).

3. Waxy coating and hairiness. Many drought-tolerant plants (eucalyptus, olive) have leaves covered with a waxy layer or hairs that reflect part of the light (Connor et al., 2011).

4. Anthocyanin coloration. Many young leaves and autumn leaves have a red or purple color due to anthocyanins, which shield chlorophyll from blue light (Hopkins & Hüner, 2009). This protects the photosynthetic apparatus until the leaf adapts or completes its life cycle.

5.7. Physiological Significance of Photoinhibition and Protection

Why does a plant need such a complex, multi-layered defense? The answer is simple: light energy is both a boon and a threat. During evolution, plants "learned" to switch quickly between modes of absorption and dissipation of energy to maximize photosynthesis while minimizing damage.

Photoinhibition is not just a disease. In its moderate form, it is a regulatory signal that allows the plant to adjust its antenna apparatus to current conditions. For example, during a transition from shade to sun, the short-term decrease in quantum yield (dynamic photoinhibition) provides time to activate the xanthophyll cycle and remodel membranes (Lambers & Oliveira, 2019). Without this, every cloud and every gust of wind would cause photooxidation.

Thus, protection against excess light is an integral part of photosynthesis. Without it, photosynthesis would be impossible in open ecosystems where the light load varies tenfold during the day.

5.8. Significance for Agronomy and Practice

Understanding the mechanisms of photoinhibition has direct practical applications:

1. Optimizing plant density. Too dense sowing creates shading for lower leaves, but upper leaves may experience light stress. Agronomic practices (seeding rate, row orientation) take these risks into account.

2. Balanced nutrition. Deficiencies of N, Mg, K, Zn, Mn weaken defense systems and increase sensitivity to light stress. Therefore, fertilization is important not only for growth but also for resilience (Connor et al., 2011; Medvedev, 2012).

3. Breeding for tolerance. Modern varieties, especially for arid regions, are selected for the ability to rapidly activate NPQ and repair D1 under high light and high temperature (Taiz et al., 2023).

4. Agrotechniques in greenhouses. Under intensive lighting (supplemental lighting), the possibility of photoinhibition must be considered, especially in combination with low humidity or CO₂ deficiency.

Final Summary of the Entire Lecture

We have completed our journey from the very beginning — understanding the nature of light as energy — to realizing how a plant manages this energy to use it for benefit and avoid harm. Here is the complete picture we have built:

1. Light is a stream of photons. A plant uses only part of the spectrum (PAR, 400–700 nm) because only these photons have the appropriate energy to excite electrons in pigments. The number of photons, not their total energy, determines photosynthetic activity.

2. The plant collects light using antenna complexes — hundreds of chlorophyll and carotenoid molecules work as a single "light collector," transferring energy to reaction centers with nearly 100% efficiency.

3. In the reaction centers (P680 and P700), charge separation occurs — light energy is converted into chemical energy in the form of excited electrons. The two photosystems work sequentially, creating a Z-shaped energy profile.

4. Electron energy is used to create a proton gradient and reduce NADP⁺. Proton flow through ATP synthase synthesizes ATP. Thus, ATP and NADPH — universal energy carriers and reducing power — are born.

5. But excess light is dangerous. If energy cannot be used, reactive oxygen species are generated, damaging membranes and proteins. The plant defends itself with three lines: thermal dissipation (xanthophyll cycle), antioxidant system, and repair of Photosystem II. This makes photosynthesis resilient in real, fluctuating conditions.

Ultimately, the light stage of photosynthesis is not just a set of biochemical reactions, but a carefully balanced system of conversion and protection that allows the plant to exist in a constantly changing light environment. Understanding this system provides us with the key to managing plant productivity in agroecosystems.