Photosynthesis and respiration

1. Introduction: The Planet's Main Process

This may sound like a bold statement, but it is absolutely accurate: almost all life on Earth depends on photosynthesis. When we talk about agriculture, we are essentially talking about managing photosynthesis. The yield of wheat, corn, potatoes — it is ultimately transformed solar energy. Understanding this process is the key to understanding how our world works and how we can influence plant productivity.

2. Why Is Photosynthesis the Foundation of Life on Earth?

Let us examine what makes photosynthesis a global process that underpins our civilisation.

A. Almost all of the planet's biomass is created by photosynthesis

The only organisms capable of creating complex organic substances from simple inorganic ones are photoautotrophs (Morot‑Gaudry et al., 2012). They use light energy to synthesise sugars from carbon dioxide and water. These sugars are the primary building material from which all other organic molecules — proteins, fats, nucleic acids — are constructed.

At its core, the carbon that makes up every cell of our body, every leaf and every grain, was once a carbon atom in a carbon dioxide molecule in the air. It is photosynthesis that fixes this atmospheric carbon, converting it from an inorganic, oxidised form (CO₂) into an organic, reduced form (carbohydrates) (Connor et al., 2011; Lambers & Oliveira, 2019). It is estimated that about 120 petagrams of carbon are fixed annually through photosynthesis. This forms the basis of all living matter on the planet.

B. Atmospheric oxygen is a product of photosynthesis

We all know that plants release oxygen. This is not merely a "by‑product" but a fundamental change in the atmosphere's composition that occurred billions of years ago.

The oxygen we breathe is the result of the splitting (photo‑oxidation) of a water molecule (H₂O) in the light reactions of photosynthesis. As classic experiments using the ¹⁸O isotope have shown, the source of the released O₂ is indeed water, not carbon dioxide (Morot‑Gaudry et al., 2012; Hopkins & Hüner, 2009). This process maintains the unique aerobic atmosphere that, in turn, allows high‑energy life forms, including us, to exist.

C. Photosynthesis is the base of all food chains

Any food we eat — be it bread, meat, milk, or vegetables — owes its origin to photosynthesis. Let us look at this through the lens of trophic chains (Connor et al., 2011).

• Primary producers are phototrophic plants that create organic matter (biomass) "out of nothing".
• Primary consumers (heterotrophs) are herbivorous animals that feed on plants, obtaining the energy stored in their tissues.
• Secondary consumers are predators and humans that feed on herbivores.

Thus, the energy accumulated in the chemical bonds of organic substances is transferred along the trophic chain. At each transfer step, a significant portion of energy is lost (mainly through respiration). That is why short trophic chains (e.g., plant → grain → human) are much more efficient than long ones (plant → grass → cow → human) from the perspective of food production. This explains the high energy efficiency of vegetarian diets.

D. The foundation for agriculture

As just mentioned, virtually any crop is transformed sunlight. The productivity of an agrocenosis is primarily determined by how efficiently plants capture light and convert its energy into the chemical energy of sugars (Lambers & Oliveira, 2019; Sadras & Calderini, 2015). Ultimately, yield is a measure of how much solar energy has been "conserved" in the form of valuable biomass.

Understanding this fact shifts the agronomist's focus. We are not just managing plants; we are managing the flow of energy. Every agronomic practice — from variety selection and plant density to irrigation systems and disease protection — should aim to maximise light capture and its efficient conversion into a commercially valuable product.

Transition to the next part of the lecture

As we see, photosynthesis is the fundamental basis, but a plant is not a static accumulator. It is a dynamic system in which two opposing processes continuously occur: energy accumulation and expenditure. It is precisely this "duality" of energy metabolism that we will discuss in the next part of our session. We will briefly examine how these two processes work and pose key questions for our entire module.

Good, let us continue. We have laid the foundation by showing that photosynthesis is the main supplier of energy on the planet. Now let us look at how this energy circulates within the plant itself. This is the subject of today's discussion — energy metabolism.

2. Plant Energy Metabolism: Two Sides of the Same Coin

A plant, like any living organism, is an open thermodynamic system. It continuously exchanges matter and energy with its environment (Hopkins & Hüner, 2009). The essence of its energy metabolism lies in two opposing but inextricably linked processes:

• Photosynthesis (anabolism) — the process of creating organic matter with the storage of chemical energy in it.
• Respiration (catabolism) — the process of oxidising organic matter with the release of stored energy to support all vital functions of the cell.

It is important to grasp immediately: plants respire all the time. Unlike animals, they are simultaneously producers (creating organic matter in the light) and consumers (using organic matter both day and night). Respiration occurs continuously, providing energy for the work of all cells, including photosynthetic ones (Morot‑Gaudry et al., 2012). Therefore, when we measure leaf gas exchange in the light, we see net CO₂ assimilation — the difference between the rate of photosynthesis (CO₂ uptake) and respiration (CO₂ release) (Lambers & Oliveira, 2019).

Let us briefly characterise each of these two processes to appreciate their opposing directions.

A. Photosynthesis — the storing process

The overall equation of photosynthesis is well known to us:

From a bioenergetic point of view, this is a classic example of an endergonic reaction. The change in free energy (ΔG⁰') for reducing one molecule of CO₂ to a carbohydrate is positive and amounts to about +475 kJ/mol (Morot‑Gaudry et al., 2012). This means that the reaction cannot proceed spontaneously. It requires a constant input of energy from outside. This external energy is supplied by photons of light absorbed by pigments — chlorophylls and carotenoids (Hopkins & Hüner, 2009).

Thus, photosynthesis is the work of "pumping" energy into the chemical bonds of organic molecules. In essence, it transfers electrons from a low energy level (in water) to a high one (in carbohydrates), converting solar energy into chemical energy.

B. Respiration — the consuming process

A fundamentally different process is cellular respiration. It is the oxidation of organic substances (primarily carbohydrates) to CO₂ and H₂O.

The overall summary equation for respiration is the reverse of photosynthesis:

Unlike photosynthesis, respiration is an exergonic process that proceeds spontaneously, releasing free energy (ΔG < 0). The energy released when bonds are broken is not dissipated as heat but is stored in the universal "energy currency" of the cell — molecules of adenosine triphosphate (ATP) and reduced coenzymes (NADH, etc.) (Hopkins & Hüner, 2009). ATP is then used for all energy‑consuming processes: protein synthesis, active ion transport, cell expansion growth, and many others.

C. Interconnection and balance

Photosynthesis and respiration form the energy cycle of the plant. During the light period, photosynthesis usually prevails over respiration, and the plant accumulates dry matter. During the dark period, respiration remains the only source of energy, and the plant consumes its stored reserves (Connor et al., 2011).

It is this balance — the ratio between photosynthetic production and respiratory costs — that ultimately determines yield. If we want to achieve maximum grain, tuber, or fruit yield, we must aim for as large a share of photosynthetic products as possible to go into forming the desired organs, rather than being spent on maintaining and growing unnecessary parts or on respiration.

Transition to the next part of the lecture

Now that we have outlined the general framework, questions naturally arise. How exactly is light converted into chemical energy? What molecular mechanisms underlie this process? And how is the obtained energy used in practice?

We will answer all these questions as we continue our study of the module. The next step is to pose the key questions that will form the backbone for all subsequent lectures.

Good, the logic is established. We have seen the global importance of photosynthesis and the opposing directions of the two main energy processes. Now it is time to set the direction for the entire module. Instead of a dry list of topics, we will formulate them as questions that we need to answer. This will turn learning into an exciting exploration.

3. Key Questions of the Module: From Light to Harvest

We have established that photosynthesis is the process of converting light energy into chemical energy, and respiration is the process of releasing it for the cell's needs. But how exactly does this happen at the molecular level? What mechanisms underlie these astonishing transformations?

Our entire further course will be devoted to a sequential and in‑depth answer to these questions. Let us formulate them so that they serve as navigation beacons in the world of plant physiology.

A. How does light become chemical energy?

This is the first and fundamental question. We know that sunlight is absorbed by pigments — primarily chlorophylls. But what happens next? How does the energy of a photon, that tiny particle of light, trigger a cascade of chemical reactions?

We will have to understand the workings of photosystems I and II, which are a kind of "solar batteries" at the molecular level (Lambers & Oliveira, 2019). We will study how the absorbed energy is transferred along the electron transport chain, and how this electron flow is used to generate two key products of the light phase: ATP (energy) and NADPH (reducing power) (Hopkins & Hüner, 2009). This is the very energy "pumped" into molecules that will be used in the next stage.

B. How is CO₂ converted into sugar?

With ATP and NADPH in reserve, the plant can undertake its main task — fixing inorganic carbon. This process occurs in the chloroplast stroma in the so‑called Calvin‑Benson cycle (Lambers & Oliveira, 2019).

At the centre of this cycle is the key enzyme — Rubisco (ribulose‑1,5‑bisphosphate carboxylase/oxygenase). It is the most abundant protein on Earth, and it performs a critical yet paradoxical job. We will learn how Rubisco catalyses the addition of CO₂ to an acceptor molecule, and how the resulting product eventually synthesises glucose (Morot‑Gaudry et al., 2012). This is the "dark phase," although in reality it is highly dependent on the products of the light reactions.

C. Why is Rubisco an imperfect enzyme?

Here we encounter a remarkable evolutionary paradox. Rubisco, as we have mentioned, is the key enzyme, but it has extremely low efficiency and, most importantly, it can catalyse not only the reaction with CO₂ (carboxylation) but also a competing reaction with O₂ (oxygenation) (Lambers & Oliveira, 2019).

This leads to photorespiration — a process in which part of the already fixed carbon is lost back as CO₂, and the energy of ATP and NADPH is wasted (Connor et al., 2011). This can reduce photosynthetic efficiency by 30‑50%. The question arises: why hasn't nature "fixed" this error? The answer lies in the history of Earth's atmosphere, and it will lead us to understand remarkable adaptations.

D. Why did C₄ and CAM plants evolve?

Photorespiration becomes a serious problem in hot and arid climates, when plants are forced to close their stomata to conserve water, thereby reducing the internal CO₂ concentration. Under these conditions, the oxygenase activity of Rubisco increases.

In response to this pressure, evolution created ingenious mechanisms to "concentrate" CO₂ around Rubisco, suppressing its oxygenase activity (Lambers & Oliveira, 2019). Thus arose C₄ photosynthesis (typical of maize, sugarcane, sorghum) and CAM photosynthesis (typical of cacti, Crassulaceae, pineapple). We will study their anatomical and biochemical features and understand the ecological and agronomic advantages they confer (Connor et al., 2011).

E. How does the plant distribute carbon?

Imagine photosynthesis as a sugar‑producing factory. But where does this sugar go next? How does the plant decide whether to direct it to root growth, fruit development, or starch storage in seeds?

This is called regulation of assimilate partitioning (sink‑source relations). We will understand how the plant "senses" its needs and redistributes carbon among various organs (sources and sinks) (Lambers & Oliveira, 2019). This knowledge is critical for the agronomist: by understanding this mechanism, we can manage it through pruning, sowing rates, growth regulators, so as to direct the maximum of carbohydrates to the commercially valuable organs — grain, tubers, fruits.

F. How does the cell get energy back?

Finally, the energy stored in sugars cannot be used directly for biosynthesis or transport. The cell needs energy in the form of ATP. This process, the reverse of photosynthesis, is called respiration, and it is no less important for life.

We will examine how, through glycolysis, the Krebs cycle, and oxidative phosphorylation, organic molecules are broken down and their energy is "converted" into ATP. We will see how light and energy metabolism are interconnected through common metabolic pathways, and why a plant cannot do without respiration even in the light (Hopkins & Hüner, 2009).

Transition to the next part of the lecture

As you can see, an exciting journey lies ahead, from photon to grain. But perhaps you have a question: why does an agronomist need all this? Is it really necessary to understand the structure of photosystems and Rubisco kinetics to obtain a harvest?

The answer is unequivocally yes. It is precisely this fundamental knowledge that underpins sound agronomic decisions. And in the next, concluding part of our introduction, we will talk about the practical significance of the studied module for agronomic practice.

Good, we have come to the most important point — the practical meaning of everything said. Now that we understand that photosynthesis is the basis of life and respiration is its engine, let us look at these processes through the eyes of an agronomist.

4. Practical Significance of the Module for the Agronomist

Dear listeners, we are beginning the study of fundamental processes, and some of you may have a legitimate question: "Why does an agronomist need to know about the structure of photosystems and the kinetics of Rubisco? Isn't it enough to have practical recommendations to get a harvest?"

The answer to this question lies at the heart of modern, science‑based agriculture. Fundamental knowledge of plant physiology is the tool that allows the agronomist not just to follow ready‑made recipes, but to understand why and how certain practices work, and to make the right decisions in every specific situation. Without this understanding, we act blindly, by trial and error.

Let us consider several illustrative examples that show how knowledge of photosynthetic physiology directly influences agronomic practice.

A. Why is yield limited by light intensity?

We have already said that photosynthesis converts light energy into chemical energy. From this simple fact follows a whole series of important agronomic conclusions.

As described by Lambers & Oliveira (2019), the photosynthetic activity of a leaf initially increases linearly with increasing light intensity, then reaches a plateau (light saturation). For the agronomist, this means that under overcast conditions or in dense stands, light becomes the main limiting factor, and even the best supply of water and nutrients will not increase yield.

From this come practical tasks:

• Choosing optimal plant density — so that plants do not shade each other.
• Breeding for crop architecture — creating varieties with more upright leaves (as has been done in modern maize hybrids), allowing light to penetrate the lower layers of the canopy (Sadras & Calderini, 2015).
• Weed control — because every weed is a competitor for sunlight.

B. Why does an elevated CO₂ concentration in the atmosphere change productivity?

We know that CO₂ is a substrate for Rubisco. In the current atmosphere, its concentration (about 420 ppm) is far from saturation for the enzyme, especially in C₃ plants (Lambers & Oliveira, 2019).

This means that an increase in atmospheric carbon dioxide generally stimulates photosynthesis, especially in C₃ crops (wheat, rice, soybean, potato). This phenomenon is known as CO₂ fertilisation. However, as we will see in the course, this effect is not infinite. Lambers & Oliveira (2019) explain that under prolonged exposure to elevated CO₂, plants may undergo "down‑regulation" of photosynthesis: the Rubisco content in leaves decreases, especially under nitrogen deficiency. The plant "realises" that it needs less enzyme and saves resources.

For the agronomist, this means that under future elevated CO₂, we may need to reconsider fertilisation systems, especially nitrogen, to avoid photosynthetic acclimation and obtain a real yield increase.

C. Why does maize love heat, while wheat does not?

This is a classic example where physiology explains fundamental differences between crops. It all comes down to photosynthesis types.

Wheat is a C₃ plant. As we have said, its Rubisco is subject to competing oxygenase reactions. As temperature rises, CO₂ solubility decreases faster than O₂, and photorespiration increases (Lambers & Oliveira, 2019). As a result, at high temperatures (above 30 °C), the photosynthetic efficiency of C₃ plants begins to decline.

Maize is a C₄ plant. It has a "pump" that concentrates CO₂ around Rubisco, and its photosynthesis hardly suffers from photorespiration even at high temperatures (Connor et al., 2011). That is why maize, sorghum, and sugarcane are crops of hot climates, and their productivity is highest under conditions of high insolation and temperature (Sadras & Calderini, 2015).

For the agronomist, this knowledge answers the questions:

• Why is it more profitable to grow C₄ crops in southern regions, and C₃ crops in temperate zones?
• Why can climate changes leading to warming shift the cultivation zones of agricultural crops?

D. Why does even slight shading sharply reduce yield?

This fact is directly related to the light response curve of photosynthesis. As shown in figures in Morot‑Gaudry et al. (2012), in the region where light is the limiting factor (the initial, linear part of the curve), photosynthesis increases proportionally to light intensity. In other words, under shading, every "lost" photon means a direct reduction in assimilate production.

Therefore, for the agronomist, the following are so important:

• Sowing dates — so that the period of active growth coincides with the longest day length.
• Control of excessive density — thinning seedlings to ensure optimal feeding area.
• Weed management — any shading of the crop plant by a weed leads to a directly proportional yield loss.

E. Why does photosynthesis not grow indefinitely? The law of the limiting factor

Earlier we discussed that photosynthesis is limited by a number of factors: light, CO₂, water, temperature, nutrients. This illustrates Liebig's law, which states that the rate of a process is limited by the factor that is in minimum supply.

We saw this in the example of the light saturation curve: at low light, light is limiting; at high light, the photosynthetic rate reaches a plateau where CO₂ concentration or enzyme activity becomes limiting (Lambers & Oliveira, 2019; Morot‑Gaudry et al., 2012).

For the agronomist, this leads to a critically important principle of balance. There is no point in increasing nitrogen fertiliser doses if the crop is too dense and plants suffer from light deficiency. There is no point in supplying more water if the root system is weak and cannot absorb it. The effectiveness of any agricultural practice is determined by whether it removes the current main limitation.

Main conclusion: The physiology of photosynthesis and respiration is not an abstract science, but a practical language in which the laws of yield formation are written. By understanding this language, the agronomist ceases to be merely an executor of instructions and becomes a true master of the process, capable of making effective and economically sound decisions.

Conclusion

We have completed our introduction. We have seen that:

1. Photosynthesis is a fundamental process that sustains life on Earth and serves as the basis of all agricultural production (Connor et al., 2011).

2. The energy metabolism of the plant is a unity of opposites: photosynthesis accumulating energy, and respiration consuming it for all life functions (Hopkins & Hüner, 2009).

3. We have to study the complex pathway of converting light and carbon dioxide into yield, from the operation of photosystems to the regulation of carbon partitioning (Lambers & Oliveira, 2019).

4. This knowledge is not speculative. It directly answers questions of agronomic practice and provides keys to managing crop productivity (Morot‑Gaudry et al., 2012; Sadras & Calderini, 2015).

References

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2. Connor, D.J., Loomis, R.S., Cassman, K.G. (2011). ‘Trophic chains’, in Crop Ecology. Productivity and Management in Agricultural Systems. Cambridge, UK: Cambridge University Press, pp. 23-43.
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