Photosynthesis in Higher Plants

Introduction

Photosynthesis is the breathtaking dance of molecular creativity, where plants, fueled by the enchanting touch of sunlight, embark on a cosmic alchemical journey to transmute ethereal photons into tangible life. It is nature’s sublime symphony, orchestrating a symposium of chlorophyll-clad artists, captivatingly choreographed to convert carbon dioxide’s gaseous whispers and water’s liquid embrace into the verdant tapestry of organic molecules, exuding the elixir of oxygen, and painting the world with vibrant hues of green. It is the quintessential fusion of celestial radiance, botanical ingenuity, and earthly sustenance, an almighty recipe whispered across epochs, igniting the eternal flame of life itself.

In simpler terms, Photosynthesis is nature’s solar-powered process where plants use sunlight to convert carbon dioxide and water into oxygen and energy-rich organic compounds, sustaining life and painting the world with green vitality.

  • Photosynthesis is called a Physicochemical process because it converts light energy into chemical energy.
  • Chlorophyll (green pigment of the leaf), light and CO2 are required for photosynthesis to occur.
  • Some pigments other than chlorophyll also participate in photosynthesis. These are called accessory pigments.
  • Some accessory photosynthetic pigments are Carotenoids, Xanthophylls, Phycobilins, and Anthocyanins. They provide different colours to the plants.
  • The presence of different photosynthetic pigments can be confirmed by paper chromatography.

Some early experiments related to photosynthesis

  • The leaf was partly covered with black paper for a few hours and allowed for photosynthesis in the sunlight. It is then tested for starch. The part of the leaf which was covered tested negative for starch and the uncovered portion tested positively for the presence of starch.
  • A leaf was partly dipped in a test tube containing KOH-soaked cotton. KOH absorb CO2 and makes it unavailable for the leaf. The leaf exposed to CO2 was able to perform photosynthesis, accumulating starch as a result. In contrast, the portion deprived of CO2 inside the test tube could not carry out photosynthesis and did not accumulate starch. This experiment demonstrates that CO2 is a significant factor in the process of photosynthesis.
  • Joseph Priestley observed that when a flame was placed in a sealed jar, it extinguished, and a mouse would suffocate due to the lack of air. However, by adding a green plant to the jar and exposing it to sunlight, the air inside became “refreshed.” This allowed the flame to burn again, and the mouse could breathe properly. Perhaps, Priestley wrote, “the injury which is continually done by such a large number of animals is, in part at least, repaired by the vegetable creation.”

In another experiment, Priestley used a 12-inch-wide glass lens to focus sunlight on reddish mercuric oxide, kept in an inverted jar producing a gas five to six times better than common air. The gas supported intense combustion and prolonged a mouse’s life compared to regular air. From his experiments, he pointed out the importance of air (oxygen) in supporting life.

  • Jan Ingenhousz’s experiment demonstrated that sunlight is essential for the production of oxygen in plants during photosynthesis. He established that plants can release oxygen when exposed to light. This experiment laid the foundation for our understanding of the role of light energy in the photosynthetic process.
  • Julius von Sachs provided evidence for the production of glucose when plants grow. He removed the green leaves from a potted plant and covered them with an airtight bell jar to limit the carbon dioxide supply. Over time, the leaves inside the jar turned pale and withered, indicating a lack of carbon dioxide for photosynthesis. Despite this, the plant continued to grow, and new leaves formed. Von Sachs detected a significant accumulation of starch in these new leaves through a starch test. This provided evidence for glucose production during plant growth, suggesting that photosynthesis occurred in other parts of the plant where light and carbon dioxide were available.
  • W. Engelmann used a prism to split light into its spectral components and illuminated a green alga, Cladophora, in the presence of aerobic bacteria. He observed that the bacteria accumulated mainly in the regions of blue and red light in the split spectrum. This experiment led to the first action spectrum of photosynthesis, resembling the absorption spectra of chlorophyll a and b. It showed the significance of blue and red light in photosynthesis.

By the mid-19th century, it was evident that photosynthesis takes place in the green parts of plants by utilizing CO2 and H2O in the presence of sunlight. The empirical formula for photosynthesis is written as

CO2 + H2O ————-(Sunlight, chlorophyll) ———–> [CH2O] + O2

  • Cornelius van Niel used green and purple Sulphur bacteria to show that CO2 is reduced by a suitable hydrogen donor. He used the analogous reaction used by green and purple Sulphur bacteria to reduce CO2.

Reaction in green and purple Sulphur bacteria

CO2 + H2S ——(Sunlight, photosynthetic pigment) ——-> [CH2O] + S + H2O

Comparing it with the reaction in plants it was concluded that the suitable hydrogen doner in plants is H2O.

It was confirmed by a radio isotopic labelling experiment. Scientists provided plants with water containing the heavy oxygen isotope, oxygen-18 (18O), instead of the regular oxygen isotope, oxygen-16 (16O).

6 CO2 + 6 H218O ——-(Sunlight, Chlorophyll) ———–> C6H12O6 + 6 18O2

From this experiment, it was confirmed that the source of oxygen evolved in photosynthesis is water.

Site of Photosynthesis

  • All green parts of plants contain chloroplast which is the site of photosynthesis.
  • In the chloroplast, the stroma and the thylakoid are the sites of actual photosynthetic reactions.
  • Photosynthesis is completed in two phases. first the light phase and second the dark phase.
  • Light phase: It takes place in the presence of light which causes the excitation of the photosystems (PS I and II) and releases high-energy electrons. It results in the production of high-energy molecules ATP and NADPH. The thylakoid membrane and stroma lamella are the sites of the light phase of photosynthesis.
  • Dark phase: It depends on the products (ATP and NADPH) of light reactions, and does not require light directly. It continues until the availability of ATP and NADPH. As it can take place in the dark also (not for a long time), it is called the dark phase. It is the actual CO2 fixation phase in which CO2 is converted into sugar. This phase takes place in the chloroplast stroma.
  • Light Harvesting Complex (LHC): The LHC is an assembly of photosynthetic pigments, including chlorophyll (b, c, d), carotenoids, phycobilin, and xanthophylls. These pigments act as antennae that capture light energy and transfer it to the reaction center.

Chlorophyll-a: Within the LHC, there is only one molecule of chlorophyll-a, which serves as the reaction center. Chlorophyll-a is the primary pigment responsible for initiating the light-dependent reactions of photosynthesis.

Photosystem (PS): The combination of the antennae (LHC) and the reaction centre (chlorophyll-a) forms the photosystem. The LHC captures light energy and funnels it to the reaction centre, where it excites electrons, initiating the process of electron transport.

  • More than one type of photopigment allows the absorption of a diverse range of wavelengths in the visible light spectra maximizing the photosynthetic efficiency.
  • There are two types of PS present in green plants, PS I and PS II. They are named in the order of their discovery.
  • Chlorophyll-a, of PS I and PS II have different absorption characteristics, resulting in different names.
  • In PSI, the reaction centre chlorophyll-a has its peak absorption at wavelength 700 nanometers (nm), giving it the name P700.
  • On the other hand, in PSII, the reaction centre chlorophyll-a has its peak absorption at wavelength 680 nm, giving it the name P680.
  • Both photosystems play crucial roles in the photosynthetic electron transport chain, with PSII functioning first and then passing the excited electrons to PSI.
  • The absorbed light energy is used to excite electrons in the reaction centre chlorophyll, initiating the flow of electrons through the electron transport chain, leading to the production of ATP and NADPH, which are essential for the synthesis of carbohydrates during photosynthesis.
Structure of PS. Coloured circles represent photopigments. Red circle represents chlorophyll-a (reaction centre). Except Chl-a, all other pigments form antennae. The antennae and reaction center together called photosystem (PS).

The Light Reaction (Non-Cyclic and Cyclic Electron Flow )

Cyclic and non-cyclic electron flow

Linear electron flow (Blue arrow pathway in the above image): In the thylakoid membranes of chloroplasts drives ATP and NADPH synthesis. Here’s a concise summary of the process:

  • Photosystem II (PS II) absorbs light, exciting an electron in P680.
  • The excited electron is captured by the primary electron acceptor, leaving P680+ (oxidized form).
  • Water is split, and its electrons replace those lost from P680+, releasing oxygen and protons. Protons accumulate in the thylakoid lumen which results in the setting of proton gradient across the thylakoid membrane.
  • Electrons move through an electron transport chain, generating a proton gradient for ATP synthesis.
  • Photoexcited electrons from PS I’s P700 are passed through a second electron transport chain.
  • NADP+ reductase catalyzes the transfer of electrons to NADP+, forming NADPH.

Cyclic electron flow (dotted circle): Under specific conditions, photoexcited electrons from photosystem I (PS I) can follow an alternative pathway known as cyclic electron flow. In this process:

  • Electrons cycle back from ferredoxin (Fd) to the cytochrome b6f complex.
  • From there, the electrons continue to a P700 chlorophyll in the PS I reaction-center complex.
  • Unlike linear electron flow, cyclic electron flow does not produce NADPH or release oxygen.
  • However, it does generate ATP.

Chemiosmotic Hypothesis of ATP generation

  • Water splitting in the thylakoid lumen leads to the accumulation of protons, while the formation of NADPH removes protons from the stroma. This creates a proton concentration difference across the thylakoid membrane.
  • The cF0-cF1 complex facilitates the movement of protons from the thylakoid lumen to the stroma, downhill the concentration gradient. This process drives the generation of ATP in the stroma.

The ATP and NADPH produced in both cyclic and non-cyclic electron flow are utilized in the synthetic phase of photosynthesis known as the Calvin (or C3) cycle. In which CO2 is reduced to sugar. As this phase depends on the products of light reaction, it is also known as dark reaction.

The Biosynthetic Phase: Calvin (or C3) cycle and C4 cycle

  • The products of the light reaction in photosynthesis are ATP, NADPH, and O2.
  • ATP and NADPH are used in the biosynthetic phase, also known as the Calvin (C3) cycle, which does not directly depend on light but requires the products of the light reaction.
  • The biosynthetic phase continues even when light is unavailable but stops after some time. When light is reintroduced, synthesis resumes.
  • The biosynthetic phase is often referred to as the “dark reaction,” but this is a misnomer since it doesn’t necessarily require darkness, only the absence of light-dependent reactions.
  • Melvin Calvin’s work with radioactive 14C in algal photosynthesis led to the discovery of the first product of CO2 fixation, 3-phosphoglyceric acid (PGA), in the Calvin cycle.
  • Some plants have a different first product of CO2 fixation, oxaloacetic acid (OAA), which is a C4 acid, leading to the C4
  • Photosynthetic plants can be categorized into two main types based on their CO2 assimilation pathway: C3 plants (PGA as the first stable product) and C4 plants (OAA as the first stable product). 
  • The unexpected discovery was that the primary acceptor molecule in the Calvin cycle is a 5-carbon ketose sugar, ribulose bisphosphate (RuBP). In C4 plants the primary acceptor is phosphoenolpyruvate (PEP).

Calvin (or C3) Cycle

It is completed in three stages. These are:

  • Carboxylation: CO2 combines with RuBP, catalyzed by RuBisCO, forming two molecules of 3-phosphoglycerate (3-PGA).
  • Reduction: Each molecule of 3-PGA is phosphorylated using 2 ATP molecules and reduced using 2 NADPH molecules to form triose phosphate (glyceraldehyde-3-phosphate, G3P).
  • Regeneration: One molecule of ATP is used to phosphorylate triose phosphate and regenerate RuBP, allowing the cycle to continue.
  • Input and output: One CO2 enter each cycle. In each cycle, 3 ATPs and 2 (NADPH + H+) are utilized. Therefore, for the production of one glucose molecule, six cycles are required in which 6 CO2, 18 ATPs, and 12 NADPH are required. On the product side after every six cycles, 18 ADPs, 12 Pi, 12 NADP+, and 1 glucose molecule is produced.
The Calvin cycle

C4 Cycle (Hatch and Slack Pathway)

  • The C4 cycle is a specialized carbon fixation pathway found in certain plants adapted to hot and arid environments.
  • Kranz anatomy is a remarkable adaptation that enhances the photosynthetic efficiency and productivity of C4 plants in challenging environmental conditions.
  • In Kranz anatomy, the bundle sheath cells surround the vascular bundles, forming several layers. These bundle sheath cells have a high concentration of chloroplasts and lack intercellular spaces. Their thick walls prevent gaseous exchange, maintaining a high concentration of CO2.
  • C4 plants use phosphoenolpyruvate carboxylase (PEPcase) to fix CO2 into a 4-carbon compound called Oxaloacetic acid (OAA).
  • 4-carbon compounds (e.g., Malic acid/Malate) are formed in mesophyll cells and transported to bundle sheath cells.
  • Bundle sheath cells surround vascular bundles and have thick walls to prevent CO2
  • In bundle sheath cells, 4-carbon compounds release CO2 and form a 3-carbon molecule Pyruvic acid/Pyruvate.
  • Released CO2 enters the Calvin cycle, and the 3-carbon molecule is sent back to mesophyll cells.
  • PEP is regenerated in mesophyll cells to complete the C4

Advantages of C4 Cycle:

  • Higher efficiency in photosynthesis, even in high-temperature and intense sunlight conditions.
  • Reduced water loss through minimized stomatal opening, suitable for arid regions.
  • Greater biomass productivity compared to C3 plants in similar environments.
  • Examples of C4 plants include maize, sorghum, sugarcane, and some grass species. The C4 cycle is a remarkable adaptation allowing these plants to thrive in challenging environments.
C4 cycle is an alternative pathway of carbon dioxide fixation

Photorespiration

  • Photorespiration is a metabolic process that occurs in plants during photosynthesis.
  • It involves the oxygenation of RuBP (ribulose-1,5-bisphosphate) by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) instead of the desired carboxylation reaction.
  • This leads to the formation of one molecule of 3-phosphoglycerate (3PGA) and one molecule of a two-carbon compound called phosphoglycolate.
  • Photorespiration takes place when the concentration of O2 is relatively high and CO2 is low, favouring the oxygenase activity of RuBisCO.
  • Unlike the Calvin cycle in photosynthesis, photorespiration does not produce sugars or ATP. Instead, it results in the release of CO2 and consumes ATP.
  • C3 plants, which include many common crops, are more susceptible to photorespiration because they lack specific adaptations found in C4 plants to minimize this process.
  • C4 plants have evolved mechanisms to concentrate CO2 at the RuBisCO active site (bundle sheath cells), which helps reduce photorespiration and enhances their photosynthetic efficiency, making them better suited for hot and arid conditions.
Photorespiration

Comparison between C3 and C4 plants:

Aspect

C3 Plants

C4 Plants

1.    Site of Calvin cycle

Mesophyll cells (MC)

Bundle sheath cells (BSC)

2.    Site of initial carboxylation

MC

MC

3.    RuBisCO’s location

MC

BSC

4.    Cell types involved in CO2 fixation

One, MC only

Two, MC and BSC

5.    Primary CO2 acceptor

RuBP (5 Carbon)

PEP (3 Carbon)

6.    Primary CO2 fixation product

3-PGA (3 Carbon)

OAA (4 Carbon)

7.    PEPcase

Not present

In MC

8.    Productivity and Yields

Low

High

9.    Tolerance to high temperatures

Low

High

10. Photorespiration

Occur at a high light intensity and high O2 concentration

Do not occur at all

Factors affecting the rate of photosynthesis

1. Light Intensity:

  • Increased light intensity leads to higher rates of photosynthesis, up to a certain point.
  • Light is the primary energy source for photosynthesis, and its availability directly influences the process.
Light intensity affecting rate of photosynthesis

2. Carbon Dioxide (CO2) Concentration:

  • Higher CO2 levels enhance photosynthesis as it is a key reactant in the Calvin cycle.
  • Adequate CO2 availability boosts the rate of carbon fixation and, consequently, the overall photosynthetic rate.
  • The rate of photosynthesis in C3 plants saturates at a CO2 concentration of 450µl/L.
  • The rate of photosynthesis in C4 plants saturates at a CO2 concentration of 360µl/L.
C3 and C4 plants affected differently by carbon dioxide

3. Temperature:

  • Photosynthesis is temperature-sensitive, with an optimal range for most plants (typically around 25-30°C).
  • Too low temperatures slow down enzyme activity, while excessive heat can lead to enzyme denaturation and reduced photosynthesis. The light phase is a non-enzymatic process hence it is less affected by temperature while the dark phase is more affected by the rise in temperature.
  • C3 plants respond to lower temperatures and show a decrease in photosynthesis at a higher temperature.
  • C4 plants respond to a higher temperature.
  • Tropical plants are adapted to a higher temperature optimum while temperate plants are adapted to a lower temperature.

4. Water availability:

  • Sufficient water is crucial for opening stomata and facilitating CO2
  • Inadequate water supply can cause stomatal closure, reducing CO2 entry and hampering photosynthesis.

5. Chlorophyll content:

  • Chlorophyll captures light energy for photosynthesis. Plants with higher chlorophyll content generally have a higher photosynthetic capacity.

6. Other factors:

  • Leaf surface: Larger leaf surface areas provide more sites for light absorption and gas exchange, positively impacting photosynthesis.
  • The duration of light exposure: Light exposure during the day affects the total amount of photosynthesis a plant can perform.
  • Nutrient availability: Adequate nutrients, such as nitrogen, phosphorus, and magnesium, are essential for enzyme function and chlorophyll production. Nutrient deficiencies can limit photosynthesis.

Summary:

Photosynthesis in higher plants is a remarkable process that sustains life on Earth by converting sunlight into chemical energy. The intricate dance of light reactions and the Calvin cycle within chloroplasts ensures the efficient capture and utilization of light energy. During light reactions, chlorophyll pigments absorb photons, initiating a cascade of events that produce oxygen and energy-rich molecules like ATP and NADPH. In the Calvin cycle, these energy carriers are harnessed to convert carbon dioxide into glucose and other essential organic compounds. This process not only serves as the foundation for plant growth but also provides oxygen, a vital component for all aerobic life forms. Photosynthesis is a complex interplay of biochemical reactions that exemplifies the incredible adaptability and resourcefulness of higher plants. The overall photosynthetic reaction takes place as:

6 CO2 + 12 H2O —-(Sunlight, Chlorophyll) ——-> C6H12O6 + 6 O2 + 6 H2O

——-END——-

 

References:

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