Photosynthesis - Definition and Examples - Biology Online Dictionary (original) (raw)

photosynthesis definition and example

Photosynthesis
n., plural: photosyntheses
[ˌfŏʊ.ɾoʊ.ˈsɪn̪.θə.sɪs]
Definition: the conversion of light energy into chemical energy by photolithorophs

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Photosynthesis is a physio-chemical process carried out by photo-auto-lithotrophs by converting light energy into chemical energy. Among the endless diversity of living organisms in the world, producers are a unique breed.

Unlike consumers (herbivores, carnivores, omnivores, or decomposers) that rely upon other living organisms for their nutritional requirements and nourishment, producers have been distinguished by their ability to synthesize their own food. This is the reason that we call producers “autotrophic or self-reliable” in nature while consumers of all the different categories are called “heterotrophic or dependent” in nature.

Now among producers, there are different categories of producers, i.e. different mechanisms via which they produce their own food.

  1. Photo-auto-litho-trophs: Since these organisms tend to derive their nutrition by channeling the sun’s light energy, they are termed phototrophic in nature. Also, since they utilize inorganic carbon and translate it into organic carbon atoms, i.e. their means of deriving food becomes autotrophic. Additionally, since the source of electrons (electron donors) here are inorganic compounds, they are specified as lithotrophic. In totality, they can be called photo-auto-litho-trophic in nature. Example: Green plants are nature’s brilliant entities that come under this category. They carry out a photosynthesis cycle by taking in carbon dioxide and fixing it into carbohydrates (energy storage molecule). Some of them also give out oxygen gas that’s vital for the other life forms to survive in the earth’s atmosphere.
  2. Chemo-auto-lithotrophs: Many of us might be unaware of the fact that there are some autotrophs that don’t utilize sunlight. Rather they derive their energy stored from a different energy source like oxidation of inorganic compounds.

The scope of today’s discussion is limited to photosynthesis and photoautotrophs. So, let’s get started and get to know the answers to these common questions: what is the photosynthesis process, what are the 3 stages of photosynthesis, what does photosynthesis produce, what is a byproduct of photosynthesis, what is the purpose of photosynthesis, is photosynthesis a chemical change, the various inputs and outputs of photosynthesis, which organisms perform photosynthesis, and many other more questions!!!

photolithotroph pioneer species features

Figure 1: From this flowchart, we can pin pointedly tell that plants are “photo-auto-litho-trophs”. Image Source: Akanksha Saxena of Biology Online.

What is Photosynthesis?

Photosynthesis definition: Photosynthesis is a physio-chemical process carried out by photo-auto-lithotrophs. In simpler language, photosynthesis is the process by which green plants convert light energy into ‘chemical energy’.

This energy transformation is only possible due to the presence of the miraculous pigment molecule chlorophyll in photosynthesis. The chemical energy as referred to before is the fixed carbon molecules generated during photosynthesis.

Green plants and algae have the ability to utilize carbon dioxide molecules and water and produce food (carbohydrates) for all life forms on Earth. There’s no doubt in the fact that life is impossible and unimaginable without green plants that photosynthesize and sustain the cycles of life.

Let’s give you a brief outline of the topic before we head forward.

photosynthesis diagram

Figure 2: Photosynthesis diagram. Image Credit: Daniel Mayer.

Watch this vid about photosynthesis:

Biology Definition:
Photosynthesis is the synthesis of complex organic material using carbon dioxide, water, inorganic salts, and light energy (from sunlight) captured by light-absorbing pigments, such as chlorophyll and other accessory pigments. Photosynthesis may basically be simplified via this equation: 6CO2+12H2O+energy=C6H12O6+6O2+6H2O, wherein carbon dioxide (CO2), water (H2O), and light energy are utilized to synthesize an energy-rich carbohydrate like glucose (C6H12O6). Other products are water and oxygen.

Etymology: from the Greek photo-, “light”, and synthesis, “putting together”
Related forms: photosynthetic (adjective)
Compare: chemosynthesis
See also: photoautotroph

Types of Photosynthesis

Plant photosynthesis and photosynthetic organisms can be classified under different categories on the basis of some characteristic features. They are:

Cyanobacteria photosynthesis and nitrogen fixation diagram

Figure 3: Cyanobacteria not only perform nitrogen fixation but are also known to be photosynthetic in nature. Image Credit: University of Essex.

Euglena parts diagram

Figure 4: Euglenas are photosynthetic in nature. As depicted in the picture, you can notice the chlorophyll-containing chloroplasts. Image Credit: ENI.

Phytoplanktons role in aquatic ecosystems diagram

Figure 5: Phytoplanktons play an important role in maintaining aquatic ecosystems. They are the major producers of oxygen in the Earth’s atmosphere. During the photosynthesis process, they release large amounts of oxygen. Image Credit: Fondriest.

Green sulfur bacteria

Figure 6: Green sulfur bacteria perform anoxygenic photosynthesis. Photo Credit:
kOchstudiO.

Photosynthesis: a two-stage process

Photosynthesis is an example of a metabolic process with 2 stages. Both the stages need light (direct or indirect sunlight). Hence, the long-claimed notion of the 2 processes being ‘absolute LIGHT and DARK reactions’ isn’t apt.

Scientific studies have pointed out that even the 2nd stage of photosynthesis requires indirect sunlight. Therefore, rather than classifying the stages as light and dark photosynthesis reactions, we’ll like to classify the 2 stages as follows:

  1. Photochemical Reaction Process: Light energy is converted to ATP; photophosphorylation process (light-dependent reactions)
  2. Carbon fixation process: Inorganic carbon is converted to organic carbon (light-independent reactions). This is an endergonic process. This process can happen in 2 ways:
    • Through Calvin cycle: In oxygenic photosynthesis as well as anoxygenic photosynthesis
    • Through Non-Calvin cycle: Only is some anoxygenic photosynthesis

Evolution of Photosynthesis Process

It is postulated that the very first photosynthetic beings and photosynthesis evolved quite early down the evolutionary timescale of life.

It is also believed that the first photosynthetic beings would have initially resorted to other available reducing agents like hydrogen ions or hydrogen sulfide in contrast to the modern-day photosynthetic organisms that utilize water as the “prime and only sources of electrons”.

It is believed that cyanobacteria would have appeared on the surface of Earth much later than the first photosynthetic beings. Once appeared they must have saturated the Earth’s atmosphere with oxygen gas and led to its oxygenation. Only after the Earth was oxygenated, the more complex forms of life would have later evolved.

Evolution of photosynthesis

Figure 7: Evolution of photosynthesis. Image Credit: Gema Lorena López Lizárraga and Juan Cristóbal García Cañedo.

Overview

When we compare photosynthesis to other metabolic processes like respiration, we can clearly notice that these two processes are almost opposite to each other. But another point to note is that both the processes in synchrony sustain life on Earth.

You cannot separate respiration from photosynthesis or photosynthesis from respiration and expect life to run normally. It is not possible that way. Let’s try to compare and list some characteristic features of photosynthesis and cellular respiration processes.

Photosynthesis vs. Respiration

  1. Anabolic vs Catabolic:
    • Photosynthesis: Anabolic process
    • Cellular respiration: Catabolic process
      By anabolic, we mean the photosynthesis process “utilizes energy to build biomolecules” like carbohydrates, starch, and sugars. These biomolecules are further utilized by both the plants and the organisms dependent on plants for their nutritional needs. On the other hand, respiration is a catabolic process. This energy is utilized to break down complex molecules to derive nutrition out of them.
  2. Site of occurrence:
    • Photosynthesis: In the chloroplasts of the eukaryotic phototrophic cells.
    • Respiration: Primarily in the mitochondria of the cell.
  3. Reactants:
    • Photosynthesis: Carbon dioxide molecules + Water molecules + Light energy
    • Respiration: Glucose + Oxygen
  4. Products:
    • Photosynthesis: Fixed carbon (carbohydrates) + Oxygen (some cases) + Water
    • Respiration: Carbon dioxide + Water +energy (ATP)
  5. Endergonic vs Exergonic/Endothermic vs Exothermic:
    • Photosynthesis: Endergonic and endothermic
    • Respiration: Exergonic and exothermic
      Just note that these terms endergonic and endothermic both convey the same meaning of “absorbing heat”. And the terms exergonic and exothermic also convey the same meaning of “releasing heat”. The only difference is that –gonics relates to “the relative change in the free energy of the system” while –thermic relates to “the relative change in enthalpy of the system”.
  6. Chemical reactions:
    • Photosynthesis: 6CO2 + 6H2O → C6H12O6+ 6O2
    • Respiration: C6H12O66 + 6O2 → 6CO2 + 6H2O

Photosynthetic Membranes and Organelles

When we begin the discussion on this topic, it’s important that we know that no photosynthesis is possible without the pigment molecules that absorb light. The absorption of sunlight is the most vital step of photosynthesis.

We should also note that the energy of photons is different for every light of different wavelengths. And the energy needed for the photosynthesis to be conducted is of “a very specific wavelength range”.

For the absorption of lights of desired wavelengths, phototrophs organize their pigment molecules in the form of reaction center proteins. These proteins are located in the membranes of the organisms. Let’s learn how these pigment molecules reside inside the organism and how they make the membranes photosynthetic in nature.

prokaryotes - pigments

Figure 8: The pigment molecules of the prokaryotes are located in the cell membranes themselves and not in specialized chloroplasts. Image Credit: Byung Hong Kim and Geoffrey Michael Gadd.

eukaryotes - pigments

Figure 9: The pigment molecules of the eukaryotes are located in the thylakoid membranes of the specialized organelle “chloroplasts”. Image Credit: Arizona.edu.net.

Photosynthetic Pigments

There are 2 types of photosynthetic pigments in the oxygenic photosynthesizing organisms. They are as follows:

Photosynthetic Pigments types

Figure 10: There is a range of photosynthetic pigments that collectively carry out photosynthesis. Image Credit: HalleyHosting.com.

  1. Chlorophyll

Chlorophyll is the green-colored pigment essential for photosynthesis. Let’s try to list its major characteristic features and roles of it.

  1. Carotenoids

Carotenoid is the photosynthetic pigment essential for working in conjunction with chlorophyll. Let’s try to list its major characteristic features and roles of it.

  1. Phycobilins

Phycobilins aren’t present in all the oxygenic photosynthetic organisms. They have a tetrapyrrole structure (no need for magnesium ion).

Organelle for Photosynthesis

What is chloroplast? In eukaryotes, photosynthesis occurs in chloroplasts as they are the designated organelles for the photosynthesis process. There are nearly 10-100 chloroplasts in a typical plant cell.

Inside chloroplasts are the thylakoids; the very specific site for the light capturing. The structure of this very unique part of the chloroplasts is briefly discussed here.

Thylakoid is a membrane-bound compartment in the chloroplasts of eukaryotic organisms. They are also present as such in the cytosol of cyanobacteria (cyanobacteria don’t have chloroplasts but they have simply thylakoids).

These thylakoids are the “primary site of the 1st stage of photosynthesis. i.e. “photochemical reaction” or popularly called “light-dependent reactions of photosynthesis”. The main components of the thylakoid are membrane, lumen, and lamellae. The chlorophyll molecules are present inside these thylakoid membranes.

parts of chloroplast

Figure 11: Chloroplast with labeled parts. Credit: Vossman, CC BY-SA 4.0. For a detailed description of the different parts of the chloroplast, read this: Structure of Chloroplast

Light-dependent Reactions

The first stage of photosynthesis is popularly called “light-dependent reactions”. We choose to call this stage the “1st stage: PHOTOCHEMICAL REACTION STAGE”. It is also called the “thylakoid reaction stage” or “hill’s reaction”.

This stage is marked by 3 essential steps of photosynthesis: Oxidation of water, reduction of NADP+, and ATP formation. The site where these reactions occur is the lamellar part of the chloroplast. The units of light-dependent reactions are quantosomes.

light reaction photosynthesis

Figure 12: Light-dependent or the photochemical reaction stage of photosynthesis. Image Credit: BlueRidgeKitties, CC BY 2.0

Let’s discuss this stage under some subheadings:

The white light that reaches Earth has subparts of different wavelengths together constituting the visible spectrum (390-760nm). But the photosynthetic organisms specifically use a subpart called PAR (Photosynthetically Active Radiation).

PAR ranges from 400-760nm. Blue light is 470-500nm while red light is 660-760nm). The green light (500-580nm) is reflected back by the plants and this is the reason that plants appear green in color. Blue-green light is not used, only blue light is used.

Photosynthetically Active Radiation

Figure 13: PAR or the Photosynthetically Active Radiation ranges from 400nm to 700nm. Image Credit: Light Science Technologies.

chlorophyll absoption spectrum

Figure 14: Different pigments absorb optimally different wavelengths of light. So the absorption spectrum of each pigment is characteristic of each one of them. Image Credit: Fondriest.

IMPORTANT NOTE: The absorption spectrum is calculated for any of the many pigments involved in photosynthesis. Contrastingly, the action spectrum is calculated only for the photochemical reaction performing pigment i.e. chlorophyll-a present at the reaction center. We identify the progress of photochemical reactions as the “evolution of oxygen gas” that primarily happens at the reaction center where only chlorophyll-a is present. Since the action is directly correlated to the specific excitation of chlorophyll-a molecule, the action spectrum is scientifically calculated only for this chlorophyll-a.

Examples:

photosynthetic action spectrum

Figure 15: Absorption spectrum shows the involved pigment in photosynthesis. As chlorophyll-a absorbs in this same exact range, the action spectrum is reflective of chlorophyll-a being the primary pigment. Image Credit: Projects.ncsu.edu.

Let’s briefly describe what actually happens here.

Photosystem II activities

Figure 16: Activities at the PS-II. Image Credit: Carc.unm.edu.

Now, if you are wondering how the first electron lost by the 1st chlorophyll is replenished to keep this cycle going, read on. The answer to this query is “photolysis of water molecules”. The chlorophyll molecule regains the lost electron when the “oxygen-evolving complex” in the thylakoid membrane carries out the photolysis of water. The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called photolysis, which releases oxygen.

Many scientists had a doubt about the source of oxygen in photosynthesis. Some speculated the oxygen atom of the CO2 gas is the source of oxygen post-photosynthesis. But it was the collective contribution of some 4 scientists that gave clarity on this topic.

C.B. Van Niel worked on purple photosynthetic bacteria (Chromatium vinosum) and found out that the source of oxygen is the oxidation of water molecules (‘indirect evidence’). While Ruben, Hassid, and Kamen carried out an isotopic study that gave ‘direct evidence’ of oxygen-evolving from H2O molecules and not CO2 molecules.

Hydrolysis of 2 molecules of water leads to the evolution of 1 molecule of oxygen gas. The photosynthesis equation for light-dependent reactions (non-cyclic electron flow) or the chemical formula for photosynthesis:

2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2

The photochemical reaction (or the light-dependent reactions) can be classified as:

Photochemical Reactions: Cyclic vs. Non-cyclic phosphorylation
Feature Cyclic phosphorylation Non-cyclic phosphorylation
Oxidation of water No Yes
Generation of oxygen gas No Yes
Reduction of NADP No Yes
Need for final electron donor/acceptor No Yes (Donor=H2O, Acceptor=NADP+)
ATP production Yes Yes
Photosystems involved Only PS1 Both PS1 and PS2

Data Source: Akanksha Saxena of Biology Online

Diagram of Z-scheme

Figure 17: Diagram of Z-scheme. Image Credit: Rajni Govindjee.

Light-Independent Reactions (Carbon-fixation Reaction)

Also called the carbon fixation process, the “light-independent reactions” is a misnomer as Science has now already proved that the second stage of photosynthesis isn’t really light-independent reactions. Though it doesn’t need direct light, indirect light is involved even in this process. We choose to label this stage of photosynthesis as the “2nd stage: CARBON-FIXATION REACTION STAGE”, which is also called:

overview of Calvin Cycle

Figure 18: Illustration for an overview of the Calvin Cycle. Image Credit: Halleyhosting.com.

The inputs for the Calvin cycle in most plants come from the previously occurred photochemical reaction.
In this cycle, the carbon dioxide produced is fixed to a glucose molecule. To be very specific, the Calvin cycle directly doesn’t produce glucose, rather it produces glyceraldehydes-5-phosphate (G-3-P). Glucose is formed after these G-3-P molecules move into the cytosol from the chloroplast.

It consists of primarily 3 steps as follows:

  1. Carboxylation: Acceptance of CO2 by RuBP which is a 5-carbon compound and the CO2-acceptor). 2 molecules of 3-phosphoglycerate are generated as the result of the carboxylation process.
  2. Reduction: Generation of 3C/4C/5C/6C/7C molecules.
  3. Regeneration of RUBP: 3 molecules of RuBP are regenerated.

In totality, 3 molecules of CO2 produce 1 molecule of G-3-P. This uses 9 ATPs and 6 NADPHs. And, 6 molecules of CO2 produce 2 molecules of G-3-P which further produce 1 molecule of glucose. This uses 18ATPs and 12 NADPHs.

The main enzyme is RuBisCo. It’s a multi-enzyme complex with 8 large and 8 small subunits. The substrates for this enzyme are CO2, O2, and RuBP. An essential ion for the biological functioning of this enzyme: Mg2+. The role of RuBisCo is that it captures carbon dioxide gas from the atmosphere and utilizes the NADPH from the 1st stage (photochemical reaction/light-dependent reaction stage) to fix the CO2.

The equation of dark reaction of photosynthesis/light-independent reaction stage/2nd stage is:
3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

The simple carbon sugars formed via the C3 cycle are utilized by the biological systems to form complex organic compounds like cellulose, precursors for amino acids synthesis and thereby proteins, precursors for lipids, and the source of fuel for respiration.

Important Point To Note: It happens in all the photosynthetic organisms as the basic carbon-fixation step.

Calvin cycle steps diagram

Figure 19: The important steps of the Calvin cycle are shown. Image Credit: TIM GUNTHER.

There are many carbon concentrating mechanisms to increase the carbon dioxide levels and the carbon fixation process like C4, CAM, etc.

C4 plant cycle

Figure 20: C4 plant cycle. Notice the separation of early and late steps in space. The spatial separation between mesophyll cells and bundle sheath cells helps in increasing the photosynthetic efficiency of the C4 plants. Image Credit: Moore et al.

CAM, C3, C4 plant cells

Figure 21: CAM plants differ from C3 and C4 as they display temporal (time) separation of the carbon fixation process steps. Image Credit: BioNinja.

Land plants display different types of photosynthesis based on their requirements and environmental constraints. They are C3, C4 +C3, and CAM+ C3 types of photosynthesis.

Aquatic plants and algae display some extra features in the photosynthetic machinery. These features further refine and define the smooth functioning and efficiency of photosynthesis.

Example: Cyanobacteria photosynthesis – cyanobacteria have carboxysomes that help in enriching the concentration of carbon dioxide around the RuBisCO enzyme. This directly increases the photosynthetic rates. The distinguished and specially enabled enzyme in the carboxysomes is called “carbonic anhydrase”. The carbonic anhydrase possesses the ability to evolve and release CO2 from the dissolved hydrocarbonate ions (HCO-). As soon as the CO2 is released, RuBisCo takes care that it doesn’t go to waste.

Cyanobacterial cell photosynthesis

Figure 22: Notice the role of carbonic anhydrase in releasing CO2 from the dissolved hydrocarbonate ions (HCO-). Image Credit: Niall Mangan.

Order and Kinetics

There are innumerable reactions and processes involved in the biological mechanism of photosynthesis. Besides the normal flow of photosynthesis, there are some plant-specific and condition-specific additional steps that further complicate the mechanism.

Since every biological mechanism has a lot of enzymes, factors, cofactors, substrates, and entities involved, photosynthesis is no different.

Let’s try to list some kinetics-specific pointers that may help.

Evolution

As discussed in the overview and starting of this article, the early photosynthetic organisms must have been primarily “anoxygenic” in nature. These bacteria used some other source than water molecules as their primary electron donors. Even the geological evidence aligns with this fact as the early atmosphere of Earth was highly reducing in nature. Some speculated organisms of the early evolutionary phase are :

After this, some filamentous photosynthetic organisms are expected to have evolved. This is scaled to be an occurrence of some 3.4 billion years old timeline. It is around 2 million years ago that oxygenic photosynthesis is believed to have evolved.

The modern and more commonly known photosynthesis in plants and most of the photosynthetic prokaryotes= Oxygenic (Electron donor= Water molecules)

Symbiosis and the origin of chloroplasts

There are some animal groups that have the ability to form and establish symbiotic relationships with photosynthetic organisms. By establishing such a relationship, these organisms can directly rely upon their photosynthetic partner for energy and food requirements. Some examples of such animal groups are:

When such symbiotic relationships are established, it’s sometimes observed that some genes of the plant cell’s nucleus get transferred to the animal cell. (Observed in some slugs).

Photosynthetic slug

Figure 27: Photosynthetic slug because of the symbiotic relationship that it forms with algae. Image Credit: Patrick J. Krug

Such symbiosis is popularly claimed to be the source of chloroplast evolution. As we notice many similarities between the photosynthetic bacteria and chloroplasts, the evolution of chloroplasts is often hinted to have occurred from these bacteria. Some of the common features between the 2 are:

It is for all these commonalities the “endosymbiotic theory” had been proposed for the evolution of chloroplasts and mitochondria in the eukaryotic cells. According to the endosymbiotic theory, the early eukaryotic cells are believed to have acquired the photosynthetic bacteria by the process of endocytosis). Those early eukaryotic cells after acquiring the photosynthetic bacteria transformed to be self-sustainable and became the “first plant cells”. (Mitochondria photosynthesis is true, they are associated with respiration!)

Endosymbiotic theory

Figure 28: Endosymbiotic theory. Image Credit: Plantlet.org.

Photosynthetic eukaryotic lineages

Photosynthetic eukaryotic lineages include:

Cyanobacteria and the evolution of photosynthesis

Almost all the prokaryotes carry out anoxygenic photosynthesis in contrast to cyanobacteria, which perform oxygenic photosynthesis. This ability to carry out oxygenic photosynthesis is speculated to have evolved at least 2450–2320 million years ago. The first photosynthetic cyanobacteria might not have been oxygenic as Earth’s atmosphere had no oxygen then.

This topic still requires more scientific study to bring out conclusive results. From the paleontological evidence, it is claimed that the 1st cyanobacteria evolved around 2000 Ma.

For the initial years of the Earth’s oxygen-rich environment (after the oxygen-evolving mechanism evolved), cyanobacteria are claimed to be the “principal primary producers of oxygen”. Even to date, cyanobacteria have been proven vital for marine ecosystems. They’re the primary producers of oxygen in oceans.

Cyanobacteria also fix nitrogen electrons fixation and play a role in biological nitrogen cycles.

Experimental History

We will list the long experimental history in deciphering the extensive photosynthesis process through the ages.

Discovery, Refinements, and Development of the concept

Find out the discovery, refinements, and development of photosynthesis as summarized in the table below:

Experimental History – Photosynthesis
No. Scientist Contribution
1 Stephen Hales Role of air and light in building a plant body.
2 Joseph Priestley Role of air in the growth of green plants.
3 Jan Ingenhousz Role of sunlight in the plant’s functioning of releasing oxygen.
4 Jene Senebier The fact that plants utilize CO2.
5 N.T. de Saussure The volume of CO2 consumed by plants=Volume of O2 liberated by plants.
6 Pelletier and Caventou Coined the term “chlorophyll”.
7 F.F. Blackman Photosynthesis= 2-step process (Light and Dark Reaction Concept), Gave the “Law of limiting factors”.
8 Warburg Flashing experiments in Chlorella.
9 Emerson and Arnold Determined that the 1st stage of photochemical/light reactions has 2 distinct photochemical processes. (led to the elucidation of 2 different pigment systems)
10 Robert Hill Proved that oxygen evolution doesn’t require the presence of CO2. The presence of sunlight, water, and a suitable hydrogen acceptor is enough chloroplasts to release oxygen. The light reaction (1st stage) is often called Hill’s reaction in his memory and honor.
11 Van Niel Proved that the source of oxygen is H2O and not CO2. Also proved that the hydrogen released from oxidizable compounds reduced carbon dioxideto carbohydrates.
12 Ruben, Hassid, Kamen Proved that the source of oxygen is H2O (by doing radioactive studies).
13 Julius von Sachs Photosynthesis site in plants= Green parts where chlorophyll is present.
14 T.W. Engelmann Described the importance of the red and blue wavelengths of light. Also described the 1st action spectrum of photosynthesis
15 Melvin Calvin Described the reactions that convert the CO2 into sugars. C3 or Calvin cycle is named in his honor.
16 M.D. Hatch and C.R. Slack Described the C4 cycle and hence the C4 cycle is also called Hatch and Slack cycle.
17 Hill and Bendall Proposed scheme of light reaction.
18 Huber et al. Studied 3-D structure of reaction center.
19 Charles Reid Barnes Coined the terms: Photosyntax and photosynthesis.

Data Source: Akanksha Saxena of Biology Online

C3 : C4 photosynthesis research

Several studies were conducted using isotopes of radioactive elements to identify the various aspects of the photosynthetic process. A number of organisms like Chlorella, Stellaria media, Cladophora, Spirogyra, Rhodopseudomonas, sulfur bacteria, green plants like maize, etc have been used to understand the photosynthesis process over the years. Gas exchange studies, isotopic studies, light spectrum studies, radioactive studies, plant anatomical and physiological studies, studies involving roles of carbon dioxide and water, etc have all together opened the gates for our deeper understanding of this topic.

Factors

The 3 main factors that directly affect the photosynthesis process are:

Although there are many more corollary factors, these 3 are the most important ones.

  1. Light intensity (irradiance), wavelength, and temperature

Light is an essential factor for photosynthesis. It directly affects the rate of it. There are 3 different parameters that we should look into:

  1. Light intensity: Optimum light intensity varies from plant to plant. There are 2 types of plants based on the intensity of light they optimally need to grow under.
    • Sciophytes: Grow under “diffuse” light. Example: Oxalis
    • Heliophytes: Grow under “direct: light. Example: Dalbergia
  2. Light quality: PAR as previously discussed is the quality or the fraction of light energy that is ‘photosynthetically active’ in nature. It ranges from 400-700nm in wavelength.
  3. Duration of light: This parameter doesn’t affect the rate of photosynthesis but affects the total photosynthetic output.

  4. Temperature

Temperature is another very important factor for photosynthesis. The carbon dioxide fixation stage (dark reactions) is affected by the temperature. The optimum temperature range is 20-25°C for the C3 plants. The optimum temperature range for C4 plants is a little higher ranging from 30-45 °C. 3. ### Carbon dioxide levels and photorespiration
Carbon dioxide concentration is the major factor in determining the rate of photosynthesis. There is no carbon-dioxide enriching system in C3 plants like the C4 plants. So, if you increase the concentration of CO2 in the system, the photosynthetic rate of C3 plants will increase as the CO2 concentration increases. On the other hand, the photosynthetic yield of the C4 plant won’t increase in such a scenario.

Featuring… “The curious case of RuBisCO and PEP Carboxylase”

Imagine an equal concentration (50-50%) of the two isotopes of carbon, C-12 and C-13, in the form of 12CO2 and 13CO2, made available to both C3 and C4 plants. Now, can you tell which isotope of the carbon will be fixed more or less by the two types of photosynthetic organisms? Can you guess if there would be a “preferable” isotope between the two? Do you think C3 plants will fix the 12CO2 and 13CO2 equally or unequally? Or do you think the 12CO2 and 13CO2 incorporation would have a biased ratio in any of the two (C3/C4 plants)????

The answer to this lies in the major carbon fixing enzyme involved.

Answer the quiz below to check what you have learned so far about photosynthesis.

References

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