Photosynthesis Process – Definition, Diagram, Reactions (Steps), Equations

We all know that Photosynthesis is a widespread process that occurs in all green plants. It is one of the reasons for the sustainability of our planet Earth. Plants, being autotrophs, carry out Photosynthesis to make their food.

Autotrophs convert the energy coming from the sun into a form that can be useful for them. Photosynthesis is a gateway reaction for converting energy from inorganic format to organic form. 

In this article, we will be dissecting every aspect of the process of Photosynthesis, its mechanism, and its importance.

Biological Definition of Photosynthesis

Photosynthesis is a process by which autotrophic organisms convert sunlight into chemical energy that plants use.

Biochemical Definition of Photosynthesis

Photosynthesis is a process in which energy-poor, inorganic, oxidized compounds of carbon (CO₂) and hydrogen (H2O) are reduced to energy-rich, reduced, organic compounds of carbohydrates (Glucose) by using light energy coming from the sun.

Types of Photosynthesis

There are two types of Photosynthesis based on the electron donors that initiate the reaction. These types are oxygenic Photosynthesis and anoxygenic Photosynthesis.

Oxygenic Photosynthesis

• In this reaction, light energy transfers electrons from water molecules, i.e., water molecules act as electron donors. Water is oxidized in this reaction.
• Oxygen is the main by-product of oxygenic Photosynthesis.
• This type of photosynthetic reaction is more common.
• It is found in green plants, cyanobacteria, and green algae.
• It is just a counter-reaction for respiration. It utilizes carbon dioxide and reintroduces oxygen molecules into the atmosphere.

The net equation for this process is: 6CO2 + 6H2O ——-(sunlight)——> C6H12O6 + 6O2

Anoxygenic Photosynthesis

• It is the phototrophic reaction in which light energy is converted to chemical energy without producing Oxygen as a by-product.
• Water is not used as an electron acceptor in this process. Other electron donors such as hydrogen sulfide are involved in these reactions.
• Organisms such as Green sulfur bacteria, phototrophic purple bacteria, phototrophic heliobacteria, etc., carry out anoxygenic Photosynthesis.

The net equation for the process is : CO2  + 2H₂S + light energy   →    (CH₂O)  + H₂O  + 2S

Site of Occurrence

• Photosynthesis is a continuous process. It takes place in the Chloroplasts present in the plants.
• Chloroplasts are double membraned organelles present in plants and green algae. 
• Chloroplasts are also found in many plant-like protists that can undergo Photosynthesis in the presence of light. For example, Euglena.
• Chlorophyll has a green pigment, i.e., chlorophyll, that captures light energy. This light energy is one of the initiators of Photosynthesis.
• There are clusters of chlorophyll present in the chloroplasts; these clusters are called photosystems.

Role of Photosystems

Photosynthetic pigments are arranged in the forms of clusters called photosystems. These photosystems are the essential unit cells for the process of Photosynthesis.

There are two types of photosystems, i.e., photosystem I (PSI) and photosystem II (PS II).

• PS-I includes chlorophyll molecules that absorb maximum light at 700 nm. It is also called P700.
• PS-II includes chlorophyll molecules that absorb maximum light at 680 nm. It is also called P680.
• Each photosystem consists of an ‘antenna complex’ and ‘reaction center.’
• Antenna complex transmits energy from one chlorophyll to another towards the reaction center.
• The Reaction center contains one or more chlorophyll molecules associated with the electron transport chain’s primary electron acceptor and electron carriers.

Steps of Photosynthesis

• Photosynthesis is a complex process that includes a highly complex series of reactions simultaneously.
• For the sake of convenience and understanding, we have divided the process into two main parts, i.e., Light Reactions and Dark Reactions.

Light Reactions

• Light reactions of Photosynthesis take place in the presence of light.
• These reactions take place in the thylakoid membrane of the chloroplast. 
• Light energy is absorbed by the thylakoid membrane in these reactions and converted into chemical energy.
• As a result, NADP and ADP get converted into ATP and NADPH.

Dark Reactions

• These reactions do not need light directly. It means they can take place in dark as well as light. 
• The NADPH and ATP produced in the light reactions act as energy sources for sugar molecules, i.e., Glucose.
• CO2 is utilized to make sugars in dark reactions.
• These reactions are also called the Calvin Cycle.

The Complete Pathway of Photosynthesis

Photosynthesis in plants and other autotrophs is a long and extensive process. Now we will discuss the complete process of Photosynthesis in green plants, starting from absorption of water, Oxygen, and sunlight and ending at the storage of sugars.

Absorption of Carbon Dioxide

Carbon Dioxide is absorbed through the stomata present in the lower epidermis of the leaves. This Carbon Dioxide enters through stomata and diffuses through air spaces between the mesophyll cells present in the leaves.

Absorption of Water

Water enters leaves through xylem vessels. This water moves up to the leaves from the soil in a chain of water molecules. The level of water uptake is determined by the rate of Photosynthesis and the rate of transpiration in the leaves.

Absorption of Sunlight

Light energy coming from the sun is absorbed by the chlorophyll molecules present in the chloroplast present in the leaves. Only 1% of the sunlight that falls on the surface of leaves is absorbed; the rest is reflected. 

Photophosphorylation 

• During photophosphorylation, light energy is absorbed by P_680, and an electron is excited to a higher level in the electron transport chain. 
• The electron, when excited, makes a hole in the photosystem. This hole is filled with electrons released by the enzymatic splitting of water molecules (photolysis).
• The oxygen ions combine to form O₂. The photoexcited electrons pass through the electron transport chain, and the energy, while moving down the electron transport chain, is used to produce ATP(photophosphorylation). 
• When it reaches the bottom of ETC, this electron fills a hole in the P₇₀₀.
• Different proteins use the photoexcited electron from P₇₀₀ is used by other proteins transfer this electron to NADP and convert it to NADPH.
• Sometimes plants switch from non-cyclic photophosphorylation to cyclic phosphorylation to balance the levels of ATP in the cells.
• This process is also called a Z-scheme.

Calvin Cycle

The Calvin cycle is the second name of light-independent or dark reactions. This cycle consists of three main parts.

1. Carbon Fixation: It refers to the incorporation of CO₂ into Ribulose Bisphosphate. An enzyme, i.e., Rubisco, catalyzes the process. The product is a highly unstable 6-carbon intermediate that breaks down into two molecules of 3-phosphoglycerate.
2. Reduction of 3-PGA: Each 3-PGA receives a molecule of ATP and becomes 1,3-bisphosphoglycerate. Each 1,3-bisphosphoglycerate accepts an electron from NADPH and becomes G3P. One Calvin cycle gives one molecule of G3P. Two processes provide a pair of G3P that can form a glucose molecule.
3. Regeneration of Ribulose Bisphosphate: The rest of the molecules of G3P rearrange themselves by a series of enzymatically catalyzed reactions to form the original molecule of RuBP, i.e., Ribulose Bisphosphoglycerate, which again is ready to accept CO₂ for the next cycle. 

Transport of Glucose

Glucose produced by Photosynthesis is then utilized in the process of respiration. The excess Glucose is then transported to different parts of the plants (sinks), where it is stored in the form of sucrose.

Factors affecting Photosynthesis

There are several physicals and internal factors in the plant body that can affect the rate of photosynthesis in the plant. We will discuss some of the factors below:

Intensity of Light

• The intensity of light is a physical factor that affects the rate of Photosynthesis. 
• Light is necessary for the light reactions of Photosynthesis. 
• This is why no photosynthesis occurs at night because there is no light to initiate the electron transport change that produces ATP and NADPH in the light reactions.
• As light intensity increases, the rate of Photosynthesis also increases. 
• It becomes constant after a certain point. After this point, the rate of Photosynthesis remains constant even if the light intensity increases.

Quality of Light

• The quality of light is an external factor that also affects the rate of Photosynthesis. 
• The photosystems present in the chloroplast absorb only a fixed wavelength of light.
• The blue color light shows a high rate of Photosynthesis because it has high energy (smaller wavelength). 
• The green light has shown the slowest rate of Photosynthesis because the chlorophyll reflects green color, so the chlorophyll does not absorb the green color.
• The red color shows the maximum rate of Photosynthesis.

Carbon Dioxide Concentration

• Carbon dioxide concentration in the atmosphere directly affects the rate of Photosynthesis in plants.
• Higher CO2 concentration will result in a higher rate of Photosynthesis and vice versa.

Temperature

• Photosynthesis is an enzyme-controlled reaction so that temperature changes can affect the efficiency of the enzymes.
• Enzymes have optimum performance at around 30-35 degrees celsius.
• For every 10 degrees increase in the temperature, the rate of Photosynthesis almost doubles.
• After about 35 degrees celsius, the enzyme denaturation starts, so the rate of Photosynthesis drops after this temperature.

Water Concentration

• Water is one of the primary reactants involved in Photosynthesis, so water concentration in the plant dramatically affects the rate of Photosynthesis.
• Higher water concentration will surely increase the rate of Photosynthesis.
• The rate of Photosynthesis drops in dry seasons because less water is available for the plant.

Dichlorophenyl Dimethylurea 

• DCMU, abbreviated as Dichlorophenyl Dimethylurea, is a herbicide. 
• It is a particular inhibitor of NADPH production in the light reactions of Photosynthesis. 
• It means it inversely affects the ability of plants to perform Photosynthesis. 
• It affects only the light reactions of Photosynthesis.

Chlorophyll concentration in leaves

• The presence of chlorophyll is the index of the plant’s photosynthetic capacity. 
• The number of chlorophyll molecules presents per unit area of the leaf gives us an idea of the capacity of the plant to perform Photosynthesis.
• Relative concentrations of different types of chlorophyll molecules are also a significant factor in the rate of photosynthesis and the plant’s photosynthetic capacity.

Age of Leaf

• As the leaves age, there is an accumulation of several by-products in the leaf.
• This significantly reduces the efficiency of the leaf to perform processes like Photosynthesis and respiration.
• The main reason behind the decrease in efficiency is the reduction of space for chloroplasts due to the accumulation of wastes in the leaf.

Significance of Photosynthesis

1. Photosynthesis is the main gateway for energy transfer from the inorganic world to the organic world. Plants convert the energy in the photons of light into chemical energy.
2. Photosynthesis is the primary source of replenishment of atmospheric Oxygen, so we can say Photosynthesis is the process that keeps equilibrium in the composition of our atmosphere.
3. Photosynthesis is the energy source for all the heterotrophs that take their food from plants. 

Frequently Asked Questions

References

1. https://www.nationalgeographic.org/encyclopedia/photosynthesis/
2. https://www.livescience.com/51720-photosynthesis.html
3. https://bio.libretexts.org/Bookshelves/
4. https://www.khanacademy.org/science/ap-biology/cellular-energetics/photosynthesis/a/intro-to-photosynthesis
5. https://microbenotes.com/photosynthesis/