4.1 Photosynthesis

Introduction

Photosynthesis is the most fundamental bioenergetic process on Earth. It is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. This process is essential for life because it produces oxygen and provides the energy-rich organic compounds that almost all living organisms depend on.

Definition

Photosynthesis: Chemically, photosynthesis is a "redox" process in which CO₂ (an oxidized form of carbon) is reduced into glucose (a reduced form of carbon). Water acts as a reducing agent which is oxidized into oxygen during this process.

Bio-energetically: Photosynthesis can be defined as an energy conversion process in which energy-poor molecules (CO₂ and H₂O) are transformed into energy-rich molecules such as glucose. The extra energy is absorbed in the form of sunlight by the photosynthetic pigments.

In simple words: Photosynthesis is the process by which plants use sunlight, carbon dioxide, and water to make glucose and oxygen.

Core Concept

• Photosynthesis is a redox process.
• CO₂ is reduced to glucose.
• Water is oxidized to oxygen.
• Light energy is absorbed by photosynthetic pigments.
• The process involves the interaction of:

Overall Reaction of Photosynthesis

6CO₂ + 12H₂O [→Chlorophyll]Light C₆H₁₂O₆ + 6O₂ + 6H₂O

• CO₂ = Carbon dioxide (oxidized form of carbon)
• H₂O = Water (reducing agent)
• C₆H₁₂O₆ = Glucose (reduced form of carbon)
• O₂ = Oxygen (oxidation product of water)
• Light = Energy source
• Chlorophyll = Photosynthetic pigment

In simple words: - Carbon dioxide + Water + Light energy → Glucose + Oxygen + Water

Detailed Explanation

Role of Light

Biological energy transformations obey the laws of thermodynamics.

• Sunlight is the ultimate source of energy.
• It flows continuously one way from the Sun to life and ecosystems.
• Sunlight is an electromagnetic form of energy.
• Light has prime importance as light energy is converted to chemical energy.
• This chemical energy is then converted to other forms of energy essentially required for life processes.
• This trapped light energy is used to convert low-energy inorganic compounds to high-energy organic compounds.

Electromagnetic Spectrum

• The full range of electromagnetic radiation in the universe is called the electromagnetic spectrum.
• Visible light is only a small part of the spectrum.
• Visible light is between 380 nm and 750 nm.
• This range is not only seen by the naked eye but is also effective for the process of photosynthesis.

Action Spectrum

Definition: A graph which shows the effectiveness of different wavelengths of light for the process of photosynthesis is called an action spectrum.

• If a plant is illuminated in different colours of light one by one, the rate of photosynthesis is measured.
• The data obtained is plotted in a graph.
• The rate of photosynthesis is variable in different colours of light.
• Analysis of the action spectrum indicates that:
• Blue (430 nm) and Red (670 nm) wavelengths of light are the most effective for the process of photosynthesis.

Role of Carbon Dioxide in Photosynthesis

• Carbon dioxide is one of the raw materials of the photosynthetic reaction.
• It acts as a carbon source for the synthesis of organic compounds in photosynthesis.
• Plants are therefore known as autotrophs because they use low-energy inorganic compounds like CO₂ for the synthesis of high-energy organic compounds by their own.
• Carbon dioxide is utilized in the dark or light-independent reaction (Calvin cycle) of photosynthesis.
• Air contains about 0.03 to 0.04 percent of carbon dioxide.
• Land plants use this atmospheric carbon dioxide for photosynthesis.
• Increase in CO₂ concentration in the environment increases the rate of the photosynthetic process.
• But when CO₂ concentration rises above 1%, the rate of photosynthesis rapidly slows down due to stomatal closing.
• Dissolved carbon dioxide, bicarbonates, and carbonates are present in water, which are used by aquatic photosynthetic organisms as carbon source.

Science Tidbits (from textbook): The rate of photosynthesis is directly proportional to the CO₂ consumed or O₂ released. Therefore, it can be measured by measuring the amount of CO₂ consumed or by measuring the amount of O₂ released during the process in a specific time.

Role of Water in Photosynthesis

• Water is another raw material for photosynthesis along with CO₂.
• Water acts as hydrogen and electron donor in photosynthesis.
• During the light-dependent phase of photosynthesis, water molecule is broken down into:
• 2H⁺
• 2e⁻
• ½O₂
• This reaction is called photolysis of water.
• Electrons released by water molecule are used to compensate the photo-excited electron lost by the chlorophyll-a molecule present in the reaction center of photosystem-II (P680).
• 2H⁺ ions and 2e⁻ are taken up temporarily by NADP⁺ to become NADPH.
• NADPH is then used to reduce CO₂ in the light-independent reaction.
• The oxygen which is produced is released into the atmosphere.

Historical Background:

• This role of water in photosynthesis was first reported by Van Niel in 1931.
• He hypothesized that plants split water as a source of hydrogen, releasing oxygen as a byproduct.
• This observation was based on investigations of photosynthesis in bacteria that make carbohydrates from carbon dioxide, but do not release oxygen.
• Van Niel's hypothesis was confirmed in 1940 when for the first time ¹⁸O in biological research was used.
• In the first experiment, water was made of ¹⁸O. The water tagged ¹⁸O was added to an alga suspension. The oxygen evolved during photosynthesis was found to be radioactive. It was separated and identified.
• In another experiment, carbon dioxide with tagged ¹⁸O was added. The oxygen evolved contained none of the isotopes.
• Thus, the source of evolved oxygen was proved to be water.

Summary Equation:

6CO₂ + 12H₂O Light→ C₆H₁₂O₆ + 6H₂O + 6O₂

Photosynthetic Pigments and Their Major Types

• Photosynthetic organisms have different types of pigments for the absorption of light.
• Pigment: Any substance that absorbs light energy for use in the food-synthesizing process.
• Algae and photosynthetic bacteria have chemically different types of pigments than plants.
• Pigments of bacteria are present in their cytoplasm.
• Photosynthetic pigments of plants are embedded in thylakoid membranes (grana lamellae) within their chloroplasts.
• Higher plants have two major groups of pigments:

Chlorophyll

• Chlorophylls absorb mainly violet, blue, orange, and red wavelengths.
• Green and yellow are least absorbed and reflected.
• Two major types of chlorophyll:

• A bluish-green pigment.
• Found in all photosynthetic organisms except photosynthetic bacteria.

• A yellowish-green pigment.
• Also found in all photosynthetic organisms except brown and red algae.

Absorption Spectrum

Definition: A graph which shows the relative absorption of different wavelengths of light by a particular pigment is called the absorption spectrum of that pigment.

• The absorption of different wavelengths of light by a particular pigment can be determined by the help of a spectrophotometer.
• The data of spectrophotometer is represented by a graph.
• The absorption spectra of different pigments indicate that they absorb different wavelengths of visible light.
• These wavelengths are not absorbed at the same rate.
• The main photoreceptors are chlorophyll-a and chlorophyll-b.
• Both show more absorption in:
• Violet-blue (400 nm to 470 nm) regions
• Orange-red (630 nm to 660 nm) regions
• Carotenoids show more absorption at 430 nm to 500 nm.

• May be related to the differences in their:
• Molecular formulae
• Structural arrangements
• Physical properties (e.g., colour)

Science Tidbits (from textbook): Carotenoids such as lutein, zeaxanthin, and meso-zeaxanthin are also found in the macula in the center of the retina and absorb up to 90% of blue and violet light. The absorption of this intense light by these pigments protects the vital and light-sensitive part of the eye (retina) from oxidative damage.

Critical Thinking (from textbook)

• How does the absorption spectrum of chlorophyll-a differ from the absorption spectrum of chlorophyll-b?
• How does the absorption spectrum of chlorophyll-a differ from the action spectrum?

Intermediate Level

Overall Reaction in Detail

6CO₂ + 12H₂O [→Chlorophyll]Light C₆H₁₂O₆ + 6O₂ + 6H₂O

• CO₂ is reduced to glucose.
• H₂O is oxidized to oxygen.

• Photosynthesis is an energy conversion process.
• Energy-poor molecules (CO₂ and H₂O) → Energy-rich molecules (glucose).
• Light energy is absorbed by photosynthetic pigments.

Role of Light in Detail

Role of CO₂ in Detail

Role of Water in Detail

Photosynthetic Pigments in Detail

Absorption Spectrum in Detail

Important Points

• 1. Chemically, photosynthesis is a "redox" process in which CO₂ (an oxidized form of carbon) is reduced into glucose (a reduced form of carbon).
• 2. Water acts as a reducing agent which is oxidized into oxygen during this process.
• 3. Bio-energetically, photosynthesis can be defined as an energy conversion process in which energy-poor molecules (CO₂ and H₂O) are transformed into energy-rich molecules such as glucose.
• 4. The extra energy is absorbed in the form of sunlight by the photosynthetic pigments.
• 5. The overall reaction of photosynthesis:
• 6CO₂ + 12H₂O [→Chlorophyll]Light C₆H₁₂O₆ + 6O₂ + 6H₂O
• 6. This process involves the interaction of sunlight, pigments, water, and carbon dioxide.
• 7. Role of Light:
• - Sunlight is the ultimate source of energy.
• - It is an electromagnetic form of energy.
• - Visible light is only a small part of the spectrum between 380 nm and 750 nm.
• - The effectiveness of a particular wavelength of light for photosynthesis depends upon its absorption in the plant body.
• - The action spectrum indicates that blue (430 nm) and red (670 nm) wavelengths are the most effective for photosynthesis.
• 8. Role of Carbon Dioxide:
• - Carbon dioxide is one of the raw materials of the photosynthetic reaction.
• - It acts as a carbon source for the synthesis of organic compounds.
• - Plants are known as autotrophs.
• - CO₂ is utilized in the dark or light-independent reaction (Calvin cycle).
• - Air contains about 0.03 to 0.04 percent of carbon dioxide.
• - Increase in CO₂ concentration increases the rate of photosynthesis up to 1%.
• - Above 1%, the rate slows down due to stomatal closing.
• 9. Role of Water:
• - Water is another raw material for photosynthesis along with CO₂.
• - Water acts as hydrogen and electron donor in photosynthesis.
• - During the light-dependent phase, water molecule is broken down into 2H⁺, 2e⁻, and ½O₂.
• - This reaction is called photolysis of water.
• - The role of water in photosynthesis was first reported by Van Niel in 1931.
• - Van Niel's hypothesis was confirmed in 1940 using ¹⁸O in biological research.
• 10. Photosynthetic Pigments:
• - Higher plants have two major groups of pigments: chlorophylls and carotenoids.
• - Chlorophyll-a is a bluish-green pigment found in all photosynthetic organisms except photosynthetic bacteria.
• - Chlorophyll-b is a yellowish-green pigment found in all photosynthetic organisms except brown and red algae.
• - Chlorophylls absorb mainly violet, blue, orange, and red wavelengths.
• - Green and yellow are least absorbed and reflected.
• 11. Absorption Spectrum:
• - A graph which shows the relative absorption of different wavelengths of light by a particular pigment is called the absorption spectrum of that pigment.
• - The main photoreceptors are chlorophyll-a and chlorophyll-b.
• - Both show more absorption in violet-blue (400–470 nm) and orange-red (630–660 nm) regions.
• - Carotenoids show more absorption at 430–500 nm.

Remember

• - Photosynthesis = redox process; CO₂ reduced, H₂O oxidized
• - Overall reaction: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
• - Role of light: Sunlight is the ultimate source; blue and red are most effective
• - Role of CO₂: Carbon source; utilized in Calvin cycle; 0.03–0.04% in air
• - Role of water: Hydrogen and electron donor; photolysis of water
• - Photolysis of water: 2H⁺ + 2e⁻ + ½O₂
• - Van Niel (1931): Proposed water is the source of oxygen
• - ¹⁸O experiment (1940): Confirmed water is the source of oxygen
• - Chlorophyll-a: Bluish-green; all photosynthetic organisms except bacteria
• - Chlorophyll-b: Yellowish-green; all photosynthetic organisms except brown and red algae
• - Absorption spectrum: Graph showing absorption of different wavelengths
• - Action spectrum: Graph showing effectiveness of different wavelengths
• - Most effective wavelengths: Blue (430 nm) and Red (670 nm)

Common Mistakes

• 1. Thinking oxygen comes from CO₂ – Oxygen comes from water (proved by ¹⁸O experiments).
• 2. Confusing absorption spectrum and action spectrum – Absorption spectrum = what is absorbed; Action spectrum = what is effective.
• 3. Thinking green light is best for photosynthesis – Green light is least effective (reflected).
• 4. Thinking chlorophyll is the only pigment – Carotenoids are also important pigments.
• 5. Confusing chlorophyll-a and chlorophyll-b – Chlorophyll-a is bluish-green; Chlorophyll-b is yellowish-green.
• 6. Forgetting the role of water – Water is the hydrogen and electron donor.
• 7. Thinking photosynthesis only occurs in light – The dark reaction (Calvin cycle) does not require light directly.
• 8. Confusing the redox process – CO₂ is reduced; H₂O is oxidized.
• 9. Thinking CO₂ concentration can increase indefinitely – Above 1%, the rate slows down due to stomatal closing.
• 10. Forgetting Van Niel's contribution – Van Niel proposed that water is the source of oxygen.
• 11. Thinking oxygen is a waste product – Oxygen is essential for aerobic life.
• 12. Confusing the role of light and pigments – Light is the energy source; pigments absorb the light.

Quick Revision

• Chemically photosynthesis is a "redox" process in which CO₂ (an oxidized form of carbon) is reduced into glucose (a reduced form of carbon).
• Water acts as reducing agent which is oxidized into oxygen during this process.
• Bio-energetically photosynthesis can be defined as an energy conversion process in which energy poor molecules i.e., CO₂ and H₂O are transformed into energy rich molecule such as glucose.
• The extra energy is absorbed in the form of sunlight by the photosynthetic pigments.
• The overall reaction of photosynthesis can be summarized as follows:
• 6CO₂ + 12H₂O → (Light, Chlorophyll) → C₆H₁₂O₆ + 6O₂ + 6H₂O
• This process involves the interaction of sunlight, pigments, water and carbon dioxide.
• Role of Light:
• Biological energy transformations obey the laws of thermodynamics.
• Sunlight is the ultimate source of energy which is continuously flowing one way from Sun to life and ecosystem.
• Sunlight is an electromagnetic form of energy.
• Light has the prime importance as light energy is converted to chemical energy and then to other forms of energy essentially required for life processes.
• The full range of electromagnetic radiation in the universe is called electromagnetic spectrum.
• Visible light is only a small part of the spectrum between 380nm to 750nm which is not only seen by naked eye but is also effective for the process of photosynthesis.
• The effectiveness of a particular wavelength of light for the process of photosynthesis primarily depends upon its absorption in plant body.
• Such a graph which shows the effectiveness of different wavelength of light for the process of photosynthesis is called action spectrum.
• Analysis of action spectrum indicates that blue (430nm) and red (670nm) wavelengths of light are the most effective for the process of photosynthesis.
• Role of Carbon Dioxide in Photosynthesis:
• Carbon dioxide is one of the raw material of the photosynthetic reaction where it acts carbon source for the synthesis of organic compounds in photosynthesis.
• Plants are therefore known as autotrophs because they use low energy inorganic compound like CO₂ for the synthesis of high energy organic compounds by their own.
• Carbon dioxide is utilized in the dark or light independent reaction (Calvin cycle) of photosynthesis.
• Air contains about 0.03 to 0.04 percent of carbon dioxide.
• Land plants use this atmospheric carbon dioxide for photosynthesis.
• Increase in CO₂ concentration in the environment increase the rate of photosynthetic process but when CO₂ concentration rises above 1% the rate of photosynthesis rapidly slow down due to stomatal closing.
• Dissolved carbon dioxide, bicarbonates and carbonates are present in water, which are used by aquatic photosynthetic organisms as carbon source.
• Role of Water in Photosynthesis:
• Water is another raw materials for Photosynthesis along with CO₂.
• Water acts as hydrogen and electron donor in photosynthesis.
• During light dependent phase of photosynthesis, water molecule is broken down into 2H⁺, 2e⁻ and 1/2O₂ so this reaction is called photolysis of water.
• Electrons released by water molecule are used to compensate the photo excited electron lost by the chlorophyll-a molecule present in the reaction center of photosystem-II (P680).
• 2H⁺ ions and 2e⁻ are taken up temporarily by the NADP⁺ to become NADPH which are then used along to reduce CO₂ in light independent reaction.
• The oxygen which is produced is released in atmosphere.
• This role of water in photosynthesis was first reported by Van Niel in 1931.
• He hypothesized that plants split water as a source of hydrogen, releasing oxygen as a byproduct.
• This observation was based on investigations of photosynthesis in bacteria that make carbohydrates from carbon dioxide, but do not release oxygen.
• Neil's hypothesis was confirmed in 1940, when for the first time ¹⁸O in biological research was used.
• In first experiment water was made of ¹⁸O.
• The water tagged ¹⁸O was added to an alga suspension.
• The oxygen, evolved during photosynthesis, was found to be radioactive.
• It was separated and identified.
• In another experiment carbon dioxide with tagged ¹⁸O was added.
• The oxygen evolved contained none of the isotopes.
• Thus the source of evolved oxygen was proved to be water.
• Photosynthetic pigments and their major types:
• Photosynthetic organisms have different types of pigments for the absorption of light.
• Pigment is any substance that absorbs light energy for the use in the food synthesizing process.
• Algae and photosynthetic bacteria have chemically different types of pigments than plants.
• Pigments of bacteria are present in their cytoplasm whereas, photosynthetic pigments of plants are embedded in thylakoid membranes (grana lamellae) within their chloroplasts.
• Higher plants have two major group of pigments i.e., chlorophylls and carotenoids.
• Chlorophyll:
• Chlorophylls absorb mainly violet, blue, orange and red wavelengths.
• Green and yellow are least absorbed and reflected.
• Two major types of chlorophyll are Chlorophyll-a and Chlorophyll-b.
• Chlorophyll-a is a bluish green pigment which is found in all photosynthetic organisms except photosynthetic bacteria.
• Chlorophyll-b is yellowish green pigment which is also found in all photosynthetic organisms except brown, red algae.
• Absorption Spectrum:
• The absorption of different wavelengths of light by a particular pigment can be determined by the help of spectrophotometer.
• The data of spectrophotometer is represented by a graph.
• Such a graph which shows the relative absorption of different wavelengths of light by a particular pigment is called absorption spectrum of that pigment.
• The absorption spectra of different pigments indicate that they absorb different wavelengths of visible light and these wavelengths are not absorbed at the same rate.
• The main photoreceptors are chlorophyll a and b and both show more absorption in violet blue (400nm to 470nm) and orange-red (630nm to 660nm) regions of the visible spectrum.
• On the other hand carotenoids show more absorption at 430nm to 500nm.
• Difference between the absorption spectrum of chlorophyll-a and chlorophyll-b may be relate to the differences in their molecular formulae, structural arrangements and their physical properties e.g. colour.
• Science Tidbits: Carotenoids such as lutein, zeaxanthin and meso-zeaxanthin are also found in the macula in the center of the retina and absorb up to 90% of blue and violet light. The absorption of this intense light by these pigments protect the vital and light sensitive part of eye i.e. retina from oxidative damage.
• Critical thinking: How does the absorption spectrum of chlorophyll-a differs from absorption spectrum of chlorophyll-b? How absorption spectrum of chlorophyll-a differs from action spectrum.

60-Second Revision

• Photosynthesis = redox process; CO₂ reduced, H₂O oxidized
• Overall reaction: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
• Role of light: Sunlight is the ultimate source; blue and red are most effective
• Role of CO₂: Carbon source; utilized in Calvin cycle; 0.03–0.04% in air
• Role of water: Hydrogen and electron donor; photolysis of water
• Photolysis of water: 2H⁺ + 2e⁻ + ½O₂
• Van Niel (1931): Proposed water is the source of oxygen
• ¹⁸O experiment (1940): Confirmed water is the source of oxygen
• Chlorophyll-a: Bluish-green; all photosynthetic organisms except bacteria
• Chlorophyll-b: Yellowish-green; all photosynthetic organisms except brown and red algae
• Absorption spectrum: Graph showing absorption of different wavelengths
• Action spectrum: Graph showing effectiveness of different wavelengths
• Most effective wavelengths: Blue (430 nm) and Red (670 nm)
• Warning: Oxygen comes from water, not CO₂. Green light is least effective. Chlorophyll-a and b have different absorption spectra. Van Niel proposed water as the source of oxygen. The ¹⁸O experiment confirmed this. Carotenoids are accessory pigments. The absorption spectrum shows what is absorbed; the action spectrum shows what is effective.