Photosynthesis – Class 10

photosynthesis complete notes

Photosynthesis is a fundamental process that occurs in green plants and certain bacteria. It is an autotrophic mode of nutrition wherein organisms synthesise their food from simple inorganic substances such as carbon dioxide and water in the presence of sunlight.

Definition of Photosynthesis

Photosynthesis is a complex biochemical process that occurs in the green parts of plants, such as leaves, stems, and sometimes in other green tissues. It is the primary mechanism by which plants produce their own food, using energy from sunlight to convert inorganic substances into organic compounds.

Modes of Autotrophic Nutrition

Autotrophic mode of nutrition refers to the mode of nutrition in which organisms synthesise their own food from simple inorganic substances present in the environment. Two common modes of autotrophic nutrition are- – 

  • Photosynthesis– Photosynthesis is the primary mode of autotrophic nutrition used by green plants and some bacteria. It is the process by which green plants synthesise organic food using carbon dioxide, water, sunlight, and chlorophyll.
  • Chemosynthesis– Chemosynthesis is another mode of autotrophic nutrition used by certain bacteria that live in extreme environments, such as deep-sea hydrothermal vents. These bacteria utilise energy obtained from chemical reactions involving inorganic compounds to produce organic food.

Significance of Photosynthesis

  • Production of Organic Food– Through Photosynthesis, green plants convert carbon dioxide and water into glucose, a simple sugar that serves as their primary source of energy. This energy is utilised for various metabolic processes required for growth, reproduction, and maintenance of cellular functions.
  • Energy Storage- Carbohydrates, primarily in the form of glucose, are produced during Photosynthesis. These carbohydrates provide energy to the plant for immediate use. Any excess carbohydrates that are not immediately utilised are converted into starch and stored as an internal energy reserve. This stored energy is available to the plant whenever required.
  • Oxygen Release- As a byproduct of Photosynthesis, oxygen is released into the atmosphere. This oxygen is essential for the survival of all living organisms, as it is used in cellular respiration to produce energy.

Also Check – Diffrence Between Photosynthesis and Respiration

Process of Photosynthesis

Photosynthesis class 10
Photosynthesis process

Photosynthesis occurs in the specialised organelles called chloroplasts, which are present in the cells of green plant tissues, particularly in leaves. The process can be divided into two main stages- 

  • The light-dependent reactions 
  • The light-independent reactions (Calvin cycle)

Light-Dependent Reactions

In the thylakoid membranes of chloroplasts, light energy is absorbed by chlorophyll, initiating the following steps-

  • Absorption of Light Energy- Chlorophyll, the green pigment present in chloroplasts, absorbs light energy from the sun.
  • Splitting of Water Molecules- The absorbed light energy is used to split water molecules (H2O) into oxygen (O2) and hydrogen ions (H+).
  • Generation of ATP and NADPH- The energy from the absorbed light is harnessed to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which serve as energy carriers for the subsequent reactions.

Light-Independent Reactions (Calvin Cycle)

In the stroma of chloroplasts, the light-independent reactions occur and involve the following steps-

  • Carbon Dioxide Fixation- Carbon dioxide (CO2) from the atmosphere combines with the hydrogen ions (H+) and energy carriers (ATP and NADPH) to form an intermediate molecule.
  • Production of Glucose- The intermediate molecule undergoes a series of enzymatic reactions, ultimately leading to the production of glucose (C6H12O6).
  • Regeneration of Reactants- Some molecules from the intermediate stage are recycled to regenerate the starting materials, ATP, NADPH, and carbon dioxide acceptors, allowing the process to continue.

Overall Equation for Photosynthesis

The process of Photosynthesis can be represented by the following simplified chemical equation-

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

In this equation, carbon dioxide (CO2) and water (H2O) are the raw materials, and in the presence of light energy, glucose (C6H12O6) and oxygen (O2) are produced.

The Raw Materials for Photosynthesis

Photosynthesis requires specific raw materials for the process to occur efficiently. The essential raw materials for Photosynthesis are carbon dioxide (CO2), water (H2O), and other mineral nutrients obtained from the soil. Let’s delve into each of these raw materials and their role in Photosynthesis-

Carbon Dioxide (CO2)

  • Carbon dioxide is a colourless and odourless gas that is released into the atmosphere as a byproduct of cellular respiration in living organisms. 
  • During cellular respiration, organisms, including plants, break down organic molecules to release energy. In turn, they release carbon dioxide as a waste product.
  • For plants, carbon dioxide enters the leaves through tiny openings called stomata. Stomata are present on the surface of leaves and allow for the exchange of gases between the plant and the environment. 
  • Carbon dioxide diffuses into the leaf through the stomata, providing the necessary carbon source for Photosynthesis.

Also Check – Mechanism of opening and Closing of the Stomata

Water (H2O)-

  • Water is another crucial raw material required for Photosynthesis. Plants absorb water through their roots from the soil. 
  • The root hairs, which are extensions of root cells, increase the surface area for water absorption.
  • Once absorbed, water is transported upward through the plant’s vascular system, specifically through a specialised tissue called xylem. 
  • The xylem vessels extend from the roots all the way to the leaves. The movement of water through xylem is driven by a combination of root pressure and transpiration, the loss of water vapour through small openings on the leaf surface called stomata.
  • During Photosynthesis, water molecules are split in a process called photolysis. This splitting occurs in the presence of sunlight and takes place in the chloroplasts of the plant cells. 
  • The photolysis of water results in the production of oxygen gas (O2) and hydrogen ions (H+). The oxygen is released into the atmosphere as a byproduct of Photosynthesis, while the hydrogen ions are used in subsequent reactions.

Other Materials- Mineral Nutrients

  • In addition to carbon dioxide and water, plants require various mineral nutrients obtained from the soil for optimal growth and Photosynthesis. 
  • These mineral nutrients include nitrogen (N), phosphorus (P), iron (Fe), and magnesium (Mg), among others.
  • Nitrogen is an essential nutrient that plants require for the synthesis of proteins and other vital compounds. Plants uptake nitrogen from the soil in the form of inorganic nitrates (or nitrites) or as organic compounds derived from symbiotic relationships with nitrogen-fixing bacteria.
  • Phosphorus is necessary for energy transfer and storage in plants. It is obtained from the soil in the form of inorganic phosphates and plays a critical role in various metabolic processes.
  • Iron is a micronutrient essential for chlorophyll synthesis. It is involved in capturing light energy during Photosynthesis. Plants absorb iron in the form of inorganic ions from the soil.
  • Magnesium is a central component of chlorophyll, the pigment responsible for capturing light energy during Photosynthesis. Plants absorb magnesium as an inorganic ion from the soil.
  • These mineral nutrients are absorbed by the plant’s roots and transported through the vascular system to the cells where Photosynthesis occurs. They are crucial for the plant’s overall growth, development, and efficient photosynthetic processes. Read More ..

The Site of Photosynthesis- Chloroplasts

Photosynthesis takes place within specialised organelles called chloroplasts, which are present in the cells of green plants. Chloroplasts play a crucial role as the primary sites of Photosynthesis. Let’s explore the structure of chloroplasts, their presence in photosynthetic cells, and the role of chlorophyll in capturing light energy.

Structure of Chloroplasts

Photosynthesis
choloroplast

Chloroplasts are unique double-membrane organelles found in the cells of plants, algae, and some other organisms capable of Photosynthesis. These organelles contain an intricate network of membranes, enzymes, and pigments that facilitate the process of Photosynthesis.

Within the chloroplasts, there are several key structures-

  • Outer Membrane- The outer membrane of the chloroplast acts as a protective barrier, separating the internal contents from the surrounding cytoplasm of the cell.
  • Inner Membrane- The inner membrane is located inside the outer membrane and encloses the stroma, a gel-like substance where the light-independent reactions of Photosynthesis occur.
  • Thylakoid Membranes- The thylakoid membranes are interconnected sac-like structures that are stacked on top of each other in a structure called a grana (plural- granum). These membranes contain specialised pigments, including chlorophyll, that capture light energy.
  • Stroma- The stroma is the fluid-filled space within the chloroplast that surrounds the thylakoid membranes. It contains enzymes and other molecules necessary for the light-independent reactions of Photosynthesis.

Also Check – Chloroplast – Definition, Structure, Distribution, function and Diagram

Presence of Chloroplasts in Photosynthetic Cells

Chloroplasts are primarily found in the photosynthetic cells of green plants, particularly in the mesophyll cells of leaves. The mesophyll cells are specialised for Photosynthesis and contain numerous chloroplasts. This arrangement maximises the surface area available for light capture and ensures efficient photosynthetic processes.

Chlorophyll and Light Absorption

  • Chlorophyll is a green pigment found within the chloroplasts that plays a vital role in capturing light energy during Photosynthesis. It is responsible for the characteristic green colour of plants.
  • Chlorophyll molecules are embedded in the thylakoid membranes of chloroplasts. These molecules have a unique structure that allows them to absorb light energy from the visible spectrum, particularly in the red and blue regions, while reflecting green light.
  • When light strikes chlorophyll molecules, they absorb photons of specific wavelengths. The absorbed light energy is then transferred to other chlorophyll molecules and ultimately to the reaction centres in the thylakoid membranes. This energy is used to drive the chemical reactions of Photosynthesis, converting light energy into chemical energy stored in the form of ATP and NADPH.

Conditions Necessary for Photosynthesis

For Photosynthesis to occur, certain conditions must be met. These conditions include the availability of sunlight and the presence of chlorophyll. Let’s explore the significance of these conditions in detail-

Sunlight

Sunlight is a crucial factor in Photosynthesis as it provides the energy needed for the process. The intensity, quality, and duration of sunlight all influence the rate of Photosynthesis.

  • Intensity- The intensity of sunlight refers to the amount of light energy reaching a given area. Higher light intensity generally leads to increased photosynthetic activity. However, there is an optimal range of light intensity beyond which the rate of Photosynthesis plateaus. Insufficient light intensity limits the amount of energy available for the process.
  • Quality– Sunlight consists of a spectrum of colours, each with a different wavelength. Plants primarily absorb light in the red and blue regions of the spectrum, while green light is reflected, giving plants their green appearance. Thus, the quality of light, specifically the presence of red and blue wavelengths, is essential for efficient Photosynthesis.
  • Duration- The duration of exposure to sunlight also affects Photosynthesis. Longer periods of light availability allow for a greater accumulation of energy, leading to increased photosynthetic activity. However, plants also have mechanisms to adapt to changing light conditions, such as adjusting the opening and closing of stomata to optimise gas exchange and reduce water loss.

Chlorophyll

Chlorophyll is a pigment found in chloroplasts, and it plays a pivotal role in capturing solar energy during Photosynthesis.

  • Trapping Solar Energy- Chlorophyll molecules are responsible for absorbing light energy from the sun. Specifically, chlorophyll molecules absorb photons of light in the red and blue regions of the spectrum. This absorbed energy is then transferred to other molecules within the chloroplast, leading to the initiation of the light-dependent reactions of Photosynthesis.
  • Chlorophyll molecules are arranged within the thylakoid membranes of the chloroplasts, maximising their exposure to light. This organisation allows for efficient light absorption and energy transfer.
  • It is important to note that other pigments, such as carotenoids, also contribute to light absorption in plants. These pigments capture light energy in different regions of the spectrum and help supplement the overall efficiency of Photosynthesis.

Events in Photosynthesis

Photosynthesis involves a series of interconnected events that occur within the chloroplasts of plant cells. These events are essential for converting light energy into chemical energy and synthesise carbohydrates. Let’s explore these events in detail-

Absorption of Light Energy-

During Photosynthesis, chlorophyll, the primary pigment in chloroplasts, plays a crucial role in absorbing light energy from the sun. Chlorophyll molecules are located in the thylakoid membranes of chloroplasts and are arranged to maximise light absorption.

When chlorophyll molecules absorb photons of specific wavelengths, they become energised. This absorbed energy is transferred to neighbouring chlorophyll molecules until it reaches specialised structures called reaction centres. These reaction centers initiate the conversion of light energy into chemical energy.

Conversion of Light Energy

The absorbed light energy is utilized in the light-dependent reactions of Photosynthesis to produce energy-rich molecules such as ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). These energy carriers serve as the “currency” for the subsequent synthesis of carbohydrates.

Additionally, during the light-dependent reactions, water molecules are split through a process called photolysis. The energy absorbed by chlorophyll facilitates the separation of water molecules into hydrogen ions (H+) and oxygen (O2). This release of oxygen is vital for the oxygenation of our atmosphere, while the hydrogen ions are further utilised in the synthesis of carbohydrates.

Reduction of Carbon Dioxide

In the light-independent reactions, also known as the Calvin cycle, the energy-rich molecules (ATP and NADPH) generated in the light-dependent reactions are used to convert carbon dioxide (CO2) into carbohydrates.

During this process, carbon dioxide molecules from the atmosphere enter the chloroplasts and undergo a series of chemical reactions. The energy from ATP and the high-energy electrons from NADPH are utilised to drive the reduction of carbon dioxide. The end result is the synthesis of simple sugars, such as glucose, which serve as a vital source of energy and building blocks for the plant.

It’s important to note that the events in Photosynthesis do not occur sequentially in a strict order. Instead, they are interconnected and continually ongoing, with various stages and reactions happening simultaneously or in response to changing environmental conditions.

Also Check – Top 10 Experiments on Photosynthesis (With Diagram)

Factors Affecting Photosynthesis

  • Temperature- Photosynthesis is temperature-sensitive, with the optimal range for most plants typically being between 20-30 degrees Celsius (68-86 degrees Fahrenheit). Higher temperatures can initially increase the rate of photosynthesis, but excessive heat can damage the enzymes involved in the process.
  • Carbon dioxide (CO2) concentration- Carbon dioxide is an important raw material for photosynthesis. Higher CO2 concentrations can increase the rate of photosynthesis, up to a certain point where it becomes saturated. Insufficient CO2 can limit the photosynthetic process.
  • Light intensity- Light is an essential factor for photosynthesis as it provides the energy needed to convert carbon dioxide and water into glucose and oxygen. Higher light intensity usually increases the rate of photosynthesis, but beyond a certain threshold the rate stagnates.
  • Light quality- Plants absorb mainly red and blue wavelengths of light, while green light is reflected, which is why plants appear green. Different wavelengths and light spectra can affect the photosynthesis rate differently. Certain plants have specific light requirements.
  • Availability of water- Water is crucial for photosynthesis as it provides the hydrogen ions necessary for the synthesis of glucose. Insufficient water supply can lead to closure of the stomata, reducing CO2 uptake and limiting the rate of photosynthesis.
  • Availability of nutrients- Plants require various nutrients, including nitrogen, phosphorus, potassium and trace elements, to carry out photosynthesis effectively. An insufficient supply of nutrients can limit the production of enzymes and other components involved in this process.
  • Chlorophyll content- Chlorophyll is the most important pigment responsible for the absorption of light energy during photosynthesis. Plants with a higher chlorophyll content tend to have a greater capacity for photosynthesis.
  • Environmental factors- Factors such as altitude, humidity and air pollutants can affect photosynthesis. Extreme conditions such as high altitude or pollution can affect the availability of light, temperature and CO2, thus affecting the rate of photosynthesis. Read More..

Adaptations and Additional Photosynthesis Information 

Photosynthesis is a remarkable process that has evolved various adaptations to suit different environmental conditions. These adaptations allow plants to optimise their photosynthetic efficiency and ensure survival in diverse habitats. Let’s explore some of these unique adaptations and additional information related to Photosynthesis-

Desert Plants Adaptation

Desert plants, such as cacti and certain succulents, have adapted to conserve water in arid environments. To minimise water loss through transpiration, they have developed a unique adaptation known as Crassulacean Acid Metabolism (CAM). CAM plants open their stomata and take in carbon dioxide during the cooler nights when evaporation rates are lower. The carbon dioxide is then stored as an intermediate compound, typically malic acid, within the cells. During the day, when the stomata are closed to prevent water loss, the stored carbon dioxide is released and used for Photosynthesis. This adaptation allows desert plants to conserve water while still performing Photosynthesis efficiently.

Also Check- What are the Adaptations of leaf for Photosynthesis

Aquatic Plants’ Adaptation

  • Aquatic plants, such as Hydrilla and Vallisneria, have adapted to living in waterlogged environments. Unlike terrestrial plants that primarily obtain carbon dioxide from the atmosphere, these plants can use dissolved carbon dioxide present in the water for Photosynthesis. They possess specialised structures that facilitate the uptake of dissolved carbon dioxide directly from their aquatic surroundings. This adaptation enables them to thrive in aquatic habitats where access to atmospheric carbon dioxide is limited.

Additional Information

Photosynthesis and Oxygen Production

Photosynthesis is responsible for oxygen production, which is vital for supporting life on Earth. The oxygen released as a byproduct during the splitting of water molecules in the light-dependent reactions of Photosynthesis contributes to the oxygenation of our atmosphere, making it suitable for aerobic organisms.

Nitrogen is taken up in the form of inorganic nitrates (or nitrites) or as organic compounds prepared by symbiotic bacteria like Rhizobium from Atmospheric nitrogen.

Photosynthetic Pigments and Leaf Coloration

Besides chlorophyll, other pigments, such as carotenoids, play a role in Photosynthesis. These pigments absorb light in different regions of the spectrum, enhancing the range of light wavelengths that can be utilised for Photosynthesis. Carotenoids also provide colours like yellow, orange, and red to leaves, contributing to the vibrant autumn foliage.

Also Check- Steps of Photosynthesis

Photosynthesis – Frequently Asked Questions and Answers

What is Photosynthesis?

Answer- Photosynthesis is a complex biochemical process that occurs in the green parts of plants, using energy from sunlight to convert inorganic substances (carbon dioxide and water) into organic compounds (glucose) as a means of producing food.

What are the two common modes of autotrophic nutrition?

Answer- The two common modes of autotrophic nutrition are Photosynthesis and chemosynthesis.

How do green plants obtain their energy through Photosynthesis?

Answer- Green plants obtain their energy through Photosynthesis by converting carbon dioxide and water into glucose, a simple sugar that serves as their primary source of energy.

What is the significance of Photosynthesis in terms of energy storage?

Answer- Photosynthesis allows plants to produce carbohydrates, primarily in the form of glucose, which provide immediate energy for the plant. Any excess carbohydrates not immediately used are converted into starch and stored as an internal energy reserve.

How does Photosynthesis contribute to the oxygen levels in the atmosphere?

Answer- As a byproduct of Photosynthesis, oxygen is released into the atmosphere. This oxygen is essential for the survival of all living organisms, as it is used in cellular respiration to produce energy.

Which organelle is responsible for Photosynthesis in green plant tissues?

Answer- Photosynthesis occurs in specialised organelles called chloroplasts, which are present in the cells of green plant tissues, particularly in leaves.

What are the two main stages of Photosynthesis?

Answer- The two main stages of Photosynthesis are the light-dependent reactions and the light-independent reactions (Calvin cycle).

What happens during the light-dependent reactions of Photosynthesis?

Answer- In the light-dependent reactions, light energy is absorbed by chlorophyll, leading to the splitting of water molecules into oxygen and hydrogen ions. The energy from the absorbed light is used to produce ATP and NADPH, which serve as energy carriers for the subsequent reactions.

What occurs during the light-independent reactions (Calvin cycle) of Photosynthesis?

Answer- During the light-independent reactions, carbon dioxide from the atmosphere combines with hydrogen ions and energy carriers (ATP and NADPH) to form an intermediate molecule, which undergoes enzymatic reactions ultimately leading to the production of glucose. Some molecules from the intermediate stage are recycled to regenerate the starting materials, allowing the process to continue.

Provide the overall chemical equation for Photosynthesis.

Answer- The overall equation for Photosynthesis is: 6CO2 + 6H2O + Light Energy → C6H12O6 + 6O2. In this equation, carbon dioxide and water are the raw materials, and in the presence of light energy, glucose and oxygen are produced.

What are the three essential raw materials required for Photosynthesis?

Answer- The three essential raw materials required for Photosynthesis are carbon dioxide (CO2), water (H2O), and mineral nutrients obtained from the soil.

What is the process called when water molecules are split during Photosynthesis?

Answer- The process of water molecule splitting during Photosynthesis is called photolysis.

Name two mineral nutrients required by plants for optimal growth and Photosynthesis.

Answer- Two mineral nutrients required by plants for optimal growth and Photosynthesis are nitrogen (N) and phosphorus (P).

Where does Photosynthesis take place within the cells of green plants?

Answer- Photosynthesis takes place within specialised organelles called chloroplasts, which are present in the cells of green plants.

What are the key structures within chloroplasts?

Answer- The key structures within chloroplasts include the outer membrane, inner membrane, thylakoid membranes, and stroma.

Which pigment is responsible for capturing light energy during Photosynthesis?

Answer- Chlorophyll is the pigment responsible for capturing light energy during Photosynthesis.

What wavelengths of light are absorbed by chlorophyll?

Answer- Chlorophyll absorbs light energy from the red and blue regions of the visible spectrum while reflecting green light.

How is the absorbed light energy used during Photosynthesis?

Answer- The absorbed light energy is used to drive the chemical reactions of Photosynthesis, converting light energy into chemical energy stored in the form of ATP and NADPH.

In which cells of plants are chloroplasts primarily found?

Answer- Chloroplasts are primarily found in the photosynthetic cells of green plants, particularly in the mesophyll cells of leaves.

What are the essential raw materials required for Photosynthesis?

Answer- The essential raw materials for Photosynthesis are carbon dioxide (CO2), water (H2O), and other mineral nutrients obtained from the soil.

How does carbon dioxide enter the leaves of plants?

Answer- Carbon dioxide enters the leaves of plants through tiny openings called stomata, which are present on the surface of leaves.

Where does water absorption occur in plants?

Answer- Water is absorbed by plants through their roots from the soil.

Which specialised tissue transports water from the roots to the leaves?

Answer- The xylem, a specialised tissue, transports water from the roots to the leaves.

What is the role of chlorophyll in Photosynthesis?

Answer- Chlorophyll is a pigment that captures light energy during Photosynthesis. It absorbs photons of specific wavelengths, particularly in the red and blue regions of the spectrum.

What are the primary factors influencing the rate of Photosynthesis?

Answer- The primary factors influencing the rate of Photosynthesis are light intensity, light quality, temperature, carbon dioxide concentration, and the availability of water and nutrients.

How does light energy get converted into chemical energy during Photosynthesis?

Answer- Light energy is converted into chemical energy during Photosynthesis through the light-dependent reactions, where chlorophyll molecules absorb light and transfer the energy to generate energy-rich molecules such as ATP and NADPH.

What is the role of ATP and NADPH in Photosynthesis?

Answer- ATP and NADPH are energy carriers produced during the light-dependent reactions of Photosynthesis. They provide the energy and reducing power needed for the light-independent reactions to convert carbon dioxide into carbohydrates.

What are some adaptations of desert plants for Photosynthesis in arid environments?

Answer- Desert plants, such as cacti, have adapted a process called Crassulacean Acid Metabolism (CAM). They open their stomata at night to take in carbon dioxide and store it as malic acid. During the day, the stored carbon dioxide is released for Photosynthesis when the stomata are closed to minimise water loss.

How do aquatic plants obtain carbon dioxide for Photosynthesis?Answer- Aquatic plants can use dissolved carbon dioxide present in the water for Photosynthesis. They have specialised structures that facilitate the uptake of dissolved carbon dioxide from their aquatic surroundings.

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