Photosynthesis in Higher Plants — Class 11 Biology Notes

Topics covered in Photosynthesis in Higher Plants

1. 1.Introduction of Photosynthesis in Higher Plants

   Short Answer

   Photosynthesis is the process by which green plants make their own food using sunlight. It's important because it's the main source of food and oxygen on Earth.

   Long Answer

   Photosynthesis is a critical process carried out by green plants, known as autotrophs, which enables them to produce their own food using the energy they capture from sunlight. This process is vital for several reasons:

   1. Source of Food: All animals, including humans, rely on plants for their food directly or indirectly. Plants are at the base of the food chain, making food through photosynthesis, which is then consumed by other organisms.

   2. Production of Oxygen: Photosynthesis is also responsible for producing and releasing oxygen into the atmosphere, which is essential for the survival of most living organisms on Earth.

   3. Energy Conversion: During photosynthesis, plants convert light energy into chemical energy in the form of glucose, which can be used by the plant or other organisms that consume the plant.

   4. Basis of Life: The energy from sunlight converted into chemical energy through photosynthesis is fundamental for the existence of life on Earth.

   This chapter will explore the structure and mechanisms of the photosynthetic machinery in plants and how it transforms light energy into chemical energy, supporting life on Earth.

2. 2.Photosynthesis Process

   Short Answer

   Photosynthesis is the process by which plants use sunlight to make food from carbon dioxide and water. This process also releases oxygen, which is essential for us to breathe.

   Long Answer

   Photosynthesis is a fascinating and vital process that occurs in green plants, algae, and some bacteria. These organisms are capable of converting light energy from the sun into chemical energy in the form of glucose (sugar), which they use to fuel their growth and activities. Here's a step-by-step overview of how it happens:

   1. Light Absorption: The process starts in the leaves, where cells contain chlorophyll, a green pigment that captures sunlight.

   2. Water Splitting: The absorbed light energy is used to split water molecules (H2O) into oxygen (O2) and hydrogen ions. The oxygen is released into the atmosphere.

   3. Carbon Dioxide Fixation: Carbon dioxide (CO2) from the air is absorbed by the plant through tiny openings in the leaves called stomata.

   4. Glucose Production: Using the hydrogen ions from water and carbon dioxide, the plant combines them through a series of reactions known as the Calvin cycle to produce glucose.

   5. Oxygen Release: As a by-product of splitting water, oxygen is released into the air, which is vital for the survival of animals and humans.

   Real-Life Examples and Applications:

   • Food Production: All the food we eat comes directly or indirectly from plants that have used photosynthesis to produce it.
   • Oxygen Production: The oxygen we breathe is a direct result of photosynthesis.
   • Energy Source: Many fuels, like wood and fossil fuels, are stored forms of energy from photosynthesis that happened millions of years ago.

   In Careers and Industries:

   • Agriculture: Understanding photosynthesis can help improve crop yields and food production.
   • Environmental Science: It's crucial for managing forests and natural resources, understanding carbon cycles, and addressing climate change.
   • Renewable Energy: Research into artificial photosynthesis aims to create new ways of producing clean energy.

3. 3.Early Experiments

   Short Answer

   Early experiments by Joseph Priestley in 1770, Jan Ingenhousz in the late 1770s, Julius von Sachs in the 1850s, T.W. Engelmann in the 1880s, and Cornelius van Niel in the early 20th century laid the groundwork for our understanding of photosynthesis. These experiments demonstrated the roles of air, light, and chlorophyll in the production of oxygen and glucose by plants.

   Long Answer

   The aim to understand the foundational experiments that have shaped our understanding of photosynthesis.
   

   Joseph Priestley's Experiment (1770)

   Objective: To demonstrate that plants can refresh air that has been "spoiled" by burning candles or animal respiration.

   Setup:

   1. Place a lit candle in a closed jar and observe how it soon extinguishes, indicating the consumption of oxygen.
   2. In a new setup, place a live mouse in a similar jar and observe how it eventually becomes lethargic or suffocates, again due to the lack of oxygen.
   3. Now, introduce a small, live plant (like mint) in the jar with a new candle. Light the candle, and once it goes out, seal the jar.
   4. Observe over a few days. Relight the candle without opening the jar. If Priestley's hypothesis holds, the plant should replenish the oxygen, allowing the candle to burn again.

   Jan Ingenhousz's Discoveries (Late 1770s)

   Objective: To show that sunlight is essential for plants to purify air.

   Setup:

   1. Repeat Priestley's setup with a plant and a candle in a jar, but this time, place the setup in a dark room for a few days, then try to relight the candle.
   2. Move the setup to a brightly lit area (preferably sunlight) for the same duration and then attempt to relight the candle.
   3. The candle should relight in the condition exposed to sunlight, demonstrating that light is necessary for the plant to produce oxygen.

   Julius von Sachs's Contributions (1850s)

   Objective: To illustrate that glucose is produced in the green parts of plants and is stored as starch.

   Setup:

   1. Grow a plant in sunlight for several days.
   2. Pluck a leaf from the plant and boil it in alcohol to remove chlorophyll, turning it white.
   3. Rinse the decolorized leaf in water and then add iodine solution. If starch is present, the leaf will turn blue-black.
   4. This experiment demonstrates the production of glucose (stored as starch) in the green parts of the plant.

   T.W. Engelmann's Experiment (1880s)

   Objective: To identify which parts of the light spectrum are most effective for photosynthesis.

   Setup (Simplified version):

   1. Place a green alga in a solution with aerobic bacteria that move towards oxygen.
   2. Illuminate the algae with a light source passed through a prism to create a spectrum.
   3. Observe the areas where bacteria gather most, indicating where the most oxygen is produced.
   4. This demonstrates that blue and red light are most effective for photosynthesis, correlating with the absorption spectrum of chlorophyll.

   Cornelius van Niel's Insights (Early 20th Century)

   Objective: To suggest that water is the source of oxygen produced in photosynthesis.

   Setup (Hypothetical, as it involves complex biochemical processes):

   1. Using isotopically labeled water (heavy oxygen, 18O18), allow a plant to undergo photosynthesis.
   2. Measure the oxygen produced by the plant and analyze its isotopic composition.
   3. If the oxygen released has the heavier isotope, it confirms that the oxygen comes from water.

   These simplified experiments offer a basic understanding of the crucial experiments that led to our current knowledge of photosynthesis. While some can be conducted with relatively simple materials, others (like van Niel's hypothesis) are more complex and require advanced techniques to directly observe.

4. 4.Where Does Photosynthesis Take Place?

   Step-by-Step Explanation of Where Photosynthesis Takes Place

   Step 1: Understanding the Site of Photosynthesis

   • In Leaves: Photosynthesis primarily occurs in the green parts of plants, predominantly in the leaves.
   • In Chloroplasts: Within the leaves, the chloroplasts are the specific sites where photosynthesis takes place.

   Step 2: Identifying Other Photosynthetic Plant Parts

   • Besides leaves, photosynthesis can occur in any green part of the plant, which may include:
     • Stems: Some plants have green stems which contain chloroplasts.
     • Green Branches: Young branches that are green can also photosynthesize.
     • Green Fruits: Certain fruits that are green also have the capability to photosynthesize.

   Step 3: Chloroplasts in Mesophyll Cells

   • Location in Cells: In the leaves, chloroplasts are found in the mesophyll cells, which contain a large number of chloroplasts for optimal photosynthesis.
   • Alignment for Light Absorption: Chloroplasts align themselves to capture maximum light:
     • Parallel Alignment: When the light intensity is normal, chloroplasts align with their flat surfaces parallel to the cell walls to capture light efficiently.
     • Perpendicular Alignment: In low light conditions, chloroplasts may adjust to a position perpendicular to the light to maximize light absorption.

   Step 4: Chloroplast Structure

   • Grana: These are stacks of thylakoids where the light-dependent reactions of photosynthesis occur.
   • Stroma Lamellae: These connect the grana and are involved in the light reactions of photosynthesis.
   • Stroma: The fluid surrounding the grana where the light-independent reactions (Calvin cycle) occur.

   Step 5: Division of Labor in the Chloroplast

   • Light Reactions: The thylakoid membranes trap light energy, which is then used to make ATP and NADPH.
   • Dark Reactions: Occurring in the stroma, these enzymatic reactions use the ATP and NADPH from the light reactions to synthesize sugars, which can be stored as starch.

   Step 6: Clarification of Terms

   • 'Light' and 'Dark' Reactions: The terms "light" and "dark" reactions can be misleading:
     • Light Reactions: These reactions require light and produce the energy carriers ATP and NADPH.
     • Dark Reactions (Calvin Cycle): Although called "dark," these reactions are light-independent in terms of direct light requirement. However, they rely on the ATP and NADPH produced by the light reactions and can occur during the day.

   This step-by-step breakdown illustrates how photosynthesis is not limited to the leaves but can occur in any green part of a plant where chloroplasts are present. Within the chloroplasts, there's a specialized division of labor with certain regions facilitating the light-dependent reactions and others facilitating the light-independent reactions.

5. 5.How Many Types of Pigments Are Involved In Photosynthesis?

   Short Answer:

   In photosynthesis, there are several types of pigments involved. The main ones are:

   1. Chlorophyll a
   2. Chlorophyll b
   3. Carotenoids (which include carotenes and xanthophylls)
   4. Phycobilins (in some algae)

   Long Answer:

   Step 1: Primary Pigments

   • Chlorophyll a: This is the primary pigment involved in photosynthesis. It absorbs light mostly in the blue-violet and red parts of the electromagnetic spectrum and reflects green, which is why plants appear green.

   Step 2: Accessory Pigments

   • Chlorophyll b: This pigment complements chlorophyll a and expands the range of light that a plant can use for photosynthesis. It has a slightly different absorption spectrum, mainly absorbing light in the blue and red-orange wavelengths.

   • Carotenoids: These are a class of pigments that include:

     • Carotenes: Purely hydrocarbon pigments (such as beta-carotene) that appear orange.
     • Xanthophylls: Oxygen-containing carotenoids (such as lutein) that appear yellow.

   Carotenoids absorb light in the blue-green to blue region of the spectrum and provide photoprotection, dissipating excess light energy that could harm chlorophyll or interact with oxygen to form reactive oxidative molecules.

   Step 3: Pigments in Algae

   • Phycobilins: These pigments are found in cyanobacteria and some algae. They include phycocyanin (which is blue) and phycoerythrin (which is red). Phycobilins absorb light in the green to yellow-green part of the spectrum, which chlorophylls and carotenoids do not absorb as well.

   Step 4: Role in Photosynthesis

   • Each type of pigment absorbs light at specific wavelengths. The energy from the light is then transferred to chlorophyll a, which acts as the primary pigment in the reaction center of the photosystems. This energy transfer allows plants to utilize a broader spectrum of light for photosynthesis.

   By working together, these pigments enable plants to efficiently capture light energy for the synthesis of organic compounds, contributing to the conversion of solar energy into chemical energy during photosynthesis.

6. 6.A Light Reaction.

   The light reaction is the stage of photosynthesis where light energy is captured and converted into chemical energy. The diagram you've provided shows a light harvesting complex (LHC), which is part of this process. Here's how the light reaction occurs in detail:

   Step 1: Light Absorption

   • Photon Capture: A photon (light energy) strikes the pigment molecules in the LHC.
   • Pigment Molecules: These are the numerous green circles in the diagram representing pigment molecules like chlorophyll and carotenoids.

   Step 2: Energy Transfer

   • Excitation: The photon's energy is absorbed by a pigment molecule, exciting electrons to a higher energy state.
   • Energy Transfer: This energy is then transferred from one pigment molecule to another, funneling towards the reaction center.

   Step 3: Reaction Center Activation

   • Reaction Center: At the core of the LHC is the reaction center, which has a special chlorophyll a molecule.
   • Chlorophyll a: This molecule in PS I is called P700 (absorbs light at 700 nm) and in PS II is called P680 (absorbs light at 680 nm).

   Step 4: Primary Electron Acceptance

   • Primary Acceptor: The excited electron from the reaction center chlorophyll a is captured by the primary acceptor, shown above the reaction center in the diagram.
   • Electron Transport: This electron is then passed to an electron transport chain.

   Step 5: ATP and NADPH Formation

   • Electron Transport Chain: As electrons move through the chain, they lose energy. This energy is used to pump protons across the thylakoid membrane, creating a gradient.
   • ATP Synthesis: ATP synthase uses this gradient to synthesize ATP from ADP and phosphate.
   • NADPH Production: Electrons at the end of the electron transport chain are used along with protons to reduce NADP⁺ to NADPH.

   Step 6: Oxygen Evolution

   • Water Splitting: In PS II, when the reaction center chlorophyll a (P680) loses an electron, it is replaced by splitting water molecules into protons, electrons, and oxygen.
   • Oxygen Release: The splitting of water releases oxygen as a byproduct.

   Step 7: The Calvin Cycle

   • Utilization of ATP and NADPH: The ATP and NADPH produced in the light reaction are used in the Calvin cycle to convert carbon dioxide into glucose.

   The light reaction, thus, sets the stage for the conversion of light energy into a stable chemical form (ATP and NADPH), driving the synthesis of organic molecules in the subsequent phase of photosynthesis.