Photosynthesis in Simple Words:

Dependence on Plants:

  • All animals, including humans, depend on plants for food. Plants, however, make their own food through a process called photosynthesis.

Autotrophs and Heterotrophs:

  • Plants are autotrophs, meaning they can make their own food. All other organisms, like animals, are heterotrophs, meaning they depend on plants for food.

Photosynthesis:

  • Photosynthesis is the process in which plants use sunlight, carbon dioxide (CO₂), and water to make their own food (organic compounds like glucose) and release oxygen (O₂) into the atmosphere. This process is vital because it is the primary source of food and oxygen on Earth.

Basic Requirements for Photosynthesis:

  • Three main factors needed for photosynthesis:
    1. Chlorophyll: The green pigment in leaves.
    2. Light: Typically sunlight.
    3. CO₂: Carbon dioxide from the air.

Experiments and Findings:

  1. Starch Formation Experiment: It was shown that photosynthesis occurs only in the green parts of leaves (where chlorophyll is present) in the presence of light.
  2. CO₂ Experiment: An experiment demonstrated that CO₂ is essential for photosynthesis since a leaf covered with KOH (which absorbs CO₂) did not form starch.

Early Discoveries in Photosynthesis:

  1. Joseph Priestley (1770): Priestley discovered that plants could “restore” the air by releasing oxygen. He showed that a candle burns out and a mouse suffocates in a sealed jar due to oxygen depletion, but when a plant is added, the oxygen is restored.
  2. Jan Ingenhousz (1779): Ingenhousz confirmed that sunlight is essential for photosynthesis and that oxygen is released only in the presence of sunlight and green parts of the plant.
  3. Julius von Sachs (1854): He showed that glucose is produced during photosynthesis and stored as starch in plants.
  4. T.W. Engelmann (1882): He discovered that blue and red light are most effective for photosynthesis.
  5. Cornelius van Niel (1930s): Van Niel concluded that the oxygen released in photosynthesis comes from water, not from CO₂, after studying purple and green bacteria.

Where Photosynthesis Takes Place:

  • Photosynthesis occurs primarily in the green parts of plants, especially in the leaves. Chloroplasts in the leaf cells (specifically in the mesophyll) contain the necessary machinery for photosynthesis.

Chloroplast Structure:

  • Chloroplasts consist of:
    • Grana: Stacks of membrane discs where light reactions occur.
    • Stroma: Fluid-filled space where dark reactions occur and glucose is made.

Pigments Involved in Photosynthesis:

  • Plants have several pigments that absorb light:
    1. Chlorophyll a: The main pigment that captures light for photosynthesis.
    2. Chlorophyll b: Assists chlorophyll a.
    3. Carotenoids: Help in light absorption and protect against damage from excessive light.

Light Reaction (Photochemical Phase):

  • Occurs in the grana of chloroplasts, where light is absorbed, water is split (releasing oxygen), and energy molecules (ATP and NADPH) are produced.
  • Photosystems: Photosystem I (PS I) and Photosystem II (PS II) are two complexes of pigments where light absorption happens. PS I absorbs light at 700 nm (P700), and PS II absorbs light at 680 nm (P680).

Electron Transport Chain (Z-Scheme):

  • In PS II, light excites electrons, which are passed through the electron transport chain to PS I, and eventually to NADP+ to form NADPH.
  • The Z-scheme describes the path of electron movement and the energy changes during the light reaction.
  1. Splitting of Water (Photolysis):
    • Water is split in Photosystem II (PS II) during photosynthesis.
    • The splitting of water produces electrons, protons (H+), and oxygen (O₂).
    • Oxygen is released as a byproduct of photosynthesis.
    • The electrons produced from splitting water replace the electrons lost from PS II.
  2. Cyclic and Non-Cyclic Photophosphorylation:
    • Photophosphorylation: The process of making ATP from ADP and inorganic phosphate using light.
    • Non-cyclic photophosphorylation:
      • Both Photosystem II (PS II) and Photosystem I (PS I) work together.
      • Electrons flow through an electron transport chain (ETC), producing both ATP and NADPH.
    • Cyclic photophosphorylation:
      • Occurs when only PS I is active.
      • Electrons flow in a cycle and are recycled, leading to only ATP production, not NADPH.
      • This type of photophosphorylation happens when light beyond 680 nm is available.
      • PS II is absent in stroma lamellae, making cyclic photophosphorylation possible there.
  3. Chemiosmotic Hypothesis:
    • ATP synthesis in chloroplasts happens through the chemiosmotic hypothesis.
    • A proton gradient is created across the thylakoid membrane, similar to how protons accumulate in mitochondria during respiration.
    • Protons are pumped into the thylakoid lumen due to:
      • Water splitting: Produces H+ ions.
      • Electron transport: Protons are transported across the membrane as electrons move through photosystems.
      • NADP reductase: Uses protons from the stroma to reduce NADP+ to NADPH, further lowering proton concentration in the stroma.
    • The proton gradient drives ATP synthesis by ATP synthase:
      • Protons flow back to the stroma through the CF0 channel of ATP synthase, which provides energy to produce ATP.
  4. ATP and NADPH Use in Biosynthesis:
    • ATP and NADPH from the light reaction are used in the biosynthetic phase (dark reaction), where CO₂ is converted into sugars.
    • These reactions continue for some time even in the absence of light but stop when the light is unavailable.
    • Calvin Cycle: The main process where CO₂ is fixed and converted into sugars.
      • Melvin Calvin discovered that the first product of CO₂ fixation is 3-phosphoglyceric acid (PGA), a 3-carbon molecule.
  5. C3 and C4 Pathways:
    • Plants can follow two different CO₂ fixation pathways:
      • C3 pathway: Produces PGA as the first product, common in most plants.
      • C4 pathway: Produces oxaloacetic acid (OAA), a 4-carbon molecule, found in plants like corn and sugarcane.
    • These pathways are important for adaptation to different environments.
  6. Primary Acceptor of CO₂:
    • The primary acceptor of CO₂ in photosynthesis is a 5-carbon molecule called ribulose bisphosphate (RuBP), not a 2-carbon molecule as initially thought.
    • RuBP combines with CO₂ to form a 3-carbon molecule, PGA.

Additional Insights:

  • ATP and NADPH are crucial for the Calvin Cycle, where the carbon from CO₂ is fixed into sugars.
  • Photosystem II (PS II) and Photosystem I (PS I) work together in the Z scheme to generate energy (ATP) and reducing power (NADPH).
  • The chemiosmotic theory explains how energy from the light reaction is stored as ATP, just like in mitochondria during cellular respiration.
  • Understanding C3 and C4 pathways is important for agriculture, especially in improving crop yield in different climates.

These concepts form the core of how photosynthesis works in plants and how energy is captured and used for the synthesis of food.

These all are the notes of chapter 11. And imporant questions are below HERE. *#THANKS FOR VISITING, VISIT AGAIN#* 😊

Q1: What is photosynthesis?
A: Photosynthesis is the process by which plants make their own food using sunlight, carbon dioxide (CO₂), and water. During this process, plants produce glucose (a type of sugar) and release oxygen (O₂) into the air.

Q2: Why do animals depend on plants?
A: All animals, including humans, depend on plants for food. Plants are the only organisms that can make their own food through photosynthesis, and animals rely on them for nutrients and oxygen.

Q3: What are autotrophs and heterotrophs?
A:

  • Autotrophs are organisms like plants that can make their own food using sunlight.
  • Heterotrophs are organisms like animals that depend on other organisms (such as plants) for food.

Key Requirements for Photosynthesis

Q4: What do plants need for photosynthesis?
A: Plants need three things for photosynthesis:

  1. Chlorophyll: The green pigment in leaves that captures sunlight.
  2. Light: Usually sunlight, which provides the energy needed.
  3. CO₂ (Carbon dioxide): From the air, which is used to make glucose.

Q5: Where does photosynthesis take place in the plant?
A: Photosynthesis mainly happens in the leaves of the plant. Inside the leaf cells, there are special structures called chloroplasts that contain chlorophyll and other machinery needed for photosynthesis.

Experiments on Photosynthesis

Q6: What did the starch formation experiment show?
A: The starch formation experiment showed that photosynthesis only happens in the green parts of leaves where chlorophyll is present and only when light is available. Starch, which is produced as a result of photosynthesis, was only found in the green areas exposed to light.

Q7: What did the CO₂ experiment demonstrate?
A: The CO₂ experiment demonstrated that carbon dioxide is essential for photosynthesis. When a leaf was covered with a substance (KOH) that absorbs CO₂, it couldn’t form starch, showing that CO₂ is needed for the process.

History and Discoveries of Photosynthesis

Q8: What did Joseph Priestley discover about plants?
A: Joseph Priestley discovered that plants could “restore” the air by releasing oxygen. He found that when a plant was placed in a sealed jar with a candle and a mouse, the candle went out, and the mouse suffocated due to a lack of oxygen. However, when the plant was added, the oxygen was restored.

Q9: What did Jan Ingenhousz contribute to the study of photosynthesis?
A: Jan Ingenhousz confirmed that sunlight is essential for photosynthesis. He showed that plants release oxygen only in the presence of sunlight and that the green parts of the plant are where this happens.

The Structure of Chloroplasts

Q10: What is the structure of a chloroplast?
A: A chloroplast has two main parts:

  1. Grana: Stacks of membrane discs where the light reactions of photosynthesis occur.
  2. Stroma: The fluid-filled space where the dark reactions (Calvin Cycle) happen and glucose is made.

Photosynthesis Process and Reactions

Q11: What happens during the light reaction of photosynthesis?
A: During the light reaction, sunlight is absorbed by the chlorophyll in the grana. This energy splits water molecules (photolysis), releasing oxygen. Energy-rich molecules, ATP and NADPH, are also produced.

Q12: What is the electron transport chain in photosynthesis?
A: The electron transport chain is the pathway through which electrons move from Photosystem II (PS II) to Photosystem I (PS I). As they move, they help create energy-rich molecules (ATP and NADPH) and also contribute to the splitting of water molecules.

Q13: What is the chemiosmotic hypothesis?
A: The chemiosmotic hypothesis explains how ATP is made in chloroplasts. Protons (H+ ions) are pumped across the thylakoid membrane, creating a proton gradient. As protons flow back into the stroma through ATP synthase, energy is released, which is used to make ATP.

Dark Reactions and the Calvin Cycle

Q14: What happens during the dark reaction (Calvin Cycle)?
A: In the dark reaction, ATP and NADPH produced during the light reaction are used to fix carbon dioxide (CO₂) into glucose. This process occurs in the stroma of the chloroplast.

Q15: What is the primary acceptor of CO₂ in photosynthesis?
A: The primary acceptor of CO₂ in the Calvin Cycle is a 5-carbon molecule called ribulose bisphosphate (RuBP). This molecule combines with CO₂ to form a 3-carbon molecule called PGA.

Types of Photosynthesis: C3 vs. C4

Q16: What is the difference between C3 and C4 pathways of photosynthesis?
A:

  • C3 pathway: This is the most common method of photosynthesis, where the first product formed is a 3-carbon molecule called PGA.
  • C4 pathway: In this pathway, the first product formed is a 4-carbon molecule (OAA). It is found in plants like corn and sugarcane, which are adapted to hot, dry environments.

Additional Knowledge

Q17: Why is photosynthesis important for life on Earth?
A: Photosynthesis is crucial because it provides the oxygen we breathe and is the basis of the food chain. All animals depend on plants for food and oxygen, making photosynthesis essential for life on Earth.

Q18: How do photosystem I and II work together?
A: Photosystem I (PS I) and Photosystem II (PS II) work together in the Z-scheme to absorb light and produce energy. PS II absorbs light at 680 nm and splits water to release oxygen, while PS I absorbs light at 700 nm to produce NADPH. Together, they help generate ATP and NADPH for the Calvin Cycle