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What is Photosynthesis?

What is photosynthesis?

Photosynthesis is the process in which plants use energy from the sunlight in order to covert water and carbon dioxide into high energy carbohydrates and oxygen. The high energy carbohydrates are sugars and starches.

 6CO2 + 6H2O + Sunlight -- > C6H12O6 + 6O2

Key Starting Materials

Products

 

Carbon dioxide

 

Sugars

Water

Oxygen

Sunlight

 

 

Photosynthesis occurs inside the Chloroplasts. Chloroplasts are double membrane bound organelles are that typically found in the green parts of the plants (leaves). Within the chloroplasts, light dependent reactions take place in the thylakoid membranes and the light independent reactions take place in the stroma.

Photosynthesis is broken down into two stages: light dependent reactions and light independent reactions.

In Light Dependent stage,

  • Energy from the sunlight is used to produce ATP and NADPH
  • Oxygen is also produced
    • Oxygen is the by-product resulting from the break down of water
  • Steps:
      • Sunlight energy hits the plant
      • Photosystem II absorbs the light.
        • There are electrons that are trapped by P680 in the photosystem II; these electrons absorb the light and become energized.
      • The energized electrons are passed through the electron transport chain and onto Photosystem I
        • As the electrons move down the electron transport chain, they start to lose energy. The energy lost is used to make ATP
          • There are a lot of H+ ions in the inner thylakoid membrane. These ions are transported to the outer membrane by ATP synthase. ATP is produced as H+ pass through the enzyme.
      • The electrons that left Photosystem II are replaced by H2O in a process called photolysis.
        • In Photolysis, water is broken into:
          • 2 electrons,  2 H+ and 1 oxygen atom
            • The electrons replace the electrons that left Photosystem II.
            • The Oxygen atom is released as by-product
            • The H+ go to the inner thylakoid membrane
      • The electrons in Photosystem I again becomes energized by the light energy.
        • NADP+ picks up the electrons from Photosystem I and converts to NADPH
          • The H+ ion comes from the outer thylakoid membrane

 

In Light Independent stage,

    • Energy of the ATP and NADPH from the light dependent reaction is used to produce high energy sugars from carbon dioxide
    • Basically, six carbon dioxide molecules are eventually converted into one six-carbon molecule (sugar)
    • Takes place in the stroma of the chloroplast
      • Steps
        • Carbon dioxide that comes from the atmosphere will enter the stroma and combine with six 5-carbon molecules to form twelve 3-carbon molecules
        • The 5-carbon molecules are called RuBP
          • RuBP carboxylase (also called Rubisco)  is an enzyme that helps to combine the carbon dioxide and RuBP
        • The 3-carbon molecules are called PGA (phosphoglycerate)
        • 12 ATP and 12 NADPH molecules convert 12 PGA into 12 PGAL by reduction