Photosynthesis

Photosynthesis

Photosynthesis is the process by which plants, some bacteria, and some protistans use the energy from sunlight to produce glucose from carbon dioxide and water. This glucose can be converted into pyruvate, which releases adenosine triphosphate (ATP) through cellular respiration. Oxygen is also released as a by-product.

Summary of Photosynthesis

  • Word Equation:
    carbon dioxide+water→glucose+oxygen\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}

  • The conversion of usable sunlight energy into chemical energy is associated with chlorophyll, the green pigment found in plants.

  • Chlorophyll is a complex molecule with various modifications present in different plants and photosynthetic organisms.

    • All photosynthetic organisms possess chlorophyll a.

    • Accessory pigments: These pigments absorb energy that chlorophyll a does not. They include:

    • Chlorophyll b, c, d, and e (in algae and protistans)

    • Xanthophylls

    • Carotenoids (e.g., beta-carotene)

  • Chlorophyll a Absorption:

    • Absorbs energy from the violet-blue (around 430 nm) and reddish-orange (around 665 nm) wavelengths, but very little from the intermediate (green-yellow-orange) wavelengths.

  • Chlorophyll Structure:

    • Comprises:

    • A lipid-soluble hydrocarbon tail ($C{20}H{39}$)

    • A flat hydrophilic head with a magnesium ion at its center; side-groups differ among chlorophyll types.

    • The tail and head are linked by an ester bond.

Leaves and Leaf Structure

  • Only plants (not all) that participate in photosynthesis have leaves, which can be viewed as solar collectors packed with photosynthetic cells.

  • Raw Materials:

    • Water and carbon dioxide enter the leaf cells.

    • Products: Sugar and oxygen leave the leaf.

  • Water is absorbed by roots and transported to leaves through xylem vessels.

  • To prevent dehydration, land plants have evolved specialized structures called stomata which allow gas exchange:

    • Cuticle: A protective waxy layer that blocks carbon dioxide from diffusing into the leaf, which enters through stomata instead.

    • Stomata are flanked by two guard cells that regulate their opening and closing.

  • Water Loss Example: Cottonwood trees can lose approximately 100 gallons (~450 dm³) of water per hour during hot, dry conditions.

Structure of the Chloroplast and Photosynthetic Membranes

  • Thylakoid: The fundamental structural unit of photosynthesis found in both photosynthetic prokaryotes and eukaryotes.

  • Eukaryotes have chloroplasts, which include a surrounding membrane.

  • Thylakoids stack in structures known as grana; the area between them is called stroma.

  • The chloroplast has three membrane systems, creating three compartments, contrasting with mitochondria, which only have two.

Stages of Photosynthesis

  • Photosynthesis can be divided into two stages:

    • Light-Dependent Reactions:

    • Occur in the grana and require direct light energy to produce energy carrier molecules.

      • Light energy is trapped by chlorophyll to form ATP (photophosphorylation).

      • Water is split into oxygen, hydrogen ions, and free electrons via photolysis:
        2H<em>2O→4H++O</em>2+4e−2H<em>2O \rightarrow 4H^+ + O</em>2 + 4e^-

      • Electrons reduce nicotinamide adenine dinucleotide phosphate (NADP) from its oxidized state (NADP+) to its reduced state (NADPH):
        NADP++2e−+2H+→NADPH+H+NADP^+ + 2e^- + 2H^+ \rightarrow NADPH + H^+

    • Light-Independent Reactions (Calvin Cycle):

    • Occur in the stroma using products ATP and NADPH to synthesize carbohydrates from carbon dioxide by reduced compound formation, initially producing glyceraldehyde 3-phosphate (GALP).

The Light-Dependent Reactions

  • Upon light absorption, electrons in chlorophyll gain energy (photoexcitation), moving to higher energy levels, leading to photoionization.

  • Electron movement involves:

    • Each chlorophyll molecule is linked to an electron donor and an acceptor forming a photosystem.

    • Photosystem II (PSII, P680) and Photosystem I (PSI, P700) facilitate energy transfer, with PSII acting first.

  • The electron transfer process creates a Z scheme, indicated by the energy changes during electron transfer which help produce ATP from ADP and phosphate.

ATP Synthesis from ADP

  • Condensation Reaction:

    • Phosphoric acid and ADP undergo a reaction:

    • Water is eliminated, leading to ATP formation through phosphorylation.

Non-Cyclic Phosphorylation (Z Scheme)

  • Produces both ATP and NADPH in PSII by:

    • Electron transfer and photolysis of water, yielding oxygen, hydrogen ions, and electrons.

  • Electrons move through an electron transport chain, with PSI as the final acceptor, reducing NADP+ to NADPH.

Chemiosmosis and ATP Synthesis

  • Located in thylakoid membranes, electrons in the transport chain pump H+ ions from stroma into thylakoid compartments, creating an electrochemical gradient.

  • H+ diffusion down the gradient drives ATP production through chemiosmosis, enabling photophosphorylation.

Cyclic Phosphorylation

  • Provides additional ATP for light-independent reactions by:

    • Using only PSI to pass excited electrons back into the electron transport chain without forming NADPH.

Light-Independent Reactions

  • Carbon dioxide combines with ribulose 1,5-bisphosphate (RuBP), resulting in an unstable six-carbon sugar that breaks down into glycerate 3-phosphate (GP).

  • ATP and NADPH are used to convert GP into glyceraldehyde 3-phosphate (GALP).

  • For each pair of GALP produced:

    • One is converted into glucose and other biological macromolecules.

    • The other is used to regenerate RuBP.

The Calvin Cycle

  • The first stable product is phosphoglycerate (PGA), a 3-carbon compound.

  • ATP and NADPH play crucial roles in converting PGA to produce glucose and cycle back to RuBP.

Factors Affecting the Rate of Photosynthesis

  • Limiting Factors:

    • Light intensity, carbon dioxide concentration, and temperature affect photosynthesis rates.

    • As light intensity rises, photosynthesis and light-dependent reactions initially increase but plateau due to other limiting factors.

  • Wavelength Importance:

    • PSI is most effective at 700 nm and PSII at 680 nm; energy concentrated in these wavelengths promotes high photosynthetic rates.

  • Carbon Dioxide Influence: Increased concentration enhances the light-independent reaction, increasing overall photosynthesis until limited by another factor.

  • Temperature Dependence:

    • Photosynthesis is enzyme-catalyzed; rates rise up to optimum temperatures, then decline if temperatures exceed the optimum.

Additional Knowledge Check

  • Engage in quizzes about Photosynthesis to reinforce understanding.