Photosynthesis

Photosynthesis Overview

• Photosynthesis is the process by which plants, some bacteria, and some protistans utilize sunlight energy to produce glucose from carbon dioxide and water.

• Resulting glucose can be converted into pyruvate, releasing adenosine triphosphate (ATP) through cellular respiration.

• Oxygen is also a byproduct.

• Summary word equation:

  • Carbon dioxide + Water -> Glucose + Oxygen

Role of Chlorophyll in Photosynthesis

• Chlorophyll is a green pigment essential for photosynthesis, acting to convert sunlight energy into chemical energy.

• Key characteristics of chlorophyll:

  • Complex molecule with several variations.

  • All photosynthetic organisms contain chlorophyll a.

  • Accessory pigments (e.g., chlorophyll b, c, d, e; xanthophylls; carotenoids like beta-carotene) capture light energy not absorbed by chlorophyll a.

  • Chlorophyll a absorbs energy from violet-blue and reddish-orange-red wavelengths inefficiently from green-yellow-orange wavelengths.

• Structure of chlorophyll:

  • Lipid-soluble hydrocarbon tail (C20H39-)

  • Hydrophilic head containing a magnesium ion; side-groups differ among chlorophyll types.

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

Leaf Structure and Function in Photosynthesis

• Leaves: Primary photosynthetic structures in plants (although not all plants have leaves).

• Leaves are developed as solar collectors filled with photosynthetic cells.

• Raw materials (water and carbon dioxide) enter the leaf cells; products (sugars and oxygen) exit.

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

• Plants evolve stomata, microscopic openings on leaves:

  • Allow CO2 to enter and O2 to exit.

  • Stomata are flanked by guard cells regulating their opening.

• The waxy cuticle covers leaves, preventing water loss but blocking gas exchange.

• Example: Cottonwood trees may lose up to 100 gallons (approximately 450 dm³) of water per hour in hot climates.

Chloroplast Structure and Thylakoid Membranes

• Thylakoids: Structural units of photosynthesis in both prokaryotic and eukaryotic organisms.

• Eukaryotes possess chloroplasts featuring surrounding membranes.

• Thylakoids are organized in stacks called grana.

• Inter-granular regions are termed stroma.

• Chloroplasts have three membrane systems forming three compartments, unlike mitochondria which have two.

Stages of Photosynthesis

Light Dependent Reactions

• When chlorophyll a absorbs light, its electron is 'excited', transferring it to a primary electron acceptor.

• Chlorophyll becomes oxidized and positive due to electron loss.

• Water splitting (photolysis) occurs:

  • 2H2O ightarrow 4H^+ + O2 + 4e^-

• Electrons react with oxidized NADP+, reducing it:

  • NADP^+ + 2e^- + 2H^+
    ightarrow NADPH + H^+

• Energy molecules ATP and NADPH are produced from light energy.

Light Independent Reactions (Calvin Cycle)

• Occurs in stroma, utilizing ATP and NADPH from the light reactions to synthesize carbohydrates (initially form glyceraldehyde 3-phosphate (GALP)).

Detailed Mechanisms of Light-Dependent Reactions

Photoexcitation and Electron Transfer

• Absorption of light causes electrons in chlorophyll to move to higher energy states (photoexcitation).

• If energy is sufficient, chlorophyll undergoes photoionization, releasing an electron and creating a positively charged ion.

• In chloroplasts, each chlorophyll associates with an electron acceptor and donor, forming a photosystem.

• Two electron transport systems are: Photosystem II (PSII) (P680) and Photosystem I (PSI) (P700), with PSII occurring before PSI in the cycle.

• The process forms a Z-scheme indicating energy releases through electron transport.

ATP Synthesis Mechanism

• Outline of ATP formation from ADP and inorganic phosphate (phosphorylation) via condensation reactions.

• Structural diagram showing the formation of ATP from ADP:

  • Condensation reaction illustrated by:
    ext{ADP} + Pi ightarrow ext{ATP} + H2O

• Key stages include pumping of H+ ions across membranes for ATP synthesis via chemiosmosis.

Cyclic and Non-Cyclic Phosphorylation

Non-Cyclic Phosphorylation

• Produces ATP and NADPH:

  • Starts with PSII, which undergoes photoionization.

  • Photolysis of water transitions an electron to chlorophyll and forms O2, H+, and electrons.

  • Electrons flow from PSII to PSI, powering NADP+ reduction to NADPH.

Cyclic Phosphorylation

• Provides additional ATP for light-independent reactions but does not produce NADPH.

• Involves only PSI where excited electrons complete the cycle back to the photosystem without NADPH production.

Light Independent Reactions and Carbon Fixation

• Carbon dioxide is captured by organisms to form carbohydrates through carbon fixation (adding hydrogen from water).

• The process starts with carbon dioxide combining with five-carbon sugar ribulose 1,5-bisphosphate (RuBP), which initially forms unstable six-carbon molecules that split into two glycerate-3-phosphate (GP) molecules per cycle.

• Glycerate-3-phosphate molecules are phosphorylated into glycerate diphosphate (G3P) using ATP, reduced by NADPH to produce glyceraldehyde-3-phosphate (GALP).

• Of two GALP produced, one forms glucose while the other is transformed back to RuBP.

Calvin Cycle Overview

• First stable product of Calvin cycle is phosphoglycerate (PGA),

• Eventually results in twelve molecules of GALP, two of which form glucose; the rest reform to continue the cycle.

Factors Influencing the Rate of Photosynthesis

• Main limiting factors include:

  • Light intensity: Generally proportional increase in the photosynthesis rate until limited by another factor.

  • Light wavelength: PSI absorbs best at 700 nm (red) and PSII at 680 nm (blue).

  • Carbon dioxide concentration: Increase enhances carbohydrate formation until limited by external factors.

  • Temperature: Enzyme-catalyzed reaction with optimal temperature for maximum rate.

Learning Check

• Engage in a quiz for reinforcement of knowledge on photosynthesis concepts and mechanisms.