Photosynthesis Overview

• Definition: Photosynthesis is a process utilized by plants, some bacteria, and protistans to convert sunlight energy into chemical energy, specifically glucose, from carbon dioxide and water.

• Word Equation: Carbon dioxide + Water 👉 Glucose + Oxygen

• Energy Conversion: Converts usable sunlight energy into chemical energy associated with chlorophyll.

The Role of Chlorophyll

• Chlorophyll: A complex molecule that absorbs light.

• Types of Chlorophyll: All photosynthetic organisms have chlorophyll a; accessory pigments include:

  • Chlorophyll b (and c, d, e)
    - Xanthophylls

  • Carotenoids (e.g., beta-carotene)

• Absorption Spectrum: Chlorophyll a primarily absorbs violet-blue and reddish-orange light wavelengths, and little from green-yellow-orange ranges.

Leaf Structure and Function

• Leaves: The primary site for photosynthesis, functioning as solar collectors.

• Function: Water and carbon dioxide enter the leaf cells, producing sugar and oxygen.

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

• Gas Exchange:

  • Stomata: Specialized structures that regulate gas entry (CO2) and exit (O2).

  • Guard Cells: Regulate the opening and closing of stomata.

  • Cuticle: Waxy layer preventing gas diffusion; CO2 enters through stomata.

• Water Loss: Significant water loss occurs, with examples like cottonwood trees losing about 100 gallons per hour.

Chloroplast Structure

• Thylakoids: Flattened sacs/vesicles in which photosynthesis occurs; stacked as grana.

• Components: Chloroplasts have three membrane systems, differing from mitochondria.

• Stroma: Fluid-filled space surrounding the thylakoids.

Stages of Photosynthesis

Light-Dependent Reactions

• Process: Occur in grana when chlorophyll absorbs light energy, exciting electrons.

• Key Steps:

  • Excited electrons are transferred to a primary electron acceptor.

  • Water splits (photolysis) to provide electrons, releasing O2, H+, and electrons.

  • ATP is produced via photophosphorylation.

  • NADP+ is reduced to form NADPH.

Light-Independent Reactions (Calvin Cycle)

• Process: Occur in the stroma using ATP and NADPH to convert CO2 into carbohydrates.

• Steps:

  • Carbon fixation via ribulose 1,5-biphosphate (RuBP).

  • Formation of glyceraldehyde 3-phosphate (GALP).

Light-Dependent Reactions Outcomes

• Electron Transfer: Light energy absorbed by chlorophyll leads to:

  • Photoexcitation: Electrons gain energy.

  • Photoionisation: Electrons are freed, resulting in a positively charged chlorophyll ion.

Non-Cyclic Phosphorylation (Z Scheme)

• Process: Involves both photosystems: PSII and PSI.

• ATP and NADPH Production: Excited electrons lead to ATP production via an electron transport chain, resulting in NADPH synthesis.

Chemiosmosis and ATP Synthesis

• Location: Thylakoid membranes.

• H+ Gradient: Electrons pump H+ ions, creating a gradient that powers ATP synthesis.

Cyclic Phosphorylation

• Description: Only involves PSI; generates additional ATP without producing NADPH.

• Function: Provides extra ATP needed for light-independent reactions.

Calvin Cycle Mechanism

• Key Steps:

  • Carbon fixation into carbohydrates using ATP and NADPH.

  • PGA formation followed by conversion to GALP.

• Outputs: Glucose production and reforming of RuBP.

Factors Affecting Photosynthesis Rate

• Limiting Factors: Light intensity, CO2 concentration, and temperature.

• Light Intensity: Direct correlation with photosynthesis rate until other factors limit it.

• Wavelength of Light: PSI (700 nm) and PSII (680 nm) are optimal wavelengths for photosynthesis.

• CO2 Concentration: Affects incorporation rates during light-independent reactions.

• Temperature: Enzyme-catalyzed reactions; increased temperatures raise rates until enzymes denature.