Photosynthesis

Interactive Visual Guide to Chloroplasts, Pigments, Light-Dependent & Light-Independent Reactions

Inside the Chloroplast: Two Stages of Reactions

Click any labeled part in the diagram to explore!
CHLOROPLAST (Two Outer Membranes) STROMA (Cytosol-like fluid surrounding thylakoids) Sunlight Energy H₂O (Water) STAGE 1: Light-Dependent Reactions (Thylakoid Membrane) O₂ Gas Released ATP & NADPH ADP + Pᵢ, NADP⁺ CO₂ (Carbon Dioxide) STAGE 2: Light-Independent Reactions (Calvin–Benson Cycle) Sugars (PGAL / Sucrose)

Overview of Photosynthesis

Interactive Guide

Photosynthesis feeds most life on Earth. Sunlight is abundant, but it cannot directly power metabolic reactions. First, it must be converted into the chemical bond energy of organic molecules.


Click on any structure inside the Chloroplast diagram on the left to see how the two stages of photosynthesis work together!

Take-Home Message:
• Stage 1 (Light-Dependent): Occurs at the thylakoid membrane. Converts light energy to chemical energy (ATP & NADPH) and splits water to release O2.
• Stage 2 (Light-Independent): Occurs in the stroma. Uses ATP and NADPH to assemble sugars from CO2 and water.

Engelmann's Experiment & Pigment Absorption

Drag the wavelength slider below!
Absorption Spectrum of Photosynthetic Pigments Light Absorbed Chlorophyll a Chlorophyll b β-carotene Theodor Engelmann's 1882 Experiment: Oxygen-Seeking Bacteria Cladophora Green Alga Strand 400 nm (Violet) 500 nm (Green) 600 nm (Orange) 700 nm (Red)
Test Wavelength of Light: 450 nm (Blue Light) Photon Energy: High Energy (Short Wavelength)

Live Pigment & O₂ Meter

Chlorophyll a (Main Pigment) 85%
Chlorophyll b (Accessory) 92%
Beta-Carotene (Accessory) 78%
O₂ Release & Bacteria Swarm Rate: Very High!
Why Blue & Red Work Best: Chlorophyll a absorbs violet, blue, orange, and red light strongly, while reflecting green light (500–580 nm). Accessory pigments like chlorophyll b and carotenoids broaden the range of wavelengths that can drive photosynthesis.
Key Concepts:
• Photons & Wavelength: Shorter wavelengths (violet/blue, ~400 nm) carry more energy per photon than longer wavelengths (red, ~700 nm).
• Light-Trapping Region: Pigments have alternating single and double carbon bonds where electrons absorb photons with the exact energy needed to jump to a higher shell.

Stage 1: The Thylakoid Membrane

STROMA (Low H⁺ concentration) Thylakoid Membrane THYLAKOID COMPARTMENT (High H⁺ concentration gradient) Photosystem II p680 1. Light 2. H₂O ½ O₂ + 2 H⁺ 3. Electron Transfer Chain H⁺ 4. H⁺ Pump Photosystem I p700 5. Light e⁻ Cycles Back! ETC II 6. NADPH NADP⁺ + H⁺ + 2e⁻ H⁺ H⁺ H⁺ H⁺ H⁺ H⁺ ATP Synthase H⁺ ATP

Noncyclic Pathway

Step 1 of 8

Photosystem II Absorbs Light Energy

Yes
O₂ Gas Released
Yes
NADPH Made
Yes
ATP Made
Why Two Pathways?
The noncyclic pathway produces both ATP and NADPH using electrons pulled from water. However, it doesn't yield quite enough ATP to balance NADPH use in the Calvin–Benson cycle—so the cyclic pathway runs to supply extra ATP and protect cells under intense light!

Stage 2: The Calvin–Benson Cycle

Summary of 3 cycles in the Stroma
Calvin–Benson Cycle (Stroma) 1. 3 CO₂ (1C each) Enzyme: Rubisco 3 RuBP (5-Carbon) 2. 6 PGA (3-Carbon) 6 ATP → 6 ADP + 6 Pᵢ 6 NADPH → 6 NADP⁺ 3. 6 PGAL (3-Carbon) 5 & 6. 1 PGAL Exported → Combines to form Sucrose 4. 5 PGAL Continue 3 ATP → 3 ADP + 2 Pᵢ

Calvin–Benson Cycle Steps

Step 1 of 5

Carbon Fixation by Rubisco

3 CO₂
Carbon Fixed
9 ATP
Used (6 + 3)
6 NADPH
Reduced
Take-Home Message:
• The light-independent reactions run on ATP and NADPH produced during the light-dependent reactions.
• Extracting carbon atoms from an inorganic source (like CO₂) and incorporating them into an organic molecule is called carbon fixation.