Photosynthesis and Chloroplasts

Photosynthesis

• Photosynthesis sustains life on Earth
• Photosynthetic organisms include plants, algae, cyanobacteria, and some protists
• These organisms transform sunlight's radiant energy into chemical energy
• Key pigment is chlorophyll, plus other pigments
• The process of photosynthesis occurs within chloroplasts
• The reaction is: 6 CO₂ + 6 H₂O (sunlight) → C₆H₁₂O₆ + 6O₂ (Carbon Dioxide + Water → Glucose + Oxygen)

Chloroplast

• Chloroplasts likely evolved from free-living photosynthetic prokaryotic cells
• This is the endosymbiotic theory
• Chloroplasts have a double membrane, inner membrane with ETC proteins
• Chloroplasts also have ribosomes, circular DNA for independent reproduction & DNA sequence similar to bacteria

Chloroplasts & Photosynthesis

• Photosynthesis in palisade cells has two stages
• Light reaction occurs in thylakoids (light energy captured by pigments, electrons released, water broken down releasing electrons and oxygen)
• Dark reaction occurs in stroma (NADPH, CO₂, ATP energy used to make glucose)

Electromagnetic Spectrum

• Sunlight is a mixture of photons with different energies
• A spectroscope separates light into different energies, creating the electromagnetic spectrum
• Most photons are invisible, but a band from about 380nm (violet) to 750nm (red) is visible light

Photosystems

• Photosystems are clusters of photosynthetic pigments embedded in thylakoid membranes
• Pigments are chemical substances produced by plants that absorb specific wavelengths of light

Plant Pigments

• Plant pigments are categorized into photosynthetic and protective pigments
• Photosynthetic pigments include chlorophylls (green), carotenoids (yellow/orange), and xanthophylls (yellow)
• Protective pigments include anthocyanins (red/purple/blue)

Absorption Spectrum

• Chlorophylls a and b absorb blue-violet and red light, reflecting green light, making plants appear green
• Other pigments absorb different wavelengths than chlorophyll, together they cover a broad range of the visible spectrum

Photosynthesis: Light Reaction

• In the thylakoid membrane, light energy is captured by pigments, which release electrons
• Water is broken down, releasing electrons and producing oxygen as a waste product
• Free electrons travel through membrane proteins (ETC), producing ATP and NADPH, which move to the dark reaction

Photosynthesis: Dark Reaction (Calvin Cycle)

• In the stroma, NADPH, CO₂, and ATP energy are used to make glucose
• The Calvin cycle includes carbon fixation, regeneration of CO₂ acceptor, and reduction

Uses of Products

• Glucose can be used in cellular respiration in the same palisade cell
• Glucose can be combined to form sucrose (transports sugars in phloem)
• Glucose can be joined into starch for storage
• Oxygen is released as a waste product, used in cellular respiration in the same palisade cell