Photosynthesis

• Photosynthetic organisms capture energy from sunlight with pigments
• Plants gather the sun’s energy with light-absorbing molecules called pigments
• The plants’ principal pigment is chlorophyll
• Chlorophyll a and chlorophyll b absorb light very well in the blue-violet and red regions of the visible spectrum, but not in the green region
• Leaves reflect green light, which is why plants look green
• Plants also contain red and orange pigments such as carotenes (carotenoids) that absorb light in other regions of the spectrum
• Photosynthesis takes place inside organelles called chloroplasts, primarily in the leaves of green plants
• Chloroplasts contain saclike photosynthetic membranes called thylakoids, which are interconnected and arranged in stacks known as grana
• Pigments are located in the thylakoid membranes
• The fluid portion outside of the thylakoids is known as the stroma
• When chlorophyll absorbs light, a large fraction of the light energy is transferred to electrons
• An electron carrier is a compound that can accept a pair of high-energy electrons and transfer them, along with most of their energy, to another molecule
• NADP+ (nicotinamide adenine dinucleotide phosphate) is a carrier molecule that accepts and holds two high-energy electrons, along with a hydrogen ion (H+), and is converted into NADPH
• NADPH can carry the high-energy electrons
• Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high-energy sugars and oxygen
• Photosynthesis involves two sets of reactions
• The first set of reactions is the light-dependent reactions (light reactions) that require the direct involvement of light and light-absorbing pigments (Chlorophyll)
• Light-dependent reactions use energy from sunlight to produce ATP and NADPH within the thylakoid membranes of the chloroplast
• Water is required as a source of electrons and hydrogen ions, and oxygen is released as a byproduct
• The second set of reactions is the light-independent reactions (Calvin cycle)
• ATP and NADPH molecules produced in the light-dependent reactions are used to produce high-energy sugars from carbon dioxide
• Plants absorb carbon dioxide from the atmosphere and complete the process of photosynthesis by producing sugars and other carbohydrates
• No light is required for the light-independent reactions, which take place in the stroma outside the thylakoids