Photosynthesis: Key Concepts

Introduction to Photosynthesis

• Photosynthesis fuels the biosphere by converting light energy into chemical energy
• Photoautotrophs, like plants and algae, use light energy to create their own nourishment
• Heterotrophs, including humans, consume plants, animals, or decaying organic matter for sustenance
• Photoautotrophs require sunlight, CO₂, water, and minerals from their surroundings to produce food

Chloroplasts and Chlorophyll

• Photosynthesis transpires inside the chloroplasts of plant cells
• Chloroplasts are enclosed by a double membrane and house thylakoids (stacked structures) within a fluid matrix called stroma
• Chlorophyll, a pigment within chloroplasts, is vital for converting solar energy into chemical energy
• Chlorophyll is concentrated in mesophyll cells within the leaf
• Gaseous exchange happens through stomata where CO₂ enters and O₂ exits of the leaf
• Water (H₂O) is absorbed through the roots and transported to the leaves via the veins

Key Concepts and Structures

• Photoautotrophs produce their own food, while heterotrophs consume photoautotrophs as a food source
• Photosynthesis is the converse of cellular respiration
• Chloroplasts reside within leaves
• Granum is a stack of thylakoids and grana refers to multiple stacks
• Photosynthesis primarily takes place in the thylakoid membrane
• Chlorophyll is a pigment that absorbs light from the sun
• Sunlight comprises different colors of light, similar to the colors of the rainbow
• Stomata, pores on the underside of leaves, facilitate gas exchange
• Mesophyll cells are packed with chloroplasts that are essential for photosynthesis.

Photosynthesis Process

• Scientists used isotopes to understand the process of photosynthesis better
• Experiment 1's equation: 6CO₂ + 12H₂O → C₆H₁₂O₂ + 6H₂ + 6O₂ was proven false
• Experiment 2's equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ is the accurate representation
• Photosynthesis can be summarized as: Light energy + 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Photosynthesis, mirroring cell respiration, is a redox process
• During photosynthesis, water (H₂O) is oxidized, and carbon dioxide (CO₂) is reduced
• Cellular respiration employs redox reactions to extract chemical energy from glucose formed during photosynthesis
• Overall photosynthesis equation: Energy + 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Light Reactions & Calvin Cycle

• Photosynthesis unfolds in two phases
• These phases are interconnected through ATP and NADPH
• Light-dependent reactions occur in the thylakoids
• This produces ATP and NADPH for the subsequent Calvin Cycle, which takes place in the stroma
• Oxygen released by plants is derived from water (H₂O) rather than carbon dioxide (CO₂)

The Equation

• The photosynthesis equation is cellular respiration flipped
• Photosynthesis is a redox reaction involving both oxidation and reduction.
• In the Calvin Cycle, carbon dioxide (CO₂) is integrated into organic compounds through carbon fixation

Light and Energy in Photosynthesis

• Light reactions transform solar energy into chemical energy
• Pigments absorb visible radiation to drive light reactions
• Sunlight, a form of electromagnetic energy, consists of various wavelengths
• Chlorophyll and other pigments absorb precise wavelengths of visible light
• Carotenoids assist in photoprotection against excessive light
• Photosystems capture solar energy where thylakoid membranes contain photosystems that are light-harvesting complexes and a reaction-center complex

The Calvin Cycle

• Melvin Calvin discovered the Calvin Cycle, which takes place in the stroma
• The Calvin Cycle does not require light
• Chloroplasts require ATP and NADPH to produce sugar from carbon dioxide in darkness
• Wavelengths are measured in lambda from one wave crest to the next
• Infrared radiation detects heat due to its high energy and close waves
• Waves are a form of radiation
• Crests and valleys describe the peaks and troughs in waveforms
• Plants appear green because they absorb almost all colors of light but reflect green light
• Plants will die if grown in only green light
• Radio waves possess low energy wavelengths, while gamma waves have very high energy.

Photosystems and Electron Transport

• Photoexcited electrons from reaction-center chlorophyll a are received by a primary electron acceptor
• Light energy is converted into chemical energy in the form of ATP and NADPH by two photosystems linked by an electron transport chain
• Electrons go from Photosystem II to Photosystem I to produce ATP and reduce NADP+ to NADPH
• Electrons are regained by Photosystem II through the splitting of water, releasing oxygen (O₂)
• Light reactions transpire within the thylakoid membranes
• During photophosphorylation, the electron transport chain pumps H+ ions into the thylakoid space.
• ATP synthase is powered by the concentration gradient, which drives H+ ions back, synthesizing ATP.

Calvin Cycle Steps

• The Calvin Cycle reduces CO₂ to create sugar, which is then powered by ATP and NADPH
  • Carbon fixation
  • Reduction
  • Release of G3P
  • Regeneration of RuBP

Photosystems and Climate

• Photosystems resemble electron transport chains, possessing hydrophilic outsides and hydrophobic trails
• Light energy moves in small packets called photons
• Electrons travel from Photosystem II to Photosystem I
• Water provides electrons to photosystems
• Photosystem II feeds electrons into Photosystem I
• ATP and NADPH are produced toward the end of the reactions, which then move into the Calvin Cycle
• The cycle uses carbon from CO₂, electrons from NADPH, and energy from ATP to produce G3P, which is used to build glucose and other organic molecules
• Alternative carbon fixation methods have developed in drier climates
• In C3 plants, if stromata close, a decrease in CO₂ and increase in O₂ will divert the Calvin Cycle into photorespiration
• C4 and CAM plants initially fix CO₂ into four-carbon compounds to supply the Calvin Cycle with CO₂ when stromata are closed during hot, dry conditions
• Photosystems provide nourishment and oxygen for almost every living thing

Final Notes

• Sugars serve as the starting materials for other organic molecules, including proteins, lipids, and cellulose
• Multiple glucose molecules are connected to create cellulose, the main structural component of cell walls
• Calvin Cycle transpires in the stroma of thylakoids
• Carbon dioxide (CO₂) enters through the stomata
• RuBP is broken down into smaller carbon molecules
• The majority of plants visible in the surroundings are C3 plants.
• Sugarcane is a C4 plant, while pineapple is a CAM plant
• Oxygen is a byproduct of plants, while carbon dioxide is ours

Climate

• Climate change can be moderated by decreasing fossil fuel use and deforestation
• The greenhouse effect is caused by CO₂ and other atmospheric gases
• Global warming is a significant factor in climate change, resulting in long-term shifts to the global climate lasting for 3 decades or more
• The Paris Climate Conference of 2015 is an international agreement that aims to decrease greenhouse gas emissions and limit the effects of global warming.
• The U.S. pulled out of this agreement in January 2017, rejoined on January 20, 2017, and then departed again in 2025, after the next election.