🔬General Biology I Unit 8 – Photosynthesis

What's Photosynthesis Anyway?

• Process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose or other sugars
• Occurs in chloroplasts, specialized organelles found in plant cells
• Requires light energy, carbon dioxide (CO2), and water (H2O) as reactants
• Produces glucose (C6H12O6) and oxygen (O2) as products
• Glucose is used by plants for energy and to make other organic compounds like cellulose and starch
• Oxygen is released into the atmosphere as a byproduct
• Photosynthesis is the primary source of energy for most life on Earth
  • Directly or indirectly, it provides the organic compounds and energy used by nearly all living things to survive

The Players: Light, Chlorophyll, and Friends

• Light energy is a key component of photosynthesis
  • Plants absorb light primarily using the pigment chlorophyll
• Chlorophyll is found in chloroplasts, specifically in the thylakoid membranes
  • Chlorophyll a is the primary pigment used in photosynthesis
  • Chlorophyll b and other pigments like carotenoids also contribute to light absorption
• Carbon dioxide enters the plant through tiny pores called stomata in the leaves
• Water is absorbed by the plant's roots and transported to the leaves via the xylem
• Enzymes play a crucial role in photosynthesis by facilitating various chemical reactions
  • RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is a key enzyme involved in the Calvin cycle
• ATP synthase is an enzyme that generates ATP (adenosine triphosphate) during the light reactions
• NADP reductase is an enzyme that catalyzes the reduction of NADP+ to NADPH during the light reactions

Step-by-Step: How Plants Make Their Food

• Step 1: Light absorption
  • Light energy is absorbed by chlorophyll and other pigments in the thylakoid membranes of chloroplasts
• Step 2: Electron excitation
  • The absorbed light energy excites electrons within the chlorophyll molecules
• Step 3: Electron transport chain
  • The excited electrons are passed through a series of proteins and molecules, forming an electron transport chain
  • This process generates a proton gradient across the thylakoid membrane
• Step 4: ATP and NADPH production
  • The proton gradient is used by ATP synthase to generate ATP
  • NADP reductase uses the high-energy electrons to reduce NADP+ to NADPH
• Step 5: Carbon fixation
  • The Calvin cycle begins with the enzyme RuBisCO fixing CO2 to a 5-carbon sugar called ribulose bisphosphate (RuBP)
  • This process produces a 6-carbon compound that splits into two 3-carbon molecules
• Step 6: Glucose production
  • The 3-carbon molecules are reduced using ATP and NADPH from the light reactions
  • These reduced molecules are then used to regenerate RuBP and produce glucose through a series of reactions

Light Reactions: Catching Some Rays

• Light reactions occur in the thylakoid membranes of chloroplasts
• Photosystems, large protein complexes, are responsible for light absorption and electron excitation
  • There are two types of photosystems: Photosystem II (PSII) and Photosystem I (PSI)
• Light energy is absorbed by chlorophyll and other pigments in the photosystems
• Excited electrons from PSII are passed to an electron transport chain
  • This process generates a proton gradient across the thylakoid membrane
• ATP synthase uses the proton gradient to generate ATP through chemiosmosis
• PSI absorbs light energy and excites electrons, which are used to reduce NADP+ to NADPH
• The splitting of water (photolysis) occurs in PSII, releasing electrons, protons, and oxygen
  • Oxygen is released as a byproduct of this process

Dark Reactions: The Calvin Cycle Shuffle

• Dark reactions, also known as the Calvin cycle, occur in the stroma of chloroplasts
• The Calvin cycle is a series of biochemical reactions that use the products of the light reactions (ATP and NADPH) to fix CO2 and produce glucose
• The cycle begins with the fixation of CO2 to ribulose bisphosphate (RuBP) by the enzyme RuBisCO
  • This process produces a 6-carbon compound that splits into two 3-carbon molecules (3-phosphoglycerate)
• ATP and NADPH from the light reactions are used to reduce 3-phosphoglycerate to form glyceraldehyde 3-phosphate (G3P)
• Some G3P molecules are used to regenerate RuBP, allowing the cycle to continue
• The remaining G3P molecules are used to synthesize glucose and other organic compounds
• The Calvin cycle is regulated by various factors, including the availability of CO2, ATP, and NADPH
  • The activity of RuBisCO is also regulated by light intensity and temperature

Factors Affecting Photosynthesis

• Light intensity
  • Higher light intensity generally increases the rate of photosynthesis until a saturation point is reached
  • Too much light can damage the photosynthetic apparatus (photoinhibition)
• Carbon dioxide concentration
  • Higher CO2 levels can increase the rate of photosynthesis, as more substrate is available for the Calvin cycle
  • However, other factors like light intensity and temperature can limit the effect of increased CO2
• Temperature
  • Optimal temperature range for photosynthesis varies among plant species
  • Extreme temperatures can denature enzymes and damage the photosynthetic apparatus
• Water availability
  • Water is a crucial reactant in photosynthesis and is needed for electron transport
  • Drought stress can lead to stomatal closure, reducing CO2 uptake and photosynthesis
• Nutrient availability
  • Essential nutrients like nitrogen, phosphorus, and magnesium are required for the synthesis of chlorophyll and other photosynthetic components
  • Nutrient deficiencies can limit photosynthesis and plant growth

Why Photosynthesis Matters

• Photosynthesis is the primary source of energy for most life on Earth
  • It directly or indirectly provides the organic compounds and energy used by nearly all living things
• Photosynthesis plays a crucial role in the global carbon cycle
  • It removes CO2 from the atmosphere and incorporates it into organic compounds
  • This process helps to regulate atmospheric CO2 levels and mitigate the effects of climate change
• Oxygen released during photosynthesis is essential for aerobic respiration in most organisms
  • The evolution of photosynthesis in cyanobacteria and plants led to the oxygenation of Earth's atmosphere
• Photosynthesis is the basis for most agricultural productivity
  • Crops and other plants rely on photosynthesis to grow and produce food for humans and other animals
• Understanding photosynthesis is crucial for developing strategies to improve crop yields and food security
  • Genetic engineering and other approaches can be used to enhance photosynthetic efficiency and resilience to environmental stresses

Real-World Applications and Cool Facts

• Artificial photosynthesis
  • Scientists are working on developing artificial systems that mimic the process of photosynthesis to produce clean energy and useful chemicals
  • These systems could potentially help address energy and environmental challenges
• Biofuels
  • Photosynthetic organisms like algae and cyanobacteria can be used to produce biofuels
  • These fuels are renewable and have a lower carbon footprint compared to fossil fuels
• Space exploration
  • Understanding photosynthesis is important for developing life support systems for long-term space missions
  • Algae and other photosynthetic organisms could be used to produce oxygen and food for astronauts
• Bioluminescent photosynthesis
  • Some deep-sea organisms like the vampire squid use bioluminescent bacteria to perform photosynthesis in the absence of sunlight
  • This unique adaptation allows these organisms to survive in the dark, nutrient-poor environments of the deep ocean
• Photosynthetic sea slugs
  • Elysia chlorotica, a species of sea slug, can perform photosynthesis by incorporating chloroplasts from the algae it consumes
  • This process, known as kleptoplasty, allows the sea slug to survive for months without eating