4.4 Photosynthesis

4 min read•august 22, 2024

Photosynthesis is the powerhouse of life on Earth. It's the process where plants and other organisms turn sunlight into food, producing oxygen as a byproduct. This amazing transformation is the foundation of most ecosystems and the reason we can breathe.

In this section, we'll break down how photosynthesis works. We'll look at the light-dependent and light-independent reactions, explore the role of pigments, and see how different factors affect the process. It's complex, but super important!

Photosynthesis: Process and Significance

Overview of Photosynthesis

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  • Photosynthesis converts light energy into chemical energy stored in sugars or other organic compounds in plants and photosynthetic organisms
  • The overall equation for photosynthesis: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2 requires carbon dioxide, water, and light energy produces glucose and oxygen
  • Photosynthesis occurs in two stages the light-dependent reactions in the thylakoid membranes of chloroplasts and the light-independent reactions (Calvin cycle) in the stroma

Importance of Photosynthesis

  • Photosynthesis provides energy used by nearly all living things to fuel cellular processes maintains atmospheric oxygen levels
  • Photosynthesis supplies all organic compounds and most energy necessary for life on Earth (glucose, other sugars, and organic molecules)
  • Photosynthetic organisms (plants, algae, cyanobacteria) form the base of most food chains and food webs
  • Photosynthesis plays a crucial role in the global carbon cycle by converting atmospheric CO2 into organic compounds

Light-Dependent vs Light-Independent Reactions

Light-Dependent Reactions (Light Reactions or Photochemical Phase)

  • Light-dependent reactions require light energy to produce ATP and NADPH used in light-independent reactions
  • Photosystems, clusters of proteins and pigments, capture light energy convert it into chemical energy
  • Light-dependent reactions occur in thylakoid membranes of chloroplasts involve splitting water molecules, releasing oxygen, transferring electrons through an electron transport chain
  • Key events include excitation of electrons in photosystems, electron transport, proton gradient formation, and ATP synthesis via chemiosmosis

Light-Independent Reactions (Dark Reactions or Calvin Cycle)

  • Light-independent reactions use ATP and NADPH from light-dependent reactions to convert CO2 into glucose
  • Light-independent reactions occur in the stroma of chloroplasts do not require light directly
  • Key enzyme RuBisCO (ribulose bisphosphate carboxylase oxygenase) catalyzes carbon fixation to produce 3-phosphoglycerate, a 3-carbon compound eventually converted into glucose
  • Other important steps include reduction of 3-phosphoglycerate to glyceraldehyde 3-phosphate (G3P), regeneration of ribulose bisphosphate (RuBP), and synthesis of glucose and other organic compounds

Role of Pigments and Electron Transport

Photosynthetic Pigments

  • Photosynthetic pigments (chlorophyll a, chlorophyll b, carotenoids) absorb light energy transfer it to reaction centers of photosystems
  • Chlorophyll a is the primary photosynthetic pigment directly involved in light-dependent reactions
  • Chlorophyll b and carotenoids are accessory pigments that extend the range of light wavelengths used for photosynthesis (blue and red light)
  • Pigments are organized in light-harvesting complexes (LHCs) that funnel energy to reaction centers

Electron Transport Chain

  • Electron transport chain is a series of protein complexes and electron carriers within the thylakoid membrane that transfer electrons from water to NADP+, generating a proton gradient and producing NADPH
  • Photosystem II (PSII) absorbs light energy, exciting electrons transferred to the electron transport chain electrons are replaced by splitting water molecules, releasing oxygen
  • Electrons move through the electron transport chain, losing energy used to pump protons (H+) from the stroma into the thylakoid lumen, creating a proton gradient
  • Photosystem I (PSI) absorbs light energy, exciting electrons transferred to NADP+ to produce NADPH
  • ATP synthase, an enzyme in the thylakoid membrane, uses the proton gradient to produce ATP through chemiosmosis (protons flow down their concentration gradient, driving ATP synthesis)

Factors Affecting Photosynthesis Rate

Environmental Factors

  • Light intensity increases photosynthesis rate until saturation point further increases do not affect the rate
  • Carbon dioxide concentration higher CO2 levels increase photosynthesis rate, but very high concentrations can be toxic
  • Temperature photosynthesis rate increases with temperature up to an optimal point (25-35°C for most plants) beyond this, the rate may decrease due to enzyme denaturation and plant tissue damage
  • Water availability essential for photosynthesis as a reactant insufficient water supply limits photosynthesis by causing stomatal closure, reducing CO2 uptake

Biological Factors

  • Nutrient availability essential nutrients (nitrogen, phosphorus, potassium) are required for synthesis of photosynthetic enzymes and pigments deficiencies limit photosynthesis rate
  • Plant species and leaf age different plant species have varying photosynthetic efficiencies younger leaves generally have higher photosynthesis rates compared to older leaves
  • Leaf anatomy and morphology leaf thickness, stomatal density, and arrangement of chloroplasts can affect photosynthesis efficiency
  • Enzyme activity and concentration higher levels of key enzymes (RuBisCO, ATP synthase) can increase photosynthesis rate, while enzyme damage or denaturation can reduce efficiency
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