C4 Plants

• Examples: Maize, sugarcane, sorghum.

• Adaptation: Found in hot, dry environments to minimize photorespiration.
• Key Enzyme: PEP carboxylase, which has a high affinity for CO₂ and does not react with O₂, reducing photorespiration.

Pathway:

• CO₂ + PEP (3C) → Oxaloacetate (4C) → Malate (4C).
• Malate is transported to bundle-sheath cells, where CO₂ is released for the Calvin cycle.

CAM (Crassulacean Acid Metabolism) Plants

• Examples: Pineapple, cacti, succulents.
• Adaptation: Found in arid environments; adapted to conserve water.
• Stomata Behavior: Stomata open at night to minimize water loss.
• Key Enzyme: PEP carboxylase, similar to C4 plants, used for initial carbon fixation at night.

Pathway:

• CO₂ + PEP → Oxaloacetate (4C) → Malate (4C), which is stored in vacuoles as malic acid overnight.
• During the day, malic acid is broken down to release CO₂ for the Calvin cycle.

Location: Takes place in the stroma of the chloroplast.

• Phases of the Calvin Cycle:

  1. Carbon Fixation:
  2. Reduction Phase:
  3. Regeneration of RuBP:

• Key Enzyme:

  • RuBisCO is the primary enzyme, acting as both a carboxylase and an oxygenase.
  • Can bind to CO₂ (carboxylation) or O₂ (oxygenation), the latter leading to photorespiration, which reduces efficiency.

• Energy Requirement:

  • For each full turn of the cycle (producing 1 G3P):
  • To produce one glucose molecule, the cycle must run twice, requiring:

• Products:

  • G3P is the direct product, which can be used to form glucose and other carbohydrates.

• Photorespiration Issue:

  • Occurs when RuBisCO fixes O₂ instead of CO₂, leading to wasteful energy loss.
  • More likely in hot, dry climates when stomata close to conserve water.

• Adaptations:

  • C4 and CAM plants have evolved mechanisms to minimize photorespiration and improve efficiency, which will be explored further.

Things To Remember

• Cyclic Photophosphorylation occurs as a means to produce additional ATP rather than ATP and NADPH. This is because the Calvin Cycle (next episode) requires more ATP than NADPH.
• PSI = Photosystem 1, PSII = Photosystem 2

Diagrams

• Noncyclic Photophosphorylation: ⁠https://cdn.kastatic.org/ka-perseus-images/faeb1179a538a3a8106fa5f3b9bd90c92f9834a7.png⁠
• Cyclic Photophosphorylation: https://cdn.kastatic.org/ka-perseus-images/d47991f2cf9c7a4fc22f055b60793af8c341991f.png

Quizzes

• https://quizlet.com/30876000/light-dependent-reactions-light-independent-reactions-flash-cards/

Key Terms, try drawing these out.

A) Leaf Anatomy

• palisade layer
• cuticle layer
• spongy layer
• vascular bundle (xylem/phloem)
• stomata
• guard cells
• epidermis (upper/lower)

B) Chloroplast Anatomy

• membrane (inner/IMS/outer)
• thylakoid membrane
• thylakoid lumen
• thylakoid lamella
• granum

C) When do stomata open and how do they do so?

Topics not covered in-depth:

• Sunlight triggers [K+] ion movement which requires ATP from the plant in order to open guard cells.