3. The Light-Independent Reactions: Securing Chemical Energy in Glucose Molecules

4. The Relationship between the Light-Dependent and the Light-Independent Reactions

5. Water, CO₂, and the C4 Pathway

6. C3 and C4 Plants are Adapted to Specific Environmental Conditions.

1. Photosynthesis: An Overview

• Leaves and chloroplasts are adaptations for photosynthesis.

  
  

2. The Light-Dependent Reactions:

Converting Light to Chemical Energy

• During photosynthesis, light is first captured by pigments in chloroplasts.
• The light-dependent reactions occur in clusters of molecules called photosystems.

Here's a section from another textbook - the important point I want you to see is the role of Active Transport in producing the energy in photosynthesis.

Photosynthesis: The Role of Light

The heart of photosynthesis as it occurs in most autotrophs consists of two key processes:

• the removal of hydrogen (H) atoms from water molecules
• the reduction of carbon dioxide (CO₂) by these hydrogen atoms to form organic molecules

The second process involves a cyclic series of reactions named (after its discoverer) the

How the System Works

• Light is absorbed by the antenna pigments of Photosystems II and I
• The absorbed energy is transferred to the reaction center pigment, P₆₈₀ in Photosystem II, P₇₀₀ in Photosystem I
• Activation of P₆₈₀ removes an electron from it
• With its resulting positive charge, P₆₈₀ is sufficiently electronegative that it can remove electrons from water
• these electrons are transferred (by way of phaeophytin and plastoquinone) to the cytochrome b and f complex where they provide the energy for chemiosmosis
• Activation of P₇₀₀ enables it to pick up electrons from cytochrome f and raise them to a sufficiently high redox potential that
• they reduce NAD⁺ to NADPH.

The saw-tooth shifts in redox potential as electrons pass from P₆₈₀ to NADP⁺ have caused this system to be called the Z-Scheme.

Chemiosmosis in Chloroplasts

The energy released as electrons pass down the gradient from Photosystem II to Photosystem I is harnessed by the cytochrome b & f complex to pump protons (H⁺) against their concentration gradient from the stroma of the chloroplast into the interior of the thylakoid (an example of active transport). As their concentration increases inside (which is the same as saying that the pH of the interior decreases), a strong diffusion gradient is set up. The only exit for these protons is through the ATP synthase complex. As in mitochondria, the energy released as these electrons flow down their gradient is harnessed to the synthesis of ATP. The process is called chemiosmosis and is an example of facilitated diffusion.

 

3. The Light-Independent Reactions: Securing Chemical Energy in Glucose Molecules

• The C₃ cycle captures carbon dioxide.
• Carbon fixed during the C₃ cycle is used to synthesize glucose.
  

4. The Relationship between the Light-Dependent and the Light-Independent Reactions

5. Water, CO₂, and the C4 Pathway

• When stomata are closed to conserve water, wasteful photorespiration occurs.

  
  
  

• C₄ Plants reduce photorespiration using a two-stage carbon fixation process.

6. C3 and C4 Plants are Adapted to Specific Environmental Conditions.