Photosynthesis Detailed Notes

Photosynthesis

• Photosynthesis is a complex series of reactions summarized by the following equation:
  $6 CO<em>2 + 12 H</em>2O + \text{Light energy} \rightarrow C<em>6H</em>{12}O<em>6 + 6 O</em>2 + 6 H_2O$

Structure of a Chloroplast

• Outer and inner membranes are separated by the intermembrane space.
• A third membrane, the thylakoid membrane, contains pigment molecules.
• The thylakoid membrane forms thylakoids.
• Thylakoids enclose the thylakoid lumen.
• A granum is a stack of thylakoids.
• The fluid-filled region between the thylakoid membrane and the inner membrane is the stroma.

The Two Stages of Photosynthesis

• Photosynthesis consists of:
  • The light reactions (the photo part)
  • The Calvin cycle (the synthesis part)

The Light Reactions

• Occur in the thylakoids.
• Convert solar energy to chemical energy.
• Split $H<em>2O$, releasing $O</em>2$.
• Reduce the electron acceptor $NADP^+$ to $NADPH$.
• Generate ATP from ADP by photophosphorylation.

The Calvin Cycle

• Occurs in the stroma.
• Forms sugar from $CO_2$, using ATP and NADPH.
• Begins with carbon fixation, incorporating $CO_2$ into organic molecules.

Photosynthesis: Light Reactions

• Chloroplasts are solar-powered chemical factories.
• Thylakoids transform light energy into the chemical energy of ATP and NADPH.
• Chlorophyll a is the main photosynthetic pigment.
• The organization of chloroplasts into photosystems allows for harvesting energy.

A Photosystem

• Consists of:

  • A reaction-center complex (a type of protein complex)
  • Surrounding light-harvesting complexes

• Light-harvesting complexes contain chlorophyll pigments.

• Energy from light is transferred from pigment to pigment.

• Energy from light is ultimately transferred to the reaction center complex.

• The reaction-center complex contains molecules capable of accepting and transferring electrons.

• Chlorophyll a molecules can transfer one of their electrons when excited to a primary electron acceptor.

• This is possible due to their molecular environment, location, and association with other molecules.

Two Forms of Photosystems in the Thylakoid

• Photosystem II (PS II) and Photosystem I (PS I) work together in linear electron flow.
• Linear electron flow involves both photosystems and produces ATP and NADPH using light energy.

Steps in Linear Electron Flow

1. A photon hits a pigment, and its energy is passed among pigment molecules until it excites P680.
2. An excited electron from P680 is transferred to the primary electron acceptor, creating $P680^+$.
3. $H_2O$ is split by enzymes, and electrons are transferred from the hydrogen atoms to $P680^+$, reducing it to P680.
   • $P680^+$ is the strongest known biological oxidizing agent.
   • $O_2$ is released as a by-product.
4. Each electron