Section 8.2 Notes

Photosynthesis is the process where most autotrophs make organic compounds, such as sugars.

Overview of Photosynthesis

During photosynthesis, light energy is converted into chemical energy. (Light energy is absorbed by the chloroplast and this energy is stored in chemical bonds in the sugars and ATP molecules.)

The following chemical equation represents what happens in photosynthesis: 6CO

2 + 6H

2O light C

6H

12O

6 + 6O

2

Photosynthesis occurs in two phases.

In phase one, the light-dependent reactions, light energy is converted to chemical energy.

Phase one reactions create and store chemical energy in ATP and NADPH molecules.

In phase two, the light-independent reactions, the ATP and NADPH that were created in phase one are used as energy sources to combine water and carbon dioxide to make glucose.

Once the glucose has been produced it can be used to make complex carbohydrates, or it can be used to help produce other biomolecules like lipids, proteins, or nucleic acids.

Chloroplasts: their structure

stroma thylakoid grana

Phase One: Light Reactions

stroma: light-independent reactions (phase two) happen here

thylakoid: light-dependent reactions (phase one) take place here

Pigments are light-absorbing molecules found in the thylakoid membrane of the chloroplasts.

The most abundant pigment found in chloroplasts is called chlorophyll.

Chlorophyll pigments reflect green light, causing leaves and many plant parts that contain this pigment to appear green.

Other pigment molecules are found in chloroplasts, but in smaller amounts than the predominant chlorophyll.

These other pigments absorb and reflect different colors of visible light than chlorophyll.

In the fall, chlorophyll molecules in the chloroplasts break down before the other pigments, revealing the color of the underlying pigments.

Thylakoid membrane structure & function:

Light energy hits the chlorophyll in photosystem II, which gives energy to electrons in the chloroplast (excites electrons).

Steps in Phase One (Light Reaction)

The energized electrons begin moving along acceptors in the thylakoid membrane.

Enzymes inside the thylakoid split water molecules, producing H+ ions (protons), O

2

molecules, and free electrons that replace those that left photosystem II.

Electrons move along the thylakoid membrane until they reach a protein that acts as a proton pump. This protein uses some of the energy from the moving electrons to pull H+ ions from the stroma into the thylakoid (against the concentration gradient).

This creates a very high concentration of H+ ions inside the thylakoid space.

High concentrations of H+ inside the thylakoid causes these protons to move through the enzyme ATP synthase in order to return to the stroma.

As the protons move through the enzyme, some of their energy is used to form ATP from ADP.

The ATP molecules and the H+ ions are now both in the stroma of the chloroplast.

When chlorophyll in photosystem I collect light energy, they re-energize the electrons which begin to move along acceptors in the thylakoid membrane.

The electrons move to an enzyme called NADP reductase, which moves two electrons to NADP+.

This causes NADP+ to become NADP-. It them combines with an H+ ion to become NADPH.

Electrons that are lost from photosystem I are replaced by the energized electrons that were moved from photosystem II (through the proton pump).

ATP and NADPH are now both in the stroma of the chloroplast. Remember that these processes are repeated continuously, so long as light is present.

Phase one of photosynthesis is now complete. The light-dependent reactions have produced ATP and NADPH molecules.

The ATP and NADPH molecules are both high-energy molecules, capable of transferring their energy to other molecules within the cell.

The energy from these molecules will now be used in phase two of photosynthesis, where the molecules required to make glucose are produced.

Phase Two: Dark ReactionsPhase two of photosynthesis – which occurs with or without light – is more commonly called the Calvin cycle.ATP and NADPH are both high energy molecules, and they are produced in phase one, but they are not stable enough to provide long-term energy storage.

This is the reason that glucose is produced – to provide a lasting energy storage molecule.

The Calvin cycle occurs in three phases: 1. the fixation phase 2. the reduction phase 3. the regeneration phase

The Fixation Phase:

In step 1 of this phase, 6 CO2 molecules are combined with 6 RuBP molecules by an enzyme.

This produces 6 molecules that each now have 6 carbon atoms.

Because these new 6-carbon molecules are very unstable, they quickly split into 12 smaller molecules, each containing 3 carbon atoms.These new, smaller molecules are called 3-PGA (3 carbons – PGA is the abbreviation for phosphoglycerate).

The carbon fixation phase is now complete.

The twelve 3-PGA molecules now use ATP molecules to add a phosphate and use NADPH to add electrons to the molecule.

Twelve ATP and twelve NADPH molecules are needed, one for each 3-PGA molecule.

These 12 reactions produce twelve new high energy molecules called G3P (glyceraldehyde 3-phosphate).

The Reduction Phase:

Two of the G3P molecules now leave the Calvin cycle and will be used to produce glucose (which contains 6 carbon atoms).

The remaining ten G3P molecules move on to begin the last phase of the Calvin cycle.

The reduction phase is now complete.

The ten G3P molecules are now converted into six RuBP molecules, which will be used to start the cycle over once again.

An enzyme called rubisco and six ATP molecules are used to make this conversion.

The Regeneration Phase:

The regeneration phase is now complete, and one full Calvin cycle has concluded.

Phase two of photosynthesis is now complete.

The 18 ADP molecules and the 12 NADP+

molecules formed in the Calvin cycle can now be used again in the light-dependent reactions of phase I of photosynthesis.

This completes the explanation of all processes involved in phase one and phase two of the photosynthesis process.