8.2 Photosynthesis

Objectives:• 4(B) Investigate and explain cellular processes, including homeostasis,

energy conversions, transport of molecules, and synthesis of new molecules

• 9(B) Compare the reactants and products of photosynthesis and cellular respiration in terms of energy and matter

Overview of Photosynthesis

Photosynthesis is a process of converting light energy into chemical energy.

Photosynthesis occurs in two phases: Light-dependent phases – light energy is

converted into chemical energy. Light-independent – chemical energy is

used to synthesize glucose.

Phase One: Light Reactions

The absorption of light is the first step in photosynthesis.

Once light energy is captured, it can be stored as ATP and NADPH.

Chloroplasts Organelles that capture light energy Contain two main compartments

Thylakoids – flattened saclike membranes.▪ Stack of thylakoids are called grana.

The fluid filled space outside the grana is called the stroma.

Pigments Light-absorbing colored molecules

called pigments are found in the thylakoid membranes.

Different pigments absorb specific wavelengths of light. Most common pigment in plants is chlorophyll.

Plants also have accessory pigments.

Electron Transport Thylakoid has a large surface area

space for a large number of electron transporting molecules and two protein complexes called photosystems.

Photosystems have light-capturing pigments.

1. Light energy excites electrons in photosystem II (PSII) releasing them down the electron transport system (ETS)

This energy also causes a water molecule to split providing replacement electrons for PSII and leaving two protons in the thylakoid space.

2. Energized electrons move from PSII to an acceptor molecule in the thylakoid membrane.

3. The electron acceptor molecule transfers the electrons along a series of electron carriers to photosystem I (PSI).

4. With an added boost from light, PSI transfers the electrons to a final electron acceptor called ferrodoxin.

5. Ferrodoxin transfers the electrons to the electron carrier NADP+, forming the energy storage molecule NADPH.

Chemiosmosis The process of splitting water and

electron transport causes a buildup of protons (H+) in the thylakoid space

The H+ moves down its concentration gradient through a transport protein called ATP synthase.

This energy is used to convert ADP to ATP.

Phase Two: The Calvin Cycle

In the second phase of photosynthesis, called the Calvin cycle, carbon dioxide (CO2) and energy from ATP and NADPH are used to make sugars.

Steps in Calvin Cycle Step 1: Carbon Fixation

CO2 molecules combine with 5-carbond molecules to form 3-phosphoglycerate (3-PGA)

Step 2 Chemical energy stored in ATP and

NADPH is transferred to 3-PGA to form glyceraldehyde 3-phosphate (G3P).

Step 3 Some G3P molecules leave the cycle to be

used for the production of glucose and other organic compounds

Step 4 An enzyme called rubisco converts the

remaining G3P molecules into 5-carbon molecules called ribulose 1,5-bisphosphate (RuBP).

These molecules combine with new CO2 and continue the cycle.

Alternative Pathways

C4 Plants The C4 pathway allows plants to maintain

photosynthesis while reducing water loss. Called C4 because they fix carbon into

four carbon compounds instead of three-carbon molecules.

Significant structural modification in the arrangement of cells within the leaves – separate CO2 uptake from location of Calvin cycle.

CAM plants Crassulacean acid metabolism (CAM)

is found in desert plants such as cacti.

Collect CO2 at night and store it in organic compounds.

During the day, release CO2 from organic compounds and enters the Calvin cycle..