Photosynthesis - Light-Dependent Reactions

Terminology and molecular details of the light-dependent stage of photosynthesis, including pigments, electron carriers, and phosphorylation types.

General Photosynthesis Equation

$$6 CO_2 + 6 H_2O \rightarrow C_6H_{12}O_6 + 6 O_2$$

Location of Light-Dependent Reaction

Thylakoid membrane within chloroplast (Grana)

Purpose of Light-Dependent Reaction

To convert light energy into chemical energy in the form of $ATP$ and reducing $NADPH$

Input for Light-Dependent Reaction

Light, Water, $ADP$, and $NADP^+$

Output of Light-Dependent Reaction

$ATP$, $NADPH$, and $O_2$

Major Complexes in Thylakoid

Photosystem-I (PS-I), Photosystem-II (PS-II), Cytochrome $b_6f$, and ATPase complex

Mobile Carriers

Plastoquinone (PQ), Plastocyanin (PC), and Ferredoxin (Fd)

P700

The reaction center for Photosystem-I (PS-I) that absorbs a maximum of $700\,nm$ light

P680

The reaction center for Photosystem-II (PS-II) that absorbs a maximum of $680\,nm$ light

Pheophytin

The primary electron acceptor for Photosystem II (PS-II)

Cytochrome $b_6f$ complex

The complex representing the rate limiting step (slowest step) of the electron transport chain process

Plastocyanin (PC)

A mobile carrier located on the thylakoid's lumen side that acts as an electron donor to PSI

Primary Acceptor of PS-I

Iron-Sulfur protein

Water Splitting Complex

Located on the luminal side of the membrane, it consists of a Manganese Cluster, Z-complex, and associated protein

Photolysis Reaction

$$2 H_2O \rightarrow 4 e^- + 4 H^+ + O_2$$

Photophosphorylation

The process of synthesizing $ATP$ using light energy during the light-dependent reaction ($ADP + P_i + light\,energy \rightarrow ATP$)

Chemiosmosis

The process where protons move from high concentration in the thylakoid lumen to low concentration in the stroma through the ATPase complex to synthesize $ATP$

ATPase complex ratio

The movement of $3$ Protons ($H^+$) through the ATPase complex produces one molecule of $ATP$

Cyclic Photophosphorylation

A temporary shift from the non-cyclic cycle starting at Photosystem 1 (P700) that occurs when the chloroplast has low $ATP$ or high demand for $NADPH_2$; it produces only $ATP$

Non-Cyclic Photophosphorylation

The process involving both PS-II and PS-I where water is split to produce $O_2$, $ATP$, and $NADPH$