Unveiling the Light Reaction Event U N D E R S TA N D I N G T H E

BASIC CONCEPT OF THE

LIGHT REACTION EVENT

in Photosynthesis
IN PHOTOSYNTHESIS
INTRODUCTION Photosynthesis –the process by which green plants, algae,
and some bacteria convert light energy, usually from the sun,
•Brief overview of into chemical energy stored in glucose (sugar). This process
photosynthesis occurs in the chloroplasts of plant cells and is vital for life
•Importance of light
reaction on Earth as it produces oxygen as a by-product, which is
essential for respiration in plants and animal
Importance of light reaction –Light reactions are essential
for photosynthesis in plants. They capture sunlight and
convert it into chemical energy in the form of ATP and
NADPH. This energy is crucial for the plant to produce
glucose during the dark reactions.
What is Photosynthesis?

•Definition -the process by which green plants, algae, and some bacteria convert light energy, usually
from the sun, into chemical energy stored in glucose (sugar). This process occurs in the chloroplasts of
plant cells and is vital for life on Earth as it produces oxygen as a by-product, which is essential for
respiration in plants and animals.
•Equation:
Overview of Photosynthesis processes

Light reaction-These occur in the thylakoid membranes of the chloroplasts

and involve the absorption of light, splitting of water, and the production of
ATP and NADPH.
Dark reaction (Calvin cycle)-These occur in the stroma of the chloroplasts
and involve the conversion of CO2 into glucose using the ATP and NADPH
produced in the light reactions.
Light reaction and Dark reaction
Light reaction

Location:
Light reactions take place in the thylakoid membranes of the chloroplasts within plant
cells. The thylakoid membranes are stacked into structures called grana, which are
surrounded by the stroma, the fluid-filled space inside the chloroplast. These specialized
membranes contain the chlorophyll and other pigments necessary to capture light energy
and initiate the process of photosynthesis.
Light Reaction Stages

Photosystem II (PSII)-Is a protein complex located in the thylakoid membrane of

chloroplasts. It plays a vital role in the light-dependent reactions of photosynthesis by
absorbing photons and utilizing their energy to split water molecules, releasing oxygen
as a by-product, and generating ATP through the electron transport chain.
Photosystem I (PSI)-is a protein complex located in the thylakoid membrane of
chloroplasts. It plays a pivotal role in the light-dependent reactions of photosynthesis by
absorbing photons and utilizing their energy to re-energize electrons and reduce NADP+
to NADPH.
Photosystem II
Structure: Photosystem II (PSII) is a multi-subunit protein complex located in the thylakoid membrane of chloroplasts. It consists of
several protein subunits and pigments, including chlorophyll a, chlorophyll b, carotenoids, and the primary electron donor P680.

Functions:

1. Light Absorption - PSII primarily absorbs light at a wavelength of 680 nm, hence it is also referred to as P680.

2. Water Splitting - The absorbed light energy is used to split water molecules into oxygen, protons (H+), and electrons. This
process is known as photolysis.

3. Electron Transport - The electrons from water are then transferred through the thylakoid electron transport chain, generating a
proton gradient across the thylakoid membrane.

4. ATP Production - The proton gradient drives the synthesis of ATP through chemiosmosis, utilizing the enzyme ATP synthase.
Electron Transport Chain(ETC)

• The Electron Transport Chain (ETC) in photosynthesis is a series of

proteins and molecules located in the thylakoid membrane of chloroplasts.
It plays a critical role in the light-dependent reactions of photosynthesis by
transferring electrons from Photosystem II (PSII) to Photosystem I (PSI),
generating a proton gradient across the thylakoid membrane, and
producing ATP through chemiosmosis.
Cytochrome b6f Complex

• Structure: The cytochrome b6f complex is a protein complex located in the thylakoid
membrane of chloroplasts. It consists of several protein subunits and cofactors,
including cytochrome b6, cytochrome f, and the Rieske iron-sulfur protein.
• Main function: The main function of the cytochrome b6f complex in photosynthesis
is to transfer electrons between Photosystem II (PSII) and Photosystem I (PSI),
facilitating the flow of electrons and generating a proton gradient across the thylakoid
membrane. This proton gradient drives the synthesis of ATP through chemiosmosis.
Photosystem I
Structure: Photosystem I (PSI) is a multi-subunit protein complex located in the thylakoid membrane of chloroplasts.
It consists of several protein subunits and pigments, including chlorophyll a and the primary electron donor P700.
Function:
1. Light Absorption- PSI primarily absorbs light at a wavelength of 700 nm, hence it is also referred to as P700.
2. Electron Re-energizing - The absorbed light energy is used to re-energize electrons that have passed through the
thylakoid electron transport chain.
3. NADPH Production - The re-energized electrons are then used to reduce NADP+ to NADPH, which is a crucial
molecule used in the Calvin cycle to fix and convert CO2 into glucose.
Photophosphorylation
• Photophosphorylation is the
process of converting ADP
(Adenosine Diphosphate) to
ATP (Adenosine Triphosphate)
using the energy of sunlight
during the light-dependent
reactions of photosynthesis.
There are two main types of
photophosphorylation:
Main types of photophosphorylation
• Photophosphorylation consists of two main types in photosynthesis: non-cyclic and cyclic
photophosphorylation. In non-cyclic photophosphorylation, both Photosystem I (PSI) and Photosystem
II (PSII) are involved. Electrons are transferred from water to PSII, then to PSI, and finally to NADP+ to
produce NADPH. This process also generates ATP from ADP and inorganic phosphate. Oxygen is released
as a by-product due to the splitting of water in PSII. On the other hand, cyclic photophosphorylation
involves only Photosystem I (PSI). In this process, electrons are cycled back from ferredoxin to the
cytochrome complex and then back to PSI. This cyclic flow of electrons generates a proton gradient,
which is used to produce ATP from ADP and inorganic phosphate. Unlike non-cyclic
photophosphorylation, cyclic photophosphorylation does not produce NADPH or release oxygen. Both
types of photophosphorylation are essential for the production of ATP and NADPH, which are then used
in the Calvin cycle to produce glucose during photosynthesis.
Comparison of ATP and ADP Production in light
reaction
• ATP- During the light-dependent reactions of • ADP-In the light-dependent reactions of
photosynthesis, the amount of ATP produced can photosynthesis, ADP (Adenosine Diphosphate) is
vary depending on the specific conditions and the converted to ATP (Adenosine Triphosphate).
organism. • The general stoichiometry for the ATP production
• Non-Cyclic Photophosphorylation: in non-cyclic photophosphorylation is 1.5 ATP
Approximately 1.5 molecules ATP molecules produced for every 1 ADP molecule
• Cyclic Photophosphorylation: consumed

Varies, generally less than in non-cyclic

Summary of light reaction
• Firstly, Photosystem II (PSII) absorbs photons and uses their energy to split water
molecules, releasing oxygen, protons (H+), and electrons. These electrons are then
transferred through the thylakoid electron transport chain, facilitated by the
cytochrome b6f complex, which generates a proton gradient across the thylakoid
membrane. Next, Photosystem I (PSI) absorbs photons and uses their energy to re-
energize electrons and reduce NADP+ to NADPH. The proton gradient generated by
the electron transport chain drives the synthesis of ATP from ADP and inorganic
phosphate (Pi) through the action of the enzyme ATP synthase in a process known as
photophosphorylation.
Light reaction Diagram
Quiz:

1).This involve the absorption of light, splitting water molecule and the
production of ATP and NADHP.
2).It absorbs photons and utilize their energy to split water molecules,
releasing oxygen as a by-product, and generating ATP through the electron
transport chain.
3).It absorbs photons and utilizing their energy to re-energize electrons and
reduce NADP+ to NADPH.
4).It transfer electrons from Photosystem II (PSII) to Photosystem I (PSI),
generating a proton gradient across the thylakoid membrane, and producing
ATP through chemiosmosis.
5).Itis the process of converting ADP (Adenosine Diphosphate) to ATP
(Adenosine Triphosphate) using the energy of sunlight during the light-
dependent reactions of photosynthesis.