Light-Dependent Reaction in the Thylakoid

Topics 8.2

Photosynthesis

8.2.1 & 8.2.2: The structure of the chloroplast

Where are the key molecules in the thylakoid found?
Photosynthesis consists of light-dependent and lightindependent reactions. It is important to understand the location of the key molecules used in the light-dependent reaction. Place your mouse on the figure to see a labelled diagram of the chloroplast structure. Also shown is the location of the thylakoids within the chloroplast and the position of PSII which contains chlorophyll P680), PSI (which contains chlorophyll P700) and ATP Synthase within the thylakoid membrane.

Placing your mouse pointer on the electron micrograph below will reveal a labelled diagram of the chloroplast.

Chloroplast Structure

8.2.3: Light-Dependent Reaction in the Thylakoid

The light dependent reaction requires light energy from the Sun which is composed of a range of wavelengths (colours). During the process light energy is converted into chemical energy by forming the energy-carrying molecule ATP and and the hygrogen-carrying molecule NADPH.

The structure of the chloroplast helps improve the production of NADPH and ATP which are both used in the light-independent reaction in the chloroplast stroma. The fluid stroma contains the enzymes for the Calvin cycle, the close association of the key molecules (e.g., PSII, PSI and ATP synthase) in the thylakoid membrane allows the efficient flow of electrons through the memebrane and the movement of protons (H+) across the membrane using carrier molecules and ATP synthase. The large number of thylakoids increases the total surface area for the absorption of sunlight and the number of key molecules used for the production of NADPH and ATP. The small size of the thylakoid lumen allows for the production of a H+ concentration gradient between lumen and stroma where the energy is used to produce ATP. The step numbers in the Table 2 correspond to the numbers in Figure 1.

light dependant reaction link light dependant reaction animation
LightDependentReaction

Table 2: Summary of the Light-Dependent Reaction

Step Description
1 Light-dependent reactions take place in the thylakoid membrane.
2 Light energy is absorbed by chlorophyll P680 (photoactivation) in photosystem (PS) II which is used to excite electrons to a higher energy level.
3 Chlorophyll P680 loses the high energy electron to an electron acceptor in PSII.
4 Light energy is also used to split water (photolysis) to produce oxygen, hydrogen and electrons which replace electrons in P680 that were lost to the electron acceptor.
5 The high energy electrons are passed along carrier molecules in the thylakoid membrane.
6 Energy lost from the electrons is used to pump H+ (protons) from the chloroplast stroma to the thylakoid lumen. This produces a H+ concentration gradient between the lumen and stroma and allows for the production of ATP.
7 Light energy is absorbed by chlorophyll P700 (photoactivation) and is used to re-excite electrons that have entered PSI.
8 The high energy electrons are passed from PSI to the final carrier molecule. In the presence of enzymes the electrons are used to reduce NADP+ to NADPH which is then used in the light-independent reaction in the chloroplast stroma.
9 H+ move down concentration gradient back to the stroma through ATP synthase (chemiosmosis). Energy from the H+ concentration gradient is used to produce ATP by forming a high energy bond between Pi to ADP (photophosphorylation). ATP is used in the light-independent reaction in the stroma.
10 Cyclic phosphorylation can occur when electrons are recycled back to carrier molecules in the thylakoid membrane which allow for the pumping of more H+ into lumen from the stroma. Then these H+ flow through the ATP synthase to produce more ATP.

8.2.4: Chemiosmosis and the Thylakoid Membrane

Chemiosmosis and photosynthesis

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