The two electrons lost from photosystem II are replaced by the splitting of water molecules. Water splitting also releases hydrogen ions into the lumen. This contributes to a hydrogen ion gradient similar to the one created by mitochondrial electron transport. After two water molecules have been split, one molecule of molecular oxygen is created. Plastoquinone Qb then transfers the two electrons to the cytochrome b6-f complex.
The two protons it picked up are released into the lumen. These transfers are coupled with the pumping of two more hydrogen ions into the lumen space by cytochrome b6-f. The electrons are next transferred to plastocyanin, another mobile carrier. This chemical energy will be used by the Calvin cycle to fuel the assembly of sugar molecules. The light-dependent reactions begin in a grouping of pigment molecules and proteins called a photosystem.
There are two photosystems Photosystem I and II , which exist in the membranes of thylakoids. Both photosystems have the same basic structure: a number of antenna proteins to which chlorophyll molecules are bound surround the reaction center where the photochemistry takes place. Each photosystem is serviced by the light-harvesting complex, which passes energy from sunlight to the reaction center.
It consists of multiple antenna proteins that contain a mixture of — chlorophyll a and b molecules as well as other pigments like carotenoids.
A photon of light energy travels until it reaches a molecule of chlorophyll pigment. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet.
The energy is transferred from chlorophyll to chlorophyll until eventually after about a millionth of a second , it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons. To replace the electron in the chlorophyll, a molecule of water is split. The replacement of the electron enables chlorophyll to respond to another photon. The oxygen molecules produced as byproducts exit the leaf through the stomata and find their way to the surrounding environment.
The hydrogen ions play critical roles in the remainder of the light-dependent reactions. In eukaryotes and some prokaryotes, two photosystems exist. The first is called photosystem II PSII , which was named for the order of its discovery rather than for the order of the function. After a photon hits the photosystem II PSII reaction center, energy from sunlight is used to extract electrons from water. As the electron passes along the electron transport chain, energy from the electron fuels proton pumps in the membrane that actively move hydrogen ions against their concentration gradient from the stroma into the thylakoid space.
This gives the atom or molecule a negative or positive charge Light-dependent reaction: the first part of photosynthesis where sun light energy is captured and stored by a plant Molecule: a chemical structure that has two or more atoms held together by a chemical bond. Water is a molecule of two hydrogen atoms and one oxygen atom H2O Protein: a type of molecule found in the cells of living things, made up of special building blocks called amino acids. Starch: made by all green plants and used to store energy for later use Thylakoid: the disk-shaped parts of a plant cell where light-dependent reactions occur The light-dependent reactions take place in the thylakoid membrane, inside chloroplasts.
Since they are light 'dependent' reactions, you can guess that these reactions need light to work. Remember that the purpose of this first part of photosynthesis is to convert sunlight energy into other forms of energy?
The light-dependent reactions of photosynthesis require sunlight. Image by Mell Plants cannot use light energy directly to make sugars.
Instead, the plant changes the light energy into a form it can use: chemical energy. Chemical energy is all around us. For example, cars need the chemical energy from gasoline to run. These two molecules are not only in plants, as animals use them as well. This water is broken apart to release electrons negatively charged subatomic particles.
When water is broken it also creates oxygen, a gas that we all breathe. The electrons must travel through special proteins stuck in the thylakoid membrane. They go through the first special protein the photosystem II protein and down the electron transport chain. Then they pass through a second special protein photosystem I protein. Wait a second That seems really confusing. Why would they name the photosystems that way?
Water molecules are broken down to release electrons. These electrons then move down a gradient, storing energy in ATP in the process. Image by Jina Lee. Photosystem I and II don't align with the route electrons take through the transport chain because they weren't discovered in that order.
Photosystem I was discovered first. Later, photosystem II was discovered and found to be earlier in the electron transport chain.
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