Plant cell

in ~ the chloroplasts of a plant cell, photosynthesis occurs in two key phases: the light-dependent and light-independent reactions.

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Within plant cells, chloroplasts are committed organelles that offer as the sites of photosynthesis. The reactions that consist of the process of photosynthesis have the right to be separated into light-dependent reactions, which take ar in the thylakoids, and also light-independent reactions (also known as dark reactions or the Calvin cycle), which take location in the stroma.

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1. Chloroplasts have a complicated internal structure, and also different reactions take place in different parts the the chloroplast.

Understanding what the inside of a chloroplasts looks choose is crucial to visualizing wherein the different reactions of photosynthesis occur.


Surrounding the chloroplasts is a double membrane, consisting of an outer membrane and an within membrane. This is similar in framework to the dual membrane that mitochondria.

Interior come the chloroplast’s inner membrane and also surrounding the thylakoids is a fluid called the stroma. The light-independent reactions of photosynthesis take location within the stroma. It has enzymes that job-related with ATP and also NADPH come “fix” carbon indigenous carbon dioxide right into molecules that have the right to be supplied to build glucose. The chloroplast’s own genetic material (separate from that of the cell) is likewise stored in the stroma.

The internal of the chloroplast contains one more membrane—the thylakoid membrane—which is folded to type numerous linked stacks the discs. Each disc is a thylakoid and each stack is a granum (pl. Grana).

The light-dependent reaction of photosynthesis take place within the thylakoids. This reactions happen when the colours chlorophyll, located within the optical membrane membranes, captures energy native the sun (photons) come initiate the malfunction of water molecules.

2. The light-dependent reactions convert light energy into chemistry energy.


The goal of the light-dependent reaction of photosynthesis is come collect energy from the sun and break down water molecule to develop ATP and NADPH. These two energy-storing molecules are then used in the light-independent reactions.

Within chloroplasts, chlorophyll is the pigment that absorbs sunlight. The is stored in the optical membrane membranes in protein complexes referred to as photosystem I and photosystem II. The collection of light-dependent reactions starts when sunshine hits a molecule that chlorophyll, located in photosystem II. This excites an electron, which pipeline the chlorophyll molecule and also travels along the thylakoid membrane via a series of carrier proteins (known together the electron move chain).

Then, something exceptional happens—photosystem II splits a water molecule to restore this lost electron and also fill the “energy vacuum” that has actually been created. This is a process humans haven’t to be able come replicate exactly in a lab!

Each water molecule division down into two hydrogen (H) atoms and also one oxygen (O) atom. The oxygen is released together a waste product—oxygen atom from disassembled water molecules sign up with up in bag to type oxygen gas (O2).

The hydrogen ions develop up in high concentration in the lumen of the thylakoid. Lock pass v an enzyme called ATP synthase, and also their movement gives the energy needed to add a third phosphate come ADP (adenosine diphosphate) to kind ATP (adenosine triphosphate). This energy-storing molecule powers numerous cellular processes. In fact, the glucose made during photosynthesis is broken down to produce an ext ATP later, throughout cellular respiration.

Meanwhile, the electron exit from photosystem II come at photosystem I, which additionally contains chlorophyll. Energy from the sunlight excites the electron again, giving it enough power to pass across the membrane and also into the stroma, where it joins through a hydrogen ion and an NADP+ to develop the energy-carrying molecule NADPH.

ATP and also NADPH relocate from the thylakoid into the stroma, where the energy they save is offered to power the light-independent reactions.

3. The light-independent reaction (Calvin cycle) use stored chemical power from the light-dependent reactions to “fix” CO2 and create a product that can be converted into glucose.


The ultimate score of the light-independent reactions (or Calvin cycle) is to rally a molecule that glucose. This is the component of photosynthesis that requires the CO2 the plant it s okay from the air.

Essentially, the plant needs the carbon from the CO2 to create the structure blocks because that glucose. An enzyme in the stroma dubbed ruBisCo combines a five-carbon molecule of RubP (ribulose biphosphate) with a molecule of carbon dioxide. This creates a six-carbon molecule the is broken down right into two three-carbon molecules (3-phosphoglycerate). This component of the light-independent reaction is referred to as carbon fixation.

Then, the energy carriers indigenous the light-dependent reactions do their contribution. ATP and NADPH provide each 3-phosphoglycerate a hydrogen atom, producing two molecules of the an easy sugar G3P (glyceraldehyde-3-phosphate). Ultimately, these two molecules that G3P are used to construct one molecule that glucose. This component of the light-independent reactions is generally referred to as reduction (or reduce the sugar) due to the fact that electrons space added.

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It is important to note that the Calvin cycle generally uses 6 molecules that carbon dioxide at a time. This means that twelve molecule of G3P space generated. However, only two of lock are provided to develop a molecule the glucose—the rest are recycled ago into RubP so that the cycle can keep running.