The Light Dependent Reactions of Photosynthesis

Living things require energy to stay alive. The main energy source for autotrophs, a group of organisms that produce their own food, achieve this by gathering natural commodities such as water and sunlight. These sources of energy are converted through a series of biochemical l processes into substances that the autotroph can use to maintain its necessary functions. The sun is the starting point for the process of photosynthesis (“photo” refers to light sunlight). The sun emits many, many different wavelengths, including X-rays, UV rays, and a spectrum of light that is visible to us. The light used by photosynthesis is the spectra of visible light.

The figure above shows the spectra of light which is absorbed by a plant. The sun is sometimes thought to emit simply a white light, but sunlight is actually reflected in a spectra similar to the one shown above. Plants appear to have a green color because this green light is not absorbed as much as other colors (with different wavelengths). As a result, this light is reflected by the plant, showing its green color.

Another feature of the above graph is the inclusion of several pigments in the typical autotroph. Organisms that undergo photosynthesis contain a variety of colored pigments, which are organized on membranes within chloroplasts. These pigments include Chlorophyll a, Chlorophyll b, and various carotenoids. Cholorophyll a is considered a primary pigments, while chlorophyll b and other light-absorbing carotenoids are referred to as accessory pigments. These accessory pigments serve mainly to broaden the spectra of absorbable light.

Photosynthetic organisms lose very little of this light energy in the form of heat. Rather, this energy is converted into a form that the plant can store and use to drive cellular reactions. Put simply, photosynthetic organisms convert light energy into chemical energy. In photosynthesis, the main energy-containing chemical produced is glucose, a simple sugar. The process also makes other chemicals such as amino acids, fats, and oils. The equation above summarizes the inputs and outputs of photosynthesis.

Carbon dioxide from the air is absorbed through the stomata of leaves. Water is absorbed through from the soil through the vascular system of the plant (xylem). Glucose is is produced, which can be stored and used for cellular respiration. Oxygen is released and leaves through the stomata. This occurs through two distinct, related stages. The light dependent phase uses electron transport (driven by light energy, as it is light dependent) to produce ATP and NADPH. This stage also requires water. The second stage, knows as the light independent phase (Calvin cycle), uses the ATP (Adenosine triphosphate) and NADPH to fix carbon from CO2 to from glucose.

The light dependent reactions takes place in the membrane of the chloroplast (thylakoids), as this location offers a large surface area to absorb light energy. The light independent reactions take place in the stroma of the chloroplast.

Setting

The structure of a plant is conducive to photosynthesis. The main photosynthetic part is the leaf. Their large surface area (flat and wide) maximizes surface area to absorb as much light as possible. Leaf cells also contain many chloroplasts, the setting of photosynthesis. In the diagram below, the sites of the light dependent and light independent reactions are shown (thylakoid and stroma). An inner membrane and outer membrane are also pictured. Plants without ample exposure to light will attempt to move as close as possible to a light source to maintain a high level of light absorption (phototropism).

Thylakoids are arranged in stacks, called grana, which increases the light-absorbing ability for the plant.

Light Dependent Reactions

These reactions occur in the membranes of thylakoids.

The process begins in photosystem II. To begin, light energy is used to split water (photolysis) which has been absorbed through xylem via transpiration. Water is broken down into hydrogen, oxygen, and electrons. The photon moves through various pigments, which become excited and unstable, quickly passing the energy to the following pigments until the photon is transported to special chlorophyll a. This pigment is also excited and releases an electron to the primary acceptor. At this point, special chlorophyll requires another electron (to replace the electron released to the primary acceptor).

Water, which is transported through the vascular system, is moved to photosystem two, where an photon splits the water molecule (hydrolysis). 2H2O becomes 2H, O, and 2e-. One electron is moved to replace the lost electron in special chlorophyll a. In photosystem I, 2 e- are passed down an electron transport chain and combined with NADP+, along with a hydrogen ion, to form NADPH (to be used in the Calvin cycle). The other two electrons, when passed through the electron transport chain, create a concentration gradient and hydrogen ions from the stroma are moved into the thylakoid compartment. This occurs as a result of the energy from the movement of the two electrons. These two electrons then enter photosystem I to replace the two electrons used to form NADPH. Hydrogen ions in the thylakoids compartment are taken by ATP synthase. The enzyme uses the hydrogen ions, with ADP, to synthesize ATP, which is later used as a part of the light-independent reactions to form glucose.

Light Independent Reactions

The light independent reactions are known as such because the reaction does not require light photons. Generally, these reactions occur at the same time as the light dependent reactions, but do not use light as a direct input for this process.

After the light dependent reactions produced ATP and NADPH, CO2 from the atmosphere is added to form G3P (Glyceraldehyde 3-phosphate).

3CO2 reacts with RuBP to synthesize 6G3P. This reaction is catalyzed by the enzyme, Rubisco. At the same time, 6 ATP from the light dependent reactions, with 6 NADPH, form 6 phosphates and 6 NADPH. The loss of electrons from ATP to ADP drives the following reaction to occur. The 6 phosphates lost from the 6ATP (now 6ADP) are added to the 6 3-phosphoglyerates (which refers to a 3-carbon structure with a phosphate bound to carbons 1 &3). NADPH from the light dependent reactions are split into NADP+ and 6 phosphates. The energy from the electron movement allows the 6 3-biophosphoglycerate and 6 phosphates to become 6G3P. Of the 6G3P, 5G3P are used to recreate RuBP (Ribulose Biphosphate, which is a 5-C molecule). 3ATP is split into 3ADP and 3 phosphates. The electron energy (from higher to lower level) drives the synthesis of 3RuBP to again react with CO2 to continue the cycle.

Stages

Carbon fixation is the change of carbon from a gaseous form (CO2) to a solid molecule, such as G3P.

Reduction refers to the release of phosphates from 6ATP to form 6ADP and 6 phosphates. With this reaction, energy from the movement of electrons from a higher to lower state create the energy needed for following reactions. The exit of a single G3P contributes to the synthesis of glucose, which is the end product of the Calvin cycle. 2 G3P makes one glucose.

Regeneration (of RuBP) shows the cyclic nature of the Calvin cycle. 5G3P, when 3ATP become 3ADP, use the 3 new phosphates with the electron energy to form 3RuBP to be used in the next cycle of these reactions.