Photosynthesis

The Fate of Sugar Produced by Photosynthesis

G3P molecules produced by the Calvin cycle are often used to make glucose and fructose, which can be combined to form sucrose. In rapidly photosynthesizing cells where sucrose is abundant, glucose can be temporarily stored in the chloroplast as starch. Because starch is not water soluble, it is broken down at night and used to make more sucrose for transport throughout the plant. The plant may convert sucrose back to starch for a more permanent storage. An example would be a corn plant storing starch within the seed.

C4 Photosynthesis

In C4 plants, which perform C4 photosynthesis, carbon fixation and the Calvin cycle occur in separate types of cells. This occurs in a three-step process: PEP carboxylase fixes CO2 in mesophyll cells. The 4-carbon organic acids produced travel to bundle-sheath  cells. The four-carbon organic acids release a CO2 molecule, which  rubisco uses to form 3-phosphoglycerate, thus initiating the  Calvin cycle.

There are two types of reaction centers:

photosystem I and photosystem II // These photosystems work together to produce an enhancement effect, in which photosynthesis increases dramatically when cells are exposed to both red and far-red light.

How does photosynthesis contrast with cellular respiration?

>> Photosynthesis is endergonic. (Reduces CO2 to sugar) >> Cellular respiration is exergonic. (Oxidizes sugar to CO2)

There are two major classes of pigment in plant leaves:

>> chlorophylls (chlorophyll a and chlorophyll b) absorb red and blue light and reflect and transmit green light. >> carotenoids absorb blue and green light and reflect and transmit yellow, orange, and red light.

Chemiosmosis and Photophosphorylation

As in the mitochondria, protons diffuse down their electrochemical gradient. Chemiosmosis results when the flow of protons through ATP synthase causes a change in its shape, driving the phosphorylation of ADP. The capture of light energy by photosystem II to produce ATP is called photophosphorylation.

Photosystem I

As with photosystem II, pigments in the antenna complex absorb photons and pass the energy to the reaction center. Excited electrons from the reaction center of photosystem I are passed down an ETC of iron- and sulfur-containing proteins to ferredoxin. The enzyme NADP+ reductase transfers a proton and two electrons from ferredoxin to NADP+, forming NADPH. The photosystem itself and NADP+ reductase are anchored in the thylakoid membrane.

Thylakoid

Flattened sacs

Electrons from Pheophytin Enter an ETC

Electrons are passed from the reduced pheophytin to an electron transport chain in the thylakoid membrane. This ETC is similar in structure and function to the ETC in mitochondria. The ETC includes plastoquinone (PQ), which shuttles electrons from pheophytin across the thylakoid membrane to a cytochrome complex.

Electrons Participate in Redox Reactions

Electrons in the electron transport chain participate in redox reactions and are gradually stepped down in potential energy. These redox reactions result in protons being pumped from one side of the membrane to the other. Proton concentration inside the thylakoid increases1000-fold.

Photosynthesis consists of two linked sets of reactions:

Light-dependent reactions produce O2 from H2O, and Calvin cycle reactions produce sugar from CO2. The reactions are linked by electrons, which are released in the light-dependent reactions when water is split to form oxygen gas and then transferred to the electron carrier NADP+, forming NADPH. The Calvin cycle then uses these electrons and the potential energy in ATP to reduce CO2 to make sugars.

Stroma

Liquid matrix

Photosynthetic Pigments Absorb Light

Photons may be absorbed, transmitted, or reflected when they strike an object. Pigments are molecules that absorb only certain wavelengths of light.

NADPH Is an Electron Carrier

Photosystem I produces NADPH, which is similar in function to the NADH and FADH2 produced by the citric acid cycle. NADPH is an electron carrier that can donate electrons to other compounds and thus reduce them.

The Location of Photosystem I and Photosystem II

Photosystem II is much more abundant in the interior, stacked membranes of grana. Photosystem I and ATP synthase are much more common in the exterior, unstacked membranes. The stroma is the site of ATP production because the proton gradient established by photosystem II drives protons into the stroma. (Both thylakoids and both found in the chloroplast)

Summary of Photosystems I and II

Photosystem II produces a proton gradient that drives the synthesis of ATP. Photosystem I yields reducing power in the form of NADPH. Although several groups of bacteria have just one of the two photosystems, the cyanobacteria, algae, and plants have both.

Granum

Stack of thylakoids

Carbon Dioxide Enters Leaves through Stomata

Stomata are leaf structures where gas exchange occurs. They consist of two guard cells that change shape to open or close. When a leaf's CO2 concentration is low during photosynthesis, stomata open to allow atmospheric CO2 to diffuse into the leaf and its cells' chloroplasts. A strong concentration gradient favoring entry of CO2 is maintained by the Calvin cycle, which constantly uses up the CO2 in chloroplasts.

Plants Must Balance Water Preservation and CO2 Delivery

Stomata are normally open during the day and closed at night. On hot, dry days, leaf cells may lose a great deal of water to evaporation through their stomata. When this occurs, they must either close the openings and halt photosynthesis or risk death from dehydration. Closing the stomata causes CO2 delivery, and thus photosynthesis, to stop. In addition, oxygen levels increase as cellular respiration continues, which increases rates of photorespiration.

The Calvin Cycle

The Calvin cycle has three phases: 1) Fixation: (gas --> solid) CO2 reacts with ribulose bisphosphate (RuBP),  producing two 3-phosphoglycerate molecules. The attachment of CO2 to an organic compound is called carbon fixation. 2) Reduction: The 3-phosphoglycerate molecules are  phosphorylated by ATP and reduced by NADPH to produce  glyceraldehyde 3-phosphate (G3P). 3) Regeneration: The remaining G3P is used in reactions that  regenerate RuBP. This cycle of reactions occurs in the chloroplast's stroma. One turn of the Calvin cycle fixes one molecule of CO2. Therefore, three turns of the Calvin cycle are required to produce one molecule of G3P. The discovery of the Calvin cycle clarified how the ATP and NADPH produced by light-capturing reactions allow cells to reduce CO2 to carbohydrate.

The Enhancement Effect

The Z scheme explains the enhancement effect: Photosynthesis is more efficient when both 680-nm and 700-nm wavelengths are available (hence the names of the pairs of reaction-center chlorophyll molecules), allowing both photosystems to run at maximum rates. Photosystem I occasionally transfers electrons to photosystem II's electron transport chain to increase ATP production, instead of using them to reduce NADP+. This cyclic photophosphorylation coexists with the Z scheme and produces additional ATP.

The Z Scheme

The Z scheme is a model of how photosystems I and II interact. First, a photon excites an electron in the pigment molecules of photosystem II's antenna complex, and resonance occurs until the energy reaches the reaction center. The electrons of photosystem II will be replaced by electrons stripped from water, producing oxygen gas as a by-product. A special pair of reaction-center chlorophyll molecules named P680 passes the excited electron to pheophytin.

The Calvin Cycle and Carbon Fixation

The energy transformation of the light-dependent reactions and the carbon dioxide reduction of the Calvin cycle are two separate but linked processes in photosynthesis. ATP and NADPH are produced by photosystems I and II in the presence of light. The reactions that produce sugar from carbon dioxide in the Calvin cycle are light-independent. These reactions require the ATP and NADPH produced by the light-dependent reactions.

What is photosynthesis?

The process of using sunlight to produce carbohydrate. This process requires sunlight, carbon dioxide, and water, and produces oxygen as a by-product. The overall reaction when glucose is the carbohydrate can be written as: 6 CO2 + 12 H2O + light energy ---> C6H12O6 + 6 O2 + 6 H2O

Electrons Become Excited When Light Is Absorbed

When a photon strikes chlorophyll, its energy can be transferred to an electron in the chlorophyll head. The electron becomes excited, raised to a higher energy state. -- In chlorophyll, red and blue photons can be absorbed and excite electrons to different states. *Red photons raise electrons to state 1. *Higher-energy blue photons raise electrons to state 2. *Green photons are of an intermediate energy level and are not easily absorbed by chlorophyll.

Photosystem II

When energy reaches the reaction center of the photosystem, the reaction center chlorophyll is oxidized when a high-energy electron is donated to the electron acceptor pheophytin, a pigment molecule structurally similar to chlorophyll. The electron is passed to an electron transport chain (ETC) in the thylakoid membrane, producing a proton gradient and driving ATP production via ATP synthase. Photosystem II triggers chemiosmosis and ATP synthesis in the chloroplast.