Dark Reactions

Dark reaction

Definition ◦ The series of biochemical reactions in

photosynthesis that do not require light to proceed, and ultimately produce organic molecules from carbon dioxide.

Supplement ◦ The energy from ATP (produced during the light

reactions) drives the dark reactions of photosynthesis. The term dark reactions does not mean the reactions happen at night or that they require darkness. It means that the reactions can proceed regardless of the amount of light available. The term is only used to identify the dark reactions with the light reactions, which obviously require light.

Light-independent Reactions

The light-independent reactions of photosynthesis are chemical reactions that convert carbon dioxide and other compounds into glucose. These reactions occur in the stroma, the fluid-filled area of a chloroplast outside of the thylakoid membranes. These reactions take the light-dependent reactions and perform further chemical processes on them. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carbon fixation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.

Despite its name, this process occurs only when light is available. Plants do not carry out the Calvin cycle by night. They, instead, release sucrose into the phloem from their starch reserves. This process happens when light is available independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and Crassulacean Acid Metabolism); CAM plants store malic acid in their vacuoles every night and release it by day in order to make this process work.

The entry and exit of gasses in plants is through small pores called stomata located on the underside of leaves. Carbon dioxide, the gas required for the Calvin cycle, is not a very abundant gas in nature. Under hot and dry environmental conditions the stomata close to reduce the loss of water vapor, but this also results in a greatly diminished supply of CO2 for the plant. Plants that normally live in dry, hot climates have adapted different ways of initially fixing CO2 prior to its entering the Calvin cycle. These pathways of carbon fixation, know as the C4 and the CAM pathways, take place in the cytoplasm of the cell.

THE C4 AND CAM PATHWAYS

The C4 pathway is designed to efficiently fix CO2 at low concentrations and plants that use this pathway are known as C4 plants. These plants first fix CO2 into a four carbon compound (C4) called oxaloacetate. This occurs in cells called mesophyll cells. First, CO2 is fixed to a three-carbon compound called phosphoenolpyruvate to produce the four-carbon compound oxaloacetate. The enzyme catalyzing this reaction, PEP carboxylase, fixes CO2 very efficiently so the C4 plants don't need to to have their stomata open as much.

The C4 pathway

The oxaloacetate is then converted to another four-carbon compound called malate in a step requiring the reducing power of NADPH. The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells. Here the four-carbon malate is decarboxylated to produce CO2, a three-carbon compound called pyruvate, and NADPH. The CO2 combines with ribulose bisphosphate and goes through the Calvin cycle while the pyruvate re-enters the mesophyll cells, reacts with ATP, and is converted back to phosphoenolpyruvate, the starting compound of the C4 cycle.

The C4 pathway is designed to efficiently fix CO2 at low concentrations and plants that use this pathway are known as C4 plants. These plants fix CO2 into a four carbon compound (C4) called oxaloacetate. This occurs in cells called mesophyll cells.

1. CO2 is fixed to a three-carbon compound called phosphoenolpyruvate to produce the four-carbon compound oxaloacetate. The enzyme catalyzing this reaction, PEP carboxylase, fixes CO2 very efficiently so the C4 plants don't need to to have their stomata open as much. The oxaloacetate is then converted to another four-carbon compound called malate in a step requiring the reducing power of NADPH.

3. The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells. Here the four-carbon malate is decarboxylated to produce CO2, a three-carbon compound called pyruvate, and NADPH. The CO2 combines with ribulose bisphosphate and goes through the Calvin cycle.

4. The pyruvate re-enters the mesophyll cells, reacts with ATP, and is converted back to phosphoenolpyruvate, the starting compound of the C4 cycle.

The CAM pathwayThe CAM pathway (Crassulacean Acid

Metabolism) is utilized by cacti, other succulents, and members of the crassulaceae. The CAM pathway uses up more energy, resulting in stunted growth, but it vital in environments where water loss is the difference between life and death. CAM plants open their stomates at night to take in carbon dioxide and close them (reducing water loss) in the day.

CAM plants live in very dry condition and, unlike other plants, open their stomata to fix CO2 only at night. Like C4 plants, the use PEP carboxylase to fix CO2, forming oxaloacetate. The oxaloacetate is converted to malate which is stored in cell vacuoles. During the day when the stomata are closed, CO2 is removed from the stored malate and enters the Calvin cycle.

C3 CARBON FIXATION PATHWAY

DEFINITION A metabolic pathway where CO2 is converted to 3-

phosphogylycerate, the first stable intermediate organic compound containing three carbon atoms.

SUPPLEMENT The C3 carbon fixation pathway is in fact the initial

phase of Calvin cycle. It starts with the enzyme rubisco catalyzing the carboxylation of Ribulose-1,5-bisphosphate by CO2 producing a highly unstable 6-carbon intermediate known as 3-keto-2-carboxyarabinitol 1,5-bisphosphate, which splits instantaneously into two molecules of the more stable 3-phosphogylycerate, an organic compound containing three carbon atoms (hence the name C3).

Plants that solely depend to C3 pathway for carbon fixation are faced with the negative effects of photorespiration, such as the wasteful loss of CO2. Photorespiration occurs under conditions of drought, high temperatures and low nitrogen or CO2 concentrations. These conditions cause the stomata to close in an attempt to prevent excessive water loss. The closure of stomata increases O2 levels, and the enzyme rubisco reacts with O2 instead of CO2, consequently losing CO2 instead of fixing CO2.

Three types of photosynthesis:C3, C4, and CAM

The three types of photosynthesis are C3, C4, and CAM. C3 photosynthesis is the typical photosynthesis that most plants use. C4 and CAM photosynthesis are both adaptations to arid conditions because they result in better water use efficiency. In addition, CAM plants can "idle," saving precious energy and water during harsh times, and C4 plants can photosynthesize faster under the desert's high heat and light conditions than C3 plants because they use an extra biochemical pathway and special anatomy to reduce photorespiration.

C3 Photosynthesis : C3 plants.- Called C3 because the CO2 is first

incorporated into a 3-carbon compound.- Stomata are open during the day.- RUBISCO, the enzyme involved in

photosynthesis, is also the enzyme involved in the uptake of CO2.

- Photosynthesis takes place throughout the leaf.

Adaptive Value: more efficient than C4 and CAM plants under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy)..

- Most plants are C3.

C4 Photosynthesis : C4 plants.- Called C4 because the CO2 is first

incorporated into a 4-carbon compound.- Stomata are open during the day.- Uses PEP Carboxylase for the enzyme

involved in the uptake of CO2. This enzyme allows CO2 to be taken into the plant very quickly, and then it "delivers" the CO2 directly to RUBISCO for photsynthesis.

- Photosynthesis takes place in inner cells (requires special anatomy called Kranz Anatomy)

Adaptive Value: Photosynthesizes faster than C3 plants under high light intensity and high temperatures because the CO2 is delivered directly to RUBISCO, not allowing it to grab oxygen and undergo photorespiration.Has better Water Use Efficiency because PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.C4 plants include several thousand species in at least 19 plant families. Example: fourwing saltbush pictured here, corn, and many of our summer annual plants.

CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism

- Called CAM after the plant family in which it was first found (Crassulaceae) and because the CO2 is stored in the form of an acid before use in photosynthesis.

- Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis

- CAM plants include many succulents such as cactuses and agaves and also some orchids and bromeliads

Adaptive Value: - Better Water Use Efficiency than C3 plants

under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).

- May CAM-idle. When conditions are extremely arid, CAM plants can just leave their stomata closed night and day. Oxygen given off in photosynthesis is used for respiration and CO2 given off in respiration is used for photosynthesis. This is a little like a perpetual energy machine, but there are costs associated with running the machinery for respiration and photosynthesis so the plant cannot CAM-idle forever.

- But CAM-idling does allow the plant to survive dry spells, and it allows the plant to recover very quickly when water is available again (unlike plants that drop their leaves and twigs and go dormant during dry spells).

CAM pathway showing carbon dioxide uptake through open stomata during night and its utilisation for the formation of malic acid which is stored in the vacuole. During day, the malic acid is decarboxylated to release carbon dioxide which is re-fixed to produce starch inside choloroplast via C3 calving cycle.