Dark Fixation of carbon Dioxide
Many species living in arid climates have thick leaves and relatively low surface-to-volume ratio and have low transpiration rates. Their cells have large central vacuoles with a thin layer of cytoplasm around them. These plants are called succulents. Another characteristic feature of these plants is that their stomata opens at night and fix CO2 into organic acids. This help them minimize water loss due to transpiration, the succulents are classified into 20 families including Cactaceae, Orchidaceae, Bromeliaceae, Liliaceae and Euphorbiaceae.
Crassulacean Acid Metabolism – CO2 Fixation in Succulents
The CO2 metabolism is succulents is unusual. It was first investigated in members of Crassulaceae, therefore, called crassulacean acid metabolism (CAM) and the plants exhibiting this metabolism are known as CAM plants. The CAM plants are usually without a well-developed palisade layer of cells, and most of the leaf or stem cells are spongy mesophyll. Bundle sheath cells are present but are quite similar to the mesophyll cells.
The CAM plants formed malic acid at night and its disappearance during daylight. The formation of acid at night can be detected by sour taste of the sap and disappearance of sugars and starch.
Mechanism of Crassulacean Acid Metabolism
The following steps are involved in Crassulacean Acid Metabolism:
- The stomata open at night and the CO2 entering leaves is combine with phosphoenolpyruvate (carboxylation), a product of starch breakdown to form oxaloacetate. Enzyme PEP carboxylase catalyze this step.
- The oxaloacetate is then reduced to malate in the presence of NADH dependent malate dehydrogenase. Most of the malate s pumped into the vacuole and stored as malic acid during night time.
- With the onset of day, the stomata close to prevent the loss of water and the entry of CO2 is also stopped along with. The malic acid diffuses out of the vacuole into the cytoplasm where it is decarboxylated.
- The CO2 released enters the chloroplasts where it fixed to 3-PGA by Calvin cycle which is then converted into sugars and starch. Rubisco is responsible for carboxylation during daylight.
Difference Between CAM Plants & C-3 Plants
In CAM plants, the fixation of carbon dioxide to malate at night and its decarboxylation to carbon dioxide and pyruvate during the day are separated in time. Whereas in C-4 plants two phases are separated spatially where the primary carboxylation to C-4 acids occurs in mesophyll and the decarboxylation and the secondary carbon dioxide fixation to PGA reactions in the bundle-sheath.
Glycolate Metabolism – C-2 Cycle
The entire process of photorespiration takes place in three organelles: chloroplasts, peroxisomes and mitochondria. The pathway involves following steps:
- In the chloroplast, the carbon dioxide is normally fixed to form two molecules of 3-phosphoglyceric acid (3-PGA). Certain level of high light intensity promotes oxidation of RuBP to a molecule of phosphoglycolic acid and one molecule of 3-PGA. The phosphoglycolic acid is converted into glycolic acid and transported to peroxisome.
- In the peroxisome, the glycolic acid is oxidized by enzyme glycolic acid oxidase to produce glyoxylic acid and hydrogen peroxide (H2O2). The glyoxylic acid liberates O2 and is converted to glycine. Two glycine molecules react to produce serine and CO2. The serine is translocated to mitochondria.
- In the mitochondria, the serine is metabolized to carbohydrates or incorporated into proteins which are transported to chloroplast.