Question

Compare the mechanisms that C_3, C₄, and CAM plants use to obtain and use carbon dioxide.

Why would you expect photorespiration on a hot, dry day to occur less in C₄ and CAM plants than in C₃ plants?

Answer 1

Mechanism of C3 ,C4 and CAM are -

C3 Plants-

The majority of plant species on Earth uses C3 photosynthesis, in which the first stable carbon compound produced contains three carbon atoms that is 3PGA(phosphoglyceric acid) . In this process, carbon dioxide enters a plant through its stomata (microscopic pores on plant leaves), where amidst a series of complex reactions, the enzyme Rubisco(most abundant enzyme) fixes carbon into sugar(Trios phosphate) through the Calvin-Benson cycle. However, two key restrictions slow down photosynthesis.

Step-1 Carboxylation - when co2 fix with RUBP in the presence of RUBISCO this step is known as fixation. And first stable compound 3PGA (phosphoglyceric acid) form.

Step-2 Reduction . In this step2 ATP amd one NADPH2 reduce First stable compound 3PGA by giving electron. Amd converted in ADP amd NADP+

Step 3- Regeneration - In this step There is need to regenerate RUBP again by using one ATP.

C3 Cycle occur only in mesophyll cells of plant hence there is no special mechanism.

C-4

\text C_4C4​start text, C, end text, start subscript, 4, end subscript plants, the light-dependent reactions and the Calvin cycle are physically separated, with the light-dependent reactions occurring in the mesophyll cells (spongy tissue in the middle of the leaf) and the Calvin cycle occurring in special cells around the leaf veins. These cells are called bundle-sheath cells.

To see how this division helps, let's look at an example of \text C_4C4​start text, C, end text, start subscript, 4, end subscript photosynthesis in action. First, atmospheric \text {CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript is fixed in the mesophyll cells to form a simple, 444-carbon organic acid (oxaloacetate). This step is carried out by a non-rubisco enzyme, PEP carboxylase, that has no tendency to bind \text O_2O2​start text, O, end text, start subscript, 2, end subscript. Oxaloacetate is then converted to a similar molecule, malate, that can be transported in to the bundle-sheath cells. Inside the bundle sheath, malate breaks down, releasing a molecule of \text {CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript. The \text {CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript is then fixed by rubisco and made into sugars via the Calvin cycle, exactly as in \text C_3C3​start text, C, end text, start subscript, 3, end subscript photosynthesis.

cycle to make sugars from \text {CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript. These pathways for fixing \text {CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript have different advantages and disadvantages and make plants suited for different habitats. The \text C_3C3​start text, C, end text, start subscript, 3, end subscript mechanism works well in cool environments, while \text C_4C4​start text, C, end text, start subscript, 4, end subscript and CAM plants are adapted to hot, dry areas.

Both the \text {C}_4C4​start text, C, end text, start subscript, 4, end subscript and CAM pathways have evolved independently over two dozen times, which suggests they may give plant species in hot climates a significant evolutionary advantage^55start superscript, 5, end superscript.

Type	Separation of initial \text {CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript fixation and Calvin cycle	Stomata open	Best adapted to
\text C_3C3​start text, C, end text, start subscript, 3, end subscript	No separation	Day	Cool, wet environments
\text C_4C4​start text, C, end text, start subscript, 4, end subscript	Between mesophyll and bundle-sheath cells (in space)	Day	Hot, sunny environments
CAM	Between night and day (in time)	Night	Very hot, dry environments

In the C4 pathway, initial carbon fixation takes place in mesophyll cells and the Calvin cycle takes place in bundle-sheath cells. PEP carboxylase attaches an incoming carbon dioxide molecul to the three-carbon molecule PEP, producing oxaloacetate (a four-carbon molecule). The oxaloacetate is converted to malate, which travels out of the mesophyll cell and into a neighboring bundle-sheath. Inside the bundle sheath cell, malate is broken down to release CO_22​start subscript, 2, end subscript, which then enters the Calvin cycle. Pyruvate is also produced in this step and moves back into the mesophyll cell, where it is converted into PEP (a reaction that converts ATP and Pi into AMP and PPi).

This process isn't without its energetic price: ATP must be expended to return the three-carbon “ferry” molecule from the bundle sheath cell and get it ready to pick up another molecule of atmospheric \text {CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript. However, because the mesophyll cells constantly pump \text{CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript into neighboring bundle-sheath cells in the form of malate, there’s always a high concentration of \text{CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript relative to \text O_2O2​start text, O, end text, start subscript, 2, end subscript right around rubisco. This strategy minimizes photorespiration.

The \text C_4C4​start text, C, end text, start subscript, 4, end subscript pathway is used in about 3\%3%3, percent of all vascular plants; some examples are crabgrass, sugarcane and corn. \text C_4C4​start text, C, end text, start subscript, 4, end subscript plants are common in habitats that are hot, but are less abundant in areas that are cooler. In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving \text {CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript from the mesophyll cell to the bundle-sheath cell.

CAM Plants-

Some plants that are adapted to dry environments, such as cacti and pineapples, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. This name comes from the family of plants, the Crassulaceae, in which scientists first discovered the pathway.

Image of a succulent.

Image credit: "Crassulaceae," by Guyon Morée (CC BY 2.0).

Instead of separating the light-dependent reactions and the use of \text{CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript in the Calvin cycle in space, CAM plants separate these processes in time. At night, CAM plants open their stomata, allowing \text {CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript to diffuse into the leaves. This \text{CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript is fixed into oxaloacetate by PEP carboxylase (the same step used by \text C_4C4​start text, C, end text, start subscript, 4, end subscript plants), then converted to malate or another type of organic acid^33cubed.

The organic acid is stored inside vacuoles until the next day. In the daylight, the CAM plants do not open their stomata, but they can still photosynthesize. That's because the organic acids are transported out of the vacuole and broken down to release \text{CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript, which enters the Calvin cycle. This controlled release maintains a high concentration of \text{CO}_2CO2​start text, C, O, end text, start subscript, 2, end subscript around rubisco^44start superscript, 4, end superscript.