Kranz Anatomy

Kranz Anatomy Definition

Kranz anatomy is a unique structure observed in C4 plants. In these plants, the mesophyll cells cluster around the bundle-sheath cell in a wreath formation (Kranz means ‘wreath or ring). Also, the number of chloroplasts observed in bundle sheath cells is more than that in the mesophyll cell. This entire structure is densely packed and plays a major role in C4 photosynthesis.

Kranz  Anatomy in C4 Plants

In C4 plants, the light-dependent reactions and the Calvin cycle occur in two different places. The light-dependent reactions take place in the bundle sheath cells whereas the Calvin cycle takes place in the mesophyll cells. This is done to overcome the energy-wasting process known as photorespiration which occurs in all C3 plants.

What happens is, in C3 plants, Carbon dioxide (CO2) is first fixed into a C3 compound, (a compound with 3 Carbons) by the photosynthetic enzyme ribulose bisphosphate carboxylase oxygenase or RUBISCO. However, this enzyme can catalyse a reaction with oxygen and create a wasteful process known as photorespiration rather than photosynthesis. 

In C4 plants such as maize enlarged bundle sheath cells (BS) surround the leaf veins (V)and are surrounded by mesophyll (M) cells. Each pair of leaf veins is separated by two BS cells and M cells in a V-BS-M-M-BS-V formation. This formation is generally regarded as Kranz Anatomy.

The Development of Kranz Anatomy can be considered in three Distinct Stages

  1.  The initiation of procambium.

  2. Bundle sheath and mesophyll cell specification.

  3. The development of chloroplast development and integration of the C4 cycle.

Now, to overcome the inefficiency of RUBISCO, in C4 plants, atmospheric CO2 is first fixed into a C4 compound or 4- Carbon compound in the mesophyll cells. This is carried out with the help of the phosphoenolpyruvate carboxylase enzyme (PEPcase). This enzyme does not react with oxygen and the new compound formed is oxaloacetate (organic acid).

Oxaloacetate is then converted to malate and transported to bundle sheath cells where it dissociates to release CO2. Here RUBISCO re-fixes the CO2 and converts it to sugars.

The two different stages of the C4 pathways are separated by morphologically distinct photosynthetic cells which allows a higher concentration of CO2 to be accumulated in RUBISCO which reduces photorespiration.

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Diagram of Kranz Anatomy

C3 Plants

About 85% of the plants in the world are C3 plants where the atmospheric Carbon dioxide is fixed by RUBISCO to produce a 3-carbon compound in the first step of the Calvin cycle and gives rise to the energy-wasting process known as photorespiration.

Example: Rice, Soybean and all trees. 

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C4 Plants

All plants where the light-dependent reactions of photosynthesis and the Calvin cycle takes place in separate locations are referred to as C4 plants. In these types of plants, the light-dependent reactions are carried out in the mesophyll cells of the leaf and the Calvin cycle takes place in the bundle sheath cells. All C4 plants show a low degree of photorespiration.

Examples: Sugarcane, Maize, Sorghum 

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CAM Plants

Plants of the Crassulaceae follow a certain metabolic pathway to reduce photorespiration. Instead of following the C4 pathway where light-dependent reactions and the Calvin cycle take place in different locations, these plants separated the processes in time.

At night they open their stomata to use the atmospheric CO2, which is fixed into oxaloacetate and then converted to malate or another organic acid. This organic acid is stored in the vacuole of the cell. The next day, the organic acid is broken down to release CO2 which enters the Calvin cycle. Even though the stomata of these plants do not open during daylight, they can photosynthesise. The pathway is known as the Crassulacean Acid Metabolism pathway and these plants are called CAM plants.

Example: Cacti, Pineapple

What are the Advantages of C4 Plants?

As the light-dependent photosynthetic reactions and the Calvin cycle of C4 plants occur in different locations within a leaf,  it gives them many advantages over C3 plants.  The following are some of the advantages -

Lower Degree of Photorespiration

 In C4 plant CO2 if fixed twice. Once with PEPcase in the mesophyll cells and the second time with RUBISCO in the bundle sheath cells. Since we know that in C3 plants during the Calvin cycle, RUBISCO can catalyse oxygen and give rise to photorespiration which wastes energy. In C4 plants., because both light-dependent reactions and Calvin cycle processes are carried out in separate locations it reduces photorespiration. 

Better Water Efficiency

Due to low photorespiration, these plants can keep their stomata closed for longer periods and avoid extra water loss. It helps them to survive in hot and dry climates. 

FAQ (Frequently Asked Questions)

Q1: What is the Advantage of Double Carbon Fixation in C4 Plants?

Ans: A double carbon fixation offers C4 plants with better photosynthetic efficiency. In a hot and dry climate, when excess water vapour diffuses out of the stomata in C3 plants, in C4 plants it’s relatively less because of the C4 cycle taking place in the bundle sheath cells of these plants. 

The stomata will be closed and the concentration of gases within cells will change. As photosynthesis takes place, CO2 will be consumed and oxygen will be generated and eventually, CO2 levels will reduce. However, because of Kranz anatomy, the CO2 levels around RUBISCO in Bundle sheath cells will always be more and it will continue to fix carbon and not give rise to photorespiration.  

This is because initially CO2 was fixed in a C4 acid by an enzyme called phosphoenolpyruvate carboxylase enzyme or PEPcase which is not inhibited by O2. The organic acid breaks down in the bundle sheath cells releasing CO2 which is used by RUBISCO.

Q2: What are the Differences between Bundle Sheath Cells and Mesophyll Cells?

Ans: 

Mesophyll Cells

Bundle Sheath Cells

Mesophyll cells are thin-walled and gas permeable

Bundle sheath cells are thick-walled and gas impermeable,

The chloroplast in mesophyll cells is randomly arranged.

The chloroplast in bundle sheath cells is centrifugally arranged.

There are no starch grains present in mesophyll cells.

Starch grain can be observed in bundle sheath cells.

Large amounts of reduced coenzymes and ATPs are produced in Mesophyll cells.

Bundle cells produce very low or no amount of ATPs and reduced coenzymes

C3 cycle does not take place in mesophyll cells due to the absence of RuBP carboxylase. 

RuBP carboxylase is available in high concentrations in bundle sheath cells which assists in the C3 cycle.

CO2 fixation does not take place in the chloroplast of mesophyll cells. It takes place in the cytoplasm of the cell with the help of PEP carboxylase.

CO2 fixation takes place in the bundle sheath cells as it is released by oxaloacetate and accepted by RuBP to enter in the C3 cycle.

Q3: Which Plants Show Krantz Anatomy?

Ans: All C4 plants show Krantz anatomy. Some examples are sugarcane, sorghum, maize, millets, switchgrass which is used as a source of biofuel.