Kranz Anatomy – An Overview

Kranz Anatomy is a distinctive arrangement found in C4 plants, characterized by mesophyll cells forming a ring around the bundle-sheath cells. Kranz Anatomy is found in angiosperms like sugarcane and grasses which undergo C4 photosynthesis. Plants growing in warm tropical or arid environments have developed this efficient photosynthetic pathway. Let us discuss Kranz’s anatomy in detail.

What is Kranz Anatomy?

In this pathway, atmospheric carbon dioxide (CO₂) is initially fixed into a four-carbon molecule (malic acid or malate), hence the name C4. Those plants typically exhibit a distinct leaf anatomy called Kranz anatomy, which involves two types of photosynthetic cells with separate morphological and functional characteristics. These are the bundle sheath cells surrounding the vascular centers and the mesophyll cells enveloping the bundle sheath cells beneath the leaf epidermis.

This distinctive wreath-like arrangement serves as a structural framework for the compartmentalization and functional segregation of the two sets of biochemical reactions necessary for this specialized CO₂-assimilation pathway. Notably, the bundle-sheath cells contain a higher number of chloroplasts compared to the mesophyll cells.

Kranz Anatomy in C4 Plants

In C4 plants, Kranz anatomy refers to a specialized arrangement of leaf cells that enhances the efficiency of photosynthesis, particularly in hot and dry conditions. This anatomical adaptation is crucial for their ability to concentrate CO₂ around the enzyme rubisco, thereby minimizing photorespiration and maximizing carbon fixation. C4 plants have two types of photosynthetic cells: mesophyll cells and bundle sheath cells. Mesophyll cells are typically arranged in a layer beneath the upper epidermis of the leaf.

These cells initially fix carbon dioxide into a four-carbon compound called oxaloacetate or malate, instead of the three-carbon compound, 3-phosphoglycerate (PGA), as in C3 plants. Surrounding the vascular bundles, which contain xylem and phloem, are specialized bundle sheath cells. These cells are characterized by having a high concentration of chloroplasts and are tightly packed. They form a protective layer around the vascular tissue.

Kranz Anatomy Diagram

The Kranz anatomy diagram is given below:

Structure of C4 Plants

The structure of C4 plants shows a distinctive Kranz Anatomy that includes:

• Mesophyll cells are uniform and arranged in concentric layers around the vascular bundles.
• Chloroplasts in mesophyll cells are fewer and do not contain starch. They possess the enzyme structure necessary for C4 photosynthesis.
• Chloroplasts in bundle sheath cells store starch and are larger. They have the enzyme structure required for C3 photosynthesis.
• Mesophyll cells and bundle sheath cells are connected through plasmodesmata, facilitating communication and exchange of materials between them.

Function of Kranz Anatomy

In most plants, carbon dioxide undergoes initial fixation into a compound with three carbon atoms (C3) through the photosynthetic enzyme ribulose bisphosphate carboxylase oxygenase, or RuBisCo, which is known as C3 photosynthesis and these plants are called C3 plants.

• However, Rubisco is inherently inefficient as it can also catalyze a reaction with oxygen, leading to an inefficient process called photorespiration instead of photosynthesis.
• To overcome this inefficiency, the C4 pathway strategically fixes atmospheric carbon dioxide into C4 acids using the enzyme phosphoenolpyruvate carboxylase, which is not responsive to oxygen.
• Subsequently, C4 acids release carbon dioxide for re-fixation by Rubisco. In many C4 plants, these two phases of the C4 pathway are spatially separated into morphologically distinct photosynthetic cell categories.
• This segregation allows for a high concentration of carbon dioxide to accumulate in the vicinity of Rubisco, promoting better photosynthetic efficiency.

Advantages of Kranz Anatomy

Kranz anatomy is advantageous because it enables plants to efficiently produce their food. This anatomical feature revolves around the veins in the leaves, forming a layer of cells that can absorb more light. This capability allows the plant to generate increased amounts of sugar and oxygen for its metabolic processes. Consequently, there are other advantages too. Some of these benefits are outlined below:

• It creates an optimal site for the concentration of carbon dioxide within the plants, particularly around the RuBisCO enzyme.
• It plays a role in preventing photorespiration, a process that can be detrimental to plant productivity.
• It facilitates the fixation of carbon dioxide twice within C4 plants, thanks to the involvement of bundle sheath cells in this specialized anatomical arrangement.

Development of Kranz Anatomy

The development of Kranz anatomy occurs in three distinct steps:

• Initiation of procambium
• Specification of bundle sheath and mesophyll cells
• Development of chloroplasts and integration of the C4 cycle

Characteristic Features of Kranz Anatomy

The features of Kranz Anatomy include:

• Mesophyll cells are uniform and arranged in concentric layers around the vascular bundles.
• Chloroplasts in mesophyll cells are fewer and do not contain starch. They possess the enzyme structure necessary for C4 photosynthesis.
• Chloroplasts in bundle sheath cells store starch and are larger. They have the enzyme structure required for C3 photosynthesis.
• Mesophyll cells and bundle sheath cells are connected through plasmodesmata, facilitating communication and exchange of materials between them.

What is the Difference Between C3 and C4 plants?

In the context of Kranz anatomy, it is worth noting the important differences between C3 and C4 plants.

Characteristic	C3 Plants	C4 Plants
Dark Reaction Pathway	Calvin cycle	Hatch-Slack pathway
Season and Habitat	Cold season, cool and wet areas	Warm season, dry areas
Environmental Conditions	Temperate environments	Tropical environments
Product of Dark Reaction	3-carbon compound (phosphoglyceric acid)	4-carbon compound (oxaloacetic acid)
Kranz Anatomy	Absent	Present
Bundle Sheath Cells and Chloroplasts	Bundle sheath cells lack chloroplasts	Bundle sheath cells contain chloroplasts
Photosynthesis in Closed Stomata	Requires open stomata for photosynthesis	Can complete photosynthesis with closed stomata
CO2 Fixation Rate	Slower CO2 fixation	Relatively rapid CO2 fixation
Location of Dark Reaction	Mesophyll cells	Mesophyll cells initiate, crucial steps in bundle sheath cells
Photorespiration	High rates of photorespiration	Absence of photorespiration
Temperature for Growth	4-7 degrees soil temperature	16-21 degrees soil temperature
Global Presence	95% of total green plants on Earth	Approximately 5% of plants on Earth
Examples	Wheat, oats, rice, sunflower, cotton	Maize, sugarcane, amaranthus

Difference between Mesophyll Cells and Bundle Sheath Cells

Mesophyll cells are located in the leaf’s interior, and responsible for initial CO₂ fixation in C4 plants, while bundle sheath cells surround vascular bundles, facilitating the Calvin cycle and minimizing photorespiration. Here are some key differences,

Characteristic	Mesophyll Cells	Bundle Sheath Cells
Location	Interior tissue of the leaf (mesophyll layer)	Surrounding vascular bundles within the leaf
Chloroplast Density	Contains chloroplasts, less densely packed	High concentration of chloroplasts, densely packed
Cell Arrangement	Loosely packed, irregular shape	Tightly packed, regular shape
Function in Photosynthesis	Initial fixation of CO2 into a four-carbon compound	Site of the Calvin cycle, breakdown of four-carbon compound to release CO2
Metabolism	Involved in C4 pathway and other metabolic processes	Primarily involved in the Calvin cycle
Adaptations to Environment	Adapted for gas exchange and light absorption	Adapted to minimize photorespiration and maximize carbon fixation efficiency

Conclusion – What is Kranz Anatomy?

The specialized molecular processes underlying Kranz Anatomy in C4 plants enable high photosynthetic rates and increased glucose production. Expanding the use of C4 plant species in agriculture, like maize and sugarcane, or transferring C4-like CO₂ fixation capabilities to C3 crops, such as rice, holds promise for enhancing agricultural productivity. Additionally, understanding C4 photosynthesis may help in developing biofuel sources like maize and switchgrass, contributing to green energy.

FAQs – Kranz Anatomy

What is Kranz Anatomy Class 11?

Kranz anatomy refers to a distinctive arrangement found in the leaves of C4 plants, wherein the tissue corresponding to spongy mesophyll cells forms a circular cluster around the leaf veins, situated outside the bundle sheath cells.

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

In C3 plants, which experience significant water loss through stomatal diffusion in hot and dry climates, C4 plants exhibit reduced water vapor loss due to the occurrence of the C4 cycle within the bundle sheath cells.

Which Plants Show Krantz Anatomy?

Kranz anatomy is a characteristic feature observed in all C4 plants. Examples of such plants include sugarcane, sorghum, maize, millet, and switchgrass etc.

What is the Function of Vascular Bundles?

The primary function of vascular bundles is to serve as a crucial component of the transport system in vascular plants. Two essential types of vascular tissues are the xylem, and phloem.

Do C3 Plants have Kranz Anatomy?

Kranz anatomy is typically absent in C3 plants. Although chloroplasts are found in the bundle sheath cells of these plants, carbon dioxide fixation occurs only once in the mesophyll cells.

What is the Anatomy of a C4 Plant?

C4 leaves showcase a unique Kranz-type anatomy, with vascular bundles encased by organelle-rich bundle sheath cells, surrounded by radially arranged mesophyll cells.

What is the Difference Between Kranz Anatomy and C3 Anatomy?

Kranz anatomy in C4 plants enhances carbon fixation, unlike C3 anatomy, which lacks this specialized leaf structure, leading to less efficient photosynthesis.