What is the Difference Between Mesophyll and Bundle Sheath Cells

The main difference between mesophyll and bundle sheath cells is that mesophyll contains chloroplasts randomly distributed along the cell wall, whereas bundle sheath cells contain chloroplasts located close to the vascular cells.

Mesophyll and bundle sheath cells are two types of photosynthetic cells that occur in C4 cells. They show different arrangements of chloroplasts or Kranz anatomy during maturation. Generally, these arrangements of chloroplasts are maintained throughout the life of the cell.

Key Areas Covered

1. What are Mesophyll
     – Definition, Structure, Function
2. What are Bundle Sheath Cells
     – Definition, Structure, Function
3. Similarities Between Mesophyll and Bundle Sheath Cells
     – Outline of Common Features
4. Difference Between Mesophyll and Bundle Sheath Cells
     – Comparison of Key Differences

Key Terms                                                                                                                  

Bundle Sheath Cells, Mesophyll

What is Mesophyll

Mesophyll is a type of photosynthetic cell that occurs in the middle of the leaf. However, the occurrence of mesophylls differs by the type of carbon fixation. Generally, there are two pathways of carbon fixation: the C3 pathway and the C4 pathway.

Mesophylls in C3 Pathway

In the C3 pathway of carbon fixation, mesophylls occur in two layers. They are the upper palisade layer and the beneath spongy layer. In general, the mesophyll cells of the upper palisade layer are vertically elongated and occur directly beneath the adaxial epidermis in one to two-cell thickness. These cells contain more chloroplasts occurring close to the cell wall, allowing for optimal exposure to light. The slight separation of cells ensures maximum carbon dioxide fixation. In addition, the amount of sunlight determines the number of layers of mesophyll cells in the palisade layer.

Figure 1: Mesophyll in C3 Pathway

Furthermore, another layer of mesophyll cells is called a spongy layer beneath the palisade layer. This layer contains cells loosely packed mesophyll cells with large intercellular air spaces connecting the stomata of the epidermis. This allows the exchange of carbon dioxide, water vapor, and oxygen gases during photosynthesis.    

C4 Pathway

Generally, the vascular bundle of the leaf exhibits Kranz anatomy in the C4 pathway. There, two layers of cells surround the vascular bundle: inner bundle sheath cells and outer mesophylls. Generally, the mesophylls of the C4 plants contain numerous well-developed chloroplasts. They also contain numerous grana supporting the light reaction of photosynthesis. However, the dark reaction of photosynthesis is less common in mesophylls of C4 plants.  

What are Bundle Sheath Cells

Bundle sheath cells are a type of photosynthetic cells that exist as a covering over the vascular bundle in C4 plants. The most significant feature of the bundle sheath cells is the presence of chloroplasts close to the vascular cells. However, the chloroplasts of the bundle sheath cells are fewer.

Figure 2: Kranz Anatomy

 Meanwhile, they contain fewer or no grana. Therefore, the light reaction of photosynthesis is less common in bundle sheath cells. But they abundantly contain RuBP carboxylase enzyme, allowing an efficient dark reaction of photosynthesis. Bundle sheath cells also contain starch granules.

Similarities Between Mesophyll and Bundle Sheath Cells

• Mesophyll and bundle sheath cells are two types of photosynthetic cells that occur in C4 plants.
• They show different arrangements of chloroplasts inside the cells.
• These arrangements of chloroplasts occur during cell maturation.
• The main function of the different chloroplast arrangements is to undergo efficient photosynthesis.  

Difference Between Mesophyll and Bundle Sheath Cells

Definition

Mesophyll refers to the inner tissue (parenchyma) of a leaf, containing many chloroplasts, while bundle sheath cells refer to the photosynthetic cells arranged into a tightly packed sheath around the vein of a leaf.

Arrangement

Mesophylls are loosely arranged cells consisting of one or more layers, while bundle sheath cells are tightly arranged cells in a sheath.

Occurrence

Mesophyll forms a protective layer around the leaf vein, while bundle sheath cells occur around the vein.

Leaf Vein

Mesophyll covers the bundle sheath cells, while bundle sheath cells cover the leaf vein.

Chloroplasts Occurrence

While mesophyll contains chloroplasts randomly distributed along the cell wall, bundle sheath cells contain chloroplasts located close to the vascular cells.

Chloroplast Organization

Mesophyll contains less-developed chloroplasts in fewer numbers, while bundle sheath cells contain well-developed chloroplasts in higher numbers.

Grana in Chloroplasts

Mesophyll contains chloroplasts with well-developed numerous grana, while bundle sheath cells contain few or no grana.

The activity of Photosystem II

Mesophyll contains highly active photosystem II while bundle sheath cells contain less active photosystem II.

RuBP Carboxylase

Mesophyll contains less RuBP carboxylase, and therefore, the light reaction of photosynthesis is more efficient, while bundle sheath cells contain abundant RuBP carboxylase; therefore, carbon dioxide fixation is more efficient.

Starch Grains

Mesophyll does not contain starch grains, while bundle sheath cells contain starch grains.

Conclusion                                                     

In brief, mesophyll and bundle sheath cells are two types of photosynthetic cells in C4 plants. Generally, mesophyll occurs in one or more layers around the bundle sheath cells. It contains chloroplasts arranged through the cell wall. In addition, the chloroplasts of mesophyll are well-developed and contain numerous grana. Moreover, the light reaction of photosynthesis is more efficient in mesophylls, and the cells do not contain starch grains. In comparison, bundle sheath cells are the photosynthetic cells occurring in a sheath around the leaf vein. The chloroplasts of bundle sheath cells occur close to the vascular cells. The chloroplasts of bundle sheath cells contain no grana, but they contain RUBP carboxylase to make the carbon dioxide fixation more efficient. Hence, the main difference between mesophyll and bundle sheath cells is the structure of chloroplasts.    

References:                

1. Maai E, Miyake H, Taniguchi M. Differential positioning of chloroplasts in C4 mesophyll and bundle sheath cells. Plant Signal Behav. 2011 Aug;6(8):1111-3. doi: 10.4161/psb.6.8.15809. Epub 2011 Aug 1. PMID: 21757999; PMCID: PMC3260704.

Image Courtesy:

1. “Leaf Tissue Structure” By Zephyris – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Cross section of maize, a C4 plant” By Ninghui Shi – Own Work (CC BY-SA 3.0) via Commons Wikimedia