The Mesophyll: The Leaf's Breathing Layer
The Anatomy of a Leaf: Where is the Mesophyll?
To understand the mesophyll, we first need to look at the general structure of a leaf. A typical leaf is like a multi-layered sandwich, with each layer playing a specific role.
- Upper and Lower Epidermis: These are the outer layers of the leaf, like the skin. Their main job is protection. They are coated with a waxy substance called the cuticle to prevent water loss.
- Stomata1: These are tiny pores, mostly found on the lower epidermis. They act as gateways, opening and closing to allow gases like $CO_2$ and $O_2$ to move in and out of the leaf. Each stoma is flanked by two guard cells that control its opening.
- The Mesophyll: This is the green tissue located between the upper and lower epidermis. It is not a single, uniform layer but is divided into two distinct sub-layers with different structures and functions.
Two Layers, One Team: Palisade and Spongy Mesophyll
The mesophyll is divided into two regions: the palisade mesophyll and the spongy mesophyll. They work together like a factory assembly line.
| Feature | Palisade Mesophyll | Spongy Mesophyll |
|---|---|---|
| Location | Just below the upper epidermis | Between palisade layer and lower epidermis |
| Cell Shape | Tall, column-shaped, tightly packed | Irregular, roundish, very loosely packed |
| Air Spaces | Very few | Numerous, creating a large network |
| Primary Function | Photosynthesis (light absorption) | Gas Exchange and some photosynthesis |
| Chloroplast Count | Very high (maximizes light capture) | Lower than palisade layer |
The palisade mesophyll is the primary power plant. Its cells are packed with chloroplasts2, the organelles that contain chlorophyll and perform photosynthesis. Their tall, tight arrangement is perfect for absorbing maximum sunlight.
The spongy mesophyll is the gas exchange hub. Its irregular, loosely packed cells create a vast network of air spaces. This labyrinth allows gases to circulate freely from the stomata to all the cells in the leaf. While these cells also contain chloroplasts and perform some photosynthesis, their main role is facilitating the movement of $CO_2$ and $O_2$.
The entire purpose of the mesophyll's structure is to enable this chemical reaction, which is powered by sunlight and occurs inside chloroplasts:
$6CO_2 + 6H_2O \xrightarrow{\text{light energy}} C_6H_{12}O_6 + 6O_2$
This reads as: Six carbon dioxide molecules plus six water molecules, using light energy, produce one glucose (sugar) molecule and six oxygen molecules.
The Journey of a Carbon Dioxide Molecule
Let's follow a molecule of $CO_2$ on its journey into a leaf to see how the mesophyll works in action.
- Entry: The stoma on the lower leaf surface opens. Our $CO_2$ molecule from the air diffuses through this pore.
- Circulation: The molecule enters the extensive air space network within the spongy mesophyll. This space is humid, which helps gases move and dissolve.
- Absorption: The $CO_2$ molecule dissolves in the thin layer of water that coats the surfaces of the mesophyll cells.
- Usage: It then diffuses through the cell wall and membrane of a mesophyll cell (either spongy or palisade) and into a chloroplast. Here, it is used as a key ingredient in the photosynthesis reaction.
- Output: Meanwhile, a waste product of photosynthesis, an $O_2$ molecule, follows the reverse path: it leaves the chloroplast, diffuses out of the cell, into the air spaces, and exits through the open stoma.
This constant exchange is why a dense forest feels so fresh—you are breathing the oxygen released by millions of mesophyll layers!
Adaptations for Different Environments
Not all plants live in ideal conditions with plenty of water. The structure of the mesophyll can adapt to help the plant survive.
- Desert Plants (e.g., Cacti): To conserve precious water, their stomata open only at night. They have a special type of photosynthesis called CAM3. Their mesophyll cells temporarily store $CO_2$ taken in at night and use it for photosynthesis during the day without opening their stomata.
- Water Plants (e.g., Water Lilies): Their stomata are on the upper epidermis because the lower surface is underwater. The mesophyll of their floating leaves has especially large air spaces (aerenchyma) that help them float and provide buoyancy, in addition to facilitating gas exchange.
- Sun vs. Shade Leaves: Leaves that grow in full sun often have a thicker palisade layer with more chloroplasts to capture intense light. Shade leaves might have a thinner palisade layer but a more developed spongy layer to maximize gas exchange in lower light conditions.
Common Mistakes and Important Questions
A: No, this is a common confusion. They are different parts of the leaf that work together. The stomata are the pores (holes) on the leaf's surface. The mesophyll is the tissue inside the leaf. Think of it like a building: the stomata are the windows and doors, while the mesophyll is all the rooms and hallways inside.
A: Plants do undergo respiration (like animals) 24/7, which uses oxygen and releases carbon dioxide. However, the term "breathing" is often used to describe gas exchange. During the day, the dominant process in the mesophyll is photosynthesis, which produces oxygen. At night, when there is no light for photosynthesis, the mesophyll's main role is gas exchange for respiration, taking in oxygen and releasing carbon dioxide.
A: Strength is not the goal here; efficiency in gas movement is. Tightly packed cells would leave no room for air spaces. The loose packing of the spongy mesophyll creates a massive internal surface area exposed to the air, much like a sponge. This maximizes the area where $CO_2$ can dissolve and diffuse into the cells and where $O_2$ can diffuse out.
Footnote
1Stomata (singular: stoma): Microscopic pores in the epidermis of leaves and stems that allow for gas exchange.
2Chloroplasts: Membrane-bound organelles found in plant cells that contain chlorophyll and are the site of photosynthesis.
3CAM (Crassulacean Acid Metabolism): A water-conserving photosynthetic adaptation where plants take in $CO_2$ at night and store it for use during the day.