Mesophyll cells are the soft green cells packed inside a leaf, sandwiched between its thin upper and lower skin. They are where most of a plant’s food is made, turning light, water and carbon dioxide into sugar. The inside of a leaf isn’t one uniform green mass, though. The cells are arranged, and even move, to manage light and air, so a leaf interior works more like a factory floor than a filling.
What Mesophyll Cells Are Made Of
The name means “middle of the leaf,” from Greek words for middle and leaf. Each mesophyll cell holds many chloroplasts, small green bodies full of chlorophyll that absorb light. Inside those chloroplasts sits a huge amount of Rubisco, the enzyme that grabs carbon dioxide from the air and starts building sugar.1PubMed Central. Structural and biochemical characterization of the C3–C4 intermediate Brassica gravinae and relatives, with particular reference to cellular distribution of Rubisco That makes mesophyll cells the place where the carbon in a plant’s body first enters from the air.
The leaf’s outer skin cells are a different story. They are mostly clear, with few or no chloroplasts, so light passes straight through them to the green cells underneath. Veins run through the mesophyll, bringing water in and carrying sugar out.
Most mesophyll cells also have a large central vacuole, a water-filled sac that takes up much of the cell. It is mostly water with dissolved salts and sugars, and its pressure pushes against the cell wall to keep the cell firm. When many cells in a leaf lose water and that pressure drops, the leaf goes limp. The vacuole also squeezes the chloroplasts into a thin layer along the inside edge of the cell, close to the wall where light and air arrive.
Light energy captured by chlorophyll powers the joining of carbon dioxide and water into sugar, which feeds the plant and builds its stems, roots and fruit. Oxygen is released as a by-product and leaves through the leaf’s pores.
In autumn, the cells start taking things apart. As leaves age, Rubisco is pulled out of the chloroplasts and broken down so the plant can reuse its nitrogen before the leaf drops.2PubMed Central. Mobilization of rubisco and stroma-localized fluorescent proteins of chloroplasts to the vacuole by an ATG gene-dependent autophagic process As Rubisco and chlorophyll are dismantled, the green fades and yellow and orange pigments that were there all along become visible.
Palisade and Spongy Layers
Many leaves have two mesophyll layers: the palisade mesophyll, made of tightly packed column-shaped cells, and the spongy mesophyll below it.3PubMed Central. Structural organization of the spongy mesophyll The palisade layer usually sits just under the upper surface, facing the sun. The spongy layer, long thought of as a random jumble of odd-shaped cells, turns out to have an organized structure when seen in 3D.4PubMed Central. Structural organization of the spongy mesophyll That kind of imaging can measure each cell’s size and the volume of air spaces between cells, both of which matter for how the leaf works.5PubMed Central. High-contrast three-dimensional imaging of the Arabidopsis leaf enables the analysis of cell dimensions in the epidermis and mesophyll
The two layers split the work, though by emphasis rather than strict separation. The palisade layer is the main light-catching zone, with chloroplasts concentrated close to the sunny surface. The spongy layer is built more for gas movement, since its loose arrangement leaves room for air to travel. It still holds chloroplasts and uses whatever light reaches it, so it is not just plumbing.
The column shape pays off. Leaves made of column-shaped palisade cells absorb more light, hold more chlorophyll and photosynthesize more per area of leaf.6PubMed Central. Palisade cell shape affects the light-induced chloroplast movements and leaf photosynthesis The columns let direct sunlight travel deeper into the leaf than scattered, cloudy-day light,7Plant, Cell & Environment. The functional significance of palisade tissue: penetration of directional versus diffuse light which helps spread light more evenly to chloroplasts throughout the leaf.8Plant, Cell & Environment. The functional significance of palisade tissue: penetration of directional versus diffuse light In sun-loving plants, light passes down through the columns and gaps so it reaches chloroplasts in the spongy layer too.9American Journal of Botany. CORRELATES OF LEAF OPTICAL PROPERTIES IN TROPICAL FOREST SUN AND EXTREME‐SHADE PLANTS The cost is that some light slips past the upper chloroplasts unused.10American Journal of Botany. CORRELATES OF LEAF OPTICAL PROPERTIES IN TROPICAL FOREST SUN AND EXTREME‐SHADE PLANTS
Leaf thickness follows the light. Leaves grown in strong sun tend to be thicker, with taller palisade cells. Shade leaves are thinner, because there’s less light worth passing deep inside. The same tree can have both, with sun leaves at the top and shade leaves inside the canopy.
How Mesophyll Cells Get Carbon Dioxide and Water
Carbon dioxide enters through the leaf’s pores and spreads through the air spaces between mesophyll cells. Those air paths are more twisted in the palisade layer than in the spongy layer, and more twisted sideways than straight up and down.11PubMed. Understanding airspace in leaves: 3D anatomy and directional tortuosity So the spongy layer works as the leaf’s main air corridor. Even so, moving through the air spaces is usually only a small hurdle for the gas.12PubMed. Resistances along the CO2 diffusion pathway inside leaves
The pores that let carbon dioxide in also let water vapor out, because wet mesophyll cell surfaces evaporate into the air spaces. On hot, dry days plants close their pores to save water, and photosynthesis slows.
The harder part is getting into the cell. Once at a cell, carbon dioxide must dissolve and cross the cell wall and several membranes to reach the chloroplast.13PubMed. Resistances along the CO2 diffusion pathway inside leaves It can only enter after dissolving in the thin film of water coating the inside of the wall, and gases move through water far more slowly than through air. Cell walls alone may account for a quarter to a half or more of this resistance.14PubMed. Resistances along the CO2 diffusion pathway inside leaves
That is why surface area matters so much. Each mesophyll cell touches the air only along part of its surface; the rest is pressed against neighboring cells. Chloroplasts lining the exposed part of the wall are best placed to receive carbon dioxide. Leaves that photosynthesize strongly press more chloroplast surface against the air spaces,15PubMed. Resistances along the CO2 diffusion pathway inside leaves and cell wall makeup and the chloroplast area facing the air spaces strongly limit how fast carbon dioxide gets in.16PubMed. Mesophyll conductance: the leaf corridors for photosynthesis Protein channels in cell membranes also help carbon dioxide cross into mesophyll cells, making Rubisco more effective.17PubMed. Carbon dioxide and water transport through plant aquaporins These channels also control water flow in and out of veins.18PubMed. Carbon dioxide and water transport through plant aquaporins
Water takes a different route. After leaving the veins, most of it moves through the mesophyll along cell walls rather than through the cells’ insides.19PubMed. The contributions of apoplastic, symplastic and gas phase pathways for water transport outside the bundle sheath in leaves The walls act as tiny channels, and cells far from any vein depend on this slow outward flow. That is why veins are spread closely throughout the leaf, so no mesophyll cell is far from a water supply. Vein layout and mesophyll structure adjust together to control how easily water flows through a leaf in different light.20Environmental and Experimental Botany. Functional consequences of light intensity on soybean leaf hydraulic conductance: Coordinated variations in leaf venation architecture and mesophyll structure
How Mesophyll Cells Protect Themselves From Too Much Sun
In strong light, chloroplasts move from the face of the cell to its side walls.21PubMed. Chloroplast avoidance movement reduces photodamage in plants That way they absorb less light, turning edge-on to the sun. Plants unable to make this move suffer faster damage, with bleached leaves and dead patches in strong light.22PubMed. Chloroplast avoidance movement reduces photodamage in plants The movement also spreads light damage more evenly through the depth of the leaf instead of concentrating it.23PubMed. Chloroplast movement provides photoprotection to plants by redistributing PSII damage within leaves
The trigger is blue light, which cells detect with special light-sensing proteins. Weak blue light draws chloroplasts toward the light, while strong blue light sends them away from it.
The effect can be large. In some crops like sorghum, midday sun is strong enough that chloroplasts move away within about 4 hours and stay there all afternoon, while photosynthesis dips.24Plant Production Science. Light stress-induced chloroplast movement and midday depression of photosynthesis in sorghum leaves In sun-grown climbing plants, chloroplasts sit on side walls most of the time, protecting them and letting light reach deeper layers.25PubMed Central. Palisade cell shape affects the light-induced chloroplast movements and leaf photosynthesis This is part of why many plants photosynthesize less at midday than mid-morning.
The move is reversible. When strong light fades, chloroplasts can slide back across the cell face to catch light again. In dim light they spread across the whole face to catch as much as possible, which is why shade plants can make the most of little light. The same cell can switch between collecting and avoiding light over hours as clouds pass or the sun moves. The cost is that, while chloroplasts are tucked at the edges, the leaf gives up some photosynthesis to avoid damage.
Mesophyll in Desert Plants, Grasses and Water Plants
CAM plants such as cacti and many succulents open their pores at night to save water. Opening pores in cool night air loses far less water than in daytime heat. Their leaves are usually thick, with large mesophyll cells and big vacuoles to store the acid made overnight.26PubMed Central. Are thick leaves, large mesophyll cells and small intercellular air spaces requisites for CAM? At night the plant takes in carbon dioxide and attaches it to a compound, forming an acid. The cells pump this acid into the vacuole, where it builds up until daylight releases the carbon for sugar-making.27New Phytologist. CAM photosynthesis: the acid test By day, with pores closed, the acid releases its carbon dioxide inside the leaf for Rubisco to use. Capture and sugar-making are separated in time. Even an ancient group of underwater plants stores acid this way.28New Phytologist. CAM photosynthesis: the acid test
This explains the thick, fleshy look. The vacuoles must hold enough acid for a whole day of sugar-making, so the cells need to be large. Large cells packed together make a thick, water-storing leaf, and thick tissue means less leaf surface per volume, which reduces water loss. The trade-off is slower growth than ordinary plants.
C4 plants like corn and sugarcane first trap carbon dioxide with a different enzyme, then pass it to Rubisco. Most arrange their leaves with a ring of cells around each vein and mesophyll outside it. Mesophyll cells grab carbon and hand it inward to the ring, concentrating carbon dioxide around Rubisco, which suits hot, sunny climates. In most C4 plants one cell type does the first capture step and a neighboring cell type does the Rubisco step, passing the carbon along as a small molecule. CAM separates the steps in time, while C4 separates them in space.
Some plants bend the rules. Certain plants in the goosefoot family do both steps inside one mesophyll cell, in separate parts of it.29PubMed Central. Species Having C4 Single-Cell-Type Photosynthesis in the Chenopodiaceae Family Evolved a Photosynthetic Phosphoenolpyruvate Carboxylase Like That of Kranz-Type C4 Species A submerged freshwater plant runs C4 photosynthesis without the usual ring-shaped leaf layout, and its mesophyll cells store lots of starch.30PubMed Central. Structural basis for C4 photosynthesis without Kranz anatomy in leaves of the submerged freshwater plant Ottelia alismoides
How Rising Carbon Dioxide Is Changing Leaves
Many plants, from trees to wheat, rice and potatoes, grow thicker leaves when carbon dioxide rises.31UW News. High CO2 levels cause plants to thicken their leaves, which could worsen climate change effects, researchers say Leaves can thicken by as much as a third, changing photosynthesis, gas exchange, cooling and sugar storage.32UW News. High CO2 levels cause plants to thicken their leaves, which could worsen climate change effects, researchers say In young wheat, extra carbon dioxide added on average one more mesophyll cell layer and larger air spaces, while the leaf skin barely changed.33PubMed Central. The Effects of Elevated CO2 Concentrations on Cell Division Rates, Growth Patterns, and Blade Anatomy in Young Wheat Plants Are Modulated by Factors Related to Leaf Position, Vernalization, and Genotype Why plants do this is still unknown.34UW News. High CO2 levels cause plants to thicken their leaves, which could worsen climate change effects, researchers say
A thicker leaf can mean extra layers of mesophyll cells, larger cells, or bigger air spaces, and these are different changes. Which one happens affects how gases move. A thicker leaf gives carbon dioxide a longer path through the air spaces before it reaches deeper cells, while more mesophyll also means more cell surface where it can dissolve and enter. The net effect on photosynthesis depends on how these opposite influences balance.
The response is common in ordinary C3 plants, and C3 photosynthesis accounts for about 95 percent of photosynthesis on Earth. So the change could affect most of the world’s plant life.