Chloroplasts are found primarily in the mesophyll cells of leaves, the soft interior tissue sandwiched between the upper and lower surfaces. But leaves are far from the only location. Chloroplasts also appear in stems, unripe fruits, flower sepals, guard cells on leaf surfaces, and even in roots under specific conditions. Where they show up, how many a cell contains, and how they behave all depend on the tissue type, the species, and the amount of light available.
The Leaf Interior Is Home Base
If you had to point to one place in a plant and say “chloroplasts live here,” the leaf mesophyll would be it. This tissue is divided into two zones. The palisade layer sits just below the upper surface and consists of tightly packed, column-shaped cells oriented vertically. Below that lies the spongy layer, with irregularly shaped cells and large air pockets between them. Both zones are loaded with chloroplasts, but the palisade layer tends to hold a higher concentration. One study measuring chloroplast water content in maple leaves found that the volume fraction of chloroplast water was roughly one-third larger in the palisade region than in the spongy region across both sun-adapted and shade-adapted leaves.1Plant, Cell & Environment. The spatial distribution of chloroplast water in Acer platanoides sun and shade leaves That makes sense: the palisade cells are the first to intercept sunlight passing through the upper epidermis, so packing them with photosynthetic machinery pays the biggest dividend.
Shade leaves, interestingly, contain a greater total volume of chloroplast material than sun leaves of the same species. That same study found the volume fraction of chloroplast water was “much larger” in shade leaves overall. The plant compensates for dimmer light by investing more heavily in the light-harvesting equipment, which is a pattern that repeats in other tissues beyond the leaf.
Guard Cells on the Leaf Surface
Most epidermal cells on a leaf’s surface are transparent and lack chloroplasts. The exception is guard cells, the paired kidney-shaped cells that open and close stomatal pores to regulate gas exchange. Guard cells typically contain several to tens of chloroplasts per cell.2Frontiers in Plant Science. Arabidopsis thaliana Leaf Epidermal Guard Cells: A Model for Studying Chloroplast Proliferation and Partitioning in Plants That number is far smaller than what a mesophyll cell holds, but the chloroplasts are functionally relevant. They contribute to the energy balance guard cells need for opening and closing, and researchers have used guard-cell chloroplast counts as a convenient marker for estimating ploidy level or identifying hybrid species.
Aquatic plants break the typical rule that epidermal cells lack chloroplasts. When the amphibious plant Rorippa aquatica is submerged, its epidermal cells develop chloroplasts in response to the underwater environment. This adaptation likely boosts photosynthetic efficiency when light has to pass through water before reaching the leaf.3PubMed. Chloroplast differentiation in epidermal cells: an environmental response supporting submerged photosynthesis in Rorippa aquatica It is one of several examples where the standard “chloroplasts are in the mesophyll, not the epidermis” textbook statement needs a qualifier.
Stems, Bark, and Even Wood
Green stems are an obvious clue that chloroplasts extend beyond leaves. Herbaceous stems, young twigs, and the green bark of many tree species all contain chloroplast-bearing cells. In the flowering ash tree (Fraxinus ornus), researchers found chlorophyll distributed through the bark’s parenchyma cells in the phelloderm, cortex, and phloem. Chlorophyll-containing cells were also detected along xylem rays deep inside the wood and around the pith, with chlorophyll concentration increasing on a gradient toward the bark surface.4PubMed. Structure and function of bark and wood chloroplasts in a drought-tolerant tree (Fraxinus ornus L.)
The light environment inside a stem is radically different from a leaf surface. Very few photons make it past the outer bark into the inner wood. Yet the chloroplasts found deep inside were still photosynthetically active and capable of generating light-driven electron transport. These stem chloroplasts showed features typical of shade adaptation: larger internal membrane stacks and shifted ratios of photosystem components compared to leaf chloroplasts. The whole photosynthetic apparatus in the stem appears tuned to wring energy out of the tiny amount of light that filters inward.
Scots pine provides another case. Its bark tissue retains functional chloroplasts even through winter, when needle photosynthesis shuts down due to cold stress. The photochemical efficiency of the bark chloroplasts remained well preserved during winter months, suggesting bark photosynthesis could play a role in the tree’s recovery from winter dormancy.5PubMed. Characterization of the photosynthetic apparatus in cortical bark chlorenchyma of Scots pine So stem and bark chloroplasts are not just vestigial; they do real photosynthetic work under conditions where leaves cannot.
Green Fruits and the Shift to Ripening
Unripe tomatoes, green peppers, and young apples are green because their cells are packed with chloroplasts. These fruit chloroplasts photosynthesize and contribute some energy to the developing fruit. As the fruit ripens, though, those chloroplasts undergo a dramatic transformation: they convert into chromoplasts, a different type of plastid that accumulates carotenoid pigments instead of chlorophyll. That is why a tomato goes from green to red.
Studies using spectral confocal microscopy on intact tomato tissue have captured this transition in detail, revealing intermediate plastids that contain both carotenoids and chlorophyll at the same time before completing the shift.6PubMed Central. Chloroplast to chromoplast transition in tomato fruit: spectral confocal microscopy analyses of carotenoids and chlorophylls in isolated plastids and time-lapse recording on intact live tissue The process is not just a passive pigment swap. It is actively controlled by a protein-degradation pathway. A plastid enzyme called SP1 acts as a trigger: boosting its activity accelerates ripening and the color change, while knocking it down delays ripening, fruit softening, and the associated metabolic shifts.7Nature Plants. The chloroplast-associated protein degradation pathway controls chromoplast development and fruit ripening in tomato Chloroplasts in fruit are temporary residents, essentially programmed to self-destruct and rebuild into something new once the fruit is ready for seed dispersal.
Roots Can Develop Chloroplasts Too
Roots normally grow in darkness and do not contain chloroplasts. Instead, their cells hold proplastids or amyloplasts, undifferentiated or starch-storing plastids that lack photosynthetic membranes. But roots are not incapable of making chloroplasts. When Arabidopsis roots are exposed to light, they turn green and develop functional chloroplasts. What makes the process especially interesting is that detached roots green up faster than roots still connected to the shoot, suggesting that the shoot actively suppresses chloroplast development in roots through signaling involving the hormones auxin and cytokinin.8PubMed Central. Regulation of Root Greening by Light and Auxin/Cytokinin Signaling in Arabidopsis
This makes practical sense. In a whole plant, investing in photosynthetic equipment underground where no light reaches would be a waste of resources. So the shoot sends hormonal signals that keep root plastids in a non-photosynthetic state. Remove that signal (by cutting the root free) and expose it to light, and the root cells readily build chloroplasts on their own. The genetic and biochemical machinery is present everywhere in the plant; it is just selectively switched off in tissues where photosynthesis would not be productive.
Seeds With Built-In Chloroplasts
Some seeds contain green embryos, and these are not merely vestiges of earlier development. The embryos of certain flowering plant groups maintain chlorophyll and appear to actively photosynthesize during seed development. These “chloroembryos,” as they have been termed, retain their chlorophyllous state until the embryo matures, and the photosynthetic activity likely plays a functional role in powering seed development.9Journal of Plant Physiology. Chloroembryos: A unique photosynthesis system Not all plants do this, and the trait varies across taxonomic groups, but it challenges the simple notion that chloroplasts are exclusively a feature of mature, above-ground organs.
Chloroplasts Move Around Inside the Cell
Chloroplasts are not bolted in place. Within a single cell, they can physically relocate in response to light intensity, a behavior called photorelocation movement. Under dim light, chloroplasts spread themselves across the cell face perpendicular to the light source, maximizing the surface area that catches photons. Under intense light, they shuffle to the cell’s side walls, minimizing their exposure and reducing the risk of photodamage.10PubMed Central. Actin-based mechanisms for light-dependent intracellular positioning of nuclei and chloroplasts in Arabidopsis In darkness, they settle to the bottom of the cell.
The movement is driven by short actin filaments that assemble along the chloroplast’s outer edge on the side facing the cell membrane. These “cp-actin filaments” form, push, and turn over rapidly, functioning like tiny rails or tracks.11PubMed Central. Short actin-based mechanism for light-directed chloroplast movement in Arabidopsis A recently identified plant-specific protein called CHUP1 regulates the polymerization of these actin structures and is essential for normal chloroplast positioning.12PubMed Central. CHLOROPLAST UNUSUAL POSITIONING 1 is a plant-specific actin polymerization factor regulating chloroplast movement
The light sensors that control this process are blue-light receptors called phototropins. One receptor primarily drives the accumulation response under weak light, while the other mediates both accumulation and the avoidance response under strong light.13PubMed Central. Phototropin 1 Mediates High-Intensity Blue Light-Induced Chloroplast Accumulation Response in a Root Phototropism 2-Dependent Manner in Arabidopsis phot2 Mutant Plants Even UV-B light can trigger chloroplast repositioning, though that response partly depends on the same phototropin pathway rather than working through the UV-specific receptor alone.14Frontiers in Plant Science. UV-B Induces Chloroplast Movements in a Phototropin-Dependent Manner These movements happen within minutes and are visible under a microscope as a measurable change in how much light passes through a leaf.
The Special Case of C4 Plants
Plants like maize and sugarcane use a modified photosynthetic pathway that splits the work between two distinct cell types: mesophyll cells and bundle sheath cells (the cells wrapping the leaf veins). Both contain chloroplasts, but the arrangement differs strikingly. In maize and sugarcane, chloroplasts occupy roughly 30 to 50 percent of mesophyll cell volume, while bundle sheath cells pack chloroplasts even more densely, filling about 60 to 70 percent of the cell volume. The mesophyll cells are lobed where chloroplasts sit, creating shapes that increase air-space exposure, while bundle sheath cells are more cylindrical with chloroplasts arranged in a ring around the cell’s outer edge.15PubMed. Exploring 3D leaf anatomical traits for C4 photosynthesis: chloroplast and plasmodesmata pit field size in maize and sugarcane This precise spatial organization is critical to the two-step COâ‚‚-concentrating mechanism that makes C4 photosynthesis more efficient in hot, bright conditions.
Plants That Have Lost Their Chloroplasts
Not every plant still relies on photosynthesis. Parasitic species like dodder (Cuscuta) obtain their energy by tapping into the vascular tissue of host plants. Over evolutionary time, this lifestyle has led to a dramatic loss of photosynthetic genes. In Cuscuta australis, 38 photosynthesis-related genes found in the model plant Arabidopsis are missing entirely, and the plastid genome has contracted from the roughly 104 genes typical of a close relative to just 81. All genes encoding the NADH dehydrogenase complex, which supports electron cycling around photosystem I under stress, have been lost.16PubMed Central. Large-scale gene losses underlie the genome evolution of parasitic plant Cuscuta australis Dodder still has plastids and retains a limited photosynthetic capability, but it is a far cry from what a self-sufficient green plant maintains. The NADH dehydrogenase genes are considered the first to go when a lineage begins evolving toward parasitism.
Variegated plants offer a less extreme case. In species and mutants that produce green-and-white patterned leaves, the green sectors contain cells with normal chloroplasts while the white sectors contain cells whose plastids are stuck at some earlier stage of development, unable to complete chloroplast biogenesis. Sometimes the green and white patches have different genetic makeups (as in chimeras), but in other cases the entire plant carries the same mutant genotype, and whether a cell ends up green or white depends on a stochastic developmental outcome.
When Chloroplasts Break Down
Chloroplasts are not permanent structures. As a leaf ages and enters senescence, its chloroplasts progressively dismantle. The internal membrane system (the thylakoids) becomes increasingly disorganized, lipid-containing droplets called plastoglobules grow larger and more numerous, and eventually the chloroplast degrades into a gerontoplast, characterized by nearly complete loss of thylakoid membranes and perforation of the outer envelope. Chlorophyll is broken down and valuable nutrients, especially nitrogen, are reclaimed and shuttled to younger tissues or developing seeds.
Acute stress can trigger similar degradation outside of normal aging. Under drought conditions, autophagy and dedicated plastid-degradation pathways selectively target severely damaged chloroplasts and remove them to maintain cellular function.17PubMed Central. Chloroplast Responses to Drought: Integrative Mechanisms and Mitigation Strategies The plant essentially triages: salvageable chloroplasts get repaired, and irreparable ones get recycled. This quality-control system keeps the cell from being overwhelmed by dysfunctional organelles that would otherwise leak damaging reactive molecules.
How Chloroplasts Get to New Cells
Because chloroplasts cannot be built from scratch by the cell, every chloroplast in a plant traces its lineage back to proplastids inherited in the egg cell. New chloroplasts arise only by division of existing ones. In algae, chloroplast division is tightly synchronized with cell division, so each daughter cell gets its share. Land plants have relaxed that synchronization: a typical leaf cell contains dozens of chloroplasts that divide independently of one another and independently of when the cell itself divides. The cell just needs to end up with enough in each daughter cell after splitting, and the somewhat loose coordination is generally sufficient.
The transition from proplastid to chloroplast requires light. In angiosperms grown in complete darkness, proplastids develop into etioplasts, which contain a distinctive lattice-like membrane structure but no chlorophyll. Only upon exposure to light do etioplasts convert into functional chloroplasts with organized thylakoid membranes and active pigment synthesis.18PubMed Central. A search for factors influencing etioplast-chloroplast transition This is why bean sprouts grown in a dark cupboard are pale yellow; their plastids are etioplasts waiting for the light signal to finish developing. Move them to a windowsill and they green up within hours as the transition kicks in.