Pigment molecules are located inside specific cellular structures, and their exact position depends on the organism. In plants, the primary pigments sit within the thylakoid membranes of chloroplasts. In animals, pigments are housed in specialized organelles like melanosomes (for melanin) or bound within proteins like hemoglobin. Each location is tightly linked to the pigment’s function, whether that’s capturing light, coloring tissue, or carrying oxygen.
Chlorophyll in Plant Chloroplasts
Chlorophyll, the green pigment responsible for photosynthesis, is embedded in the thylakoid membrane inside chloroplasts. Chloroplasts themselves have a double outer membrane, but it’s the third, inner membrane system, the thylakoids, where the action happens. These thylakoids form long, folded sheets within the organelle, and chlorophyll molecules sit within protein complexes anchored in that membrane. The fluid-filled space surrounding the thylakoids is called the stroma, which handles a different stage of photosynthesis but contains no pigment.
The positioning is precise. Within light-harvesting complexes, each chlorophyll molecule is locked into a fixed orientation by the surrounding protein scaffold. X-ray crystallography shows that pigment molecules in these complexes are spaced just angstroms apart, close enough to pass energy from one molecule to the next like a relay. When light hits a chlorophyll molecule, it energizes electrons that are immediately passed into an electron transport chain running through the same thylakoid membrane. Chlorophyll a is the primary pigment, but several other types of chlorophyll plus red, brown, and blue accessory pigments also reside in these membranes, broadening the range of light wavelengths the plant can use.
Carotenoids and Other Membrane-Bound Pigments
Carotenoids, the yellow-orange pigments in plants, are also embedded in thylakoid membranes alongside chlorophyll. They serve a dual role: capturing light energy that chlorophyll misses and protecting the cell from damage caused by excess light. Under high-light conditions, some organisms increase carotenoid production specifically for this protective function.
Anthocyanins in the Vacuole
Not all plant pigments live in membranes. Anthocyanins, the water-soluble pigments responsible for deep red, purple, and blue colors in flowers, fruits, and leaves, are stored inside vacuoles. They’re synthesized on the surface of the endoplasmic reticulum in the cytoplasm, then transported into the vacuolar lumen. The vacuole’s acidic environment is essential: the low pH intensifies their color and prevents them from being broken down by oxidation. Inside the vacuole, anthocyanins either float freely in solution or cluster into dense bodies called anthocyanin vacuolar inclusions.
Melanin in Human Skin Cells
In human skin, pigment molecules are made and stored in organelles called melanosomes, which are found inside melanocytes. Melanocytes sit in the basal layer of the epidermis, the deepest layer of the outer skin. Each melanocyte forms a functional unit with 30 to 40 surrounding keratinocytes, the cells that make up most of the skin’s surface.
Melanosomes go through four distinct maturation stages. Stage I melanosomes are small, round vesicles with no pigment. At stage II, an internal protein scaffold forms, creating a fibrillar matrix that will hold melanin, but pigment production still hasn’t started. Melanin synthesis begins at stage III, as pigment is deposited onto those protein fibrils. By stage IV, melanin completely fills the organelle, and the melanosome is ready for transport. The process depends on tightly controlled internal chemistry: membrane proteins regulate the pH and ion balance inside the melanosome, and copper must be delivered to activate the enzyme that drives melanin synthesis.
Once fully loaded with melanin, melanosomes are shuttled along the cell’s internal skeleton from melanocytes into surrounding keratinocytes. The melanin granules accumulate above each keratinocyte’s nucleus, forming a protective cap that shields the DNA from ultraviolet radiation. As skin cells mature and move toward the surface, they carry their melanin with them and eventually shed.
Melanin in Hair and Eyes
Hair gets its color from melanin deposited into the hair shaft during growth. Melanocytes in the hair follicle bulb, specifically in a region called the matrix just above the dermal papilla, are the only ones that actively produce pigment. These melanocytes transfer melanin granules into immature keratinocytes that will become the hair’s cortex and medulla. Unlike skin, where melanin is mostly broken down as cells mature, melanin in hair cortex cells stays largely intact, which is why hair maintains a consistent color along its length.
In the eye, pigment is concentrated in two places. The retinal pigment epithelium (RPE) is a single layer of hexagonal cells sitting between the light-sensing photoreceptors and the blood supply at the back of the eye. These cells are packed with melanin granules that absorb stray light of various wavelengths, reducing glare and protecting the retina from oxidative damage caused by ultraviolet exposure. The iris also contains melanocytes whose pigment density determines eye color.
Hemoglobin in Red Blood Cells
The red color of blood comes from heme, a pigment molecule embedded inside hemoglobin proteins within red blood cells. Each hemoglobin molecule is a tetramer, made of four protein subunits (two alpha, two beta), and each subunit contains one heme group tucked into a binding pocket formed by specific protein folds. That means every hemoglobin molecule carries four pigment molecules.
Each heme consists of a porphyrin ring, a flat, ring-shaped organic structure, with an iron atom held at its center by four nitrogen atoms. The iron is also anchored to the protein by a histidine amino acid on one side, while the other side remains open as the binding site for oxygen. This arrangement is what lets hemoglobin pick up oxygen in the lungs and release it in tissues. Since mature red blood cells are essentially sacs filled with hemoglobin (around 270 million molecules per cell), the pigment is distributed throughout the entire cytoplasm rather than confined to a specific organelle.
Chromatophores in Cephalopods
Squid and octopuses can change color in milliseconds, and they do this using specialized cells called chromatophores. Inside each chromatophore, pigment granules are held within an elastic sac called the cytoelastic sacculus. Radial muscle fibers radiate outward from this sac. When the muscles contract, they stretch the sacculus into a thin, flat disc, spreading the pigment across a wide area and making the color visible. When the muscles relax, the elastic energy stored in the stretched sacculus snaps it back into a small sphere, concentrating the pigment into a tiny, nearly invisible point.
This is a fundamentally different system from anything in plants or mammals. The pigment’s location doesn’t change at the molecular level (it stays inside the sacculus), but its physical distribution within the cell shifts dramatically, creating the appearance of rapid color change.
Pigments in Photosynthetic Bacteria
Photosynthetic bacteria handle pigment storage differently from plants because they lack chloroplasts. In green sulfur and green filamentous bacteria, the main light-harvesting pigments (bacteriochlorophylls) are housed in structures called chlorosomes: oblong bodies attached to the inner surface of the cell’s plasma membrane. Inside chlorosomes, bacteriochlorophyll molecules self-assemble into tightly packed aggregates arranged in lamellar (layered) sheets, with spacing of 2 to 3 nanometers between layers. In some species, these aggregates form concentric nanotubes with pigments arranged in helical spirals.
The chlorosome connects to the plasma membrane through a protein baseplate, which funnels captured light energy into the membrane-bound reaction centers where photosynthesis occurs. This setup is remarkably efficient: the dense pigment packing allows these bacteria to photosynthesize in extremely low-light environments, like the deep layers of lakes and microbial mats.