Plastids are double membrane-bound organelles found in plant cells and algae, all derived from a common progenitor called the proplastid. They are classified based on their pigment content and storage function. Chromoplasts: contain carotenoid pigments (carotenes and xanthophylls); responsible for yellow, orange, red, and purple colours in flowers, fruits, and roots; lack chlorophyll; may arise from chloroplasts during fruit ripening. Chloroplasts: contain chlorophyll and carotenoids; site of photosynthesis; found in green parts of plants. Leucoplasts (colourless plastids): three main types based on storage: Amyloplasts (store starch), Aleuroplasts/Proteinoplasts (store proteins), Elaioplasts/Lipidoplasts (store oils/lipids). All plastid types can interconvert — for example, chloroplasts can develop into chromoplasts (as in ripening tomatoes or autumn leaves), and amyloplasts can develop into chloroplasts when potato tubers are exposed to light (they turn green).

Carotenoids are a large family of lipid-soluble pigments found in plants, algae, and many other organisms. They are divided into two main classes: Carotenes: pure hydrocarbons (contain only C and H). Alpha-carotene, beta-carotene (most important — precursor of Vitamin A), lycopene (red colour in tomatoes). Beta-carotene in chromoplasts gives carrots, mangoes, and papayas their orange colour. Xanthophylls: oxygenated carotenoids (contain O in addition to C and H). Lutein (most abundant xanthophyll in plants), zeaxanthin, violaxanthin, neoxanthin, fucoxanthin (in brown algae). Xanthophylls give yellow colours to marigolds, corn, egg yolk (chickens acquire lutein and zeaxanthin from plant feed). Carotenoids serve dual roles: in chromoplasts, they are structural pigments for attraction of pollinators/seed dispersers; in chloroplasts, they are accessory photosynthetic pigments that absorb wavelengths of light not efficiently absorbed by chlorophyll (blue and blue-green), as well as protecting against photodamage (carotenoids quench harmful singlet oxygen).

Chromoplasts develop in various ways depending on the plant tissue and conditions. In fruits: chloroplasts are initially present in green unripe fruits (allowing some photosynthesis to supply developing fruit). As fruits ripen, chloroplasts are converted into chromoplasts through a developmental transition involving chlorophyll degradation, dissolution of the thylakoid membrane system, and accumulation of carotenoid-containing structures. In tomatoes ripening, the dramatic colour change from green to red involves this chloroplast-to-chromoplast conversion, with lycopene (a carotene) being the primary accumulating pigment. In flowers: chromoplasts develop directly from proplastids or from chloroplasts in petal cells, providing the yellow, orange, and red colours that attract pollinators. In roots: carrots accumulate beta-carotene in chromoplasts in root storage tissue. The carotenoids in chromoplasts are packaged in various structures including plastoglobuli (lipid droplets), membranous whorls, and crystalline arrays, depending on the specific pigments and the plant species.

Carotenoids have substantial economic and human health significance. Provitamin A activity: Alpha- and beta-carotene and beta-cryptoxanthin are provitamin A carotenoids — they can be converted to vitamin A (retinol) in the human intestine. Vitamin A deficiency is the leading cause of preventable childhood blindness worldwide, particularly in developing countries. Golden Rice, a genetically engineered rice variety expressing beta-carotene in the endosperm (chromoplast formation induced in otherwise achlorophyllous cells), was developed to address this deficiency. Macular health: Lutein and zeaxanthin accumulate specifically in the macula of the human retina (obtained entirely from dietary plant sources since humans cannot synthesise carotenoids de novo) where they act as natural UV light filters, and their adequate dietary intake is associated with reduced risk of age-related macular degeneration. Antioxidant activity: Carotenoids are potent antioxidants, quenching reactive oxygen species; lycopene consumption (primarily from tomatoes and tomato products) has been associated with reduced risk of certain cancers and cardiovascular disease. Food colouring: Beta-carotene (E160a) and other carotenoids are widely used as natural food colorants.