Carotenoids are a type of pigments in chromoplasts and chloroplasts of higher plants. Ripe fruits and flowers are the best material for observing them. Aging leaves turn yellow in autumn because chlorophyll is degraded and the hidden carotenoids are visible now.
Colored chromoplasts: attracting animals, antioxidants
Their main task is to synthesis and store the fat-soluble carotenoids. They are distributed in flowers, fruits, leaves, and some plants roots, giving vibrant colors to these parts. For example, red tomatoes, orange carrots, and yellow corn contain different types of carotenoids. One theory suggests that the bright chromoplasts in flowering plants attract insects and birds for pollination and seed dispersal. These pigments easily accumulate in the food chain. If you consume a lot of orange or yellow vegetables and fruits like carrots, oranges, and pumpkins during a long period, the skin will turn yellow, as a great deal of lipophilic carotenoids accumulate in subcutaneous fat. Carotenoids from algae (which are not produced in chromoplasts) are also the reason for the red or orange coloration of flamingos, salmon, and lobsters.
Since plants can’t seek shelter like animals to avoid high-energy photons in sunlight, they have evolved pigments that absorb harmful radiation. Carotenoids are accessory pigments in light energy transfer. As they pass through hundreds of chlorophylls and carotenoids, photons with wavelengths of 400-700 nanometers (visible light) degrade to 700 or 680 nanometers at reaction center where they are absorbed by the photosystem precisely.
During photosynthesis, the easily leaked high-energy electrons result in reactive oxygen species, which steal electrons from everything around them and destroy biological functions. They are neutralized by these pigments to reduce oxidative stress and cellular damage. Additionally, humans and animals that consume vegetables and fruits rich in these pigments, such as carrots, tomatoes, and bell peppers, gain antioxidant benefits. Compared to water-soluble vitamin C, fat-soluble carotenoids have a long half-life ranging from several days to weeks. Chronic diseases, inflammation, and skin health can all be improved. Carotenoids are also precursors of vitamin A.
Morphology and differentiation of chromoplast
They are highly heterogeneous organelles and have various forms in different plants. The membrane system in matrix is reticular, lamellar, and tubular. There are also crystalline pigments. Differentiation from chloroplasts is the default pathway for chromoplasts, but it is rare to develop from proplastids directly. Chromoplasts in tomatoes have been widely studied. Tomatoes change from white to green and then to red when fully mature. In this process, proplastids differentiate into non-pigmented leucoplasts, which then differentiate into green chloroplasts. During maturation, the thylakoid membranes disintegrate, and chlorophyll begins to disappear. Plastoglobules start to appear in the plastid matrix and gradually enlarge. The first synthesized pigment is phytoene. Then it transforms into lycopene, the intermediate in other carotenoid synthesis pathways. In fully ripe tomatoes, the thylakoids and grana completely disappear, and their chromoplasts are filled with plastoglobules and needle-like carotenoid crystals. The large tomato cells in fleshy part may contain up to 2000 chromoplasts.