Plastid (2): Chromoplast Function: Antioxidant, Attracting Animal

Anec  > Biology > Organelle

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.

Frequently Asked Questions

Are chromoplasts the only place for pigments storage in plants?

The central vacuole is also responsible for plant vibrant colors. The common water-soluble pigments found in plant vacuoles are anthocyanins whose precursor is glucose. Their color is determined by pH. When cell sap is acidic, anthocyanins are red. They are purple in neutral environments and turn blue in alkaline conditions. The color of fruits and flowers is determined by anthocyanins and chromoplasts. If they are yellow or orange, it's likely due to the yellow carotenoids in the chromoplasts, such as in mangoes and pumpkins. If they are purple or blue, maybe there are some anthocyanins in the vacuoles, such as in blueberries. Both pigments can make plants appear red, as seen in tomatoes and autumn maple leaves. Additionally, there are other pigments in plant central vacuoles. Betalains in red beets give the roots a magenta color. Betalains are so abundant in dragon fruit that eating only one can turn your urine red, leading someone to think they have kidney disease mistakenly.

Can chromoplasts convert back into chloroplasts?

Chromoplasts can revert to chloroplasts under certain conditions. If the weather is warm, sunlight and water is adequate, and soil is fertilized with nitrogen, the orange may turn green again because these factors favor chlorophyll formation. Orange carrots exposed to sufficient sunlight will turn green. However, this doesn’t always happen, as some plants have irreversible chromoplasts, such as fully ripe tomatoes and peppers.

Anec  > Biology > Organelle

More