# Biology

By [claren1314](https://paragraph.com/@claren1314) · 2021-10-29

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Autumn [leaves](https://en.wikipedia.org/wiki/Leaves), yellow flowers, bananas, oranges and other yellow fruits all contain [carotenoids](https://en.wikipedia.org/wiki/Carotenoid), yellow and red organic pigments that are found in the [chloroplasts](https://en.wikipedia.org/wiki/Chloroplast) and [chromoplasts](https://en.wikipedia.org/wiki/Chromoplast) of plants and some other [photosynthetic](https://en.wikipedia.org/wiki/Photosynthetic) organisms like [algae](https://en.wikipedia.org/wiki/Algae), some bacteria and some fungi. They serve two key roles in plants and algae: they absorb light energy for use in [photosynthesis](https://en.wikipedia.org/wiki/Photosynthesis), and they protect the green [chlorophyll](https://en.wikipedia.org/wiki/Chlorophyll) from photodamage.[\[3\]](https://en.wikipedia.org/wiki/Yellow#cite_note-fasebj.org-3)

In late summer, as [daylight](https://en.wikipedia.org/wiki/Daylight) hours shorten and temperatures cool, the [veins](https://en.wikipedia.org/wiki/Vein) that carry fluids into and out of the leaf are gradually closed off. The water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. It is during this time that the chlorophyll begins to decrease. As the chlorophyll diminishes, the yellow and red carotenoids become more and more visible, creating the classic [autumn leaf color](https://en.wikipedia.org/wiki/Autumn_leaf_color).

Carotenoids are common in many living things; they give the characteristic color to carrots, maize, [daffodils](https://en.wikipedia.org/wiki/Daffodil), [rutabagas](https://en.wikipedia.org/wiki/Rutabaga), [buttercups](https://en.wikipedia.org/wiki/Buttercup) and bananas. They are responsible for the red of cooked [lobsters](https://en.wikipedia.org/wiki/Lobster), the pink of [flamingoes](https://en.wikipedia.org/wiki/Flamingoes) and [salmon](https://en.wikipedia.org/wiki/Salmon) and the yellow of [canaries](https://en.wikipedia.org/wiki/Domestic_canary) and [egg yolks](https://en.wikipedia.org/wiki/Egg_yolk).

[Xanthophylls](https://en.wikipedia.org/wiki/Xanthophylls) are the most common yellow [pigments](https://en.wikipedia.org/wiki/Pigment) that form one of two major divisions of the carotenoid group. The name is from Greek _xanthos_ (ξανθος, "yellow") + _phyllon_ (φύλλον, "leaf"). Xanthophylls are most commonly found in the leaves of green plants, but they also find their way into animals through the food they eat. For example, the yellow color of chicken egg yolks, fat, and skin comes from the feed the chickens consume. Chicken farmers understand this, and often add [xanthophylls](https://en.wikipedia.org/wiki/Xanthophyll), usually [lutein](https://en.wikipedia.org/wiki/Lutein), to make the egg yolks more yellow.

Bananas are green when they are picked because of the chlorophyll their skin contains. Once picked, they begin to ripen; [hormones](https://en.wikipedia.org/wiki/Hormones) in the bananas convert amino acids into ethylene gas, which stimulates the production of several [enzymes](https://en.wikipedia.org/wiki/Enzymes). These enzymes start to change the color, texture and flavor of the banana. The green chlorophyll supply is stopped and the yellow color of the carotenoids replaces it; eventually, as the enzymes continue their work, the cell walls break down and the bananas turn brown.

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*Originally published on [claren1314](https://paragraph.com/@claren1314/biology)*