Why do leaves turn yellow in winter?

In autumn, the green pigment slowly fades, and the red pigment and yellow pigment are exposed. All leaves contain green chlorophyll. Trees use chlorophyll to capture light energy and store it in the form of chemicals such as sugar with the help of other substances in leaves.

Besides chlorophyll, many leaves also contain other pigments, such as yellow, orange and red. Although these pigments can't carry out photosynthesis like chlorophyll, some of them can transfer the captured light energy to chlorophyll.

In spring and summer, the content of chlorophyll in leaves is much richer than other pigments, so the leaves show the green color of chlorophyll, while the colors of other pigments are invisible.

Extended data:

Chlorophyll is the main pigment of plant photosynthesis, and it is a kind of lipid-containing pigment family, which is located on thylakoid membrane. Chlorophyll absorbs most red light and purple light but reflects green light, so chlorophyll appears green and plays a central role in light absorption of photosynthesis.

Chlorophyll is a magnesium porphyrin compound, including chlorophyll a, b, c, d, f, protochlorophyll and bacterial chlorophyll. Chlorophyll is not very stable and will be decomposed by light, acid, alkali, oxygen and oxidant.

Under acidic conditions, chlorophyll molecules can easily lose magnesium in porphyrin ring and become pheophytin. Chlorophyll has many uses, such as hematopoiesis, providing vitamins, detoxification and disease resistance.

References:

Baidu encyclopedia-chlorophyll