The Real Reason Leaves Change Color In the Fall

Chọn chế độ học:

3000 Oxford Words4000 IELTS Words5000 Oxford Words3000 Common Words1000 TOEIC Words5000 TOEFL Words

Phụ đề (29)

0:01You've probably heard that leaves of deciduous trees change color in autumn because they

0:05lose their green chlorophyll molecules, allowing the underlying yellow-orange pigments to shine

0:09through. However, this doesn’t explain why the leaves change color in the first place.

0:14Deciduous trees drop their leaves each year to avoid the high cost of winterizing them,

0:19but this seasonal shedding would be too wasteful to be worth doing if it meant/far too wasteful

0:22if the trees lost all of the valuable nutrients they laboriously extracted from the soil to

0:26build their leaves earlier in the year.

0:28So each fall, deciduous trees recycle their leaves before dropping them. That means taking

0:33apart cells and photosynthetic apparatus from the inside out in order to recoup their nitrogen

0:37and phosphorus and store it in twigs and branches until next spring.

0:41This is actually really tricky, because as the disassembly starts, the chlorophyll molecules

0:46that absorb sunlight for photosynthesis are still absorbing the sun’s energy. But with

0:50no photosynthesis happening, they end up passing the unused energy along to oxygen molecules,

0:55which become dangerously reactive when energized. These molecules wreak havoc, damaging the

1:00parts of the leaf that are in the process of recovering and transporting nutrients back

1:04to the rest of the tree.

1:06To keep this destruction to a minimum, leaves break down their chlorophyll into less dangerous

1:10molecules that are typically transparent, though sometimes yellow. Either way, with

1:14the bright green molecules gone, yellow and orange pigments that were there all along

1:18(helping with photosynthesis), are no longer overshadowed, and ta-da! Yellow and orange

1:22leaves.

1:23Some trees take an extra autumnal precaution against chlorophyll-induced-destruction: as

1:28the leaf dismantling starts, they build new, special pigments to shade chlorophyll from

1:32sunlight until it can be broken down. These new pigments tend to be red or purple in color,

1:37so trees that use them have red leaves in the fall – sometimes insanely bright red.

1:42In the end, these exquisite gold and russet displays help deciduous trees recover as much

1:47as 50% of the nitrogen and phosphorus from their old leaves to help grow fresh new green

1:52ones in the spring. They are perhaps the world’s prettiest

2:27recycling plants.