Question Video: Recalling the Primary Function of the Waxy Cuticles | Nagwa Question Video: Recalling the Primary Function of the Waxy Cuticles | Nagwa

Question Video: Recalling the Primary Function of the Waxy Cuticles Biology • Second Year of Secondary School

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In plant leaves, what is the primary function of waxy cuticles?

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Video Transcript

In plant leaves, what is the primary function of waxy cuticles? (A) They prevent the loss of minerals by diffusion from the plant. (B) They capture the sunlight required for photosynthesis. (C) They provide a surface area for reactions to take place. Or (D) they prevent excessive water loss by evaporation from the leaf.

To answer this question, let’s review the key facts about the structure of the leaf and narrow down our options as we do so. The leaf is specially adapted for its role, which is to carry out photosynthesis. Photosynthesis is the process by which plants make their own food, and it occurs in the chloroplasts. Here, carbon dioxide and water are converted into sugars, such as glucose, and oxygen in the presence of sunlight. The chloroplasts contain a green pigment called chlorophyll, which absorbs the sunlight energy. This light energy is then converted into chemical energy, which is stored in the glucose molecules produced.

Of course, it is not just photosynthesis that takes place inside the cells. There are also other reactions, such as respiration, production of enzymes, and hormone secretion. These reactions occur in the cytoplasm and inside other organelles of the cell. Now we have found out about where the reactions take place, we can rule out option (C), as it is the organelles inside the cell that provide a surface area for reactions to take place.

We have covered the energy, but what about the substrates that go into the reaction at the beginning? Carbon dioxide enters the leaf through small pores, mainly in the bottom of the leaf, called stomata. This is the plural, stoma if it is just one. The carbon dioxide diffuses into the air spaces of the spongy mesophyll layer and then into the photosynthetic cells. The water is brought up from the soil by the xylem vessels. Mineral ions are carried with the water to be used to make other molecules, such as amino acids, from the carbohydrates produced by the plant. Water evaporates from the leaf through the open stomata. So, at night, the stomata shut to try to reduce water loss.

If too much water is lost, not only will photosynthesis cease, but the plant will wilt and eventually die. So the leaf has another adaptation to reduce water loss: the waxy cuticle, so called because it is made out of a water-repellent substance called cutin. This cuticle is secreted by the upper epidermis, shown here. It is on the upper epidermis because this is the side that faces the Sun so is more likely to get hot. Not all water loss can be controlled by just closing the stomata. If the temperature or wind gets too high, then plants start to lose water through the upper epidermis. The waxy cuticle reduces this water loss as much as possible.

Both the upper epidermis and waxy cuticle are transparent and colorless to allow the sunlight through to the palisade cells beneath, which are the main photosynthesizing layer. If these top layers are colorless and transparent, it means that they cannot contain the green pigment chlorophyll and so cannot absorb sunlight energy. Therefore, we can rule out option (B).

Now we have looked back at the structure of the leaf and how it is specialized for photosynthesis, let’s return to our question. We’ve already ruled out options (B) and (C). We’ve also learned that waxy cuticle is a water-resistant layer so can reduce water loss from the surface of the leaf. Therefore, the correct answer is (D). In plant leaves, the primary function of waxy cuticles is that they prevent excessive water loss by evaporation from the leaf.

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