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Lower Epidermis of a Leaf: Function & Concept

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  • 0:05 Epidermal Cells
  • 0:47 Cuticle
  • 1:20 Guard Cells and Stoma
  • 2:52 Stomata Function
  • 4:09 Trichomes
  • 4:53 Lesson Summary
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Lesson Transcript
Instructor: Jennifer Szymanski

Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.

As the primary means for capturing the sunlight necessary for photosynthesis, the upper surface of leaves is crucial for plant survival. But what about the underside? In this lesson, we'll examine the leaf's lower epidermis structure.

Epidermal Cells

Humans and animals have specialized skin that covers and protects their bodies. In plants, we call this specialized skin the epidermis. The epidermal cells that make up this skin are transparent. As most epidermal cells lack chloroplasts, they can't perform photosynthesis, or the use of sunlight to convert carbon dioxide and water into glucose and oxygen.

The life of an epidermal cell begins in the protoderm, one of the three types of primary meristematic, or undifferentiated, tissue that plants possess. The epidermis also has other special features that help the plant survive in its environment. Let's look at three features common to the lower leaf epidermis.

Cuticle

The cuticle covers both the upper and lower parts of the leaf epidermis, made mostly of lipids and waxes. The cuticle tends to be thicker on the top of the leaf, since that's the part that's most exposed to the sun. The cuticle protects the leaf's photosynthesizing cells from danger, such as bacteria, fungal spores, viruses and other disease-causing dangers. The cuticle also protects the leaf's photosynthesizing cells from environmental toxins, excessive ultraviolet radiation and too much water loss.

Guard Cells and Stoma

The most important structure on a leaf's lower epidermis is the mouth-shaped opening called the stoma. There are many stomata on each leaf - up to one million per square centimeter, and they have two main functions: to regulate gas exchange and to help prevent water loss.

More than 90% of the water taken in by the roots of a plant evaporates from the leaves into the air as water vapor. Since water molecules stick together due to their molecular structure, those leaving water molecules create a continuous flow of water through the plant. Most of the water that exits the plant does so through the stomata. If a plant faces dry conditions, whether through a longer period of drought or just a hot afternoon, the stomata will partially close to prevent excessive evaporation, thanks to the actions of specialized guard cells.

Each stoma is flanked by guard cells. Kidney-shaped guard cells are unusual epidermal cells because they have chloroplasts. In wet conditions, the guard cells take in potassium ions, a high concentration of which creates a high osmotic potential. Osmotic potential means that water moves from an area of high concentration outside the guard cells to an area of lower concentration inside the guard cells. When this happens, the guard cells become turgid, or swollen, and the stomata open. When the potassium ions leave the guard cells, the reverse happens and the stomata close.

Stomata Function

Stomata are crucial in regulating gas exchange, or releasing oxygen from the plant, as well as admitting carbon dioxide. Remember - the plant must keep and use some oxygen for cellular respiration. Once the guard cells are open, the cells within the leaf responsible for photosynthesis are in contact with the surrounding air.

Gas exchange takes place through diffusion, where both oxygen and carbon dioxide move from areas of higher concentration to lower concentration, with oxygen mostly moving out and carbon dioxide mostly moving in. Stomata usually open in the morning and close overnight. This makes sense because the first part of photosynthesis does not occur in the absence of sunlight, and, therefore, neither water loss nor gas exchange happens very quickly.

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