Leaf Functions

Leaves are the primary site of photosynthesis in most plants. Mesophyll cells in leaves use light energy, carbon dioxide, and water to make sugars. Light energy also is used by mesophyll cells to synthesize amino acids and a variety of other organic molecules. Sugars made in a leaf can be used by the leaf as an energy source or as building blocks for cells. They also may be transported to other parts of the plant, where they are either stored or used for energy or building blocks.

A major limitation to plant photosynthesis is insufficient water due to transpiration. For example, up to 98 percent of the water that is absorbed by a corn plant's roots is lost through transpiration. However, transpiration may benefit the plant by cooling it and by speeding the transport of mineral nutrients through the xylem.

Modifications for Capturing Light

Leaves absorb light, which, in turn, provides the energy for photosynthesis. The leaves of some plant species have adapted to their environment to maximize light interception. On the same tree, leaves that develop in full sun are thicker, have a smaller area per leaf, and have more chloroplasts per unit area. Shade-leaf chloro-plasts are arranged so that shading of one chloroplast by another is minimized, while sun-leaf chloroplasts are not.

In dry environments, plants often receive more light than they can use. Structures have evolved in these plants that reduce the amount of light absorbed. For example, many desert plants have evolved dense coatings of hairs that reduce light absorption. The cut window plant shown in Figure 29-16 protects itself from its dry environment by growing underground. Only its leaf tips protrude above the soil to gather light for photosynthesis.

figure 29-15

Cells from all three tissue systems are represented in a leaf. The epidermis is part of the dermal system, and the vascular bundle (vein) is part of the vascular system. The mesophyll is ground tissue made of parenchyma cells, and it generally contains chloroplasts.

figure 29-16

The cut window plant (Haworthia truncata) has flat-topped leaves that look as if they have been cut. The transparent leaf tips are sacs that diffuse sunlight before channeling it to the plant's buried leaves.

figure 29-17

These micrographs of monocot and dicot leaves show how the arrangement of stomata in each is different. (a) In the corn leaf, the stomata are in a parallel arrangement, which is typical of monocots. (b) In the potato leaf, the stomata are in a random arrangement, which is typical of dicots.

figure 29-17

These micrographs of monocot and dicot leaves show how the arrangement of stomata in each is different. (a) In the corn leaf, the stomata are in a parallel arrangement, which is typical of monocots. (b) In the potato leaf, the stomata are in a random arrangement, which is typical of dicots.

Gas Exchange

Plants must balance their need to open their stomata to receive carbon dioxide and release oxygen with their need to close their stom-ata to prevent water loss through transpiration. A stoma is bordered by two kidney-shaped guard cells. Guard cells, shown in Figure 29-15 on the previous page, are modified cells on the leaf epidermis that regulate gas and water exchange. Figure 29-17 shows how the stomata are arranged differently in monocots and dicots.

The stomata of most plants open during the day and close at night. The opening and closing of a stoma is regulated by the amount of water in its guard cells. When epidermal cells of leaves pump potassium ions (K+) into guard cells, water moves into the guard cells by osmosis. This influx of water makes the guard cells swell, which causes them to bow apart and form a pore. During darkness, potassium ions are pumped out of the guard cells. Water then leaves the guard cells by osmosis. This causes the guard cells to shrink slightly and the pore to close.

Stomata also close if water is scarce. The closing of stomata greatly reduces further water loss and may help the plant survive until the next rain. However, stomata closure virtually shuts down photosynthesis by cutting off the supply of carbon dioxide.

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