Leaf Anatomy Supports Photosynthesis

We can think of roots and stems as important supporting actors that sustain the activities of the real stars of the plant body, the leaves—the organs of photosynthesis. Leaf anatomy is beautifully adapted to carry out photosynthesis and to support it by exchanging the gases O2 and CO2 with the environment, limiting evaporative water loss, and exporting the products of photosynthesis to the rest of the plant. Figure 35.23a shows a typical eudicot leaf in three dimensions.

Most eudicot leaves have two zones of photosynthetic parenchyma tissue referred to as mesophyll, which means "middle of the leaf." The upper layer or layers of mesophyll consist of elongated cells; this zone is referred to as palisade mesophyll. The lower layer or layers consist of irregularly shaped cells; this zone is called spongy mesophyll. Within the mesophyll is a great deal of air space through which carbon dioxide can diffuse to reach and be absorbed by photosynthesizing cells.

Vascular tissue branches extensively throughout the leaf, forming a network of veins (Figure 35.23b). Veins extend to within a few cell diameters of all the cells of the leaf, ensuring that the mesophyll cells are well supplied with water and minerals. The products of photosynthesis are loaded into the phloem of the veins for export to the rest of the plant.

Covering the entire leaf on both its upper and lower surfaces is a layer of nonphotosynthetic cells, which constitute the epidermis. The epidermal cells have an overlying waxy cuticle that is highly impermeable to water. But this impermeability poses a problem: While keeping water in the leaf, the epidermis also keeps carbon dioxide—the other raw material of photosynthesis—out.

The problem of balancing water retention and carbon dioxide availability is solved by an elegant regulatory system that will be discussed in more detail in the next chapter. Guard cells are modified epidermal cells that change their shape, thereby opening or closing pores called stomata, which serve as passageways between the environment and the leaf's interior (Figure 35.23c). When the stomata are open, carbon dioxide can enter and oxygen can leave, but water vapor can also be lost.

Guard cell Stoma

Guard cell Stoma

Spongy Cell

Cuticle

Upper epidermis

Palisade mesophyll cell

Bundle sheath cell

Xylem Phloem

Lower epidermis

SPongy mesophyll cells

35.23 The Eudicot Leaf (a) This three-dimensional diagram shows a eudicot leaf. (b) The network of fine veins in this maple leaf carries water to the mesophyll cells and carries photosynthetic products away from them. (c) These paired cells on the lower epidermis of a eudicot leaf are guard cells; the gaps between them are stomata, through which carbon dioxide enters the leaf.

Cuticle

Upper epidermis

Palisade mesophyll cell

Bundle sheath cell

Xylem Phloem

Lower epidermis

SPongy mesophyll cells

Pleural Stoma

Guard cells A

Guard cells A

Stoma \

35.23 The Eudicot Leaf (a) This three-dimensional diagram shows a eudicot leaf. (b) The network of fine veins in this maple leaf carries water to the mesophyll cells and carries photosynthetic products away from them. (c) These paired cells on the lower epidermis of a eudicot leaf are guard cells; the gaps between them are stomata, through which carbon dioxide enters the leaf.

Stoma \

Pleural Stoma

In Chapter 8 we described C4 plants, which can fix carbon dioxide efficiently even when the carbon dioxide supply in the leaf decreases to a level at which the photosynthesis of C3 plants is inefficient. One adaptation that helps C4 plants do this is their modified leaf anatomy (see Figure 8.16). The pho-tosynthetic cells in the C4 leaf are grouped around the veins in concentric layers, forming an outer mesophyll layer and an inner bundle sheath. These layers each contain different types of chloroplasts, leading to the biochemical division of labor illustrated in Figure 8.17.

Leaves receive water and mineral nutrients from the roots by way of the stems. In return, the leaves export products of photosynthesis, providing a supply of chemical energy to the rest of the plant body. And, as we have just seen, leaves exchange gases, including water vapor, with the environment by way of the stomata. All three of these processes will be considered in detail in the next chapter.

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Responses

  • tyler
    Is the palisade mesophyll adjacent to the lower epidermis?
    8 years ago
  • olli-pekka
    What are the openings in the lower epidermis through which oxygen and carbon dioxide pass through?
    8 years ago
  • jonathan
    Does water for photosynthesis enter or leave the leaf in the lower epidermis?
    8 years ago
  • rosita ferri
    How does carbon dioxide enter the palisade mesophyll in the leaf?
    8 years ago
  • mia
    What are stoma cells in leaves?
    8 years ago
  • Hagosa
    What is a cell membrane of the spongy cell?
    8 years ago
  • Vanessa
    Which layer are the stomata and guard cells located?
    8 years ago
  • teresa
    What is bundle sheath in a leaf anatomy?
    8 years ago
  • stephanie
    What are spongy cells?
    8 years ago
  • minna
    Does mesophyll cell have cell membrane?
    7 years ago
  • Kevin
    How raw materials for photosynthesis get into a palisade cell?
    7 years ago
  • Tyyne Piili
    How does co2 gwt into palisade cells?
    7 years ago
  • anke
    How does water enter the mesophyll?
    7 years ago
  • Gianfranco
    Which leaf layer does carbon dioxide enter?
    7 years ago
  • TANJA
    What are the mesophyll layers in eudicots?
    7 years ago
  • lelia
    How does carbon dioxide in the air reach mesophyll cells inside the leaf?
    7 years ago
  • Mosco Burrows
    How does co2 reach the palisade mesophyll cells in a leaf?
    7 years ago

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