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Plant Structure and Function Leaves

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					Plant Structure and
Function - Leaves
Objectives of today’s class:
Learn the typical structure of plant leaves

Develop an understanding of how these
structures support photosynthesis in the
leaf
  Photosynthesis


Photosynthesis can be described in this series
of coupled reactions:

         H 2O              ADP &            (CH2O)
                           NADP
 Light     Chlorophyll

          O2               ATP &            CO2 & H2O
                           NADPH
                  LIGHT            CALVIN
                REACTION           CYCLE
Design features for a leaf
• Exposed to sunlight
• Large surface area
• Capable of exchanging gases
  • CO2 in, O2 out
• Import minerals and water
  • Not obtained from atmosphere
• Export fixed carbon to “sinks”
• Control water loss
• Resist biotic and abiotic stresses
A “typical” leaf

              stem

                    axil with
                   axillary bud


                          leaf
                         blade
internode
    node           petiole
   The leaf blade

Broad expanded part of
  the leaf frequently has
  the following
  characteristics:
• Large surface area
• Thin with a small
  distance between
  upper and lower
  surfaces
   The leaf blade

These anatomical features:
• Maximize the surface area while minimizing
  volume
• Reduce the distance that gases must diffuse
  through the leaf
Leaf structure varies to allow plants to survive
  and grow under diverse conditions.
    Leaf tissues - epidermis

• Epidermis
    • epi - upon
    • dermis - skin
• The outer layer
    of cells
•   Comprised of a
    number of
    different cell
    types
Leaf tissues - epidermis

• Cell types of the epidermis
• Epidermal cells
  • Most abundant, arranged in a number of
    ways
  Leaf epidermis

• Epidermal cells usually lack chloroplasts
• Epidermis is also covered by a waxy
 cuticle
  • Secreted from epidermal cells
  • Impermeable to water
  Leaf epidermis

• There are other specialized cells in the
 epidermis
  • Guard cells, forming stomata
  • Trichomes, leaf hairs
  Stomata and Guard Cells

• Stomata are pores in the
  epidermis that lead to
  intercellular spaces in the leaf
  (from the Greek “stoma”,
  meaning mouth)
• Found on both upper and
  lower surfaces of the leaf,
  more prevalent on bottom
• Formed by specialized guard
  cells
Stomata and Guard Cells

• Crescent shaped cells
  • Inner wall is thickened
  • When guard cells are turgid, stomata are
   open; pores close when cells are not turgid
    Stomata and Guard Cells

• Guard cells regulate
    gas exchange and
    water loss from the
    leaf
•   Guard cells open and
    close depending on
    environmental and
    developmental signals
Why are stomata important
for photosynthesis?
• Photosynthesis requires efficient gas
    exchange through stomata
•   Interior of the leaf is moist, so a large
    amount of water is lost through stomata
    • >90% of water loss occurs via stomata
    • Cuticle is impermeable to water
• When plants cannot get enough water,
    stomata close to preserve water at the
    expense of photosynthesis
Why are stomata important
for photosynthesis?

• Plants using C3 photosynthesis open
    stomata during the day, close them at
    night
•   Some plants have a mechanism that
    allows them to fix CO2 at night (stomata
    open) in order to minimize water loss
    during the day - CAM plants
 Other specialized
 epidermal cells

Trichomes and glands
• Cellular protrusions from
  epidermis
• These have a variety of
  forms and serve a number
  of protective functions
• Stinging hairs can prevent
  predation, e.g. on nettles
 Other specialized
 epidermal cells
Trichomes and glands
• Globular trichomes release compounds
  that are toxic to insects
• Secretory hairs allow plants to secrete
  compounds
Mesophyll Tissue

            • Occupies most of the
                internal tissue of the
                leaf
            •   Comprised of two cell
                types
                • Palisade parenchyma
                  cells
                • Spongy parenchyma
                  cells
  Mesophyll Tissue

• Palisade parenchyma
 cells
  • Elongated cells
  • One to three cell layers
    thick
  • Contain many chloroplasts
  • Primary site for
    photosynthesis
  Mesophyll Tissue

• Spongy parenchyma
 cells
  • More randomly
    arranged
  • Air spaces between
    cells
  • Fewer chloroplasts
Mesophyll Tissue

• Mesophyll tissue
 is designed for:
  • Interception of
    light energy
  • Fixation of CO2
  • Exchange of
    gases
     Vascular tissue

• Visible as veins
    distributed throughout
    the leaf
•   Required for transport
    of material to and
    from the leaf
    • Water and nutrients
      in, photosynthetic
      products out
Monocot vs. Dicot Leaf Veins
Vascular tissue

• Organized as bundles containing xylem
 and phloem
Vascular tissue

• Xylem
  • Distribution of water and minerals transported
   from root
• Phloem
  • Transport of fixed carbon compounds from
   the leaf to the rest of the plant
 Leaf morphology and
 arrangement
• Leaf morphology varies between species
  • A stable characteristic that can be used for
   plant identification
Simple leaves
  • Single leaf blade at each node
Compound leaves
 Multiple leaflets
Leaf morphology and
arrangement
Other characteristics used to describe
 leaves include:
  • How they are attached to the stem
  • Shape of leaves
  • Leaf margin
Leaf Modifications
Leaves can be modified to serve other
  functions:
• The scales of many bulbs (e.g. daffodil)
  are leaves that serve as storage tissue
• Tendrils (on pea, morning glory, etc.) are
  modified leaves that allow plants to vine
• Spines or thorns on some plants are
  modified forms of leaves that protect the
  plant from predation
Leaf Modifications

Leaves can be modified to serve other
  functions
Organ    Tissue      Cell Type


                     Epidermal
        Epidermis    Guard cells
                      Hair cells

                      Palisade
Leaf    Mesophyll
                      Spongy

                       Xylem
        Vascular
                      Phloem
        System
                    Sclerenchyma

				
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posted:10/14/2011
language:English
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