Plant Structure and Function Leaves

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Plant Structure and Function - Leaves

• Objectives of todays class
• Learn the typical structure of plant leaves
• Develop an understanding of how these structures
  support photosynthesis in the leaf

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Photosynthesis
Photosynthesis can be described in this series
of coupled reactions
ADP NADP
(CH2O)
H2O
Light
Chlorophyll
ATP NADPH
CO2 H2O
O2
LIGHT REACTION
CALVIN CYCLE
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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

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A typical leaf
stem
axil with axillary bud
leaf blade
internode
node
petiole
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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

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The leaf blade

• These anatomical features
• Maximize the surface area while minimizing volume
• Reduce the distance that gases must diffuse
  through the leaf
• Remember that plant species are adapted to a wide
  variety of environments. Leaf structure varies to
  allow plants to survive and grow under diverse
  conditions.

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Leaf tissues - epidermis

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

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Leaf tissues - epidermis

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

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Leaf epidermis

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

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Leaf epidermis

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

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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 (adaxial) and lower (abaxial)
  surfaces of the leaf
• Formed by specialized guard cells

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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

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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

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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
• gt90 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

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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

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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

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Other specialized epidermal cells

• Trichomes and glands
• Globular trichomes release compounds that are
  toxic to insects
• Secretory hairs allow plants to secrete compounds

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Mesophyll Tissue

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

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Mesophyll Tissue

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

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Mesophyll Tissue

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

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Mesophyll Tissue

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

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Vascular tissue

• Visible as veins distributed throughout the leaf
• Required for transport of material to and from
  the leaf
• Water and nutrients in, photosynthate out

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Vascular tissue

• Organized as bundles containing xylem and phloem

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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
• Schlerenchyma
• Thickened fibers that provide structural strength
  to the leaf

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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

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Leaf morphology and arrangement

• Other characteristics used to describe leaves
  include
• How they are attached to the stem
• Shape of leaves
• Leaf margin

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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

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Leaf Modifications

• Leaves can be modified to serve other functions

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Organ
Tissue
Cell Type
Epidermis
Leaf
Mesophyll
Vascular System