Transcript Plant Structure and Function Leaves

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
Light
ADP &
NADP
(CH2O)
ATP &
NADPH
CO2 & H2O
Chlorophyll
O2
LIGHT
REACTION
CALVIN
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
Leaf
Mesophyll
Palisade
Spongy
Xylem
Vascular
System
Phloem
Sclerenchyma