Biology
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23–4 Leaves
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23-4 Leaves
Leaf Structure
Leaf Structure
How does the structure of a leaf enable
it to carry out photosynthesis?
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23-4 Leaves
Leaf Structure
The structure of a leaf is optimized for
absorbing light and carrying out
photosynthesis.
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23-4 Leaves
Leaf Structure
To collect sunlight, most leaves have thin, flattened
sections called blades.
Blade
Simple leaf
Leaflet
Petiole
Bud
Stem
Compound leaf
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23-4 Leaves
Leaf Structure
The blade is attached to the stem by a thin stalk
called a petiole.
Blade
Simple leaf
Leaflet
Petiole
Bud
Stem
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Compound leaf
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23-4 Leaves
Leaf Structure
Simple leaves have only one blade and one petiole.
Blade
Simple leaf
Leaflet
Petiole
Bud
Stem
Compound leaf
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23-4 Leaves
Leaf Structure
Compound leaves have several blades, or leaflets,
that are joined together and to the stem by several
petioles.
Blade
Simple leaf
Leaflet
Petiole
Bud
Stem
Compound leaf
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23-4 Leaves
Leaf Structure
Leaves are covered on the top and bottom by
epidermis.
Epidermis
Epidermis
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23-4 Leaves
Leaf Structure
The epidermis of many leaves is covered by the
cuticle.
Cuticle
Epidermis
Epidermis
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23-4 Leaves
Leaf Structure
The cuticle and epidermal cells form a waterproof
barrier that protects tissues inside the leaf and limits
the loss of water through evaporation.
The vascular tissues of leaves are connected directly
to the vascular tissues of stems.
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23-4 Leaves
Leaf Structure
In leaves, xylem and phloem tissues are gathered
together into bundles that run from the stem into the
petiole.
In the leaf blade, the vascular bundles are
surrounded by parenchyma and sclerenchyma cells.
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23-4 Leaves
Leaf Structure
All these tissues form the veins of a leaf.
Xylem
Phloem
Vein
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23-4 Leaves
Leaf Functions
Leaf Functions
Most leaves consist of a specialized ground tissue
known as mesophyll.
Palisade
mesophyll
Spongy
mesophyll
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23-4 Leaves
Leaf Functions
The layer of mesophyll cells found directly under the
epidermis is called the palisade mesophyll. These
closely-packed cells absorb light that enters the leaf.
Palisade
mesophyll
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23-4 Leaves
Leaf Functions
Beneath the palisade mesophyll is the spongy
mesophyll, a loose tissue with many air spaces
between its cells.
Spongy
mesophyll
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23-4 Leaves
Leaf Functions
The air spaces connect with the exterior through
stomata.
Stomata are porelike openings in the underside of the
leaf that allow carbon dioxide and oxygen to diffuse
into and out of the leaf.
Stoma
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23-4 Leaves
Leaf Functions
Each stoma consists of
two guard cells.
Guard cells are
specialized cells that
control the opening and
closing of stomata by
responding to changes
in water pressure.
Guard cells
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23-4 Leaves
Leaf Functions
Transpiration
The surfaces of spongy mesophyll cells are kept
moist so gases can enter and leave the cells
easily.
Water evaporates from these surfaces and is lost
to the atmosphere.
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23-4 Leaves
Leaf Functions
Transpiration is the loss of water through its leaves.
This lost water is replaced by water drawn into the
leaf through xylem vessels in the vascular tissue.
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23-4 Leaves
Leaf Functions
Gas Exchange
How does gas exchange take place in a
leaf?
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23-4 Leaves
Leaf Functions
Plant leaves allow gas exchange between air
spaces in the spongy mesophyll and the
exterior by opening their stomata.
Plants keep their stomata open just enough
to allow photosynthesis to take place but
not so much that they lose an excessive
amount of water.
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23-4 Leaves
Leaf Functions
Guard cells are specialized cells that control the
stomata.
Stomata open and close in response to changes in
water pressure within guard cells.
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23-4 Leaves
Leaf Functions
When water pressure within guard cells is high, the
stoma open.
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23-4 Leaves
Leaf Functions
When water pressure within guard cells decreases,
the stoma closes.
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23-4 Leaves
Leaf Functions
Plants regulate the opening and closing of their
stomata to balance water loss with rates of
photosynthesis.
Stomata are open in daytime, when photosynthesis is
active, and closed at night, to prevent water loss.
In hot, dry conditions stomata may close even in
bright sunlight, to conserve water.
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23–4
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23–4
A compound leaf is one that has
a. a blade attached by several petioles.
b. two or more blades.
c. a blade that is divided into many leaflets.
d. many blades, each with its own petiole.
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23–4
The layer of cells in a leaf that absorb light is the
a. phloem.
b. vein.
c. palisade mesophyll.
d. epidermis.
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23–4
The structure of a leaf allows it to
a. maximize sun exposure and maximize water
loss.
b. maximize sun exposure and minimize water
loss.
c. minimize sun exposure and maximize water
loss.
d. minimize sun exposure and minimize water
loss.
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23–4
A process in which water is lost through the
leaves of a plant is called
a. transpiration.
b. photosynthesis.
c. glycolysis.
d. cellular respiration.
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23–4
Gas exchange in a leaf occurs through the
a. cuticle.
b. epidermis.
c. mesophyll.
d. stomata.
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