Biology
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23-4 Leaves 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
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Compound leaf Slide
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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
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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
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.
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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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