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Plant Structures
Leaves
Structure and function
Leaves...
All leaves are responsible for:
• Absorbing energy from the sun in organelles called
___________
• The majority of photosynthetic production (which
can take place in any green part of a plant; eg.
Cactus),
• Taking in ____________and releasing __________
and water vapour (gas exchange)
• Offering protection from_______________
• Using osmotic pressure to draw
water up from the _______________
Parts of the leaf
Blade: Flattened area of leaf.
Petiole: Stalk-like structure
that attaches leaf blade to
stem.
Midrib and veins: Contains
the vascular tissue.
*there are many different types of leaves – simple,
compound, etc.
** monocots have parallel veins and dicots have
branching veins
Photosynthesis
1. Leaves are the major site for photosynthesis
Chlorophyll, found in chloroplasts,
is the most common
photopigment, which captures
the red and blue lightwaves
(they reflect green light) from the sun to
make food (sugar).
Photosynthesis is the
process in which carbon
dioxide (CO2) and water
(H2O) are used to
produce carbohydrates
and oxygen (O2) in the
presence of light and
chlorophyll.
Stomata
• Photosynthesis require a constant supply of
carbon dioxide. The product of this process –
oxygen – must be released.
• Gas exchange in plants occurs in the leaves
and is regulated by small pores called stomata.
Typical internal leaf structure
Epidermal Cells
• Tightly packed in a single layer covered by a
waxy coating called the cuticle.
• Cuticle:
– Prevents water loss
– Provides a physical barrier against bacteria,
mould and insects.
• Don’t contain chloroplasts.
• But are transparent so light can pass through.
Mesophyll
• Located in the mid region of the leaf.
• Primary site of photosynthesis in the leaf.
• There are two types:
1.
Palisade mesophyll (note how the cells are
shaped): These cells contain many chloroplasts.
Primary site for photosynthesis.
2.
Spongy mesophyll (note the irregularly shaped
and spaced cells): These cells have fewer
chloroplasts. There are many air spaces which
allows carbon dioxide to diffuse into cells and
oxygen out of cells.
Stoma (stomata)
• An opening in the epidermis of a leaf (on the
underside of the leaf in land plants) through
which gases pass out and in. It also regulates
water loss
• In vascular aquatic plants (e.g. Water lilies)
stoma are on the upper epidermis
• Water plants that do not float have no
stomata
Stomata continued.....
• When stomata are open water is lost
(transpiration) and the plant can exchange
oxygen for carbon dioxide through diffusion.
• When stomata are closed the plant conserves
water, but can not exchange gases.
• Guard cells located on either side of the
stomata regulate the opening and closing of
the stomata (open in sunlight and high
humidity).
Leaf adaptation/specialization
• Evolution favours plants with leaves adapted
to their environment (abiotic)
• Eg. Conifers have thin leaves (needles) to shed
snow readily and conserve water; evergreen
for short growing season
• Eg. Cacti grow in low rainfall areas – have very
reduced leaves (spines) for minimal water loss
and very few stomata
• Leaves are vulnerable to being eaten –
evolution favours plants with leaves that are
less appealing (two main types of defenses –
structural and chemical)
• Eg. Leaves that are hairy or modified into
spikes like a cactus.
Chemical defense is most common
Eg.
Nicotine in tobacco
Toxin in milkweed
Hydrangea
Poison Ivy
Other leaf specializations
• Onion bulb – has modified leaves for water
and nutrient storage
• Aloe vera – fleshy leaves to store water
• Sundew leaves
– sticky to trap insects