Plant StructuresLeaves

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 tobaccoToxin in milkweedHydrangeaPoison 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