BIO201 Leaves Slides
Leaves
A. Common forms of Eudicot and Monocot Leaves
Eudicot Features:
Blade
Petiole
Axillary bud
Node
Stem
Monocot Features:
Sheath
Blade
Leaf Arrangement
(a) Alternate Leaves: Leaves are arranged alternately along the stem, allowing maximum exposure to sunlight.
(b) Opposite Leaves: Pairs of leaves arise at each node, directly across from each other.
(c) Whorled Leaves: Three or more leaves grow from a single node in a circular arrangement.
(d) Rosette: Leaves form a cluster at the base of the plant, allowing for greater survival under low-light conditions.
Variation in Leaf Structure
Simple Leaf: A single undivided blade.
Compound Leaf: Multiple leaflets attached to a single petiole.
Doubly Compound Leaf: Leaflets are further divided, creating a complex leaf structure.
Needles: Adaptations for conserving water in certain environments (pine trees).
Leaf Anatomy
Ground Tissue System: Contains mesophyll, where photosynthesis occurs.
Vascular Tissue System: Xylem and phloem that transport water, nutrients, and sugars.
Dermal Tissue System: Includes epidermis and guard cells with chloroplasts for regulating photosynthesis.
Leaf Adaptations
C4 Photosynthesis: Adaptation for plants in hot, dry climates to efficiently fix carbon dioxide.
Xerophytes: Plants adapted to conserve water, often having specialized adaptations like thicker cuticles.
Summary of Modified Leaves
Modified Leaves | Example |
|---|---|
Bulbs | Onion leaves store food |
Stem Leaves | Red poinsettia leaves attract pollinators |
Succulent Leaves | Aloe vera leaves store water |
Tendrils | Pea tendrils aid in climbing |
Traps | Pitcher plant leaves trap insects… |
Digestive | Enzymes produced break down the trapped insects. |
Guard Cells and Water Movement
In the presence of light, potassium ions (K⁺) are actively transported into guard cells, causing them to swell and open.
Loss of light or drought conditions lead to K⁺ ions diffusing out, increasing water concentration externally, causing water to move in from the surrounding cells, controlling transpiration.
Movement of Water in Plants
A 50 ft tree can lose over 50 gallons of water an hour.
Water moves through vascular tissue: the xylem transports water upwards against gravity, predominantly through capillary action and cohesion.
The energy source for water movement comes from solar energy that drives transpiration (evaporation of water from leaves).
Cohesion-Tension Theory of Transpiration
Key Processes:
Water vapor diffuses out of leaves due to concentration gradients with the atmosphere.
Evaporation within leaves driven by heat from solar radiation pulls water from the xylem, root cortex, and soil into roots.
Trichomes
Structures on leaf surfaces that:
Slow water loss by increasing the boundary layer of still air around leaves.
Protect leaf tissue from herbivores and extreme environmental conditions.
Photosynthesis: The Basics
Light energy and carbon dioxide are used to produce oxygen and glucose in the leaves, primarily occurring in chloroplasts.
Photosynthesis Chemical Equation:
Components of Photosynthesis
Light-Capturing Reactions: Convert light energy into chemical forms.
Calvin Cycle: Utilizes ATP and NADPH to convert CO₂ into sugar (G3P).
Leaf Structure in Photosynthesis
Leaf surfaces typically contain stomata, regulated by guard cells.
Carbon dioxide diffuses through stomata into leaf interiors where it is utilized in photosynthesis.
Calvin Cycle and Key Enzymes
RuBisCO: Enzyme that catalyzes the fixing of carbon dioxide in the Calvin Cycle, the most abundant protein on Earth.
Breakdown of CO₂ during photorespiration leads to the formation of phosphoglycolate, which repurposes energy inefficiently.
C4 Photosynthesis
C4 Photosynthesis Mechanism:
Different enzyme (PEP carboxylase) initiates carbon fixation, resulting in 4-carbon organic acid formation, improving CO₂ concentration in the process.
Advantages of C4 Plants:
Greater resistance to drought conditions compared to C3 plants.
Leaf Anatomical Adaptations to Dry Conditions
Adaptations include:
Stomata located within pits to reduce exposure.
Thick, waxy cuticles aiding in moisture retention.
Thick epidermal layers and trichomes for additional protection against water loss.
Venation Patterns in Leaves
Net or Pinnate Venation: Typically observed in dicots, displaying a branching pattern for increased vascular support.
Parallel Venation: Characteristic of monocots, with veins running parallel to each other, improving structural integrity.