Leaf structure and adaptations

Chp 7

what is the function of stomata?

• Gas Exchange:
  Stomata allow carbon dioxide (CO₂) to enter the leaf for photosynthesis and let oxygen (O₂) exit as a byproduct.

• Water Regulation:
  Stomata control water loss through a process called transpiration by opening and closing.

• Controlled by:
  Guard cells, which swell or shrink to open or close the stomatal pore based on environmental conditions (like light, humidity, and CO₂ levels).

what is the stoma?

• A stoma (plural: stomata) is a tiny pore or opening found primarily on the underside of leaves.

what are guard cells?

• Definition:
  Specialized kidney-shaped cells that surround each stoma.

• Function:
  Control the opening and closing of the stoma, regulating gas exchange and water loss.

what are the steps involved in the guard cells opening?

1. Light or low internal CO₂ triggers activity.

2. Potassium ions (K⁺) are actively pumped into the guard cells.

3. Water follows by osmosis, entering the guard cells.

4. Guard cells swell and curve outward, opening the stoma.

what are the steps involved in the guard cells closing?

1. Darkness, high CO₂, or water stress signals closure.

2. Potassium ions (K⁺) are pumped out of guard cells.

3. Water exits by osmosis.

4. Guard cells become flaccid, and the stoma closes.

what are crypts?

• Definition:
  Sunken stomata located in small chambers or pits on the leaf surface.

• Function:
  Reduce water loss by trapping moist air and protecting stomata from wind.

what triggers the stoma opening and closing?

• Opening Triggers:

  • Light (especially blue light)

  • Low internal CO₂

  • Adequate water availability

• Closing Triggers:

  • Darkness

  • High CO₂ concentration

  • Drought or water stress (detected via abscisic acid, a stress hormone)

what effect do environmental stresses have on stoma? what are examples of these?

• Stress: Drought, high heat, high salinity, wind

• Effect:
  Stomata close to conserve water, which reduces photosynthesis due to limited CO₂ intake.

what does leaf orientation have an affect on? what are the types of orientation, what do they increase/decrease, what types of environments would you find these in?

• Can affect the rate of transpiration and photosynthesis in plants. Trade off of maximizing light capture and minimizing

• Horizontal Orientation:

  • Maximizes light capture

  • Increases transpiration

  • Common in temperate plants

• Vertical Orientation:

  • Reduces water loss and sunlight exposure

  • Found in arid environments (e.g., succulents)

• Effect on Plant:
  Influences photosynthesis and transpiration, cooling and water efficiency

what is a cuticle?

• A waxy, protective layer that covers the epidermis of leaves and stems.

what is the purpose of a cuticle?

Function:

• Prevents water loss through evaporation, directing transpiration to stomata

• Acts as a barrier to pathogens and environmental damage

what does the thickness of a cuticle reveal?

• Thicker cuticle:
  Adaptation to dry, hot, or windy environments → helps conserve water.

• Thinner cuticle:
  Found in humid, shaded, or aquatic environments where water loss is less of a concern.

• provides info about a plants ability to retain water and therefore its natural habitat

what kind of cuticles might be on plants in a hot/dry climate vs a cold/moist climate vs aquatic?

• Hot/Dry Climates (Desert Plants):

  • Very thick cuticle to minimize water loss.

  • Example: Cacti, succulents.

• Cold/Moist Climates (Temperate Forest Plants):

  • Moderate cuticle; still protects from wind and water loss.

• Aquatic Plants:

  • Very thin or no cuticle on submerged leaves; water loss is not a concern.

when are stomata open/closed, why?

Typically open during the day and close at night (to minimize water loss while plant is not photosynthesizing)

what might effect stomata density/stomata placement?

• Environmental Factors:

  • Humidity: Low humidity = fewer or sunken stomata.

  • Light intensity: High light = higher stomata density.

  • Water availability: More water = more stomata.

• Plant Type:

  • Aquatic/Floating plants: Stomata mostly on upper surface.

  • Desert plants: Fewer, sunken stomata, often on lower surface.

  • Shade plants: Fewer stomata to reduce water loss

where are stomata located and in what density in salal, succulents, duckweed, and elodea?

Plant Type	Stomata Location	Stomatal Density	Notes
Salal (terrestrial shrub)	Mostly on lower surface	Moderate	Adapted to forest floor—reduced water loss
Succulents	Fewer stomata, often sunken or on underside only	Low	To conserve water in dry climates
Duckweed (floating aquatic)	Upper surface only	Low	Exposed to air above water
Elodea (submerged aquatic)	No stomata	None	Gas exchange occurs directly through the surface of the plant

what is the density of stomata in aquatic/floating plants? why? how are their stomata effected?

• Low density of stomata, often only on the upper surface of the leaf.

• Reason: Water contact on the underside reduces the need for stomata there.

• Floating plants (like duckweed) only exchange gases with the air above, so stomata are concentrated on top.

how do you calculate the density of stomata?

• Stomatal density = Number of stomata per unit area

• Example: Count the number of stomata in a field of view (under a microscope) and divide by the area observed (usually in mm² or cm²).

what is the function of leaf veins?

• Transport: Veins transport water (via xylem) and sugar (via phloem) throughout the leaf.

• Support: They also help support the leaf’s structure.

what are mesophyll cells? how many layers are in mesophyll, what are they called?

• Photosynthetic cells found in the middle layer of the leaf.

• Includes both palisade and spongy mesophyll cells.

• There are two layers in the mesophyll:

  1. Palisade mesophyll (upper layer)

  2. Spongy mesophyll (lower layer)

where is the Palisade Mesophyll located? what is its structure, what is its main function?

• Located in the upper part of the leaf, just below the upper epidermis.

• Made of elongated, tightly packed cells.

• Cells are rich in chloroplasts for efficient light absorption.

• Photosynthesis: It is the primary site of photosynthesis in the leaf because it contains the most chloroplasts.

where is the spongy Mesophyll located? what is its structure, what is its main function?

• Location: In the lower part of the leaf, beneath the palisade mesophyll layer and above the lower epidermis.

• Made up of loosely arranged cells with large air spaces between them.

• Cells are irregular in shape and contain chloroplasts, but fewer than palisade cells.

• Gas exchange: Allows carbon dioxide and oxygen to diffuse between cells and air spaces.

• Also plays a minor role in photosynthesis.

which cells exchange gases with the air in leaves?

• Spongy mesophyll cells exchange gases with the air in the intercellular air spaces.

• Gases enter and exit the leaf through stomata on the lower epidermis.