Transport in Plants

Transport in Plants

• Water is lost through the leaves in a process called transpiration.
• This loss of water is replaced by water drawn into the leaf.
• Evaporation is the major moving force behind water movement.
• As water is lost, osmotic pressure moves water out of the vascular tissue.
• This pulls water up from the stem to the leaves.
• Transpiration is affected by heat, humidity, and wind.

Controlling Transpiration

• Opening the stomata increases water loss.
• Closing the stomata decreases water loss.

Water Transport

• Active transport and root pressure cause water to move from the soil to the roots.
• Capillary action, combined with active transport and root pressure, moves materials throughout the plant.

Capillary Transport

• Capillary transport results from both cohesive and adhesive forces.
• Water molecules are attracted to one another, and water is also attracted to the xylem tubes in the plant.
• This causes water to move from the roots to the stem and upward.

Nutrient Transport

• Most nutrients are pushed through plants.
• Nutrient movement takes place in the phloem.

Source to Sink

• Source – any cell that produces sugars.
• Sink – any cell where sugars are used.
• The pressure-flow hypothesis explains how sugars move from source to sink.

Transpiration and Wilting

• Osmotic pressure keeps plants semi-rigid.
• Wilting is a result of high transpiration rates.
• The loss of water causes a drop in osmotic pressure.
• This loss of rigidity conserves water.

Leaves - Function

• Leaves are the main site of photosynthesis.
• Oxygen and carbon dioxide intake and release occur in the leaves.
• Leaves are also important for waste removal and the movement of water.
• Transpiration and guttation are two ways water moves through leaves.

Leaves - Structure

• Leaves typically consist of a stalk, a flattened blade, a network of veins, and stipules.
• Green leaves capture light energy.
• Stomata regulate gas exchange between the leaf interior and the atmosphere.
• Stomata also regulate the evaporation of water.
• Leaves can be simple or compound:
  • Simple leaves: Have a single blade.
  • Compound leaves: Blade is divided into leaflets.
    • Pinnately compound leaves: Leaflets in pairs along rachis (petiole).
    • Bipinnately compound leaves: Leaflets subdivided.
    • Palmately compound leaves: All leaflets attached at the same point at the end of the petiole.

Internal Structure of Leaves

• Veins carry water and nutrients to the leaf.
• Mesophyll is the photosynthetic tissue of the leaf.
• Epidermis is the outer layer of the leaf.

Stomata

• Stomata are bordered by two guard cells that regulate gas exchange and water evaporation.
• They can inflate or deflate to control the opening and closing of the stomata.

Leaf Arrangements and Types

• Leaves are attached to stems at nodes.
• Stem regions between nodes are known as internodes.
• Phyllotaxy: Arrangement of leaves on the stem.
  • Alternate: One leaf per node.
  • Opposite: Two leaves per node.
  • Whorled: Three or more leaves at a node.

Leaf Venation and Types

• Venation: Arrangement of veins in the leaf.
  • Monocot: Leaf veins parallel.
  • Dicot:
    • Reticulate venation: Network-like veins.
    • Palmate venation: Veins radiate from a single point.
    • Pinnate venation: Veins branch off from a central vein.
    • Dichotomous venation: Veins fork repeatedly.

Mesophyll and Veins

• Most photosynthesis takes place in the mesophyll between the epidermal layers.
  • Palisade mesophyll: Tightly packed, barrel-shaped parenchyma cells, usually in two rows. Contains most of the leaf's chloroplasts.
  • Spongy mesophyll: Loosely arranged parenchyma cells with abundant air spaces.
• Veins (vascular bundles) are scattered throughout the mesophyll.
  • Consist of xylem and phloem tissues surrounded by a bundle sheath of thicker-walled parenchyma.
  • Monocots: Usually do not have the mesophyll differentiated into palisade and spongy layers.
  • Monocots often have bulliform cells on either side of the main central vein. These cells partly collapse under dry conditions.

Specialized Leaves

• Leaves have adapted for different purposes.
  • Sun leaves: Larger and thicker than shade leaves to capture more sunlight.
  • Shade leaves: Have larger surface areas, but thinner to allow for maximum light absorption in low light conditions.
  • Tendrils: Modified leaves that help the plant climb.
  • Spines: Modified leaves for protection.
  • Window leaves: Function as light collectors in arid environments.
  • Prickles: Sharp outgrowths from the epidermis, used for protection.
  • Thorns: Modified stems used for protection.
  • Flower-pot leaves: Act as reservoirs for water and nutrients.
  • Insect-trapping leaves: Modified leaves to catch insects.
    • Sundew: Sticky hairs trap insects.
    • Venus’s flytraps: Trap insects in hinged leaves.
    • Pitcher plants: Trap insects in a pitcher-like structure.
  • Colorful leaves: Some leaves are brightly colored to attract pollinators.
  • Clary's sage: Have colorful bracts surrounding the flowers. - Poinsettia: Has colorful bracts that appear to be flower petals. - Air plants: Specialized leaves for epiphytic life.

Autumnal Changes in Leaf Color

• Chlorophyll is the dominant pigment in leaves.
• Other pigments including carotenoids (yellows) and anthocyanins or betacyanins (reds and blues) are also present.
• As chlorophyll breaks down in the fall, other colors become more visible.

Abscission

• Deciduous plants drop leaves seasonally.
• Abscission is the process by which leaves are shed.
• This occurs due to changes in the abscission zone near the base of the petiole:
  • A protective layer forms, with cells coated and impregnated with suberin.

Explore the fascinating mechanisms plants use to transport water and nutrients. This quiz covers transpiration, osmotic pressure, and the role of stomata in regulating water loss. Understand the importance of capillary action and how it aids in the movement of water within the plant system.