Bio Review Notes
NUTRITION and TRANSPORT in PLANTS
Performance Objectives:
Leaves are the principal organs of photosynthesis in most plants.
The light reactions of photosynthesis take place most efficiently in the palisade layer, and the light-independent (or "dark") reactions in the spongy mesophyll.
Vascular tissues (xylem, phloem) help transport materials within the plant.
  • Leaves are made of:
    • Upper epidermis, coated with a waxy cuticle.
    • Palisade layer, containing the highest density of chloroplasts.   The "light" (light-dependent) reactions of photosynthesis are most efficient here.
    • Spongy mesophyll:   The "dark" (light-independent) photosynthesis reactions are most efficient here because air spaces facilitate gas exchange.
    • Veins (extensions of xylem + phloem) run through the spongy mesophyll layer.
    • Lower epidermis, coated with a waxy cuticle, contains pores for gas exchange called stomates or stomata.


  • Vascular tissues:
  • Phloem transport:   The phloem tissue transports photosynthetic products from the leaves to other parts of the plant, principally downward through the stem. The principal transport cells in phloem are seive tube cells.
  • Xylem and transpiration:
    • Water and dissolved minerals (ions, including K+, Ca2+, Mg2+, NO3-, PO43-) ascend from roots through stems to upper parts of the plant, traveling through tube-like tracheids of the xylem.
    • Loss of water from leaves is called transpiration.
    • The ascent of sap seems to be governed largely by transpiration pull, or reduced pressure from above, a process requiring long, unbroken chains of fluid with no air bubbles. Root pressure also helps a bit.