Water Movement Through Vascular Plants & the Transpiration-Cohesion-Tension Model

along with vascular tissue came the evolution of specialized organs

• roots: absorb water and minerals

  • xylem ‘starts’ here, and the pipes run upward through the stem to the leaves

• leaves: make sugars by photosynthesis

  • phloem ‘starts’ here, and the pipes run downward through the stem to the roots

• stems: support the plant to elevate the leaves and spread them out to maximize photosynthesis

general structure of vascular plants (tracheophytes)

• vascular plants (tracheophytes) are characterized by a dominant sporophyte generation with specialized tissues—xylem for water transport and phloem for nutrient transport—organized into three main vegetative organs: roots, stems, and leaves. These structures allow efficient transport, structural support, and photosynthesis, enabling growth on land

vascular tissue

• found in the center of stems and roots

• vascular cyclinder: xylem & phloem in the center of roots and stems

tracheids (xylem cells)

• tracheids: long zylem cells, full of holes called pits, empty and dead at maturity

• water moves between tracheids in continuous flow

  • all tracheophytes have xylem cells called tracheids

vessel elements

• vessel elements: a specialized type of xylem cell found in angiosperms

• specialized, non-living, tube-like cells in the xylem of angiosperms and some gymnosperms that conduct water and nutrients from roots to leaves

water and ions move through roots to the xylem via 2 pathways (apoplast & symplast)

the apoplast includes water inside:

• the continuous meshwork of cellulose within cell walls

• intercellular spaces

• tracheids and vessel elements

  • in the apoplastic pathway, water and solutes never cross a membrane

the symplast includes water within:

• the cytoplasm of living cells connected by plasmodesmata

  • plasma membranes control movement of water and ions in the symplastic pathway

leaf anatomy

• Leaf blade (lamina): Broad, flat part; main site of photosynthesis

• Petiole: Stalk that attaches the leaf to the stem; positions leaf for light

• Cuticle: Waxy outer layer; reduces water loss

• Upper epidermis: Protective outer cell layer; lets light pass through

• Lower epidermis: Protective layer; usually contains most stomata

• Stomata: Small pores for gas exchange (CO₂ in, O₂ out)

• Guard cells: Control opening and closing of stomata

• Mesophyll: Middle tissue layer specialized for photosynthesis

  • Palisade mesophyll: Tightly packed cells with many chloroplasts

  • Spongy mesophyll: Loosely packed cells with air spaces for gas exchange

• Veins (vascular bundles): Transport materials

  • Xylem: Transports water and minerals

  • Phloem: Transports sugars (food)

what can transpiration cause?

• transpiration generates tension, which draws water from xylem of the nearest vein into the apoplast surrounding the mesophyll cells

• the drawing and removal of water from the veins establishes tension on the entire column of water

• turgor pressure: keeps plants upright

  • ex: the central vacuoles of a plant, when filled with water, make each cell firm. the cells have good turgor pressure and are turgid

    • loss of turgor pressure causes wilting

basic root structure

• Root cap: Protects the growing tip; helps root push through soil

• Apical meristem: Region of active cell division for root growth

• Zone of elongation: Cells lengthen, increasing root size

• Zone of differentiation (maturation): Cells specialize into different tissues

• Epidermis: Outer layer; absorbs water and minerals

• Root hairs: Tiny extensions of epidermal cells; increase surface area for absorption

• Cortex: Stores food and transports water inward

• Endodermis: Selective barrier controlling movement of water into vascular tissue

• Pericycle: Gives rise to lateral roots

• Vascular cylinder (stele): Central transport system

  • Xylem: Conducts water and minerals upward

  • Phloem: Conducts sugars throughout the plant

the role of the root in water absorption and movement within the plant

• Root hairs absorb water from soil by osmosis

• Water moves through the epidermis and cortex

• Endodermis (Casparian strip) controls what enters

• Water enters the xylem

• Xylem carries water upward to the rest of the plant

phloem function

• phloem sap is mostly sucrose in solution. (glucose from photosynthesis is converted into sucrose for transport in the plant.)

• phloem cells conduct in different directions, traveling from source (where sugars are made) to sink (where sugars are needed in non-photosynthetic cells)

phloem cells: sieve tube elements

• sieve tube elements: long phloem cells, living at maturity, that have perforated ends in the cell wall

• sugars must move across the cell membrane from sieve cell to sieve cell through the sieve plate perforations

• sieve tube elements have no organelles and depend upon companion cells for metabolism

• at a source cell, sucrose is actively transported into the phloem sieve tubes

• now these sieve tubes higher sucrose concentration, thus water enters from xylem by osmosis

• this causes increased pressure in the sieve tubes, which pushes the sucrose solution towards the sink via pressure and solute gradients

• at sink cells, sucrose is unloaded: water then flows from the sieve tubes back into the xylem