aice bio - unit 7

Roots

• Casparian strip: band of “waxy” material (suberin) along the endodermis; PREVENTS WATER FROM TRAVELLING VIA THE APOPLAST PATHWAY

Stems

• pith: a tissue in the stems of vascular plants

Roots & stems

• Epidermis: “skin” of the plant; aerial parts secrete a waxy cuticle

• Endodermis: inner “skin” layer separating the tissue layers

• Cortex: tissue of cells between the epidermis and the stele/vascular bundle

• Stele/vascular bundle: Xylem & phloem; part of the transport system of the plant

Monocot vs. dicot

Roots

Stems

• Monocots- vascular bundles scattered in cortex

• Dicots- vascular bundles in a ring; cortex outside bundles, pith in the middle

Leafs/Flowers

• Monocots: Multiples of 3 (flowers), parallel veins (leaves)

• Dicots: Multiples of 4 or 5 (flowers), net like veins

Seeds

Apoplast and Symplast pathways

Water (and solutes! Ions, organic compounds) are taken up by root hairs and cross the

cortex to get to the xylem vessels. They can take two possible pathways: apoplast and symplast.

• Apoplast Pathway: water enters and moves through the cell wall (intercellular spaces)

• Symplast Pathway: Water enters the cytoplasm and travels via the plasmodesmata

*NOTE THAT THE APOPLAST PATHWAY IS BLOCKED BY THE CASPARIAN STRIP.

• From the endodermis inwards, water is moving towards the xylem

  ONLY via the symplast pathway from the endodermis inwards, water is moving towards the xylem ONLY via the symplast pathway

Xylem

• Function: transport water around the plant, therefore must have little resistance to flow and be as open as possible.

• Made of dead cells with hard walls for support

• Reinforced with lignin: strong, waterproof material

• Usually at the CENTER of vascular bundles

• Usually bigger

Part 2:

Phloem: Transports food (sugars, proteins, hormones, mineral elements, etc.)

• Two-way flow

• Made of: Sieve tube elements

  • Alive to allow for active transport

  • Lack most organelles because it would interfere with sap flow and add resistance

  • Arranged end to end to form tubes.

  • Stay alive thanks to companion cells

    • Perform vital functions that the sieve tube elements cannot.

    • high number of mitochondria/chloroplast to create enough ATP for both itself and sieve tube cell (metabolically active)

    • Modified parenchyma cells

    • dense cytoplasm

    • Small vacuoles

Part 3: Movement of sugar and water in the plant

• Translocation: the process of moving materials from the leaves to other parts of a plant.

• In the phloem, transport of assimilates happen through mass flow, source to sink

  • source: anywhere where sugar is produced or stored

  • sink: anywhere sucrose is needed (“roots and shoots”)

• TO GET SUCROSE INTO THE PHLOEM: Sucrose is actively loaded into sieve elements from companion cells with the help of proton pumps and co transporter proteins.

  • Sucrose is produced in the parenchyma of photosynthesizing leaves

  • Active transport is needed because there is a higher concentration in the phloem than in the leaves, so we want sugar to go against the concentration gradient. This is the only instance of active transport in this process.

  • PROCESS: Proton pumps pump protons out of the cell, establishing a proton gradient. When protons diffuse back in through facilitated diffusion, they bring sucrose with it. This allows the cells to carry sucrose into companion cells against the concentration gradient. From the companion cells, the sucrose diffuses into the sieve tube element.

• Water leaves the phloem as well, which helps to set up a hydrostatic gradient as water flows from high to low.

• HOW SUGAR MOVES: HYDROSTATIC PRESSURE GRADIENTS

  

  • High solute concentration = low water potential at the source

  • As sugar flows into the phloem, a lower water potential is also created in the phloem.

  • This draws water from the adjacent xylem into the phloem. (Water flows from high water potential to low water potential.)

  • As water moves into the phloem, it creates a positive pressure in the phloem (Hydrostatic pressure).

  • The high pressure forces mass flow movement of phloem sap from source to sink.

  • These sugars being moved by mass flow are removed from the phloem by the sink.

  • Removal of the sugars causes low solute concentration = high water potential, which causes water to leave the phloem and return to the xylem.

• Sucrose moves out of the phloem via diffusion.