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.