Plant Transport Chapters 28 & 29

REVIEW Transport – movement of molecules Passive transport – down a concentration gradient with out energy Osmosis – water Aquaporins – assist water through bilayer Facilitated Diffusion - large / polar molecules with transport protein Simple diffusion – small molecules directly through bilayer Active transport – against a concentration gradient with energy Endocytocis – engulf large molecules Exocytosis – remove large molecules Pumps – ions / large molecules

Proton Pumps Cotransport Creates a proton concentration gradient using ATP so other molecules can passively enter the plant cell. Sugar, NO3-

Water Potential and Osmosis Water moves from a higher water potential to a lower water potential Ψ = Ψs + Ψp Ψs = solute potential (osmotic potential) Adding solutes to a solution lowers the solute potential and there fore the overall water potential Ψp = pressure potential

Osmosis and Plant Cells Isotonic – external solute concentration the same as the internal solute concentration Water moves in and out at the same rate Dynamic Equilibruim Flaccid or wilty Hypertonic – external solute concentration greater than the internal solute concentration Water moves out of cell Plasmolyszed Hypotonic – internal solute concentration greater that external solute concentration Water moves in Turgid Healthy plant

s = –0.9 (a) 0.4 M sucrose solution:  = 0 s = –0.9  = –0.9 MPa  = 0 s = –0.7  = –0.7 MPa Initial flaccid cell:  = 0 s = 0  = 0 MPa Distilled water: Plasmolyzed cell at osmotic equilibrium with its surroundings  = 0  = –0.9 MPa  = 0.7 s = –0.7  = 0 MPa Turgid cell at osmotic Initial conditions: cellular  > environmental . The cell loses water and plasmolyzes. After plasmolysis is complete, the water potentials of the cell and its surroundings are the same. Initial conditions: cellular  < environmental . There is a net uptake of water by osmosis, causing the cell to become turgid. When this tendency for water to enter is offset by the back pressure of the elastic wall, water potentials are equal for the cell and its surroundings. (The volume change of the cell is exaggerated in this diagram.) (b)

Plant Cell Cell wall Cytoplasm Vacuole Plasmodesta (plasmadesmata) Vacuolar membrane is called the tonoplast Transport proteins in the plasma membrane regulate traffic of molecules between the cytosol and the cell wall. the vacuolar membrane regulate traffic of molecules between the cytosol and the vacuole. Plasmodesma Vacuolar membrane (tonoplast) Plasma membrane Cell wall Cytosol Vacuole Cell compartments. The cell wall, cytosol, and vacuole are the three main compartments of most mature plant cells. (a) Figure 36.8a

Movement of materials Symplast Apoplast Bulk Flow Movement of materials through the cytoplasm of the cell through the plasmodesta Must cross cell membrane Apoplast Movement of material through the cell wall and extracellular spaces Doesn’t have to cross cell membrane until it reaches the Casparian strip in the endodermis Bulk Flow Long distance transport Xylem or phloem

Roots to Xylem Root Pressure – pushing of water from the root to the xylem Water and minerals travel through The epidermis of the root The cortex The stele (inside the endodermis) which contains the tracheary elements Xylem - tracheids – dead cells

Symplast vs. Apoplast Symplast Path Apoplast Path In cytoplasm (protoplasm) In cell walls Water enters the root cells by crossing the plasma membrane Water does not enter the root cells, but stays in the cell walls Not blocked by the Casparian strip Blocked at the endodermis by the Casparian strip Can continue to xylem Cannot continue to xylem unless it passes into the root cells through the plasma membrane

Key Symplast Apoplast Transmembrane route The symplast is the continuum of cytosol connected by plasmodesmata. The apoplast is the continuum of cell walls and extracellular spaces. Transmembrane route Symplastic route Apoplastic route Transport routes between cells. At the tissue level, there are three passages: the transmembrane, symplastic, and apoplastic routes. Substances may transfer from one route to another. (b)

Bulk Flow – Water and Minerals Transpiration – “pulling” of water from the roots to the leaves of plants Hydrogen Bonding Cohesion – keeps H2O together as it is pulled upward Adhesion – keeps H2O from falling with gravity Sticks to the sides of the xylem

Transpiration – pg 589 Chain Reaction Water evaporates through open stomata Pulls water from mesophyll cells to the stomata Plant cells shrink (plasmolysize) due to the loss of water Tension is created which pulls the water in the xylem up the plant to replace the water in the plasmolysized cells

Water potential gradient Xylem sap Outside air Y = –100.0 MPa Leaf Y (air spaces) = –7.0MPa Leaf Y (cell walls) = –1.0 MPa Trunk xylem Y = – 0.8 MPa Water potential gradient Root xylem Y = – 0.6 MPa Soil Y = – 0.3 MPa Mesophyll cells Stoma Water molecule Atmosphere Transpiration Adhesion Cell wall Cohesion, by hydrogen bonding Root hair Soil particle Cohesion and adhesion in the xylem Water uptake from soil

Factors that affect transpiration Heat – increases rate CAM plants only have stomata on the underside of the leaves to conserve H2O Wind – increases rate Humidity – decreases rate # of stomata – increases rate

Transpiration Regulation Guard cells around the stomata in the epidermis of the plant leaf CO2 enters, O2 leaves and H2O evaporates Light stimulates the stomata to open to allow CO2 to enter for photosynthesis

20 µm

Xylem vs. Phloem Transport – Bulk Flow Dead Cells – Tracheids Living Cells – Sieve Tube Members Water and Minerals Organic Compounds (sugar) Unidirectional Movement (up) Bi-Directional Movement (down, up, side to side) Fast – max rate 15 meters / hr Slow – max flow rate 1 meter / hr NO ATP ATP Transpiration Translocation

Translocation Photosynthesis occurs in the leaves and produces organic compounds (sugar) for the plant. These organic substances travel via translocation in the phloem from source to sink Source – an organ that produces more sugar than it requires (leaf) Sink – an organ that does not make enough sugar for its own needs (flower, developing bud, root, or fruit)

Source (Leaf) to Sink (Organ of need) Mesophyll cell produces sugars via photosynthesis Sugars are transported through bundle sheath cells and companion cells before entering the sieve tube members. This process occurs via the symplast through the plasmodesta of the cell walls. Once in the sieve tubes, sugars are transported to the organ of need (sink).

Mesophyll cell Cell walls (apoplast) Plasma membrane Plasmodesmata Companion (transfer) cell Sieve-tube member Phloem parenchyma cell Bundle- sheath cell

Pressure Flow Model – pg 594 Sugars are pumped by active transport into companion cells and sieve tube members (phloem). As sugars (solutes) accumulate, water enters the sieve tube via osmosis from nearby xylem. Turgor pressure builds in the sieve tube at the source end and the fluid is pushed to the sink organ, which has a lower turgor pressure.

Pressure Flow Model – pg 594 Sugars are removed using active transport at the sink cells. Removed sugars are either used (metabolized) or converted into starch for storage. Left over water in the phloem is pulled into the nearby xylem and transported back up the plant.