MTO 3.3 TRANSPORT IN PLANTS Flashcards
What’s a Dicotyledonous Plant?
- make seed that contain two cotyledons (organs).
-act as food stores for developing embryo plant and form initial leaves in germination.
Vascular Bundle [Roots]
- Xylem and Phloem found together.
- found in centre of root.
- ring of endodermis around vascular bundle.
- inside endodermis are ring of meristem cells called pericycle.
Vascular Bundles [Stem]
- found near outer edge of stem.
- xylem near inside of bundle.
- phloem near outside.
- in between xylem and phloem -> Cambium. Layer of meristematic cells.
Vascular Bundle [Leaves]
- form veins of leaves.
- xylem are above phloem.
Xylem Tissue
- transports water and dissolved minerals up the plant. (Transpiration)
- consists of xylem vessels, fibres and parenchyma cells.
Structure of Xylem Vessels
Continuous, hollow tubes with no end walls —> water movement much faster. More space for water to flow due to lack of content.
Walls reinforced with Lignin —> strengthens wall (prevents collapse under tension). Water-proof walls reduce lateral flow of water. Increases capillarity.
Lignification in Spiral Pattern —> allows flexibility and stretching of stem.
Bordered pits (pores) in walls of vessels —> allow lateral flow of water between vessels to get around blockage (e.g. air bubbles).
Narrow Lumen —> more effective capillary action.
Phloem
- transports sucrose up and down the plant.
- made of sieve tube elements and companion cells.
- sucrose transported as sap (dissolved in water).
Structure and Function of Sieve Tube Elements
Little Cytoplasm + most organelle absent —> less resistant for transport and more space for transport.
Sieve Plates —> connect sieve tube elements to allow sucrose through.
Joined end to end to form tube —> allows continuous transport.
Bidirectional Flow —> allows sucrose to go both up and down plant.
Living —> allows active transport.
Structure and Function of Companion Cells
Many Mitochondria —> lots of respiration. Provides large amounts of ATP for active processes.
Nucleus —> controls functions of both companion cell and sieve tube elements.
Plasmodesmata —> allows continuation of cytoplasm between companion cell and sieve tube element - transport of molecules such as protein and ATP.
Osmosis
Passive Movement of water molecules from region of higher water potential to region of lower water potential across a partially permeable membrane.
Turgid
Plant cells swell as lots of water enters it through osmosis.
Plasmolysed
Cell membrane pulls away from cell wall.
Root Hair Cells Adaptations to their Functions
- found on epidermal layer of plant roots.
- hair-like projection in soil —> large SA for osmosis and mineral uptake.
- thin wall —> short diffusion pathway.
- many mitochondria —> energy for active transport of minerals.
Pathways for Osmosis [Apoplast]
Apoplast Pathway (Cell Wall)
-> water travels through the cell walls in gaps.
-> doesn’t cross membranes and so can’t enter the cytoplasm.
Pathways for Osmosis [Symplast]
-> water crosses the cell surface membranes via osmosis (through aquaporins) and can then move through plasmodesmata.
Pathways for Osmosis [Vacuolar Pathway]
-> similar to symplast but water also moves through through the vacuoles not just the cytoplasm.
The Casparian Strip
- Water pot lowest in xylem than anywhere else in root. Causes osmosis of water from root hair cell from cortex to endodermis.
- Lowers water pot in xylem. Water now crosses the cell surface membranes from the cortex to the endodermis.
- On cell walls of the cells of the endodermis strip of waterproof material called Suberin known as Capsparian Strip.
- Casparian Strip blocks Apoplast pathway between the cortex and xylem - water now must take the symplast pathway.
- Minerals must be actively transported from the cytoplasm into the xylem - water must take the symplast pathway.
Transpiration
Loss of water by evaporation out of plants leaves via stomata. Happens at same time as gaseous exchange through the stomata - as oxygen leaves the leaf, so does water.
3 Ways Water is helped to move up the xylem vessels from the root:
- Root Pressure (push of water entering xylem vessel in roots. Doesn’t move water far.)
- Capillary Action (adhesion (forces of attraction) of water molecules to lignin in narrow xylem vessels can pull the water up the sides of the vessel.)
- Transpiration Pull (most of the driving force.)
Why water molecules are cohesive and adhesive
- Adhesive because they form hydrogen bonds with lignin.
- Cohesive because they form hydrogen bonds with each other.
Factors affecting Transpiration
Number of Leaves -> more leaves = more water loss. Larger Surface Area for Evaporation.
Number and Size of Stomata -> more stomata = more water loss.
Waxy Cuticle Present -> less water loss. Hydrophobic.
Light -> more water loss. Stomata open wider for photosynthesis so more SA for water loss by diffusion.
Temperature -> more water loss when higher temp. More kinetic energy. Water diffuses faster.
Humidity -> more water loss = high humidity. Air more saturated with water therefore water potential gradient is shallower.
Wind -> more water loss = more wind. Carries water that has diffused out away maintaining high water potential gradient.
Water Availability -> more water loss (water in soil). Can’t replace water that’s been lost.
Xerophytes
Plant adapted to reduce water loss by transpiration so that it can survive in very dry conditions. E.g. Maram Grass.
Adaptations of Plants to reduce Water Loss
Waxy Cuticle (hydrophobic)
Stomata on underside of Leaf (reduce evaporation due to sun)
Stomata close at Midnight (no light for photosynthesis)
Deciduous Plant lose leaves in winter (when may not be able to photosynthesise.)
Hydrophyte
Plant that is adapted to living in water or where the ground is very wet.
