3.3) Transport in plants
Terms in this set (50)
What are the functions of phloem?
-Dissolved assimilates, Up or down
What are the functions of xylem?
-Water and minerals, up
-Fibres to help support the plant
-Parenchyma cells which act as packing tissue to separate and support the vessels
What is vascular tissue?
Made up of cells specialised for transporting fluids by mass flow
Why is demand for oxygen low in plants?
Plants aren't very active and their respiration rate is low
How are xylem and phloem arranged in young root?
-Central core of xylem often in X shape
-Phloem is found in between the arms of the X shaped xylem tissue
-This arrangement provides strength to withstand the pulling forces to which roots are exposed
-Sheath of cells around vascular bundle called endodermis
How are xylem and phloem arranged in the stem?
-Vascular bundles found near the outer edge of the stem
-Xylem found towards the inside and phloem towards the outside
-Layer of cambium in between xylem and phloem
-Ring of vascular tissue under bark provides strength and flexibility
How are xylem and phloem arranged in the leaves?
-Vascular bundles form midrib and veins of a leaf
-Xylem is on top of the phloem
What does lignin do?
-Impregnates the walls of the cells making them waterproof
-This kills the cells
What are bordered pits?
-Places where lignification is not complete
-Allow water to leave one vessel and pass into the next
-Allow water to leave the xylem and pass into the living parts of the plant
How is xylem adapted to its function?
-Made from dead cells aligned to form a continuous column
-Tubes are narrow so the water column doesn't break easily
-Bordered pits allow water to move sideways
-Lignin in the walls
-No cell contents
What is phloem made up of?
-Sieve tube elements
How are sieve tube elements adapted?
-No nucleus and little cytoplasm
-Perforated cross-walls called sieve plates
How are companion cells adapted?
-Large nucleus and dense cytoplasm
-Lots of mitochondria to produce ATP needed for active processes
What are the gaps in cells walls between 2 cells called?
What are the three pathways water can take?
-Apoplast (through spaces between cell walls)
-Symplast pathway (Water enters through plasma membrane and then passes through plasmodesmata from one cell to another)
-Vacuolar pathway (Similar to Symplast but water passes through vacuoles as well as cytoplasm)
What is the water potential of pure water?
What happens when you put a plant cell in pure water?
-Water moves into the water by osmosis
-water potential of cell lower than of water
-Cell becomes turgid
What happens when you place a plant cell in a salt solution?
-Water moves out of the cell
-Water potential of the cell higher than the solution
-Tissue becomes flaccid
-Cytoplasm no longer pushes against cell wall, plasmolysis
What happens when 2 cells with different water potentials are next to each other?
Water moves by osmosis down its concentration gradient into neighbouring cell until water potentials are equal
What is a potometer?
A device that can measure the rate of water uptake as a leafy stem transpires
What is transpiration?
The loss of water vapour from the aerial parts of a plant, mostly through the stomata in leaves
What happens during transpiration?
-Water enters leaf through xylem and moves by osmosis into the cells of the spongy mesophyll
-Water evaporates from cell walls of the spongy mesophyll
-Water vapour moves out of the leaf by diffusion through the open stomata
Why is transpiration important?
-Transports useful mineral ions up the plant
-Maintains cell turgidity
-Supplies water for growth, cell elongation and photosynthesis
-Supplies water that evaporates and can cool down the plant
How does light intensity affect transpiration rate?
-In light stomata open to allow gaseous exchange for photosynthesis
-Higher light intensity increases transpiration rate
How does temperature affect transpiration rate?
-Increase rate of evaporation from cell surfaces so there's a higher water potential in the leaf
-Increase rate of diffusion through stomata because water molecules have more kinetic energy
-decreases water potential in air so water vapour diffuses out of stomata quicker
-Increases transpiration rates
How does relative humidity affect transpiration rate?
-Smaller water potential gradient between air in leaf and outside leaf
-Decreases transpiration rate
How does air movement affect transpiration rate?
-Air moving will carry away water which has just diffused out of the leaves
-This will maintain a higher water potential gradient
-Increases transpiration rate
How does water availability affect transpiration rate?
-If there is little water in the soil then water lost can't be replaced
-Stomata will close and plant will wilt
-Decreases transpiration rate
How does water get from the soil across the root into the stem?
-Root hair cells absorb mineral ions and water from the soil (mineral ions actively transported)
-Water moves by osmosis down its water potential gradient to the endodermis
-Can only move by Symplast pathway after casparian strip because it blocks the apoplast pathway
-Mineral ions actively transported into medulla and water follows because water potential is lower
What is the role of the endodermis?
-Caspian strip blocks apoplast pathway and ensures water and mineral ions enter the cytoplasm
-Plasma membrane contain transporter proteins
What is mass flow?
Flow of water and mineral ions in one direction
What is root pressure?
-Moving minerals into medulla by active transport draws water into the roots by osmosis
-Pressure in the medulla builds up and forces water into the xylem
-Can only push water a few metres up the stem
What is the transpiration pull?
-Loss of water from leaves must be replaced from below
-Water molecules attracted to each other by cohesion, this holds water molecules in a long chain
-As molecules are lost the chain is pulled up, this creates tension
What is capillary action?
-Adhesion attracts water molecules to the sides of the xylem vessels
-Xylem vessels are narrow so water can be pulled up the sides of the xylem
What is adhesion?
The attraction between water molecules and the walls of the xylem vessels
What is cohesion?
The attraction between water molecules caused by hydrogen bonds
What is a hydrophyte?
A plant adapted to living in water or where the ground is very wet
What is a xerophyte?
A plant adapted to living in dry conditions
How do plants reduce water loss?
-Waxy cuticle will reduce water loss through the epidermis
-Stomata found on the under surface of leaves
-Stomata close at night
-Some plants loose leaves in winter when the ground is frozen
-Stomata in pits which are flooded and covered by hairs
-Dense spongy mesophyll
-Lower water potential in leaves (high salt concentration)
How is marram grass adapted?
-Rolled leaves so air is trapped inside
-Thick waxy cuticle
-Stomata on inside of leaf in pits covered by hairs
-Dense spongy mesophyll
How are cacti adapted?
-They are succulents (store water in their stems)
-Leaves reduced to spines to reduce surface area
-Stem is green for photosynthesis
-Roots are very widespread
How are water lilys adapted?
-Many large air spaces in leaves to make them float and absorb sunlight and then allow oxygen to diffuse quickly to the roots for aerobic respiration
-Stomata on upper epidermis so they're exposed to air
What are hydathodes?
Special structures which can release water droplets that can then evaporate
What is a sink?
A part of the plant where materials are removed from the transport system
What is a source?
A part of the plant that loads materials into the transport system
What is translocation?
The transport of assimilates throughout a plant
What is active loading?
-H+ ions pumped out of cells using ATP
-H+ ions diffuse (facilitated diffusion) back in bringing sucrose molecules with them (can only get through cotransporter proteins if attached to sucrose)
-Sucrose concentration in companion cell higher so it diffuses into phloem through plasmodesmata
How does sucrose move through phloem?
-By mass flow
-Flow caused by difference in hydrostatic pressure between 2 ends of the tubes
What direction does the sucrose flow in?
-Any direction (not all at once)
-Water leaving tube at sink reduces pressure
-Water entering at source increases pressure
What areas could be sources and sinks?
-Anywhere in plants
-Example of source could be roots
-Example of sink could be leaves or cells that are using sucrose