Topic 3: Exchange + Transport: Transport in Plants p2 Flashcards
Function of the bordered pits in xylem vessels
Bordered pits in xylem vessels allows for the lateral movement of water and dissolved ions out of the xylem and into specific tissue
Why do large multicellular plants need a transport system?
Large multicellular plants need a transport system:
-Small surface area to volume ratio > diffusion not fast enough -> for substances eg sucrose
-Long distance from external surface to cells
Adaptations of plants with no vascular bundles containing xylem
Adaptations of plants with no vascular bundles containing xylem:
-No/thin waxy cuticle -> wax production is a waste
-Large surface area to increase photosynthesis as transpiration is not an issue
-Many stomata to increase gas exchange
-Stomata on the top surface, as gas concentration is higher in air than water
Why is nile blue stain used for observing plants under a microscope?
Nile blue stain - increases contrast/to make nuclei visible/ to show no nuclei in sieve tubes
How can water still be lost from cut stem when all leaves have been covered by petroleum jelly?
Water can still be lost from cut stems due to evaporation from upper leaf surfaces
Possible sources of error in transpiration practical
Sources of error:
-Not all lower leaf surface covered
-Leaks in apparatus
-Shoot not cut under water
-Error in reading position of meniscus
Movement of water into root cells and into symplast pathways
Water moves into root cells and into the symplast pathway by osmosis
Movement of water up the stem in the xylem and in the apoplast pathway
Water moves up the stem in the xylem and in the apoplast pathway by mass flow
Movement of water out leaf via stomata
Water diffuses out of the leaf via stomata
Why is it beneficial for carbohydrates to be in the form of sucrose when transported within plants?
-Sucrose is soluble and so can be transported in sap
-Metabolically inactive so not used/removed during transport
Similiarities in the transport within the phloem and the transport within the xylem
Similarities in the transport within the phloem and the transport within the xylem:
-Solutes carried in solution in both
-Both carry mineral salts
-Both use mass flow/generated hydrostatic pressure
Differences in transport within the phloem and transport within the xylem
Differences in transport within the phloem and transport within the xylem:
-Transport in phloem can take place in different directions but transport in the xylem only takes place up in the plant
-Phloem carries carbohydrates and xylem does not
-Phloem transports using living cells and xylem does not
-Xylem uses capillary action/cohesion and adhesion, whereas phloem does not
How does the casparian strip prevent ions reaching the xylem in the apoplast pathway?
-Casparian strip does not allow solutions and water through
-Forces the solutions and water to pass through a plasma membrane
-Phospholipid bilayer repels ions/charged particles
How do plants that live in soil with low water potential eg in salt marsh communities (halophytes)?
Halophytes (plants that live in soil with low water potential):
-Lower water potential inside root hair cells
-Actively transport ions into root hair cells
How does mass flow of the phloem sap occur in plants with a vascular system?
How mass flow of the phloem sap occurs in plants with a vascular system:
-Sucrose/assimilates in sieve tube elements
-Assimilates enters sieve tube/phloem (at source) and lowers water potential in sieve tube
-Water enters sieve tube by osmosis
-Assimilates leave sieve tube and increases water potential in sieve tube
How does increased air movement increase the loss of water vapour from leaves?
Increased air movement:
-Water vapour around stomata blow away
-Reduces water vapour potential around the stomata
-Increases water vapour potential gradient between air space in leaf and outside
How does not having vascular tissue affect the size to which plants can grow?
Effect on not having vascular tissue on plant growth:
-Cannot grow large
-No support from xylem
-Uses only diffusion - no mass flow/rapid transport
-Diffusion too slow (short diffusion pathway, large surface area to volume ratio)
Parts of the leaves where water may be lost, other than the leaf/stomata
Parts of the leaves where water may be lost, other than the leaf/stomata:
-Epidermis/Cuticle
What increases the cohesion-tension theory (capillary action) in plant?
Evaporation at top of plant creates tension in the xylem
Types of cells that can be found in the phloem tissue
Types of cells that can be found in the phloem tissue:
-Parenchyma, companion cells, sieve tube elements
Benefits to plants of internal transport systems
Benefits to plants of internal transport systems:
-Surface area to volume ratio originally too small -> insufficient diffusion
-Transport system ensures molecules reaches all tissues
-Allows for a high metabolic rate
Guard cell location
Guard cells are located at the leaves and contain chloroplast
Substances transported by companion cells
Companion cells transport sucrose
Substance example transported by root hair cells
Nitrate ions are transported by root hair cells
Translocation definition
Translocation = the movement of nutrients around a plant - sugars, amino acids and other organic molecules
Sources in phloems
Sources - these provide assimilated to the plant from reactions or storage for transport
Examples of sources in plants
Sources:
-Green leaves and stems
-Storage organs eg tubers and root taps
-Food stores in seeds
Sinks in plants
Sinks - these use assimilates from the plant in various processes
Examples of sinks in plants
Sinks:
-Growing roots
-Active process in the roots and stem
-Meristem cells activity - dividing
-Developing stores eg seeds, fruits or storage organs
Two routes by which assimilates are moved into the phloem
Two routes by which assimilates are moved into the phloem:
-Symplast route
-Apoplast route
The symplast route by which assimilates move in the phloem
The symplast route by which assimilates move in the phloem:
-Assimilates are moved through the cytoplasm of mesophyll cells into the sieve tubes across connecting plasmodesmata
-Passive provess
-Assimilates are moved by changes in water potential of cells
The apoplast route by which assimilates move in the phloem
The apoplast route by which assimilates move in the phloem:
-Assimilates diffuse through the cell wall and intermembrane spaces
-When they reach the companion cells, they are actively transported across the membrane into sieve cells cytoplasm
-Hydrogen ions act as co-transporters and actively move assimilates across the membrane
Process of active loading in plants
Active loading in plants:
-Hydrogen ions are actively pumped out of companion cells
-Flows back in down a concentration gradient, through a co-transporter protein + brings sucrose in with it
Mass flow in the phloem
Mass flow in the phloem:
1). Sucrose actively loaded into sieve tube element and reduces the water potential
2). Water flows in by osmosis and increases the hydrostatic pressure in the sieve tube element
3). Water moves down sieve tube from higher hydrostatic pressure at source to lower hydrostatic pressure at sink
4). Sucrose is removed from the sieve tube by the surrounding cells and increases the water potential in the sieve tube
5). Water moves out of the sieve tube and reduces the hydrostatic pressure
Evidence of mass flow in the phloem
Evidence for mass flow in the phloem:
-Advances in microscopy - allows us to see adaptation of companion cells for active transport
-Poisoning mitochondria - translocation stops
-Flow of sugars in phloem much faster than it would be by diffusion alone - shows active process is involved
-Aphids
How can aphids be used as evidence for mass flow in the phloem?
Aphids - shows there is a positive pressure in the phloem that forces the sap out through a stylet - the pressure and flow rate in phloem is lower closer to sink than near the source - concentration of sucrose in phloem sap is also higher near source than sink
Argument against the use of mass flow of substances in the phloem
Not all solutes in the phloem move at the same rate, and no one is sure yet of the use of sieve plates in the process
Sieve tube elements in phloems
Sieve tube elements:
-No organelles to form hollow tube that transports substances
-Each has a companion cell - companion cells have the organelles and carries out the metabolic reactions
Sieve plates in the phloem
Sieve plates in the phloem:
-Sieve plates have large pores that allow sap to move through the sieve tube elements.
-Sieve plates allow sugars to be transported through the phloem.
Why is sucrose concentration low at sinks?
Sucrose concentration is low at sinks because it is being used up as it moves along the plant from source to sink
