9 - Transport in plants Flashcards
Why do multicellular plants need a transport system?
- substances such as glucose and oxygen need to be transported to cells that do not photosynthesise.
- waste products of cell metabolism need to be removed.
- small SA:V ratio
- relatively big with high metabolic rate.
- exchange by diffusion alone would be too slow to meet metabolic needs.
- hormones need to be transported to where they are required.
What two types of tissues are part of the vascular system?
- xylem
- phloem
How are the vascular bundles arranged in a stem?
- phloem outside
- xylem inside
- cambium in between
vascular bundle around edge to give strength and support.
cambium layer contains meristem cells.
How are the vascular bundles arranged in a root?
- xylem in x structure in middle
- phloem in 4 sections around xylem
- root hair surrounding
vascular bundle in middle to help plant withstand tugging strains (e.g wind).
How are the vascular bundles arranged in a leaf?
- xylem on top
- phloem on bottom
midrib (largest middle part of leaf) is the main vein carrying the vascular tissue.
Function of xylem
- Transports water and dissolved mineral ions
- one direction movement (upwards). Water moves from roots towards leaves.
- provides structural support
- passive process
- TRANSPIRATION
Functions of phloem
- Transports organic solutes and dissolved sugars.
- bidirectional movement.
- from leaves to rest of plant.
- active process
- TRANSLOCATION
what cells are present in xylem?
- xylem vessels
- xylem fibres
- xylem parenchyma
what cells are present in phloem?
- sieve tube elements
- companion cells
- parenchyma
- phloem fibres
explain the structure of the xylem
- xylem vessels are long hollow structures formed from cells (dead) joined end to end.
- no end walls between the cells, forming a continuous hollow tube.
- thick lignified walls (lignin) help to support the xylem vessels and prevent them from collapsing inwards under the transpiration pull.
- lignin deposited in walls as spirals or distinct rings.
- water and mineral ions move in and out of xylem vessels through non-lignified pits.
explain the structure of phloem
sieve tube elements:
living cells joined end to end to form a long, hollow structure.
- sieve plates are between the cells, and are perforated to allow phloem contents through.
- no nucleus, very thin cytoplasm, very few organelles (unusual for a living cell).
Companion cells
- 1 companion cell for every sieve tube element.
- they are linked by many plasmodesmata.
- they have a nucleus and organelles.
- they carry out the living functions for both themselves and the sieve tube cells.
- e.g provide energy for active transport of solutes.
- no lignified walls
Sieve tube cells are living cells. Why are they unusual?
- no nucleus
- very thin layer of cytoplasm
- vey few organelles.
What is the phloem filled with?
- phloem sap
Which kind of plants have a vascular system consisting of xylem and phloem?
herbaceous dicotyledonous plants
Why do plants need water?
- to transport mineral ions and sugars in aqueous solution.
- water is a reactant photosynthesis
- cooling effect by transpiration
- turgor pressure.
What are the adaptations of a root hair cell?
- microscopic means they can penetrate between soil particles.
- large SA:V ratio as they are microscopic in size.
- thin surface layer provides a short diffusion and osmosis distance.
- concentration of solutes in cytoplasm of root hair cells maintains a water potential gradient between soil water and cell.
What is osmosis?
the movement of water molecules from an area of higher water potential to an area of lower water potential across a partially permeable membrane.
What are the 3 different pathways water can take from the root to the xylem?
- symplast pathway
- apoplast pathway
- vacuolar pathway
Symplast pathway (cytoplasm)
- water travels through the living parts of cells (cytoplasm)
- the cytoplasm of neighbouring cells are connected by plasmodesmata.
- osmosis
apoplast pathway (cell wall)
- water travels through the non-living parts of cells (cell walls).
- water can from carry solutes and move area of high hydrostatic pressure to areas of low hydrostatic pressure.
vacuolar pathway
through vacuoles as well as cytoplasm.
Explain the casparian strip
- water travels through symplast, apoplast, vacuolar pathways and reach the endodermis.
- path is blocked by the casparian strip.
- it is a waxy (suberin) strip which is impenetrable.
- water is forced through the symplast pathway.
- useful as water has to go through partially permeable cell surface membrane.
- controls which substances get through to xylem.
- toxic solutes are removed and water moves on to the xylem.
How does water leave the plant?
- water leaves xylem and move into mesophyll cells by apoplast pathway.
- water evaporates from the cell walls to large air spaces between cells in the leaf.
- when stomata in the leaf open, water diffuse out of the leaf (down water potential gradient).
- loss of water from a plant’s surface is called transpiration.
How does water move up the plant against gravity?
Cohesion and tension:
- as water evaporates from the leaves, this creates tension, which pulls up more water into the leaves.
- cohesion means that when some water molecules are pulled into the leaf, other water molecules follow.
- means that water moves up the xylem in a continuous stream.
Adhesion:
- water molecules are attracted to the walls of the xylem vessels.
- this helps water molecules to rise up the xylem vessels..
evidence for cohesion tension theory
- changes in tree diameter (high transpiration rates during day diameter decreases due to increased tension) and vice versa
- broken xylem vessel: stops drawing up water as air is drawn in breaks the transpiration stream.
Factors affecting transpiration rate
- Light intensity
- Relative humidity
- temperature
- Wind
- soil-water availability
How does light intensity affect transpiration rate
- at higher light intensity
- more stomata open
- evaporation from leaf increases
- rate of transpiration increases
How does relative humidity affect transpiration rate
- at higher relative humidity
- water potential gradient between inside leaf and surroundings becomes shallower
- rate of transpiration decreases
How does temperature affect transpiration rate
- at higher temperatures
- kinetic energy of water molecules increases, so water evaporates from the leaves quicker.
- water potential gradient between inside leaf and surroundings increases.
- rate of transpiration increases.
How does wind affect transpiration rate
- at higher wind speeds
- when water is lost from a leaf, a thin layer of water forms outside the leaf around stomata. This decreases the water potential gradient.
- wind blows this layer away, increasing the water potential gradient between the inside of leaf and surroundings.
- rate of transpiration increases.
How does soil-water availability affect transpiration rate
- at higher soil-water availability
- rate of transpiration increases.
In a potometer, how do you work out the volume of water taken in by the plant?
πr²h
In a potometer, how do you work out the rate of transpiration and units?
mm3min-1
πr²h (mm3) / time taken (mins)
What are xerophytes and examples?
plants adapted to living in dry conditions
- cacti
- marram grass
What are hydrophytes and examples?
plants that live in or on water.
- water lilies
How are xerophytes adapted to their environments?
- sunken stomata: sheltered from wind.
- hairs and pits: traps moist air. Reduces the water potential gradient between the inside and outside the plant.
- rolling leaves: traps moist air (reduces water potential between outside and inside plant), reduces exposed surface area for transpiration to occur.
- thick waxy cuticle
- densely packed mesophyll
How are hydrophytes adapted to their environments?
- very thin or absent waxy cuticle. (don’t need to conserve water)
- many always-open stomata on upper surfaces.
- reduced structure of plant (water supports the leaves and flowers)
- wide, flat leaves: to increase surface area for photosynthesis.
Define transpiration
Transpiration is the passive process of the loss of water vapour by evaporation from the surface of leaves and stems of a plant.
Define translocation
Translocation is the active process to transport assimilates, especially sucrose, in the phloem between sources and sinks.
what is the sugar transported as in translocation
glucose is transported as sucrose.
source examples?
- green leaves (can be both source and sink) and green stems
- tubers and tap roots
- food stores in seeds during germination
sink examples?
- growing roots
- meristems
adaptations of companion cells for translocation?
- lots of mitochondria to provide ATP for active pumping of H+ ions and sucrose across its membrane
- nucleus to control the activities of itself and the sieve tube element.
two routes which assimilates are loaded into the phloem?
- symplast route (passive - through plasmodesmata)
- apoplast (active)
What are the three stages of translocation?
- active loading
- mass flow
- unloading at the sink
Stages of active loading
- hydrogen ions are actively pumped by proton pumps from the companion cells into the source by active transport (ATP provides energy).
- hydrogen ion concentration increases outside the companion cell.
- hydrogen ions re-enter companion cells along with sucrose molecules via carrier proteins by facilitated diffusion (passive).
- sucrose molecules then diffuse into phloem sieve tube elements via the plasmodesmata.
Stages of mass flow
- increase in solute concentration in the sieve tube decreases water potential.
- water enters sieve tube by osmosis from xylem.
- hydrostatic pressure inside sieve tube at the source increases.
- water and solutes move towards sink down a hydrostatic pressure gradient along the phloem.
stages of unloading at the sink
- solutes leave the sieve tube at sink by diffusion
- water potential inside sieve tube increases.
- water moves out of sieve tube by osmosis to xylem.
- water potential and hydrostatic pressure decreases. Creates a low hydrostatic pressure at sink compared to high hydrostatic pressure at source.
evidence of mass flow theory
- advances in microscopy
- if mitochondria in companion cell are poisoned, translocation stops.
- feeding aphids
- rate of flow of sugars in phloem (10000 times faster than if diffusion alone, suggesting an active process for mass flow).
