transport in plants Flashcards
What is vascular tissue made up of
- xylem
- phloem
What is xylem responsible for
- transport of water and mineral ions
- providing support
What is the phloem responsible for
- translocation of organic solutes eg sucrose and amino acids
Describe the arrangement of the vascular bundle in the root and why it is arranged this way
- xylem is arranged centrally into a star shape with phloem outside it
- helps to anchor the plant into the soil and resist pulling forces
Describe the arrangement of the vascular bundle in the stem and why it is arranged this way
- arranged towards the periphery in a ring
- provides support to resist bending
Describe the arrangement of the vascular bundle in the leaf and why it is arranged this way
- arranged in the midrib
- resistance to tearing and flexibility
angiosperms
- flowering plants
What is the structure of xylem and where are they present
- water is conducted through vessels and tracheids whcih are dead cells due to lignin deposition in the walls
- fibres provide support and xylem parenchyma acts as packing tissue
- tracheids are present in flowering plants, ferns and conifers
- vessles are only present in flowering plants
Describe water uptake by the roots
- water enters the root hair cells via osmosis because soil solution has a higher water potential than the vacoule of the hair cell whcih contains ions and sugars
How are root hair cells adapted for uptake of water
having a large surface area
What are the three pathways of water across the cortex of the root from epidermis towards xylem in the centre
- apoplast pathway
- symplast patheway
- vacuolar pathway
What is the apoplast pathway
- the most signficant route
- involves water moving between spaces in the cellulose cell wall
What is the symplast pathway
- water moves through cytoplasm and plasmodesmate (strands of cytoplasm through cell wall pits)
Describe the vacoular pathway
is a minor route involves water passing from vacoule to vacoule
Describe the movement of water through the three pathways
- in the root pericycle is surrounded by a single layer of cells called the endodermis which forms a ring surrounding the vascular tissue in the centre of the root
- cell walls of the endodermis are impregnated with suberin forming an impermeable band called the casparian strip that drives water from apoplast pathway into the cytoplasm
- water potential of endodermal cell is raised by water being forced into them by the casparian strip and the acctive transport of sodium ions into the xylem
- this lowers the water potential of fluid in the xylem forcing water into the xylem by osmosis - root pressure
What does the endodermis help do
- endodermis helps regulate movement of water, ions and hormones into and out of xylem
How is water prevented from entering the xylem via the apoplast pathway
- prescence of lignin in cell walls of the xylem vessels water proofs them
- prevent water from entering the xylem via the apoplast pathway
describe the uptake of minerals
- includes nitrates and phospjates
- activley transported into the root hair cells against there concentration gradient
- also pass along apoplast pathway in solution
- once they reach casparian strip they enter the cytoplasm via active transport and then pass via diffussion or active transport into the xylem
transpiration
the evaporation of water vapour from the leaves or other above ground parts of the plant out through stomata into the atmosphere
adhesive forces
are created between charged on water molecules and their attraction with the hydophilic lining of the vessels
cohesive forces
are created by the attractive forces between water molecules due to their dipolar charges forming hydrogen bonds
capilliarity
movement of water up narrow tubes by capillary action
describe the movement of water from roots to leaves
- cohesion tension theory explains how water moves up xylem
- main mechanism that pulls water up the stem is transpiration which is a passive process
- transpiration pull relies on adhesive forces between water molecules and xylem and cohesive forces between water molecules, root pressure and capilliarity are also involved but alone would not be sufficient to raise water up the xylem to any signigficant height
How is the transpiration pull created
- water evaporate from the leaf air space through the stomata (diffusion through the cuticle as well) water is drawn from the inside the cells lining the space by osmosis
- these cells now have a lower water potential and so draw water from adjacent cells via osmosis
- this continues across the leaf until water is drawm from thee adjacent xylem vessel
- as water is drawn out of the xylem water moleculea are pulled up to replace those lost due to cohesive forces that exist between water molecules
- water molecules enter the xylem to replace those moving up by osmosis from the endodermal cells and water crosses the cortex from the root hair by the same method as in the leaf cells
How do plants balance water loss
transpiration with the need to get water and mineral ions to the leaves themselves so water loss is inevitable
Describe how temperature affects the rate of transpiration
- increasing the temperature causes the water molecules to gain more kinetic energy therefore increasing the rate of diffusion out into the atmosphere through the stomata
Describe how humidity affects the rate of transpiration
- as humidity of the air outside the leaf increases the difference between the inside and outside of the leaf decreases
- reducing the diffusion gradeint
Describe how air movement affects the rate of transpiration
as air speed increases saturated air is removed from the leaf surface more quickly therfore increasing the diffusision gradient
Describe how light intensity affects the rate of transpiration
- increasing light intensity will increase stomatal opening
What conditions will produce the hgihest rates of transpiration
hot, bright, dry windy day
What is a potometer and how does it work
- actually measures the rate of water uptake
- some water will be used in photosynthesis but if cells turgid the uptake rate is approximated as the transpiration rate
What is important to do when setting up a potometer
- cut the stem and fit it tp the potometer underwater as it prevents the formation of any air bubbles in the xylem vessels
- seal all joints with vaseline to prevent air entry
- blot leaves dry as any water on lead surface could create a humid layer
Describe what to do after setting up the potometer correctly
- introduce air bubble at the end of the capillary tube and measure the distance it travels in a set period
- the volume can be calculated easily if diamter of capillary tube is known
- carry out repeats
What are mesophytes
- live in temperate regions with adequate water supply but must survive times of the year when water is scarce of unavaliable
What are adaptations of mesophytes
- closing stomata is water scarce - cannot maintain turgor in guard cells
- shedding leaves and become dormant during winter
- overwintering beneath the ground as bulbs or corms
- annual plants producing seeds that can overwinter
What are xerophytes
plants adapted to living in dry environments by reducing water loss
What is an example of a xerophyte
marram grass
What are adaptations of xerophytes
- sunken stomata which trap humid air reducing water potential gradient between air spaces inside the leaf and outside air
- hairs around stomata which trap water vapour reducing water potential gradient between the leaf and the air
- rolled leaves whcih reduces the surface area over whcih transpiration occurs some plants reduce leaves to spines and use stem to photynthesise
- thick cuticle which further reduces water loss from leaf surface
What is hydrophyte
grow partially or fully submerged in water so lack of water is never a problem but ensuring they receieve adequate light and carbon dioxide for photosyntehsis is
Give an example of a hydrophyte
water lily
Give adaptations of a hydrophyte
- having stomata on the upper leaf surface which is in contact with the air
- stems and leaves have large air spaces providing buoyancy and a resovoir of oxygen and carbon dioxide
- having poorly developed xylem tissue no need to transport water as it is all around
- leaves have little or no cuticle as water loss is not a problem
- support tissue not needed as water is a supportive medium
What is translocation
- products of photsynthesis are transported in the phloem as sucrose from where they are produced (the source) to where they are used or stored as insoluble food reserves (the sink)
- phloem also transportes amino acids
What are the three types of cells the phloem contain
- sive tubes
- companion cells
- phloem parenchyma acting as packaging tissue
What are sieve tubes
- walls perforated with pores to produce longitudinal tubes that contain cytoplasm but no nucleus and most organelles disintergrate during their development
- the end walls so not break down but instead become perforated by pores forming the end plates
What are comanion cells
dense cytoplasm with nucleus and many mitochondria and are connected to each sieve tube by plasmodesmata
Describe evidence that phloem is the vessel involved in translocation
- radioactivley labelling carbon dioxide using C14 products and their paths can be traced by exposing the plant to x ray film
- these are called autoradiographs
- ringing experiments also been used wherebu the outer ring of stem is cut to remove phloem whilst leaving the xylem behind
- bulge forms above ring suggesting sugar moves down the stem in the phloem
- aphids allowed to feed on plants then were anaesthesised before removing the head leaving the feeding stylet in pleace
- analysis of the liquid extruding from the stylet showed that it was sucrose
Describe the disadvantage of mass flow theory
- sucrose and amino acids treanported at different rates in opposite directions in same phloem vessle
- how transport occurs thousands of times faster than is possible by diffusion
Describe the mass flow theory
- photosynthesising cells (source cells) produce glucose whcih is converted into sucrose which lowers the water potential of the cells
- as water enters the cell by osmosis hydrostatic pressure forces sucrose into the phloem sieve tube
- by increasing the level of solutes in the phloem the water potential is lowered and water moves in from the adjacent cells and xylem by osmosis down a water potential gradient
- raises the hydrostatic pressure in the phloem so that it has a higher pressure
- sucrose and dissolved soluted move by mass flow from a high to a low hydrostatic pressure down a pressure gradient
- at roots growing points (sink ells) the sucrose diffuses into the cells down a concentration gradient so is therefore removed from sieve tubes
- in sieve tubes it is converted to starch for storage or converted to glucose to be respired
- loss of sucrose from phloem raises the water potential higher than in xylem and adjacent cells
- water enters the xylem by osmosis
- water also moves from phloem to xylem down a water potential gradient causing a reduction in the hydrostatic pressure
- water moves up the xylem by transpiration
What do the theories involve that account for bidirectional transport
protein filaments
cytoplasmic streaming
