Transport in plants

Question 1

What are the two types of vascular tissue and briefly describe their roles

Answer 1

• xylem and phloem
• xylem is responsible for the transport of water and mineral ions as well as providing support
• phloem is responsible for translocation of organic solutes eg sucrose and amino acids

Question 2

Name the layers in the root working from the outside to the inside of the cell

Answer 2

• root hair
• epidermis
• cortex
• endodermis
• pericycle
• phloem
• xylem

Question 3

Where is the casparian strip found

Answer 3

• endodermis

Question 4

What does the stele contain

Answer 4

• phloem
• xylem
• pericycle

Question 5

Describe the structure of the xylem

Answer 5

• water is conducted through vessels and tracheids which are dead cell due to lignin deposition in walls
• fibre provides support and xylem parenchyma acts as a packaging tissue
• tracheids are present in flowering plants (angiosperms) ferns and conifers vessels only presnet in flowering plants

Question 6

Describe the process of water uptake by the roots

Answer 6

• root hair cells have adaptations uptak of water by having large surface area
• water enters root hair cells by osmosis because soil solution has higher water potential than vacuole of hair cell whcih contains ions and sugars
• water moves across cortex of root from epidermis towards xylem in the centre via three different pathways
• prescence of lignin in cell walls ox xylem vessels waterproofs them
• prevent water entering xylem via apoplast pathway
• in root pericycle surrounding single layer of cells called endodermis form ring surrounding vascular tissue in centre of root
• cell walls of endodermis impregenating with suberin forming impermeable band known as casparian strip driving water from apoplast into cytoplasm
• endodermiss helps to refulate movement of water ions and hormones into and out of xylem
• water potential of endodermal cells raised by water being forced into them by casparian strip and active transport of sodium ions into xylem. loweirng the water potential of fluid in xylem forcing water into xylem by osmosis : root pressure

Question 7

What are the three different pathways i which water travels through root cells to xylem

Answer 7

• apoplast pathway - most significant route involves water moving between spaces in cellulose cell wall
• symplast pathway - water moves through cytoplasm and plasmodesmata (strands) of xytoplasm through cell wall pits
• vacular pathway- minor route involves water passing from vacuole to vacuole

Question 8

How is minerals uptaken by plants

Answer 8

• minerals eg nitrates and phosphates activley transporteed into root hair cells against their concentration gradient
• pass along apoplast pathway in solution
• Once reach casparian strip enter cytoplasm via active transport and then pass via diffusion of acitve transport into xylem

Question 9

Describe the movement of water from roots to the leaves

Answer 9

• cohesion tension theory - moves water up xylem
• main mechanisms pull water up stem is transpiration - passive process
• transpiration pull relies on adhesive forces between water molecules and xylem and cohesive forces between water molecules root pressure and capillary
• transpiration pull created water evaporates from leaf air speace through stomata (diffusion through cuticles) water drawn from inside cells lining space by osmosis
• cells now have a lower water potential draw water from adjacent cells by osmosis and continues across leaf until water is drawn from adjacent xylem vessel as water is drawn out of xylem water molcules are pulled to replace the lost due to cohesie forces exist between water molecules
• water molecules enter xylem to replace those moving up by osmosis from endodermal cells and water crosses the cortex from root hair by same method as in leaf cell

Question 10

What is transpiration

Answer 10

• plants have to balance water loss by transpirtation with need to get water and mineral ions to leaves themsselves water loss is inevitavle
• bright , hot , dry , windy dayss , hgihest rate of transpirtation

Question 11

How does temperature effect transpiration

Answer 11

• water molecules gain more kinetic energy increase rate of difffusion out into atmosphere through stomata
• increase transpiration

Question 12

Describe how humidity effects transpiration

Answer 12

• humidity increases difference between inside and outside reducing diffusion gradient
• transpiration decreases

Question 13

Describe how air movement effects transpiration

Answer 13

• air speed increases saturated air is removed from leaf surface more quickly
• increasing diffusion gradient
• transpiration increases

Question 15

Describe how light intensity affect transpiration

Answer 15

• increase light internsity increase stomatal opening
• rate of transpiration increases

Question 16

Describe the potometer practical

Answer 16

• meassures rate of uptake
• some water will be used in photosynthesis if cells are turgid uptake rate approximated as transpiration rate
• cut stem and fit into potometer under water prevents formation of any air bubbles in xylem
• blot leaves dru as any water of leaf surface can create humid layer
• introduce air bubble at end of capillary tibe and measure distance it travels in set period volume calculated if capillary tube known
• carry out repeats

Question 17

What are mesohpytes and how are they adapted to live in their environments

Answer 17

• live in temperate regions with adequate water supplu but most survive time where water scarce or unabailable eg water frozen
• closing stomata if water is scarce as cannot maintain turgot in guard cells
• shedding leaves become dormant in winter
• overwintering beneath fround in bulbs or corns
• annual plants produce seed that can over winter

Question 18

What are xerophytes and how are they adapted to live in their environments

Answer 18

• eg marram gras plants adapted to dry environment by reducing water loss
• sunken stomata - trap humid air reducing water potential gradient between air spaces inside leaf and outside air
• hairs around stomata - trap water vapour to reduce water potential fradicent between lead and air
• rolled leaves - reduces surface area over transpiration occurs some plants take extemes reduce leaves to spine and use stem to photosynthesise
• thick cuticle further reduces water loss from leaf surface

Question 19

What are hydrophytes and how are they adapted to live in their environments

Answer 19

• partially or fully submerged in water lack water is never a problem ensyring revieve adequate light carbon dioxide for photosynthesis ef - water lily
• have stomata on upper leaf surface contact with air
• stems and leaves have large air spaces provide buoyancy and a resovoir of O2 + CO2
• having porrly developed xylem tissue there no need transport water as it is all around
• leaves have little or no cuticle as water loss not a problm
• support tissue is not needed as water is a supportive medium

Question 20

What is translocation

Answer 20

• porduct of photosynthesis are transported in phloem as sucrose from were produced (sources) wheere ued or stored as insoluble food resrvess eg starch (sink)
• phloem also transports amino acids

Question 21

Describe the structure of phloem

Answer 21

• living tisue made three types of cells
• sieve tubes - walls perforated with pores to produce longitudinal tubes that contain cytoplasm no nucleus and most organelles disintegrate during development
• end walls don’t break down but instead becoe perforated by pores forming end plates
• companion cells - dense cytoplasm with nucleus many mitochondira and are connected to each sieve tube by plasmodesmata
• phloem parenchyma acts as a packaging tube

Question 22

What is evidence that the phloem is involved in translocation

Answer 22

• radioactive labelling carbon dioxide products and paths can be traces exposing plant to x ray film called autoradiographs
• ringing experiments used whervy outer rind of stem is cut to remove phloem whilst leaving xylem behind
• bulge forms above ring sugar moves up and down stem in phloem
• aphids allowed feed on plants anathesised before removing head and leaving feeding stylet in place
• analysis of liquid extruding stylet showed it was sucrose

Question 23

Describe the limitations of the mass flow theory

Answer 23

• fail to explain how sucrose and amino acid are transported at ifferent rates in opposite directions in same phloem vells or how transport occurs thousands time faster than by diffusion

Question 24

Decribe the mass flow theory

Answer 24

• photosynthesising cells (source cell) produce glucose whcih is converted into sucrose which lowers the water potential of the cell as water enters cell via osmosis hydrostatic pressure forces sucrose into phlowm sieve tube
• increasing levels of solutes in phlowm water potential is lowered and water moves into adjacent cells and xylem via osmosis down a water potential gradient raises hydrostatic pressure in phloem so higher pressure
• sucrose and dissolved solutes move by mass flow from high to low hydrostatic pressire down a pressure gradient
• at roots / growing points (sink cells ) the sucrose diffuses into cells down a concentration gradient therefore removed from sieve tube sieve tubes it is converted to starch for storage of converted to glucose to be respired
• loss of sucrose from phloem raises water potential higher than in xylem and adjacent cells
• water enters xylem via osmosis
• water also moves from phloem to xylem down water potential gradent cause reduction in hydrostatic pressure
• water moves up xylem via transpiration