Transport in plants

• why do plants need a transport system

  long diffusion distance from external sources to cells

  substances like sucrose need to be transported from source to sink within the plant

  they have a smaller surface area to volume ratio which means simple diffusion is not suitable
• what are the benefits of plants developing internal transport systems

  surface area to volume ratio is too small

  so diffusion from the outer surface is not efficient

  therefore they need internal transport systems to ensure sufficient mineral ions and water reach the cells

  which allows a higher metabolic rate
• example of a substance transported in the root hair cells

  ammonium ions
• xylem and phloem in the transverse root (diagram)

  the xylem is like an X in the middle

  with little candy corn shaped phloem in each of the four corners

  the medulla is the large space

  the endodermis is the outer layer
• xylem and phloem in the transverse stem (diagram)

  both arranged on top of each other in candy corn structures that go around the outside in a circle

  the xylem is closest to the middle

  the cambium is the second layer

  the phloem is the third layer
• xylem and phloem in the transverse leaf

  arranged in a macaroni shape

  xylem is closest to the upper part of the leaf

  phloem is underneath it
• structure of the phloem

  sieve element -> living cells, no nucleus or organelles, perforated end walls

  companion cells -> contain organelles to provide ATP for active transport
• structure of the xylem

  dead and hollow

  no organelles

  no end walls creates a continuous column of water

  lignin deposited in the cell walls makes them waterproof so the water has no osmotic effect on them
• how does the casparian strip prevent ions and water moving through it

  its impervious to water and mineral ions

  it forces them into the symplast pathway through the plasma membrane

  however the phospholipid bilayer repels ions as they are charged particles and need specific carrier and channel proteins to cross the membrane
• how do hydrogen bonds form

  water is a polar molecule

  because of the different electronegativities of hydrogen and oxygen, oxygen more strongly attracts the pair of electrons which causes it to have a partially negative charge and the hydrogen a partially positive charge

  hydrogen bonds then form between the oxygen of one water molecule and a hydrogen atom of another water molecule
• how does active transport of mineral ions allow water to move into cells

  ions enter against their concentration gradient by active transport

  lowers water potential of cells

  causing water to move in via osmosis down its water potential gradient
• process of viewing vascular tissues under a microscope

  cut a thin transverse cross section

  add a stain such as TBO and observe under a light microscope under low power
• how would the lignin in xylem differ between hydrophytes, xerophytes and deciduous woodland plants

  hydrophytes have more lignified walls

  xerophytes have less lignified walls
• how would thickness of xylem walls differ between hydrophytes, xerophytes, and deciduous woodland plants

  hydrophytes have a thicker xylem wall

  xerophytes have a thinner xylem wall
• differences between the xylem and phloem

  xylem contains lignified walls whereas phloem only contains cellulose

  xylem has a wide lumen whereas phloem has a small lumen
• similarities between the xylem and the phloem

  both made up of cells joined end to end

  xylem and sieve tube elements both contain no organelles

  both are complex tissues made up of more than one cell type
• label A B and C of a diagram of xylem tissue

  A -> vessel wall

  B -> vessel lumen

  C -> bordered pit

  
• which of these diagrams is the student more likely to have seen and why

  B would more likely have been seen as light microscopes cannot provide the high level of detail of organelles as seen in A such as the rough endoplasmic reticulum - the sieve tube elements are also hexagonal in structure

  
• what is resolution

  the ability to see two seperate objects close together as individual objects
• what stain should a student use when viewing phloem under a light microscope

  Nile blue to increase contrast and to show there is no nuclei present in the sieve tube elements
• what are the function of the bordered pits in xylem

  lateral movement of water
• precautions of setting up a potometer

  1. do not allow air to enter the cut end of the shoot to ensure there is a continuous column of water
  2. keep abiotic factors controlled such as humidity as it affects the rate of evaporation of water vapour
  3. keep the screw clip closed to prevent entry of water whilst measuring

  
• results of a standard deviation mean

  the higher the results the more spread about the mean the data is

  less precise and repeatable
• how could a student ensure that the estimate of leaf area covered on a graph paper was accurate

  1. flatten the leaves
  2. account for partially covered squares
  3. double leaf area to account for both leaf surfaces
• how do you go from mm3 min-1 to cm3 hr-1 cm-2

  1. change from min-1 to hr-1 by multiplying by 60
  2. change from mm3 to cm3 by dividing by 1000
  3. divide the answer by the cm-2 length
• how could a student plan an investigation into the rate of transpiration in two species of plant that would allow valid data to be collected (method)

  pick two plant species the student would like to compare

  set up a potometer and measure the rate of transpiration by measuring the distance the air bubble has travelled at regular intervals

  repeat the experiment at least three times for each plant species

  this would allow a mean to be calculated and for anomalies to be ruled out

  plotting a graph for distance against time for the two plant species would allow the data to be compared visually
• how could a student plan an investigation into the rate of transpiration in two species of plant that would allow valid data to be collected(controlling variables)


  make sure variables that affect the rate of transpiration are controlled such as temperature by using a thermostatically controlled room

  try and use a similar sized leaf and control lighting by closing the blinds
• another method is by using plastic bags left for six hours, using a syringe to collect any water inside, and recording volume of water - what are the issues with this

  some water vapour might not have condensed so they should record the mass of the bag before and after

  water accumulating in the bag affects the water vapour potential gradient which impacts the rates of transpiration so the time for recording should be less

  leaves are different sized so they should pick leaves of similar sizes, measure the surface area, and divide by two for the two sides
• conclusions made from standard deviation error bars that largely overlap

  probably no significant difference

  as the difference in --- is very small

  and the error bars have a very large overlap suggesting that the data is very spread out
• how could a potometer be set up to calculate a more accurate rate of transpiration

  air tight

  dry the leaves

  cut shoot underwater

  measure the distance the air bubble travels per timed interval

  calculate volume of water uptake

  maintain constant conditions and controlled abiotic factors such as temperature and humidity
• when doing spearman's rank which number is ranked as 1

  the highest number
• what does n stand for in spearman's rank

  the number of columns

  for example: in the picture below n would be 10

  
• how do leaf hairs enabled the plant to conserve water at differing water availabilities at different distances from the river

  further away from the river means there is less water available

  transpiration causes water loss from the plant

  the leaf hairs trap water vapour which reduces the water vapour potential gradient from inside to outside the leaf
• what does validity mean in an experiment

  controlling the variables

  for example:

  time of day

  time of year

  similar size, age, and height
• how to analyze the results of a spearman's rank test

  if the value is below the critical value, you accept the null hypothesis as there is no significant correlation between the two data sets and is more likely that any correlation is due to chance
• how do you calculate minimum rate on a graph

  change in y / change in x
• if the stomata are covered in petroleum jelly how is water being lost from the leaf

  evaporation from the upper side of the leaf
• how could a valid comparison made between the rate of transpiration of these two shoots

  account for leaf area by calculating the rate of transpiration per unit area

  
• how can you see what the effect an anomaly has on the mean

  identify the anomaly

  state whether it has made the mean higher or lower

  re-calculate the mean without the anomaly present

  then minus the two means to see how much of an increase or decrease the anomaly has had on the mean
• suggest a reading for an anomalous result in the potometer experiment


  air bubble not returned to starting position

  misread scale

  timed for a longer interval than the others

  abiotic condition wasn't controlled