09 November 2014

#42 Summary of Transport in multicellular plants

1. Multicellular organisms with small surface area to volume ratios need transport systems.

2. Water and mineral salts are transported through a plant in xylem vessels. Movement of water is a  passive process in which the water moves down a water potential gradient from soil to air.

 3. The energy for this process comes from the Sun, which causes evaporation of water from the wet walls of mesophyll cells in leaves. Water vapour in the air spaces of the leaf diff uses out of the leaf through stomata, in a process called transpiration. This loss of water sets up a water potential gradient throughout the plant.

 4. Transpiration is an inevitable consequence of gaseous exchange in plants. Plants need stomata so that carbon dioxide and oxygen can be exchanged with the environment.

 5. The rate of transpiration is affected by several environmental factors, particularly temperature, light intensity, wind speed and humidity. It is difficult to measure rate of transpiration directly, but water uptake can be measured using a potometer.

 6. Plants that are adapted to live in places where the environmental conditions are likely to cause high rates of transpiration, and where soil water is in short supply, are called xerophytes. They have often evolved adaptations that help to reduce the rate of loss of water vapour from their leaves.

7. Water enters the plant through root hairs by osmosis. Water crosses the root either through the cytoplasm of cells (the symplast pathway) or via their cell walls (the apoplast pathway), and enters the dead, empty xylem vessels. It also moves across the leaf by symplast and apoplast pathways.

8. Water moves up xylem vessels by mass flow, as a result of pressure diff erences caused by loss of water from leaves by transpiration. Root pressure can also contribute to this pressure diff erence.

9. Mineral salts are essential nutrients. Examples are nitrate, which is needed for the synthesis of a wide variety of organic compounds, and magnesium, which is a constituent of chlorophyll.

10. Translocation of organic solutes such as sucrose occurs through living phloem sieve tubes. The 
phloem sap moves by mass flow from a region known as the source to a region known as the sink. Sucrose is produced at the source (e.g. photosynthesising leaves) and used at the sink (e.g. a flower or a storage organ).

 11. Mass flow occurs as a result of pressure differences between the source and the sink. Active loading of sucrose into the sieve tubes at the source results in the entry of water by osmosis, thus creating a high hydrostatic pressure in the sieve tubes.

 12. Both xylem vessels and phloem sieve tubes show unique structural features which are adaptations to their roles in transport.

1. Multiple-choice test

1. Which feature is seen in both sieve tube elements and xylem vessel elements?

A   elongated cells arranged end to end
B   end walls perforated by large pores
C   lignified walls with pits
D   thin lining layer of cytoplasm

2. Part of the stem of a plant is heated to kill living vascular tissues.

How will this treatment affect transport through phloem and xylem?

3. Which description is correct?

A   Companion cells have no nuclei and are not metabolically active.
B   Pits are part of the cellulose cell wall of a xylem vessel element where no lignin has been deposited.
C   Sieve tube elements are dead and have no cytoplasm or organelles.
D   The lignified wall of a xylem vessel element is an adaptation to the high pressure inside the element.

4  What is responsible for the upward movement of water in xylem vessels in plants?

A  active loading of water against the water potential gradient in roots and osmosis in the xylem vessels
B  increasing water potential at the top of the xylem vessels and osmosis in the roots
C decreasing water potential at the top of the xylem vessels and cohesion of water in the vessels
D translocation in the leaves and capillarity in the xylem vessels

5. The movement of water in the apoplast pathway takes place outside living cells, whereas the symplast pathway involves living cells.

What occurs in the apoplast and symplast pathways?

6. What may magnesium ions and nitrate ions be used to make in a plant?

7. Which of the following are adaptations shown by xerophytes?

1  leaves covered by hairs
2  leaves covered in a layer of wax
3  leaves reduced to spines
4  stems that store water
5  stomata sunken into pits

A   1, 2, 3, 4 and 5
B   1, 2 and 3 only
C   2, 3 and 4 only
D   1, 4 and 5 only

8 Translocation moves sucrose from sources to sinks.
Which row shows a source and a sink?

9 Which description of translocation is not correct?

A   Loading sucrose into a sieve tube element increases the water potential of the sap inside it.

B   Sucrose is actively loaded into a sieve tube element at a source through a companion cell.

C   Sucrose is removed from a sieve tube element at a sink and converted into other sugars.
D   When sucrose is loaded into a sieve tube element water follows, moving down a water potential gradient by osmosis.

10. The rate of transpiration from a plant was measured in different 
conditions. One of three environmental factors was varied at a time. 
The results are shown in the graph.

Answers to Multiple choice test
1. A
2. B
3. B
4. C
5. B
6. A
7. A
8. B
9. A
10. C

2. End-of-chapter questions

1 If sucrose is actively loaded into a sieve tube, which combination of changes take place in the sieve tube?

2  Which  of the following rows correctly describes the hydrostatic  pressure of the two types of elements?

3. The diagram shows the effect  of light  intensity on the  rate  of transpiration     from the upper and lower epidermis of a leaf. Other environmental factors were kept constant. What could explain the differences in transpiration rates from the two surfaces?

 A   Higherlight intensities   are associated   with   higher   temperatures.
 B   Thepalisade mesophyll   cells have  fewer  air  spaces  than   the  spongy   mesophyll    cells.
 C   Theupper epidermis  has  fewer  stomata.
 D   Theupper epidermis  is more  exposed  to  light.

4. Explain how water  moves  from:
a   the soil into a root  hair  cell.
b   one root cortex  cell to  another.
c   a xylem vessel into  a leaf  mesophyll  cell.

5. Name three cell types found   in:
i    xylem
ii     phloem.
b   State the functions of the cell types you have  named.

6  a  The effect of increasing size on surface  area:  volume  ratio can be shown  most  easily using  a cube. Copy and complete the  following table for cubes with  the dimensions  indicated (units are not needed):

b. Explain the  relevance   of these  dimensions and ratios to transport in large multicellular organisms.

7. Arrange the following in order of water potential.  Use the symbol > to mean  'greater than'. 
dry atmospheric air; mesophyll cell; root hair cell; soil solution; xylem vessel contents

8. Figure a shows changes  in the relative humidity of the atmosphere during the  daylight hours of one  day.

Figure b shows changes in the tension in the xylem of a tree during the same  period.

a   Describe  and  explain  the  relationship between relative humidity and xylem  tension.

b   Describe and explain the differences observed in xylem tension between the  top of the tree and the bottom of the tree.

9    An instrument  called a dendrogram can be used to measure small changes in  the diameter of a tree trunk.

Typically, the  instrument  reveals daily changes,  with the diameter at its lowest during daylight hours and at its greatest at night.

Suggest an explanation for these observations.

10.   Copy the table and place a tick  or  a cross  in  the  appropriate box  to indicate whether  nitrogen  or magnesium may be in the biochemicals  shown.

11. The figure is a graph showing  the  relationship   between  rate  of transpiration  and  rate  of water  uptake  for  a particular prlant.

a   Define the term  transpiration.      [2]
b   State the two  environmental  factors which are most likely to  be responsible  for the changes  in  transpiration rateshown in  the  figure.   [2]
c   Describe the  relationship  between  rate  of transpiration and rate of water  uptake  shown  in  the  figure.                                                     [2]
d  Explainthe relationship.                    [4]

[Total:   10]

12   The  figure  is a light  micrograph  of a transverse   section   through    the  leaf of marram   grass  (Ammophila), a xerophytic plant.

a   Identify   three  xerophytic   features  visible  in the  light  micrograph.  [3]
b    Explain   how  each  of the  features  you  have identified   helps  the  plant   to conserve  water.                                                                                                                              [6]
[Total: 9]

13   Explain   how  active  loading   of sucrose  into  sieve elements   accounts   for  the  following   observations:
a    The  phloem   sap has  a relatively   high  pH  of about   pH  8.  [1]
b    The  inside  of sieve element/companion      cell units  is negatively   charged   relative  to  the  outside.   (There  is a difference   in electrical   potential    across  the  cell surface  membrane,    with  a potential    of about  -150  mV on  the  inside.) [2]
c    ATP  is present   in  relatively   large  amounts    inside  sieve tubes.[1]
                                                                                                                                              [Total: 4]

14  Figure  below shows  a sieve element with  red-stained  'triangles' of callose  at each  end.  These  triangles  indicate  the  positions  of the  sieve plates.

a   Assuming    the  magnification  of the  micrograph  is x 100, calculate  the  length  of the  sieve element.   Show  your working.    [3]

b   Scientists   were  puzzled   for  many  years  by the  fact  that  sieve plates  were  present   in sieve elements,   because sieve plates  increase   the  resistance   to flow. This  contrasts   with  xylem  vessel elements,   which  have  open  ends, redud resistance  to flow.

   i  Calculate   how  many  sieve plates  per  metre  a sucrose  molecule   would   have  to cross  if it were  travelling in  the  sieve tube  identified  in a above.  Show  your  working.   (Assume  all the  sieve elements   are the  same size as the  one  measured   in  the Figure above.)     [2]
   ii  What   is the  function  of the  sieve plates?                                  [1]
   iii   What   feature   of the  sieve plates  allows  materials   to cross  them?     [1]

c Flow  rates  in sieve tubes  range  from  0.3  to  1.5  m h-1  and  average  about 1 m h-1.. If the  flow rate  in the sieve element  shown   in Figure above  were  1 m h-1, how  long  would   it take  a sucrose  molecule   to  travel through    it? Show  your  working.             [Total: 3]

15. Translocation of organic  solutes  takes  place  between   sources   and  sinks.

a   Briefly explain under  what   circumstances:
   i   a seed could  be a sink                     [1]
   ii   a seed could  be a source                [1]
  iii  a leaf could  be a sink                      [1]
  iv  a leaf  could  be a source                [1]
   v   a storage organ  could   be a sink   [1]
  vi  a storage organ  could   be a source. [1]

b  Suggest two possible  roles  for  glucose  in  each  of the  following   sinks:
   i  a storage organ      [2]
   ii a growing bud.      [2]

 [Total: 10]

3. End-of-chapter answers 

1   A
2   B
3   C
5   a i vessel elements; tracheids; parenchyma; fibres;
        ii sieve (tube) elements; companion cells; parenchyma; fibres;
     b  vessel element: transport of water/support/transport of mineral ions;
          tracheid: transport of water/support/transport of mineral ions;
         sieve element: transport of, sucrose/organic solutes;
         companion cells: loading/unloading, phloem (sieve element)/forms functional unit with sieve element;
        parenchyma: storage/gas exchange;
        fibres: support/mechanical strength;

        b as size increases, volume increases faster than surface area;
            therefore as size increases, the surface area : volume ratio decreases;
            can no longer rely on diff usion to satisfy transport needs;

 soil solution > root hair cell > xylem vessel contents
     > mesophyll cell > dry atmospheric air

8 a the lower the relative humidity, the higher the tension/the lower the hydrostatic pressure, in the xylem;
     more evaporation from leaf (mesophyll cells) when low relative humidity;
    results in lower water potential in leaf (mesophyll cells);
    therefore more water moves from xylem (vessels to replace water lost from leaf);
    down a water potential gradient;
     sets up tension in the xylem vessels;

  b lowest/most negative, hydrostatic pressure is at the top of the tree;
     because water is being lost at the top of the tree;
    this sets up a tension which is greatest at the top of the tree;
    there is a, hydrostatic pressure/tension, gradient in the xylem vessels;
   some pressure is (inevitably) lost on the way down the tree;

 9   transpiration/loss of water vapour/loss of water by evaporation, from the leaves occurs during the day;
     because the stomata are open;
    this results in tension in the xylem (vessels);
    walls of xylem vessels are pulled slightly inwards/vessels shrink slightly;
    overall eff ect is for diameter of whole trunk to, shrink/get smaller;
   stomata close at night, so no transpiration at night;

 11 a the loss of water vapour;
         from the leaves/from the surface of a plant; [2]
     b light intensity;  temperature; [2]
     c rate of water uptake shows the same pattern as rate of transpiration; AW
      but there is a time delay, with changes in rate of transpiration occurring before changes in water uptake; AW [2]
    d transpiration causes water uptake;
       loss of water (by transpiration) sets up a water potential gradient in the plant;
      water potential in roots is lower than water potential in soil;
      therefore water enters plant through roots;
      time delay between rate of transpiration and rate of water uptake is due to time taken for effect of transpiration to be transmitted through the plant; AW [max. 4]
[Total: 10]

 12 a thick cuticle (on lower epidermis/outer surface when rolled);
         leaf rolled up (due to activity of hinge cells);
          hairy upper epidermis/leaf is hairy;
         stomata absent from lower epidermis/stomata only present in upper epidermis;
         sunken stomata/stomata in pits/stomata in grooves (in upper epidermis);       [max. 3]
  b thick cuticle:
          cuticle contains a (fatty and relatively) waterproof substance called cutin;
          the thicker it is, the more eff ective;
leaf rolled up:
          encloses a humid atmosphere/allows a humid atmosphere to build up;
         hairs trap a layer of (still) moist air next to the leaf;

      stomata absent from lower epidermis:
        reduces/prevents, transpiration from, lower epidermis/exposed surface;
        sunken stomata:
        allows a humid (still) atmosphere to build up around the stomata;

     Allow 1 mark on one occasion only for ‘reduces the steepness of the water potential gradient from leaf to air inside the (rolled) leaf’ if relevant; [max. 6]
[Total: 9]

       a hydrogen ions are actively transported out of the, sieve element/companion cell; [1]
      b there are more hydrogen ions/there is a build-up of hydrogen ions, outside the sieve element–companion cell units compared with inside;
             hydrogen ions are positively charged; [2]
    c ATP is needed for the active transport of hydrogen ions out of the tubes; [1]
[Total: 4]

14 a actual length = observed length/magnifi cation,
                                   A = I:M
 observed length of sieve element = 51 mm (allow ±1 mm);
 actual length = 51 mm/150 = 0.51 mm; accept
conversion of mm to μm: answer = 510 μm [3]

  b i 1 metre = 1000 mm;
         1000/0.51 = 1961(to nearest whole number);
       1 metre = 1 000 000 μm;
       1 000 000/510 = 1961 (to nearest whole number); [2]

 ii to maintain the pressure gradient inside the sieve tubes;
       without the sieve plates the diff erent pressures at source and sink would quickly equilibrate;                                                                                                                                            [max. 1]
 iii sieve pores;                                                                                                                [1]

c (sieve element is 0.51 mm long)
     (1 hour = 3600 seconds)
      3600 seconds to travel 1 metre;
     0.51/1000 × 3600 seconds to travel 0.51 mm;
      = 1.8 seconds (to one decimal place);
 Accept 510 μm and 1 000 000 (μm) instead of 0.51 mm and 1000 (mm). [3]
[Total: 10]

 15 a when seed is forming/just after fertilisation; [1]
     b germination; [1]
     c young immature leaf/leaf that is still growing; [1]
     d mature photosynthesising leaf; [1]
     e when food is being accumulated/when storage organ is growing (in size)/developing/end of plant’s growing season/just before winter; [1]
    f when plant starts to grow (using food from the storage organ); [1]
     g i to make starch;
        respiration; [2]
     ii to make cellulose;
      respiration; [2]
[Total: 10]