A level Biology 7.1 Structure of transport tissues – Exam style question – Paper 2

Question

 When a leaf is first formed it is described as a sink for carbohydrate. As the leaf continues to grow, it starts to photosynthesise and becomes a source of carbohydrates and other assimilates.

    Fig. 3.1 shows the changes that occur to the structure of plasmodesmata in the leaf as it grows.

  (a) Suggest the advantage of complex plasmodesmata between cells in leaves.[2]

  (b) Assimilates are transported into phloem sieve tubes.

         Explain how assimilates in phloem sieve tubes move from the veins in a mature leaf to sinks, such as flowers and fruits, in the rest of the plant.[4] [Total: 6] 

Answer/Explanation

Ans:

3 (a) increased/faster, movement/ diffusion, of, assimilates / amino acids / sucrose/water/ solutes / ions / molecules ;
          I substances / particles / carbohydrates
          I freely / easily / efficiently
          I osmosis

          (because) more, (symplast) pathways / passages /AW ;
                  accept in context of blockage of some plasmodesmata

          correct ref. to symplast pathway in context of an advantage ;

          e.g. of complex plasmodesmata ;
                    from companion cell into sieve tube (elements)/when loading sucrose into phloem
          AVP ; e.g. selectivity / control/regulation, of movement

    (b) 1 mass flow ; A pressure flow
            2 sucrose/ solutes / assimilates / sugars, decreases, water potential/ solute potential ; A symbol(s) Ψ
            3 water enters (sieve tubes), down water potential gradient/ by osmosis ;
            4 increase in/ high(er), hydrostatic pressure ;
            5 unloading/removal, of sucrose at the sink lowers the (hydrostatic) pressure ;
            6 movement (from source to sink) is by gradient in (hydrostatic) pressure ;

Question

 Marram grass, Ammophila arenaria, is a xerophyte adapted to grow in sandy soils in exposed locations.
    Fig. 4.1 is a light micrograph of a section through a leaf of marram grass.

(a) A. arenaria is adapted to reduce transpiration.

     (i) State what is meant by the term transpiration.[3]

 (ii) One adaptation of A. arenaria is the curled leaf.

             Give one other adaptation, visible in Fig. 4.1, and explain how this reduces transpiration.[3]

             adaptation

             explanation
(b) A student investigated the rates of transpiration and absorption of water by two species of plants, P and Q, over an 18 hour period between 0600 and 2400. The environmental conditions for the two species were the same.

      The results are shown in Fig. 4.2.

     (i)Use Fig. 4.2 to calculate the difference between the rates of transpiration and water absorption for species P at time 1400.

          answer [2]

     (ii) Describe and explain the patterns of transpiration and water absorption for species P. [4]

     (iii) Suggest why the pattern of transpiration for species Q is not the same as for species P. [2] [Total: 14]

Answer/Explanation

Ans:

4 (a) (i) loss from, leaves / aerial parts of plant ;
                 of water vapour ; link to first point

                 plus one from:
                 evaporation of water, from surface of spongy mesophyll cells / into air
                 spaces ;
                 diffusion of water vapour, out/ to atmosphere ; R evaporation
                 movement/ diffusion, (out) through (open) stomata ; R evaporation
                 water vapour moves (out) down the water potential gradient ;

          (ii) adaptation for 1 mark, explanation to max 2

                 thick (waxy) cuticle ;
                 explanation
                 idea that wax is, (mainly) impermeable to water/hydrophobic / barrier to
                         water vapour movement ;
                 reduces, water loss from parts with no stomata/ uncontrolled water
                         loss / cuticular transpiration ;
                 idea that increased distance decreases rate of diffusion of water vapour
                 or
                 reflective cuticle ;
                 explanation
                 reduces heat load ;
                 reduces evaporation (from spongy mesophyll cells surfaces) ;
                 reduces rate of diffusion of water vapour (through cuticle) ;
                 or
                 folded inner surface/AW ; A trichomes / hairs ;
                 explanation
                 traps water vapour /AW ;
                 reduces, diffusion/water potential, gradient ;
                 (water potential gradient) between sub-stomatal air space and outside /AW ;
                 or
                 no stomata (visible) on the, outer/ exposed, surface ;
                 explanation
                 idea that stomata are main route for water loss ;
                 idea that reduces area where there is a high rate of water loss ;
                 surface directly exposed to air currents has no stomata ; ora

                 R curled or rolled given as adaptation but allow explanation to max 2
                 explanation
                 stomata on inside ;
                 no/ away from, air currents ; A increases humidity within enclosed space/AW
                 reduces, diffusion/water potential, gradient (between sub-stomatal air space and outside);

    (b) (i) 18 g h–1 ;;

                  one mark if no units given
                  one mark if incorrect answer but correct values extracted from Fig. 4.2
                  (60–42 g h–1)
           (ii) describe to max 3
                  rate of, transpiration/water absorption, increases and decreases /reaches a
                  peak ;
                  time delay between high rates of transpiration and water absorption/AW ;
                  lower values for water absorption until (approx.) 1645 ; ora A 1630 to 1700
                  data quote to support ;

                  explain to max 3
                  ref. to daylight and night and stomatal, opening/ closure/AW ;
                  higher light intensity / greater stomatal opening, higher rate of transpiration ;
                  ora
                  idea that transpiration drives water absorption ;
                  further detail ; e.g. explanation in terms of water potential gradient
                  ref. to cohesion-tension from leaf to root [max 4]

           (iii) xerophyte ;
                  example of xeromorphic feature ; A ref. to adaptation(s) (for dry areas)
                  high light intensity during middle of day /AW (for species P) ;
                  idea that loss of water during the day needs to be minimised ;
                  suggestion that (most) stomata, closed during the day / only open at night ;

Question [Maximum marks: 7]

Plants have two transport tissues, xylem and phloem.

(a) Describe and explain two ways in which the structure of xylem vessels is adapted to their
       function.

(b) Describe two differences between the vessels that transport phloem sap in flowering plants
       and the vessels that transport blood in mammals.

       Fig. 6.1 shows root hair cells.

(c) Explain why the root hair cells in Fig. 6.1 have more mitochondria than the other root cells
       shown.

Answer/Explanation

Answer:      6(a) mark first two statements given
                                  cellulose, cell wall / lining, allows adhesion of water ;

                                  thick (cellulose) cell wall prevents collapse / idea of providing support (under tension) ;

                                  (cell wall impregnated with) lignin, for waterproofing / prevents water loss ;

                                  lignin, rings / spirals / thickening / AW (of walls), prevents collapse / idea of providing support (under tension) ;
                                  A lignified walls

                                 no cytoplasm / lack of contents / hollow / empty (lumen), gives, less resistance to / unimpeded / uninterrupted / unhindered /
                                 ease of, flow ; A greater volume per unit time / faster rate R continuous, smooth R dead

                                 ack of end walls / continuous tube, so, less resistance to / unimpeded / uninterrupted / unhindered / ease of, flow ; AW
                                 R continuous, smooth

                                 pits / pores, for lateral movement / for movement around air bubbles / supplies (water) to (surrounding), cells / tissues ;
                                 R holes

                                 wide / large diameter / large lumen, so large volume of water can be transported ;

                                 if max 2 not gained, allow one mark for two correct explanations or descriptions where accompanying descriptions and
                                 explanations are in context

6(b) two from                                                                                                                                                                                   accept phloem vessels from sieve tube                                                    only sieve tubes / one type, v, arteries, veins and capillaries (any two) A three types;

          sieve tubes are composed of cells v blood vessels composed of tissues ; A ref. to named tissues

          sieve tubes, have cytoplasm or blood vessels are, hollow / AW;

          sieve tubes have sieve plates ; ora

          sieve tubes have companion cells (to fully function) ;

          veins have valves ; ora

          ref. to blood vessels and vasoconstriction ; ora

           idea of transport open (phloem) v closed (blood vessels) ; A sieve tubes have plasmodesmata

           ref. to unidirectional flow in blood vessels ; ora

6(c) three from
          mitochondria, synthesise ATP / carry out aerobic respiration ;

          more ATP needs to be synthesised / increased ATP synthesis ; A need to produce large quantities of ATP

          to provide more (metabolic) energy ; R in context of water uptake

          for active transport / active uptake ; in context of mineral ions A to move mineral ions against the concentration gradient

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