2 Which of the following rows correctly describes the hydrostatic pressure of the two types of elements?
Thepalisade mesophyll cells have fewer air spaces than the spongy mesophyll cells.
4. Explain how water moves from:
5. Name
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):
11. The figure is a graph showing the relationship between rate of transpiration and rate of water uptake for a particular prlant.
State the two environmental factors which are most likely to be responsible for the changes in transpiration rateshown in the figure. [2]
Describe the relationship between rate of transpiration and rate of water uptake shown in the figure. [2]
Explainthe relationship. [4]
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]
13 Explain how active loading of sucrose into sieve elements accounts for the following observations:
[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]
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;
7 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;
hairy:
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]
13
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);
or
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;
therefore:
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]
Insulin is a hormone produced by the pancreas. Insulin allows your body to use the carbohydrates (sugar) it gets from food sources. It acts as a “regulator” for carbohydrates in your body and prevents your blood sugar levels from becoming too high or too low.
