Epidermis
Endodermis
Root hair
cell
Cortex
NOTE:
Xylem (dead) cells
Phloem cells
Cambium cells
may also count.
Pericycle
Green - apoplast
Pink - symplast
During osmosis, water flows from higher to lower water potential. A decreasing
water potential gradient is maintained as water is taken in from the root cells
to the xylem, facilitating steady flow of water through the root cells.
At 100% light intensity, there was a decrease from 5.5 to approx. 3.8 in
transpiration rate -- a difference of 1.7.
At 25% light intensity, there was a decrease from approx. 3.9 to approx. 3.8
in transpiration rate -- a difference of only 0.1.
It can be seen that in both these light intensities, transpiration rates appear
to be similar at 75 days after installation.
Transpiration rates are higher during a sunny day to the higher light intensity
and temperature. Guard cells become turgid and allow stomata to return open
to facilitate gaseous exchange as well as transpirational pull, which assists
with temperature regulation (cooling).
At night-time, the guard cells become flaccid and the stomata close, greatly
reducing the intake of carbon dioxide due to a pause in photosynthesis. As a
result, transpiration rates are very low.
A source is a photosynthetic organ capable of producing
sugars in excess, for transport. Sucrose is unloaded from
sources into the phloem (e.g. leaves).
A sink is a non-photosynthetic organ that does not produce
sugars but needs them to meet their own requirements.
Sucrose is loaded into sinks from the phloem. (e.g. roots)
Solutes are loaded into the phloem by active transport, thus
ATP is needed to transport the solutes against a conentration
gradient.
Using ATP, H+ ions are pumped out of companion cells to
adjacent cells. As H+ ions move back into the companion cell
via transport proteins, sucrose moves in with them through a
protein co-transporter.
Pressure changes in the phloem results in mass flow of solutes along
the phloem.
1. Solute is loaded into the phloem via active transport. This
allows sucrose to be pumped into the sieve tube, thus
decreasing water potential there.
2. As a result, osmosis occurs, causing water to flow from the
xylem to the phloem.
3. This creates hydrostatic pressure that pushes the solute through
the sieve tube due to mass flow.
4. As the solute reaches the sink (roots), the sucrose is unloaded
from the phloem. This increases water potential.
5. Water is absorbed from the soil into the xylem.
6. The increased water potential in the phloem allows water to
flow into the base of the xylem via osmosis.
7. Transpirational pull, capillarity and root pressure all contribute
to the movement of water up the narrow xylem tubes. The
adhesive and cohesive forces of water also assists.
It should be noted, however, that this mass flow theory only accounts
for unidirectional movement of solute in the phloem.
(12 marks)
2.
(a)
Figure 2A shows a cross-section of phloem tissue and Figure 2B shows a longitudinal
section of phloem tissue.
Figure 2A. Cross-section of phloem
tissue
(i)
Figure 2B. Longitudinal section of
phloem tissue
In the space below, draw and label a diagram of the TWO phloem cell types shown
in Figure 2B.
02207020/CAPE 2018
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(ii)
Describe TWO features of EACH phloem cell type shown in Figure 2B.
.................................................................................................................................
Sieve element - Lacks a nucleus and mitochondria. Are book-ended by
.................................................................................................................................
sieve plates. Elongated shape. Large empty lumen for sucrose transport.
.................................................................................................................................
Companion cell - Contains prominent nuclei and many mitochondria to provide for the
.................................................................................................................................
metabolic needs of sieve tube elements. Attached to side of sieve element.
.................................................................................................................................
Dense cytoplasm.
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
[2 marks]
(iii)
State the major function of EACH of the TWO phloem cell types.
.................................................................................................................................
Sieve element - Transport of sucrose from source to sink (e.g. leaves to roots)
.................................................................................................................................
Transport of other assimilates and amino acids around the plant (bidirectional).
.................................................................................................................................
Companion cell - Facilitates movement of sucrose into phloem tube cells by
.................................................................................................................................
diffusion. Providing ATP to the sieve tube element to allow active transport
.................................................................................................................................
and uptake of sucrose.
.................................................................................................................................
.................................................................................................................................
.................................................................................................................................
[2 marks]
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- 10 (b)
Figure 4 shows the results of some experiments in which the effect of the closure of stomata
on transpiration rate in a plant is studied. Two series of experiments are conducted, one
in still air and the other under windy conditions.
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In still air
20
10
Stomatal aperture (pm)
Figure 4. Rate of transpiration relative to stomatal aperture
(i)
With reference to the structure of stomata, explain the nature of the relationship
between stomata and transpiration.
Stomata are pores mainly found along the leaf's lower epidermis. They are
surrounded by guard cells, which control the opening and closing of the stomata.
When guard cells are open, transpiration rates increase, allowing the diffusion of water
vapour out of plant leaves. The guard cells regulate the rate of transpiration in response
to various environmental factors, such as temperature.
[3 marks]
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02207020/cAPE 2017
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(ii)
Using the data given in Figure 4, compare the general trend in transpiration rates
of the plant in still air and under windy conditions.
In still air, rate of transpiration increases gradually with stomatal aperture
while in wind, there is a steep increase in the rate.
The overall rate of transpiration is lower in still air when compared with wind.
In wind, the rate of transpiration increases steadily with opening of stomatal
aperture. However, in still air, the rate begins to level off around 5 micrometer
stomatal aperture.
[3 marks]
(iii)
Suggest an explanation for the results obtained for the experiment done under
windy conditions.
In windy conditions, regions of humidity (saturated with water vapour)
are carried away from the leaf, increasing rates of water diffusion out of the leaf.
[1 markl
Total 15 marks
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02207020/cAPE 2017
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0220702011
I
1. Measure the distance travelled by the bubble in a given time interval.
2. Take readings several times for the duration of the experiment.
3. Calculate the average reading. The rate of transpiration would be
obtained from the average distance travelled per unit time.
1. Xylem tissue has a narrow lumen that facilitates transpirational pull due to the
adhesion of water molecules.
2. Xylem contains dead lignin, required for support.
3. Xylem vessels are hollow (no cytoplasm), allowing rapid transport of water.
4. Xylem vessels are elongated and arranged end to end to form a continuous column.
There is no effect. Potassium uptake is unchanged, most likely due to
respiration rates being very low at such a low temperature.
Potassium cyanide causes rate of uptake to plateau, most likely due to
inhibiting energy production for the active transport of potassium ions.
(a)(i) Translocation is the transport of soluble products of photosynthesis (e.g. sucrose) via the phloem.
Transport occurs from a photosynthetic organ (source) to a non-photosynthetic organ (sink).
(ii) Phloem sap, containing sugars, flow from leaves (source) to roots (sinks). The spiral slash
prevents the bulk flow of phloem sap to the roots, instead diverting the sap to the other sinks such as
the fruits. Thus, more sugars are transported to the fruit tissues, making them sweeter.
- 6 2.
(a)
Figure 4 is a photomicrograph of a stained, longitudinal section of vascular tissue showing
phloem and xylem vessels.
I
II
III
IV
Figure 4. Photomicrograph of longitudinal section of vascular tissue
(Source: http://www.wellcome.ac.uk/en/bia/gallery.htm)
(i)
In the space below, make a detailed drawing of the region highlighted by the box in
Figure 4. Your drawing should be the actual size shown in Figure 4.
[5 marks]
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02207020/CAPE 2013
- 7 (ii)
Identify the structures highlighted by the lines labelled I, II, III and IV in Figure 4.
Phloem vessel/ sieve tube element
I __________________________________________________________________
II _________________________________________________________________
III ________________________________________________________________
IV ________________________________________________________________
[2 marks]
sieve plate
parenchyma/companion cell
xylem vessel
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02207020/CAPE 2013
-8(b)
Figure 4 is a schematic representation of the transverse section through a dicotyledonous root.
=,n^[JIW
Epidermis
Cortex
Figure 4. Schematic representation of the transverse section through a dicotyledonous root
(D
Use arrows labelled A and B to indicate on Figure 4, TWO geparate locations where
the movement of ions across the root is by active transport.
[2 marksl
(ii)
For EACH of the two locations identified in (b) (i) above, outline the movement of
ions into and out of the cells.
A
Active transport from soil into root hair, and from root hair into cortex
B
Active transport from cortex into endodermis, and from endodermis into pericycle
[2 marks]
(iii)
Explain the relationship between the movement of ions and the movement of water
across the root.
High concentration of ions in the pericycle creates a more negative water
potential in the pericycle cells,
This causes water to move into the pericycle from the cortex by osmosis.
[2 marksl
Total 15 marks
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022070201cAP8 2012
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2.
(a)
5
-
The apparatus in Figure 3 is used to investigate the effect of environmental factors on the rate
of transpiration.
-Retort stand
Leafy _
shoot
Syringe containing water for pushing air
-back to right-hand end of capillary tube
Rubber
bung
Graduated capillary tube
Water -
-Air
Conical flask
-Rubber tubing
Figure 3. Apparatus for investigating the effect of
environmental factors on transpiration rate
(i)
What is the role of the syringe?
[1 mark]
(ii)
State TWO precautions that must be taken when setting up the apparatus in Figure 3.
Ensure that there are no air bubbles in the apparatus
Dry the surface of the leaves before start of the experiment
Select shoot with healthy leaves
[2 marks]
(iii)
State TWO measurements that must be recorded to calculate the rate of transpiration.
Length/Distance moved and time
[1 mark]
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02207020/CAPE/2011
- 6 -
(iv)
State what should be done to ensure reliability of the results.
Repeat the experiment (under sane conditions)
___________________________________________________________________
Take multiple readings under the same conditions
___________________________________________________________________
[1 mark]
(v)
Describe how this apparatus could be used to investigate the effect of sunlight on
transpiration. Suggest a possible control for the experiment.
Exclude sunlight by covering the shoot with dark polythene bag/material/put in a dark cupboard.
___________________________________________________________________
___________________________________________________________________
Control:
As a control cover a second identical shoot with a
_________________________________________________________
transparent bag/keep in a lit room.
_________________________________________________________
[2 marks]
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02207020/CAPE/2011
- 16 5.
(a)
(i)
Explain
(b)
(i)
Briefly explain the term ‘translocation’ as applied to plants.
(ii)
Of the many mechanisms of translocation which have been proposed the mass
(pressure) flow hypothesis has gained some support from experimental work.
[2 marks]
Outline the principle of mass flow and discuss, using TWO examples, experimental
evidence in support of mass flow as a possible mechanism of translocation.
[5 marks]
Total 15 marks
Write your answer to Question 5 here.
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02207020/CAPE/2011
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02207020/CAPE/2011
-
2.
(a)
5
-
Figure 2 shows apparatus that was used to investigate the distribution of stomata on the
upper and lower parts of a leaf.
Retort stand
Leafy shoot
- Syringe containing water for pushing air
back to right-hand end of capillary tube
Rubber bung
Graduated capillary tube
-Air
Water
- Rubber tubing
Conical filter
flask
Figure 2. Apparatus for investigating stomata distribution in a leaf
Biological Sciences 1 and 2, 3rd Edition, p . 439
(i)
Explain FOUR precautions that should be taken when setting up and conducting
this experiment.
[ 4 marks]
(ii)
From which surface will the water loss most likely be GREATER?
Lower surface/lower part of leaf.
[ 1 mark ]
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02207020/CAPE/2010
-
(iii)
6 -
Suggest an appropriate control for the experiment.
same experiment set up but without any covering to the leaves
same experiment set up but with both sides covered.
[ 1 m ark ]
(b)
Identify THREE factors that affect the closing and opening of stomata, and explain how
these factors function.
Light stimulates guard cells which accumulate water from neighboring cells, becoming
turgid.
Temperature: If high stomata opens, if low stomata may close
Water availability in soil: short supply; stomata close
Low humidity: high transpiration rate: stomata open
High humidity : low transpiration rate: stomata close
[ 3 marks]
(c)
Figure 3 shows an electron micrograph of a plant structure.
X
Y
Figure 3. Electron m icrograph o f a plant structure
Roberts M., Reiss M. and M onger B., Advanced B iology.
Nelson 2000, p . 259.
(i)
Identify the structures labelled X and Y in Figure 3.
companion cell
X : ________________________________________________________________
sieve tube
Y : ________________________________________________________________
[ 1 m ark ]
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02207020/CAPE/2010
- 7 (ii)
Discuss TWO structural differences between the cells labelled X and Y and
describe how, in spite of these differences, they function as a unit.
Y (sieve tube ) has no nucleus/ribosomes/cytoskeleton/little cytoplasm /
but these are present in X ( companion cell)
Y (sieve tube) has fewer/ less mitochondria /ER than X(companion cell)
Y (sieve tube) dependent on X (companion cell) for support and energy
for movement of sugars and amino acids.
X connected to Y via plasmodesmata hence function as a unit.
[ 4 marks]
(iii)
Name ONE route by which substances move from the chloroplast in leaves to
the phloem.
Apoplastic route / symplastic route
_______________________________________________________________
[ 1 mark ]
Total 15 marks
02207020/CAPE/2010
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- 4 -
2.
(a)
State FOUR structural features of xylem vessels.
(i)
(ii)
(iii)
(iv)
Tubular in Shape
Walls impregnated with lignin
Formed by fusion of several cells
Tubular rn shape
Have thickened walls . Formed by fusion of several ceLls
. fofr. long tubes known as vessels (elongated)
. Cells fuse (alignerl) end to end o End walls have perforat.ion pl
Pits present
[ 4 marks]
(b)
Xylem vessels play an important role in the conduction of water and mineral salts in
plants. Using THREE of the features listed at (a), explain how EACH feature faci litates
the conduction of water and mineral salts in plants.
Tubular shape facilitates movement of water
Fusion of end walls forms a long tube for movement of material
Presence of pits enable water movement from vessel to vessel
Presence of lignin makes vessel wall rigid, prevents walls from collapsing under pressure.
Vessels are empty therefore no obstruction to flow
[ 3 marks]
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02207020/CAPE/2009