Recapping Gas exchange and water transport in plants
Name_______________
1. a) Label this leaf cross section
Upper Epidermal cell
Palisade mesophyll cell
Mesphyll cell
Air space
Spongy mesophyll cell
Xylem vessel
phloem
stoma
Colour key
b) Show a possible path of movement of liquid water
from the xylem into mesophyll cells where it is used in photosynthesis or evaporated from
the mesophyll cell wall.
c) Show the direction of net carbon dioxide movement:
There is a net movement of carbon dioxide into the leaf (from the external environment)
via open stomata along a carbon dioxide concentration gradient. (not the carbon dioxide
produce in aerobic respiration is also used in photosynthesis)
d) Show the direction of net oxygen movement
net movement of oxygen is out of the leaf (via stomata) along an oxygen concentration
gradient.
e) Show the direction of net water vapour movement
water evaporates for the cell wall of the mesophyll cells and then as water vapour diffuses
from the air spaces in the leaf via the stomata to the environment. It diffuses along a water
vapour concentration gradient.
2.
a)(i) What gases are taken up by a woody stem? oxygen
(ii) Via what structures do they enter? lenticels
(ii) What process is this gas used in? aerobic respiration
(iii) Give an example of a cell type in which this process would occur. Any living cell in the
woody stem. Eg. Sieve tube cell, companion cells, cambium cell, parenchyma cell.
b) (i) What gas is absorbed into cells of the roots? oxygen
(ii) What process is the gas used for? aerobic respiration
(ii) From where is this gas absorbed? The air spaces in the soil(between the soil crumbs)
(iii) Name an active process that occurs in the cells of a root. Any process that requires
ATP for example; active uptake of minerals, mitosis, production of enzymes
3. Which of the following cells is:
a)Turgid? A
b) Flaccid? B
c) Which would have more potassium ions within? A ( K uptake by these cells causes water
to move into these cells by osmosis, making the guard cells turgid)
d) Use a red pen to indicate the more flexible cell wall of the guard cell
e) Use a blue pen to indicate the thicker cell wall of the guard cell
A
B
4. Examine this small section of a leaf epidermis from the Kalanchoe plant pictured
Guard cell
chloroplast
a)
b)
c)
d)
Epidermal cells
Label an epidermal cell
Label a chloroplast
Label a guard cell
Are these stomata open of closed? Justify your choice. Open but not very
wide. The guard cells are slightly bent and a gap is evident between the
guard cells which is an opening through which gases and water vapour
can pass.
e) Is this more likely to be the upper or lower epidermis? Justify your choice.
Answer: More likely to be the lower epidermis.
Justifcation: Images shows stomata are abundant.(three on this small
area of epidermis) and in horizontally orientated leaves most (if not all)
stomata are located on the lower epidermis, as this enables gas
exchange whilst minimizing water lost.
5. a) Define transpiration: The evaporation of water from the surface of a plant,
mostly via stomata in the leaves.
6.
b)(i) In which vascular tissue does water transport occur? Xylem
(ii) Is this tissue living or dead? dead
c) What is the most significant factor driving water movement up a plant?
Transpiration
Briefly explain its role in moving water through a plant.
Evaporation of water from the cell walls of the mesophyll cells creates a negative
pressure or “suction” that draws more water molecules up the xylem in a
continuous column. (this is assisted by the fact that water molecules are cohensive)



d) Outline how each of the following assists in transporting water through a plant
Root pressure: the positive pressure of water entering a root by osmosis creates a
small push that assists to move water upwards in the xylem
Cohesion:
The attraction of water molecules to each other means that a pulling or pushing
force exerted on one water molecule will be transferred along the column to the
other water molecules in the xylem.
Adhesion and capillarity
The attraction of the water molecules to the cellulose wall of the xylem tubes
draws the water a little way up the tube and stops the water column falling down
the tube. This attractive force is enhanced by the fact that the xylem vessels are
extremely narrow and this means there is a large surface area of wall in contact
with the water molecules in the column.
e)Through which structures do plants lose water? stomata
6. Of the following pairs which plant is likely to lose water at a faster rate.( all other
factors being the constant between them except for differences indicated) Note qu
doesn’t say explain! So you didn’t need to.
A
B
C
D
A or B: B;( it has bigger leaves and so will
have more stomata through which water
vapour can diffuse out form the leave.)
C or D: C (it has more leaves and so more
stomata through which water vapour can
diffuse out form the leave.)
E 30 ⁰C
G In a terrarium
F 20 ⁰C
E or F: E (temp is higher in E and thus water
vapour molecules will have more kinetic
H On the window sill
G or H: H; (it will be more humid in the
terrarium than on the window still and this
energy and be moving faster than in F. This
results in a faster rate of diffusion of water
vapour and a faster rate of water loss.)
will mean that rate of water loss in the
terrarium will be slower. The air on the
window sill is drying thus the water vapour
gradient between the airspaces in the leaf
and the environment will be greater than
for the plant in the terrarium. Thus water
vapour will diffuse out of the plant faster
on the sill.)
7. a) What is translocation?
The movement of organic compounds mainly sucrose and amino acids, through a
plant. ( also mineral ions)
b) (i) In which tissue does this occur?phloem
(ii) Is this tissue living or dead? Living
(iii) Name the main two cell types that make up this tissue. sieve tube cells and
companion cells (also parenchyma cells but these are just packing cells)
c) Sucrose is transported in a plant from “source” to “ sink”
(i) Give an example of source: leaves
(ii) Give an example of a sink: roots, fruits
(iii) What is responsible for providing the force to transport sucrose?
Active transport of sucrose into the sieve tube cells causes water to move into
the sieve tube cells via osmosis. This causes a positive PRESSURE that provides the force
to transport sucrose to an area of lower pressure. This ideas is known as the pressure flow
hypothesis.
(iv)
(v)
(vi)
Give an example of when the roots of a plant would be considered a ” source”
In a deciduous tree in the winter the roots are a source.
Give an example of when the roots of a plant would be considered a “sink”
In the summer and spring when a deciduous plant has leaves and is
photosynthesis the roots are a “sink”. Sucrose is transported to them for
storage.
Outline one fate of the sucrose that is transported.
Either of


Sucrose can be changed into starch and stored in amyloplast
Surcrose can be converted to glucose and used as a reactant in respiration.
8. a) Distinguish between the location/distribution of the xylem and the phloem in the
root of a dicot and a monocot.
In the monocot root the xylem and phloem for a ring near the centre of the root. In
the middle of the ring is the pith.
This is a tissue drawing showing the general arrangement of the vascular tissue.
pith
Epidermis
Phloem
Xylem
cortex
In a dicot root the xylem usually forms an X shape right it the middle of the root
and the phloem is in bundles just outside the xylem.
cortex
Epidermis
Phloem
Vascular bundle
Xylem
b) Distinguish between the location/distribution of the xylem and the phloem in the
stem of a dicot and a monocot.
Stem of dicot, vascular bundles are in a ring towards the epidermis of the stem.
Stem of a monocot the vasulcar bundles are scattered.
c) Describe the location of the xylem and phloem in the vascular bundle of a leaf
The Xylem is on the side of the bundle closest to the centre of the stem. The phloem is on
the side of the bundles closest to the epidermis.(remember the ring barking)