Download Unit 4 - Plant Nutrition

Unit 4 – Plant Nutrition
4.1 – Photosynthesis, 4.2 – Leaf structure, 4.3 – Mineral nutrition
SUFEATIN SURHAN  BIOLOGY MSPSBS  2009
SYLLABUS CHECKLIST
Candidates should be able to:
a) understand that photosynthesis is the fundamental process by which plants manufacture carbohydrates from
raw materials;
b) investigate the necessity for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate
controls;
c) state the equation (in words or symbols) for photosynthesis;
d) investigate and state the effect of varying light intensity, carbon dioxide concentration and temperature on
the rate of photosynthesis (e.g. in submerged aquatic plants);
e) understand the concept of limiting factors in photosynthesis;
f)
describe the intake of carbon dioxide and water by plants;
g) understand that chlorophyll traps light energy and converts it into chemical energy for the formation of
carbohydrates and their subsequent storage;
h) explain why most forms of life are completely dependent on photosynthesis;
i)
identify and label the cuticle, cellular and tissue structure of a dicotyledonous leaf, as seen in cross-section
under the microscope, and describe the significance of these features in terms of function,
i.e.
distribution of chloroplasts – photosynthesis;
stomata and mesophyll cells – gas exchange;
vascular bundles – transport;
j)
understand the effect of a lack of nitrate and magnesium ions on plant growth.
4.1 – Photosynthesis
 When water reaches the veins of the leaf, the water
moves into the mesophyll cells in response to an
osmotic gradient.
 Autotrophic nutrition involves the building up or
synthesis of complex organic substances (food) from
simple inorganic chemicals, using a source of energy.
 Green plants are autotrophs that synthesise food by
using light energy from the sun. This anabolic
process is called photosynthesis.
Definition of photosynthesis:
Photosynthesis is the process by which green plants
use chlorophyll to convert light energy into
chemical energy which is in turn used to produce
sugars from water and carbon dioxide, liberating
oxygen as a by-product.
Raw materials of photosynthesis
 The raw materials
needed are carbon
dioxide and water.
These are simple,
low-energy
containing inorganic
chemicals.
 Carbon dioxide is
obtained from air. It
diffuses into the
leaves through tiny
Figure 2 Simple representation of
pores called stomata.
Photosynthesis
 Water is obtained
from the soil. It enters the plant through the root
hairs by osmosis and travels up the xylem vessels to
the leaves.
 The numerous root hair cells provide a very large
surface area for the uptake of water ions from the
soil. Osmosis continues in the adjacent cortex cells
until water reaches the xylem vessels.
Figure 1 Osmosis of water into the mesophyll cells
Conditions of photosynthesis
 Light and chlorophyll are the necessary conditions
for photosynthesis. Light energy comes from the
sun while chlorophyll is found in the chloroplasts of
plant cells.
 When sunlight falls on green leaves, chlorophyll
absorbs the light energy and converts it to chemical
energy to fuel the synthetic reactions that occur in
the chloroplast.
 Chlorophyll is a green-coloured pigment that
contains magnesium.
 Chlorophyll can only absorb light of wavelengths in
the blue and red region. Green light is not
absorbed at all and is reflected away from the leaf.
This light is absorbed by our eyes, which is why
leaves appear green in colour.
Figure 3 Absorption spectrum and action spectrum of light
 Chloroplasts are organelles abundant in leaf cells
and so leaves are the organs of a plant for carrying
out photosynthesis. Enzymes for photosynthesis are
present in chloroplasts.
Millions of chlorophyll
molecules are arranged on a series of membranes
like library books stacked on shelves.
Figure 4 Osmosis of water from the root hair cell to the
Xylem vessels
 The xylem vessels transport water up the stem by
transpiration pull.
sufe/bio/mspsbs/2009
Figure 5 Chloroplasts and Chlorophyll
Page 2 of 14
Products of photosynthesis
 Sugar (glucose) is the main product of
photosynthesis.
 Glucose is a complex, high-energy containing
organic compound. The chemical energy in sugar
comes from the absorbed light energy. The sugar is
almost immediately converted to starch in the
leaves.
 Glucose cannot be stored because it is very soluble.
Therefore, glucose must be converted into sucrose
before it is transported away from the leaf via
phloem tissues by translocation.
 Excess glucose is converted into starch and stored as
starch grains inside chloroplasts.
 Note: Leaves are usually tested for the presence of
starch to indicate that photosynthesis has occurred.
However, the product of photosynthesis is the
simple sugar, glucose, not starch.
 Oxygen is a by-product of photosynthesis. Oxygen
from the mesophyll cells diffuse down the
concentration gradient into the intercellular spaces
and out through the stomata of the leaf down the
concentration gradient into the atmosphere
surrounding the leaf.
Word equation for photosynthesis
Symbol equation for photosynthesis
Fate of glucose in green leaves after photosynthesis
 Glucose may be used to release energy in the leaf
during tissue respiration for cellular activities.
 Glucose may be turned into starch and stored in
the leaf, stem tubers, root tubers and corms. Starch
is a polysaccharide which is large and complex, not
reactive, insoluble and can be easily made into
granules and stored in the chloroplasts for future
use.
 Glucose may be used to make other organic
substances like sucrose, cellulose and oils. In the
actively growing root tips and shoot tips, sugar is
converted to cellulose to produce cell walls or
converted to protein to produce enzymes needed
for growth.
Addition of minerals containing
nitrogen and sulphur can lead to the formation of
amino acids. The amino acids will then combine to
form proteins for making protoplasm, chlorophyll,
enzymes and vitamins. Excess amino acids are
stored as proteins in the leaves.
 Glucose is transported to other parts of the plant.
Glucose is first converted to sucrose to be
transported via phloem tissues to storage organs of
the plant. Sucrose molecules are small, soluble and
less reactive than glucose.
Sucrose can be
converted back to glucose and then used as a
respiratory substrate to release energy.
 Glucose is used to make cellulose cell wall.
 Glucose can be converted to fats and stored in the
storage organs of the plant.
Figure 6 (Below) Fate of glucose in green leaves after
photosynthesis
sufe/bio/mspsbs/2009
Page 3 of 14
Importance of photosynthesis
 As a source of energy (food) for practically all
living organisms. Photosynthetic organisms are the
most important food producers in ecosystems.
 Supplies oxygen to the air which is essential to the
respiration process in most life forms.
 Removes carbon dioxide thus playing an important
role in maintaining low levels of atmospheric carbon
dioxide.
 Energy stored in coal (fuel) comes from
photosynthesis.
Photosynthesis and Respiration
 In daylight:
 plants carry out respiration.
 plants carry out photosynthesis producing
oxygen and glucose.
 the level of oxygen in the atmosphere around
the plant rises because the rate of production of
oxygen during photosynthesis is greater than it
is being used up during respiration.
 the level of carbon dioxide in the atmosphere
around the plant will fall during the daytime.
Carbon dioxide is used up during photosynthesis
and will rise at night (produced by respiration).
 In the dark or absence of light:
 plants
perform
respiration
only
while
photosynthesis stops.
 the level of oxygen will fall as oxygen is used up
during plant respiration and not produced due
to the absence of photosynthesis in the dark.
 the level of carbon dioxide will rise due to the
production of carbon dioxide during respiration.
o
Enables corrections to be made to the results
which may be due to variations in experimental
factors or unnatural conditions of the
experimental set-up.
2. Destarching the leaves of a plant
 Leaves of a plant used for studying photosynthesis
must be free of starch at the start of the
experiment. To remove starch (destarch) from
leaves, the plant is placed in the dark for at least 24
hours.
 In the dark (or at night), the starch in leaves is
converted to sucrose and transported to other parts
of the plant.
Figure 8 Destarching a plant
3. Testing for starch
 Presence of starch in leaves indicates that
photosynthesis has occurred. A leaf is tested for
starch as shown:
1. If the leaf is too big, cut the leaf measuring
approximately 1cm by 1cm.
2. Boil the leaf in hot boiling water for 2 minutes
to kill and soften it, destroy the enzymes thus
preventing further chemical changes and make
it more permeable to iodine solution.
Important procedures in experiments on
photosynthesis
1. Setting up control experiments
 We study the effect of a factor (eg. Light) on a
process (eg. Photosynthesis) that occurs in a living
organism by eliminating or varying the factor.
Usually, a similar experimental set-up with normal
conditions is also prepared.
This is the control
experiment.
 Doing this is important, as a control:
o Gives standard condition for comparison with
the experimental one;
sufe/bio/mspsbs/2009
Figure 9
Leaf in boiling water
3. Boil the leaf in alcohol using a hot water bath
till the leaf is decolourised. Chlorophyll is
removed because it is soluble in alcohol. A hot
water bath must be used as alcohol is
flammable.
Figure 7
Decolourisation of leaf
Page 4 of 14
4.
5.
6.
7.
8.
Dip the leaf in hot water to soften it.
Place the leaf on a clean white tile.
Add a few drops of iodine solution.
If the leaf turns blue black, starch is present.
If the leaf remains white or stained yellow with
iodine solution, starch is absent.
Figure 10
Testing with iodine
Investigation on photosynthesis
How can we find out whether sunlight is
necessary for photosynthesis?
1. Destarch a potted plant by placing it in the dark for
two days.
2. Remove one leaf and test it for starch. This is to
make sure that the plant has been completely
destarched.
3. Sandwich a leaf, which is still attached to the plant,
between two pieces of black paper. Fasten the
papers with paper clips. Place the plant in strong
sunlight.
4. After a few hours, remove the leaf and test it for
starch.
5. Observation: Drawing of the leaf after it has been
tested for starch:
Blue-black
regions
sufe/bio/mspsbs/2009
6. Conclusions:
 The parts of the leaf exposed to sunlight turned
blue black due to the presence of starch. This
indicates that the areas exposed to sunlight
have undergone photosynthesis.
 The parts of the leaf covered with the black
paper remained white indicating the absence of
starch.
 This experiment shows that sunlight is necessary
for photosynthesis.
How can we find out whether chlorophyll
is necessary for photosynthesis?
1. Destarch a plant with variegated leaves by placing it
in the dark for two days. In a variegated leaf, only
certain parts are green. The green parts are the
only parts that contain chlorophyll.
2. Expose the plant to strong sunlight for a few hours.
3. Remove one leaf and make a drawing to show the
distribution of the green parts, i.e. the parts that
contain chlorophyll.
4. Decolurise the leaf and test it for starch.
5. Make a drawing of the leaf to show the distribution
of the blue-black colour. Compare this with your
drawing in step 3.
6. Observation:
7. Conclusion:
 The parts of the leaf that were originally green
contained chlorophyll. This area turned blue
black indicating the presence of starch.
 The parts of the leaf that were originally
white/yellow did not contain chlorophyll. This
area remained white indicating the absence of
starch.
 This shows that chlorophyll is necessary for
photosynthesis.
Page 5 of 14
How can we find out whether carbon
dioxide is necessary for photosynthesis?
First Method
1. Destarch two potted plants by placing them in the
dark for two days.
2. Enclose the pots in polythene bags. Secure the bags
to the plant stems.
3. Place on pot in the bell jar as shown in the figure.
The plant does not have a supply of carbon dioxide
from the air because soda lime and potassium
hydroxide solution rapidly absorb carbon dioxide.
Leave the whole apparatus in strong sunlight for a
few hours.
4. Set a control using pebbles and water in place of
soda lime and potassium hydroxide solution
respectively as shown in the figure. Leave the
control apparatus in strong sunlight for a few hours.
Second Method
1. Destarch two potted plants by placing them in the
dark for two days.
2. Remove one plant to check that the plant has been
completely destarched. The leaf must not turn dark
blue.
If starch is absent then continue the
experiment.
3. Set up the apparatus as shown in the diagram. Make
sure that air cannot enter the flasks by smearing the
rubber stopper with Vaseline to make an air tight
seal
4. Place the plant in bright sunlight for 4 - 6 hours.
5. Test each leaf for starch.
6. Observation:
 The leaf from conical A turned dark blue when
tested for starch.
 The leaf from conical B remains white when
tested for starch.
7. Conclusion:
 Carbon dioxide is necessary for photosynthesis.
How can we find out what gas is given off
during photosynthesis?
1. Set up the experiment below using some freshwater
plants, e.g. Hydrilla or Elodea as shown in the figure
below:
5. Remove a leaf from each plant and test them for
starch.
6. Observation:
 The leaf in Bell jar A remained white indicating
that starch is absent.
 The leaf in Bell jar B turned blue black
indicating that starch is present.
7. Conclusion:
 Carbon dioxide is necessary for photosynthesis.
sufe/bio/mspsbs/2009
Page 6 of 14
2. Dissolve a little sodium hydrogencarbonate in the
water in the beaker. Sodium hydrogencarbonate
provides carbon dioxide to the plant.
3. Place the apparatus in strong sunlight for a few
hours.
4. Prepare another set-up but place this set-up in a
dark place. This is the control experiment.
5. You will notice that the gas bubbles form on the
leaves in the beaker placed in sunlight. These
bubbles will rise up to the test tube and displace
the water downwards. When the tube is about half
filled with the gas, remove the tube by placing a
thumb over its mouth.
6. Test the gas with a glowing splinter.
7. Observation:
 The glowing splint rekindled in the tube from
the experiment exposed to sunlight.
 No gas bubbles are formed and collected in the
control experiment.
8. Conclusion:
 Oxygen is produced during the process of
photosynthesis.
Some other factor
is limiting here
Carbon dioxide is the
limiting factor here
3. Temperature
 Rate increases with temperature up to an optimal
temperature (usually 370C).
Then the rate
decreases.
This is related to the effect of
temperature on activity of enzymes.
370C
Factors affecting the rate of
photosynthesis
1. Light intensity
 Photosynthetic rate increases with light intensity
until a certain point when some other factor
becomes a limiting factor and prevents further
increase in rate.
Some other factor
is limiting here
LIMITING FACTORS
 Limiting factor is any factor that directly affects the
rate of a process if its quantity is changed.
 Example 1:
Light is the limiting
factor here
2. Carbon dioxide concentration
 Rate increases with increasing carbon dioxide
concentration. Under natural conditions, carbon
dioxide is an important limiting factor since
atmospheric carbon dioxide remains constant at
around 0.03% by volume.
sufe/bio/mspsbs/2009


The photosynthetic rate increases greatly when
the carbon dioxide concentration is increased
from 0.03% (graph A) to 0.13% (graph B) at 200C.
This shows that carbon dioxide is an important
limiting factor after point X in graph A.
Page 7 of 14
 Example 2:
How can we investigate the effect of
different light intensities on the rate of
photosynthesis?
1. Set up the apparatus as shown in the figure below
with the cut end of the water plant facing upwards.
1. Graph 1:
 As the light intensity increases, the rate of
photosynthesis increases from O to A. Light
is a limiting factor.
 Light is no longer a limiting factor beyond
point A since the rate remains a constant
even though light intensity increases.
 Temperature
or
carbon
dioxide
concentration may be the limiting factor
that causes the leveling off of the graph
along AB.
2. Graph 2:
 Increasing the temperature from 200C to
300C with the carbon dioxide remaining a
constant does not really affect the rate as
shown in Graph 2.
 Therefore temperature is not a limiting
factor.
3. Graph 3:
 When the temperature remains a constant
and the carbon dioxide concentration of
the environment is raised to 0.13%, the
rate of photosynthesis increases as shown
in Graph 3.
 Therefore carbon dioxide concentration is
a limiting factor in AB in Graph 1.
4. Graph 4:
 The limiting factor in EF in Graph 3 is the
temperature of its surroundings.
 Increasing the temperature from 200C to
300C causes an increase in the rate of
photosynthesis as shown in Graph 4 although
the carbon dioxide concentration remains
constant at 0.13%.
sufe/bio/mspsbs/2009
2. Note: The same concentration of dilute sodium
hydrogencarbonate solution is used in all
investigation.
Therefore,
carbon
dioxide
concentration remains a constant.
3. Place a 60W lamp 50cm away from the plant.
4. Air bubbles are given off from the cut end of the
plant. Allow some time for the plant to adapt to
the conditions provided before taking readings.
5. When the bubbles are produced at a regular rate,
count the number of bubbles over a period of 5
minutes. Repeat this a few times to obtain the
average rate.
6. Repeat step 5 with the light source closer to the
plant, e.g. 40cm, 30cm, 20cm, and 10cm. Note: the
nearer the light source is to the beaker, the higher
the light intensity that the plant is exposed to.
7. Record your results in a table. Plot a graph to show
the rate of bubbling per minute against the distance
between the lamp and the plant (light intensity).
8. Observation:
 As the distance between the water plant and
lamp decreases, the rate of air bubbles
produced by the water plant increases until it
reaches a maximum rate and reaches a constant
9. Conclusion:
 As the light intensity increases, the rate of
photosynthesis increases until it reaches a
maximum rate.
Page 8 of 14
How can we investigate the effect of
different temperatures on the rate of
photosynthesis?
1. Set up the apparatus as shown in the figure below
with the cut end of the water plant facing upwards.
2. Note: The same concentration of dilute sodium
hydrogencarbonate solution is used in all
investigation.
Therefore,
carbon
dioxide
concentration remains a constant.
3. Place a lamp (e.g. 60W bulb) 10cm away from the
plant. Keep this distance constant throughout the
investigation.
4. Add ice cold water to the water bath to keep the
temperature at 50C.
5. Allow some time for the plant to adapt to the
conditions provided before taking the readings.
6. Count the number of bubbles over a period of 5
minutes. Repeat this a few times to obtain an
average rate.
7. Repeat step 6 at different temperatures, e.g. 150C,
250C, 350C, 450C, 550C, 650C and 750C.
8. Record your results in a table and plot a graph to
show the rate of bubbling per minute against
temperature.
9. Observation:
 The rate of bubbling increases with a rise in
temperature to a maximum of about 400C.
Beyond this maximum temperature the rate of
bubbling decreases rapidly.
10. Conclusion:
 The rate of photosynthesis increase with a rise
in temperature to a maximum temperature of
about 400C. Beyond this maximum temperature
photosynthesis decreases rapidly.
sufe/bio/mspsbs/2009
How can we investigate the effect of
different carbon dioxide concentrations on
the rate of photosynthesis?
1. Set up the apparatus as shown in the figure below
with the cut end of the water plant facing upwards.
2. Place a lamp (e.g. 60W bulb) 10cm away from the
plant. Keep this distance constant throughout the
investigation.
3. Conduct the investigation at room temperature.
4. Use
different
concentrations
of
sodium
hydrogencarbonate solutions, e.g. 0.01M, 0.02M,
0.03M, 0.04M, 0.05M, 0.06M, up to 0.1M. (These are
proportional to the carbon dioxide concentrations in
the solution).
5. When the bubbles are coming out at a regular rate,
measure the rate of bubbling for each concentration
of the sodium hydrogencarbonate solution.
6. Plot a graph to show the rate of bubbling against
the concentration of the solution.
7. Observation:
 As carbon dioxide concentration increases, the
rate of bubbling increases proportionally form
point O to A on the graph. Carbon dioxide is the
limiting factor here.
 Beyond point A, carbon dioxide concentration is
no longer a limiting factor since the rate of
bubbling remains a constant even though carbon
dioxide concentration increases.
Page 9 of 14
8. Conclusion:
 As carbon dioxide concentration increases, the
rate of photosynthesis increases proportionally
from point O to point A on the graph. Carbon
dioxide is the limiting factor here.
 Beyond point A, carbon dioxide concentration is
no longer a limiting factor since the rate of
photosynthesis remains a constant even though
carbon dioxide concentration increases.
4.2 – Leaf Structure



 The leaf is an organ specialized for photosynthesis.



known as the bundle sheath. This is why veins
normally appear lighter in colour than the rest of
the leaf.
The leaf is surrounded by a tough, continuous,
protective epidermis. The epidermis contains pores
called stomata (singular: stoma) which allow
gaseous exchange.
The mesophyll layer contains chloroplasts. The
palisade (upper) mesophyll cells, which contain
the most chloroplasts, can be thought of as the
power house of the photosynthetic leaf.
The palisade and spongy (lower) mesophyll cells
receive water from the xylem vessels of the vein
network.
Carbon dioxide diffuses into the surface film of
water around the mesophyll cells.
Water evaporates from all internal surfaces and
water vapour diffuses out of the stomata.
The sugar formed in photosynthesis is transported to
the phloem sieve tubes of the vein network and is
translocated away.
 It consists of a lamina (leaf blade)
connected to a steam by a petiole
(leaf stalk).
 The lamina is a thin structure in
which many cells are held in well
illuminated positions.
 The whole leaf is supported by a
system of branching veins that form a
fine network throughout the lamina.
 A vein is a vascular bundle
surrounded by a few parenchyma
cells normally without chloroplasts,
sufe/bio/mspsbs/2009
Page 10 of 14
sufe/bio/mspsbs/2009
Page 11 of 14
Layers of the leaf
The stomata (singular: stoma)
 Leaf structure consists of:
CELL
LAYER
Upper
Epidermis
Palisade
mesophyll
layer
Spongy
mesophyll
layer
Veins
composed
of:
Xylem
and
Phloem
Lower
Epidermis
STRUCTURE
FUNCTION
Uppermost layer
of cells.
Protects inner layers of
cells.
Usually secretes
waxy cutin which
forms the cuticle.
Cuticle is waterproff
therefore reduces and
prevents excessive water
loss by evaporation. It also
protect inner layers of cells
from mechanical damage.
Chloroplasts
absent.
Allow sunlight to penetrate
into inner layers.
Closely packed
long cylindrical
cells.
Exposes many cells to light
rays.
Numerous
chloroplasts
containing
chlorophyll
present.
Allows absorption of
sunlight by chlorophyll.
Chloroplasts move around
by cytoplasmic streaming to
receive the highest light
intensity for maximum
photosynthesis.
Allows diffusion of carbon
dioxide into leaf, and
oxygen and water vapour
out of leaf.
Loosely arranged
cells with
intercellular air
spaces.
Presence of
chloroplasts
containing
chlorophyll.
Extend throughout
the leaf within
short distance of
every mesophyll
cells.
Allows absorption of
sunlight by chlorophyll by
photosynthesis.
Cylindrical,
hollow, thickwalled cells.
Transports water to leaf
and provide mechanical
support.
Cylindrical, thinwalled with
cytoplasm.
Transports products of
photosynthesis (food) from
leaf to all parts of the
plant.
Similar in
structure and
function to upper
epidermis but
includes:
Presence of guard
cells of the
stomata.
Presence of
chlorophyll.
sufe/bio/mspsbs/2009
Supports leaf.
 The cells in the epidermis are made up of guard
cells and epidermal cells.
 A pair of guard cells will make a stoma.
 In most dicotyledons the stomata occur only in the
lower epidermis while in monocotyledons e.g.
grasses the stomata are equally distributed on both
sides of the leaf.
 The guard cells of the stomata are bean-shaped and
that the cell wall near the stoma is thicker than
elsewhere in the cell.
 The guard cells also contain chloroplasts so they
can manufacture food (sugar) by photosynthesis.
 Stomata are open during the day to allow carbon
dioxide to diffuse into the leaf where it is required
for photosynthesis.
 Stomata are closed during the night, carbon
dioxide supply to the leaf and photosynthesis stops.
In Daylight






Regulate opening and
closing of stomata and so
regulate the rate of
diffusion of carbon dioxide,
oxygen and water vapour.


Due to photosynthesis, the concentration of
potassium ions and glucose in the guard cells
increases.
This reduces the water potential in the guard cells.
This causes water to enter the guard cells by
osmosis from the neighbouring epidermal cells.
This increases the turgor pressure inside the guard
cells causing the cells to swell up and become more
curved.
Since the guard cells have thicker cell wall on one
side of the cell (the side around the stomatal pore),
this thicker cell wall is least likely to expand
compared to the thinner side of the cell wall.
This cause the guard cells to curve in such a way
that the stomatal pore between them is opened.
This allows loss of water by transpiration.
Transpiration pull is set up and allows water and
mineral salts to be transported by the xylem vessel.
Photosynthesis occurs in
chloroplasts.
Page 12 of 14
At Night






Due to the absence of photosynthesis, the
concentration of potassium ions and glucose in the
guard cells decreases.
This increases the water potential in the guard
cells.
This causes water to leave the guard cells by
osmosis to the neighbouring epidermal cells.
This decreases the turgor pressure inside the guard
cells causing the cells to shrink, become flaccid,
straightened and become less curved.
The thick inner walls pull the guard cell inward
towards the stomatal pore.
This closes the stomata and helps to reduce the
excessive loss of water by transpiration from the
plant.
How do you demonstrate that there are
more stomata on the lower surface than
on the upper surface of the leaf?
 Procedure:
 Immerse a green leaf in a beaker of very hot
water for one minute.
 Observe the upper and lower surface of the leaf
while it is immersed in the hot water.
 Observation:
 Many large air bubbles appear on the lower
surface of the leaf.
 No air bubbles are observed on the upper
surface of the leaf.
 Conclusion:
 There are many stomata on the lower surface of
the leaf.
 The air present in the air spaces of the spongy
mesophyll layer expands when heated causing
the air bubbles to appear directly outside the
stomata of guard cells.
sufe/bio/mspsbs/2009
How is the leaf adapted for
photosynthesis?
 Leaf is adapted for photosynthesis, gaseous
exchange and distribution of its photosynthetic
products.
STRUCTURE
Petiole (leaf stalk)
Large, flat surface
of lamina
Thin lamina
Waxy cuticle on
upper and lower
epidermis
Stomata present in
the epidermal layers
Chloroplasts
containing
chlorophyll, in all
mesophyll cells
More chloroplasts in
the upper palisade
tissues
Interconnecting
system of air spaces
in the spongy
mesophyll
Extensive vein
system containing
xylem and phloem
Guard cells
FUNCTION
Holds leaf in position to absorb
maximum light energy.
To provide maximum surface
area for light absorption.
Allows maximum absorption of
light energy.
Allows carbon dioxide to reach
inner cells rapidly.
Enables sunlight to reach all
mesophyll cells.
Reduces water loss throgu
evaporation from the leaf.
Open in sunlight, allowing carbon
dioxide to diffuse in and oxygen
to diffuse out of the leaf.
Chlorophyll absorbs and
transforms light energy into
chemical energy used in the
manufacture of sugars.
More light energy can be
absorbed near the leaf surface.
Allow rapid diffusion of carbon
dioxide into mesophyll cells.
Xylem transports water and
mineral salts to mesophyll cells.
Phloem transports sugars away
form the leaf.
Able to open and close stomata
to regulate gaseous exchange.
4.3 – Mineral Nutrition
 Carbon, hydrogen and oxygen are incorporated into
the plant during photosynthesis. Besides these,
plants need other elements especially nitrogen,
potassium, phosphorous and magnesium for tissue
formation and healthy growth.
 These elements are present as mineral salts (which
exist as ions) dissolved in soil water.
 They enter the plant by diffusion and active
transport when root hairs absorb soil water.
Page 13 of 14
 Nitrogen is absorbed by the plant as nitrates or
ammonium ions.
 Nitrates are required for the synthesis of amino
acids and proteins.
Nitrogen-containing ions
combine with carbohydrates to form amino acids,
proteins, nitrogen bases of nucleic acids and
chlorophyll.
 Deficiency symtoms of nitrates (nitrogen) includes:
 Stunted plant growth.
 Few, small and yellow leaves.
 Plant eventually wither and dies.
 Magnesium is absorbed by the plant as magnesium
salts (e.g. magnesium sulphate).
 Magnesium is the central atom in a chlorophyll
molecule.
If the plant lacks magnesium,
chlorophyll cannot be synthesized.
 Deficiency
symptoms
of
magnesium
ions
(magnesium) includes:
 Small leaves.
 Chlorosis occurs: leaves turn yellow.
 Plant eventually dies.
 Phosphorous is absorbed by the plant as salts
(phosphates).
 Phosphorous is essential for synthesis of proteins
and nucleic acids and for the release of energy by
cellular respiration.
 Defeciency symptoms of phosphates (phosphorous)
includes:
 Stunted growth.
 Small leaves.
 Dull green leaves.
 Thin weak stem.
 Poorly developed roots.
 Plant eventually dies.
sufe/bio/mspsbs/2009
How do we investigate whether nitrogen,
phosphorous and magnesium are essential
for plant growth?
1. Take four gas jars and label them from A to D. Fill
gas jar A with a complete culture solution and each
of the gas jars B to D with a culture solution in
which one of the essential elements are absent.
2. Select four seedlings of balsam of about the same
size. Wash the roots with distilled water. Fit the
mouth of each jar with a three-holed cork. Place
the seedling in the central hole and hold it with a
piece of cotton wool such that the roots are
immersed in the culture solution as shown in the
following figure:
3. Put the gas jars in a suitable place so that they
receive adequate sunlight.
4. Leave then to grow for about two months.
5. Examine the seedlings. Note the colour and size of
the leaves, length of the main and branch root and
the stem.
6. Record the total surface area of the leaves using the
graph paper method.
7. Record the total length of the main and branch
roots.
8. Observation and conclusions:
 The water culture experiment shows that
healthy plant growth can only take place if the
plant is provided with the essential elements.
 The experiment shows that magnesium, nitrogen
and phosphorous occurs as inorganic compounds
in dilute solutions which are absorbed by the
roots.
Page 14 of 14