2007 Autotrophic Nutrition
P.1
Autotrophic Nutrition (Autotrophism)
Autotrophism
carried out by organisms which can manufacture their own food.
synthesis of complex organic compounds from simple inorganic substances (usually CO2 and
H2O)
A.
Chemosynthesis
-
synthesise complex organic compounds from inorganic substances but they obtain energy
from oxidation of various inorganic materials (H2S, NH3, iron)
-
Examples
1.
Colourless sulphur bacteria
2.
Iron bacteria :
3.
Nitrifying bacteria:
Convertion of :
NH4+
---> NO22NO
---> NO3Energy from the reactions is used to drive the synthesis of carbohydrates.
[HKALE 86 I]
1.
Distinguish between
(c)
chemosynthetic and photosynthetic autotrophic nutrition.
(2 marks)
1.
(c)
Photosynthetic autotrophic nutrition refers to the synthesis of organic compounds from
CO2 and H2O using sunlight as the source of energy and chlorophyll (or some other
closely related pigment) for trapping the light energy.
Chemosynthetic autotrophic nutrition refer to the synthesis of organic compounds from
CO2 and H2O with the energy supplied by special methods of respiration involving the
oxidation of various inorganic material such as hydrogen sulphide, ammonia and iron.
(2)
[HKALE 88 II]
7. (c) Give another example of autotrophic nutrition and describe how it differs from
photosynthesis. Name an organism in which your example is found.(3 marks)
(c)
Chemosynthesis
½
In photosynthesis sunlight is used as the source of energy
and chlorophyll or some other closely related pigment
is used for trapping the light energy.
In chemosynthesis energy is derived from the oxidation of
compounds.
e.g.
iron bacteria
sulphur bacteria
nitrifying bacteria
1
1
½
½
½
any 1
(3)
2007 Autotrophic Nutrition
P.2
[HKALE 94 II]
1. (c) Using an annotated flow chart, illustrate how energy flows and carbon is cycled
through photoautotrophic and chemoheterotrophic organisms. Indicate the roles of
respiration and photosynthesis in the flow chart. (10 marks)
(c)
- route of energy flow showing loss at each level
- route of carbon showing recycling
- annotations ( marks indicated in brackets in the above flowchart )
- respiration ( suitably put in thee flowchart )
- death and consumption/feeding ( suitably put in the flowchart )
Note: - route of carbon should include autotrophs, animals and
decomposers.
2
2
5
1
1
2007 Autotrophic Nutrition
B.
P.3
Photosynthesis
energy source : sunlight
chlorophyll (or some closely related pigment) : trapping light source
All green plants or chlorophyll-containing plants.
6 CO2
+ 12 H2O*
light
(chemical energy)
(byproduct)
-----------------> C6H12O6 + 6 H2O + 6 O2* 
chlorophyll
glucose
I.
The Importance of Photosynthesis to Life
1.
as a producer
- absorb and convert solar energy / light (by chloroplasts) into chemical energy and stored in
the form of organic food substances
===> start the food chain
2.
purify the air ---
release O2 into atmosphere
absorb CO2 from atmosphere
Q: Discuss what would happen to the living world if all photosynthetic organisms disappeared from the earth.
II.
Site of Photosynthesis --- Chloroplasts [HKAL 82-I-2, HKALE 84 II-2]
2007 Autotrophic Nutrition
P.4
- light transducer:
light (solar energy) ------> chemical energy
1.Lamellae:
- thylakoids, intergranal lamellae
---> hold chlorophyll molecules at a suitable position for trapping
the maximum amount of light inside chloroplast
- Stacking arrangement : greatest economy of space
(largest surface area)
2.Stroma :
- watery matrix
- site with enzymes for reduction of CO2 in Calvin Cycle.
在每一個類囊體的裏面有很多撮葉綠素， 稱為quantasomes 。 通常一個葉綠體有約60 基粒， 每一個基粒約有50
片類囊體， 每個類囊體上有約500 個quantasomes ， 每一個quantasum 中約有230 個葉綠素的分子。 在膜之間
也充滿了液體， 稱為基質stroma 。
2007 Autotrophic Nutrition
P.5
3. Pigments in chloroplasts:
- locate in the membranes (thylakoids, intergranal lamellae) inside chloroplasts
(a) Primary Pigment :
i.
(b)
chlorophyll a
- most abundant photosynthetic pigment
- with a porphyrin group containing magnesium (Mg)
- absorb light from the blue and red regions of the spectrum
Accessory Pigment :
- not directly involve in photosynthesis
- act as antennae to Photosystems (PSI PSII)
----> absorb light energy from various regions of the light spectrum
---> then pass on to the primary pigments
(allowing plants to use light of different wavelengths)
ii.
others chlorophylls: chlorophyll b , chlorophyll c (green), chlorophyll d,
bacteriochlorophyll a-d (pale blue)
iii.
xanthophyll
iv.
carotene
2007 Autotrophic Nutrition
[HKALE 94 II]
1. (a) Outline the role of accessory pigments in photosynthesis.
pigments and state its absorption spectrum.
1. (a)
P.6
Name one group of accessory
(3 marks)
Absorb light energy from various regions of thee light spectrum and then pass on to
chlorophylls, thus allowing plants to use light of more different wavelengths than
could be trapped by chlorophylls alone.
Carotenoids.
Absorb light from the blue and blue-green regions of the spectrum.
-
2
½
½
pigments can be separated by paper chromatography
[HKALE 95 I]
2. You are provided with three samples of leaf pigment mixtures, two of which are identical.
Name and outline the method you would use in the school laboratory to determine which two
samples contain the same mixture of pigments.
(3 marks)
Ans.
2.
Separate the pigments by * chromatography, (½) spot the extracts onto 3
chromatographic/ filter paper, (½) develop in appropriate developing solvent (½) by
hanging paper vertically in a chromatography jar / boiling tube/ beaker. Identical
extracts should have similar pigment pattern or similar Rfs for (½) the different
pigment sports (½). Describe the developing process.
(3)
2007 Autotrophic Nutrition
III.
P.7
Light Absorption by the Leaves
sunlight
83%
12%
5%
Leaves absorb
reflect
transmit
- mostly red and blue light
- 4% used during photosynthesis
the rest (96%) dissipated as heat
mainly green light
mainly green light
2007 Autotrophic Nutrition
P.8
* The absorption and action spectra for chlorophyll pigments extracted from plant show that the
wavelengths of light absorbed by chlorophyll pigments, namely red and blue light, are very
similar to the wavelengths that cause photosynthesis. The absoption and action spectra
match quite well. So the wavelengths (blue and red light) optimally absorbed are the ones that
provide most energy for photosynthesis.
2007 Autotrophic Nutrition
Leaf Structure of dicotyledonous plants in relation to photosynthesis:
P.9
2007 Autotrophic Nutrition
P.10
Adaptations for obtaining energy (sunlight)
1.
phototropism causes shoots to grow towards the light in order to allow the attached leaves to receive
maximum illumination
2.
etiolation causes rapid elongation of shoots in dark, to ensure that the leaves are brought up into the light as
soon as possible
3.
mosaic arrangement of leaves reduces the degree of shading of one leaf by another
4.
leaves are flat (supported by a frame work of vascular bundles) and have a large surface area to receive
light
5.
leaves are thin that light can penetrate to each cell layers
6.
the cuticle and epidermis are transparent to allow light reaching mesophyll cells
7.
palisade mesophyll cells have a lot of chloroplasts and are closely packed
8.
chloroplasts within the mesophyll cells can move to the best position to absorb light
9.
ordered arrangement of chlorophyll within grana of chloroplasts
- to maximizes the amount of chlorophyll to absorb light
- brings the chlorophyll in close proximity to other pigments and substances which are
necessary for its functioning
Some plant species are adapted to positions of full sunlight and others to
permanently shaded positions are sometimes referred to as ‘sun’ and ‘shade’
plants respectively.
Shade plants have a lower rate of respiration than that of sun plants. One
reason for the lower rate of respiration is that the shade plant builds thinner
leaves with fewer palisade mesophyll layers.
Consequently the shade plant reaches its compensation point at a quite low
light intensity, and much sooner than the sun plant does.
The sun plants invest much more energy in the construction and maintenance
of thicker leaves for the benefit of trapping much more of the incident light
energy and so increase its carbohydrate production. The manufacture of the
extra sugar sustains more growth, reproduction and seed production.
2007 Autotrophic Nutrition
P.11
2007 Autotrophic Nutrition
P.12
[HKALE 97 I]
10. (a) Figure 1 presents the results of an experiment about the effect of light intensity on the net CO2
fixation rate of two flowering plants A and B. This experiment was conducted in a greenhouse under controlled conditions.
(i) State two variables that must be kept constant to achieve the aim of this experiment. (1 mark)
(ii) What is meant by net CO2 fixation ?
(1 mark)
(iii)Account for the net CO2 fixation rate of plants A and B at light intensities below 40 arbitrary
units.
(3 marks)
(iv) Compare the net CO2 fixation rate of plants A and B at light intensities above 40 arbitrary
units.
(4 marks)
(v) Based on this experiment, which habitat, shady or sunny, is the natural habitat of each of the
two plant, A and B ? (N.B. Habitat with a light intensity below 100 arbitrary units are consider
shady.)
(1 mark)
(b)
Figure 2 shows the cross sections of two different leaves, X and Y , taken from the same tree.
They are of equal magnification and are at the same stage of maturity.
(i) State the structural differences between the photosynthetic tissue(s) of these two leaves.
(ii) The net CO2 fixation rate of leaves X and Y show a pattern similar to that of plants A and B in
part (a). Match the leaves with plant A and B. Give reasons for the match.
(2 marks)
(iii)Suggest the possible positions of these two leaves on the tree. How is the structure of leaf Y
related to its possible position on the tree ?
(2 marks)
Total : 15½ marks
2007 Autotrophic Nutrition
Answer:
a
(i)
(ii)
(iii)
P.13
temperature (½), carbon dioxide concentration (½)
amount of CO2 fixed in photosynthesis less CO2 released from respiration /
photosynthetic CO2 fixation less CO2 released from respiration
(1)
(1)
For both plants A and B, light intensity increases, rate of net CO2 fixation increase (½) as
light is the limiting factor (½)
(1)
Plant A :
 no net CO2 fixation occurs (½) because its rate of respiration exceeds its rate of
photosynthesis (½) which is very slow.
Plant B:
 at light intensity below 10 units, no net CO2 fixation occurs (½) , because its rate of
respiration exceeds its rate of photosynthesis (½)
 between 10 and 40 units, net CO2 fixation occurs (½) , because photosynthetic CO2
fixation exceeds respiratory CO2 production (½)
(iv)




(b)
between 40-100 arbitrary intensity units : A has a slower rate of net CO2 fixation
than B/ B has a higher rate of net CO2 fixation than A.
at 100 arbitrary intensity units, the rate of net CO2 fixation for A and B is the same
(½)
between 100 -500 arbitrary intensity units, the rate of net CO2 fixation for A
increases with increasing light intensity (½) and is higher than that of B (½). The rate
of net CO2 fixation for B reaches a maximum (½) and stays unchanged despite of
increasing light intensity (½).
between 500-600 arbitrary intensity units, the rate of net CO2 fixation for A reaches a
maximum (½) and stays unchanged despite of further increases in light intensity (½).
(v)
Plant A : sunny (½)
Plant B: shady (½)
(i)
X:
(ii)
X: plant A (½)
Y: plant B (½)
longer palisade cells (½) and 2 layers of palisade cells, only 1 layer in Y(½) ,
denser chloroplasts in both mesophyll layers (½), (the reverse comparison starting
the features of the features of Y is also acceptable )
(1)
(2)
Max.
(3)
(1)
(½)
(2)
(1)
Max.
(4)
(1)
(1½)
1
1
Plant A could achieve a higher net CO2 fixation rate (½) , leaf X with more chloroplasts /
photosynthetic tissue to absorb more light for CO2 fixation (½)
OR
Leaf X - more chloroplasts/more photosynthetic tissue
to fully utilize light of a wide intensity range for CO2 fixation (½)
Leaf Y - less chloroplasts/ less photosynthetic tissue /
thinner leaf limits light utilization to fix CO2 at high intensity (½)
(iii)
leaf X : upper/ (canopy) and outer layer (½) / exposed
leaf Y: lower and inner / centre of tree (½)
leaf Y receives less light (½), less photosynthetic tissue is required for capturing light
for photosynthesis / this results in a reduction of photosynthetic tissue for photosynthesis
(½)
(Alternative: Y is in a sheltered position and experiences less water loss, thus a thinner
cuticle is observed)
½
½
(2)
1
1
(2)
2007 Autotrophic Nutrition
P.14
Adaptations for obtaining and removing gases
1.
numerous stomata on epidermis of leaves which
- when opened, permit diffusion of gases into and out of the leaf: favour photosynthesis
- when closed, reducing considerably the loss of water: photosynthesis is unfavoured
* -- in the presence of light, stomatal pores open widely and some water loss is
unavoidable.
-- at times of considerable water loss (e.g. shortage of water, strong wind) , guard cells
loss turgidity and close the stomatal pore, regardless of the demands for carbon
dioxide.
- usually more in lower epidermis than the upper epidermis
---> reduce excess water loss through stomata on the directly shined upper surface
2.
Spongy mesophyll possesses many air spaces, hence diffusion of gases between the atmosphere and the
palisade mesophyll would not be interrupted by the relatively thick spongy mesophyll tissue
Adaptations for obtaining water and removing the carbohydrates produced
Extensive finely branching network of vascular tissue through the leaf
- xylem: water supply
- phloem: removing sugars (mainly sucrose in dicotyledonous leaf)
2007 Autotrophic Nutrition
P.15
[HKALE 91 I-7]
7. Why are leaves usually thin and flat? Name ONE environment where there are many exceptions
and explain why this is so.
(7 marks)
Answer
7. Leaves are the main organs of photosynthesis. In order to function efficiently, the chloroplasts 1
must be supplied with adequate light, carbon dioxide and water.
The flatness (an orientation) of the leaf maximises the area exposed to light
2
while the thinness maximises penetration (minimises distance) of light to the mesophyll tissue
where the chloroplasts are.
The flatness and thinness of the leaf maximises the surface/volume ratio and so maximises the 2
rate of diffusion of carbon dioxide into the leaf through the cuticle or stomata (rate of diffusion
proportional to area) and
minimises the distance which the carbon dioxide has to diffuse.
However, thin, flat shape also maximises the surface area for water loss.
3
In dry environments, many plants (xerophytes or halophytes) have thick or even cylindrical
leaves or rolled leaf margins. These are less efficient for photosynthesis but the lower
surface/volume ratio reduces water loss.
Xerophyte leaves may also be thicker because of the presence of water storage tissue
(succulents).
(max 7)
(Alternatives: turbulent aquatic environment where thin, flat leaves liable to mechanical damage)
[HKALE 96 II]
2.
(a)
Explain how the structural features of a dicot leaf make it an efficient organ for
photosynthesis.
(8 marks)
Answer:
2.
(a)
*a flat blade increases surface area to capture sunlight (1)
* blade connected to the stem by the petiole, which positions the blade for maximum 8
exposure to the sun (1);
* veins to support the blade(1);
* veins transport water to the mesophyll (1);
*stomata allow for diffusion of CO2 (1);
*chloroplasts of leaf are concentrated in the palisade mesophyll (1);
* columnar palisade mesophyll cells are closely packed beneath the epidermis,
maximize capture of sunlight (1):
*spongy mesophyll cells have large air spaces between them to facilitate CO2
diffusion (1).
2007 Autotrophic Nutrition
P.16
IV. Mechanism of Photosynthesis (Photochemical Reactions)
- Photosynthesis is a 2-stage process:
1. Light reaction (Photolysis)
2. Dark reaction (C Fixation)
(Photolysis)
- photochemical reaction
 require light and chlorophyll
(C Fixation)
- light not required
ATP from photolysis and enzymes are
required
Light Reaction
Dark Reaction
- temp. insensitive
- temp. sensitive
-water is oxidized
-CO2 is reduced to carbohydrates
(split into H+ and O2 )
(O2 come from H2O proved by using isotope 18O)
-occur in grana of chloroplast
-occur in stroma of chloroplast
Overall Equation :
6 CO2
+
12 H2O
light energy
------------> C6H12O6
chlorophyll
+
6 H2O
+
6 O2
1. Light Collection and release of electron from chlorophyll a
- electrons of pigment can absorb light of certain wavelength to a higher energy level or even be released.
-
Light of various wavelengths is captured by antennae complexes. Energy is funneled to the primary
pigments --- chlorophyll a.
-
After absorbing light energy of specific wavelengths, electrons in these chlorophyll a are excited.
-
release of excited electrons from chlorophyll a cause cyclic- or non-cyclic- photophosphorylation
 energy from these excited electrons generate ATP during the processes
2007 Autotrophic Nutrition
P.17
2. Cyclic Photophosphorylation
- Chlorophyll : donor and ultimate acceptor of electrons.
- Light strikes at the photopigments to raise the energy level of an electron from the chlorophyll a
- activated electron then passes through the electron transport system with ATP produced from ADP and
inorganic phosphate
- electron is returned through the cyclic chain of carriers and accepted by the previous chlorophyll
molecule
- the neutrality and stability of the chlorophyll molecule is restored
 ATP production only
Cyclic Photophosphorylation
3. Photolysis of water and Non-Cyclic Photophosphorylation
Light energy is finally converted to chemical energy in ATP (Photophosphorylation) and a reduced
coenzyme,NADPH2.
-
Products: ATP, NADPH2 and O2
(NADP : Nicotinamide adenine dinucleotide phosphate)
- When light strikes at the photopigments, an electron of a particular chlorophyll is raised to a
higher energy level and is taken up by an electron acceptor and finally passed to NADP for
production of NADPH (H+ is obtained from splitting of H2O).
- the electron lost from the chlorophyll is replaced (instead of accepting the returned electron,
i.e. non-cyclic process) by another electron from another illuminated chlorophyll via an
electron acceptor and the electron carrier system (with ATP produced).
- The electron lost from later chlorophyll molecule is replaced by an electron from the hydroxyl
ion (OH-) derived from the splitting of H2O (Photolysis).
2007 Autotrophic Nutrition
-
P.18
Results of the non-cyclic photophosphorylation in Light Reaction
(syl: construct a flow chart to show the process of photochemical reactions)
light
H2O ----------------->
Chlorophyll
NADP
O2
+
H+
NADPH
[ air ]
electron carrier system
ADP + Pi
ATP
-
-
[Dark reaction ]
Importance of photochemical reactions
- ATP molecules produced can act as the energy source in the Dark Reaction.
- H2O molecules are broken down to give H+ (in NADPH) for the reduction of CO2 to form
sugar.
Photolysis of water provides
-H for reduction of NADP
-oxygen release to atmosphere
[HKALE 88 II]
7. (a) Discuss the role of light in photosynthesis.
(8 marks)
Ans.: Light excites an electron in the chlorophyll molecule
½
The electron is emitted and passes through a series of electron carriers.
½
The last electron carrier returns the electron to the chlorophyll molecule. ½
During the electron transfer, energy is released for the synthesis of ATP
which provides the energy for subsequent synthesis of carbohydrates.
1
½
Water dissociates to yield H+ and OH-.
1
H+ combines with an e- to form a H atom. The H atoms are taken up by
NADP to form NADPH.
1
NADPH participates in the reduction of CO2.
1
OH- donates an e- to chlorophyll and the resulting OH forms water and
oxygen.
1
Involvement of photosystem I and photosystem II and their interaction.
1
(8)
2007 Autotrophic Nutrition
P.19
2007 Autotrophic Nutrition
2.
The Dark Reaction
P.20
(Calvin Cycle) (C Fixation)
-
occur in stroma
-
CO2 is fixed by a 5-C compound (ribulose bisphosphate / RuBP) and lead to the formation of 2
molecules of a 3-C compound (phosphoglyceric acid / PGA).
-
the 3-C compound is reduced by NADPH2 (which provide H for reduction) to triose
phosphate (TP), a 3-C sugar], energy (ATP)is required.
-
Products of Dark Reaction : TP
-
The fates of TP:
i. after a series of reactions, the 5-C compound RuBP is regenerated from majority of TP
 provide a continous supply of the 5-C carbon dioxide acceptor
ii. some of the TP combine to yield hexose phosphate which is subsequently metabolized to
glucose and other product, e.g. sucrose and starch
2007 Autotrophic Nutrition
P.21
[HKALE 96 II]
2.
(b)
Illustrate the Calvin cycle (carbon fixation) using a flow diagram and state which
intermediate is used for the synthesis of carbohydrates. Name one product of this
synthesis.
(6 marks)
Ans
(b)
5
[HKALE 85 II]
8. Discuss photosynthesis with reference to
(b) the principal stages involved in the production of monosaccharides; and
(c) its significance to plant and animal life.
8. (b)
(9 marks)
(6 marks)
Light stage in quantasomes of lamella :
Photoactivation of chlorophyll molecules and emission of high energy electrons.
1
Transfer of high energy electrons by a series of carriers in electron transport system
back to chlorophyll molecules in cyclic photophosphorylation with the synthesis of
ATP during the transfer from one carrier to another.
1
+
Alternative combination High energy electrons with H (from spontaneous dissociation
of water molecules) to produce H atoms in non-cyclic photophosphorylation. H atoms
then reduce NADP to NADPH2 for later use in dark stage.
1
OH- (from spontaneous dissociation of water molecules) donates an electron to
chlorophyll molecules via the electron carrier system, with formation of ATP during the
transfer. The remaining OH forms water and oxygen.
1
The significance of the light stage is to generate ATP and NADPH2 for use in the dark
stage for carbon fixation.
1
Dark stage in stroma :
Combination of CO2 with ribulose diphosphate, incorporating it into the cycle, to form
an unstable 6-C compound.
1
Immediate splitting of each unstable 6-C compound to two molecules of 3-C 1
phosphoglyceric acid.
1
Reduction of phosphoglyceric acid to triose phosphate by BADPH2, using energy
provided by ATP produced in the light stage.
1
Triose phosphates combine to form 6-C sugar through a series of intermediate steps,
with energy provided by the ATP formed in the light stage.
(9)
(Diagrams may be used to supplement description. However, a diagram alone without
explanation is NOT acceptable).
2007 Autotrophic Nutrition
8. (c)
P.22
Significance to plant and animal life
1. Nutrition :
- process by which green plants build up sugar from simple inorganic compounds for
energy.
- process from which green plants build up other organic compound (e.g. amino acids,
fatty acids and glycerol etc.) for repair and growth.
11/2
2. Ecology :
- process which enables the existence and supply of producers in grazing food chains in
nature. The availability of producers enables existence of the other members of food
chains.
- process which man can make use of to produce food for himself through cultivation. 11/2
Energy transfer :
3. - process on earth which can capture sun’s energy and convert it into a form which can
be transferred from organism to organism to maintain life on earth.
11/2
Gaseous balance :
- process can remove excess CO2 in atmosphere produced in respiration of plants and
4. animals, burning of fuels etc. and keep atmospheric CO2 level at a balanced level for
plant and animal survival.
- process can add O2 to the atmosphere for respiration of plants and animals, burning 11/2
etc. and keep atmospheric O2 level at a balanced level for plant and animal survival.
(6)
(Any other reasonable answer - 1 mark each)
[HKALE 94 II]
1.
(b)
Contrast the “light reaction” with the “dark reaction” in photosynthesis.
(b)
"Light reaction"
"Dark reaction"
Location in Grana/Thylakoids
Stroma
chloroplast
Reactions (i) Photochemical i.e. requires light (i) Does not require light and
and chlorophyll
chlorophyll
(ii) Energy is captured.
(ii) Energy is used up in synthesis
Light energy is converted to
chemical energy via chlorophyll.
(photophosphorylation)
(iii) Water is split into hydrogen
ions and oxygen (photolysis)
(iii) CO2 is fixed by ribulose
biphosphate (RuBP) to form 2 x
phospho-glyceric acid (PGA)
(iv) PGA is reduced to a sugar (PGAL)
using the NADPH2. RuBP is
reformed via the Calvin cycle
Products
ATP, NADPH2 and oxygen
( answers must be in pairs ) (7 marks)
PGAL/trios phosphate/3C sugar
½,½
½,½
1, ½
1,1
0,1
½,½
2007 Autotrophic Nutrition
P.23
[HKALE 88 II]
7.
(b)
How are starch, lipids and proteins derived from the primary product (triose
phosphate / phosphoglyceraldehyde) of photosynthesis?
(9 marks)
(b)
Formation of starch
Starch is synthesised in a process which is essentially
the reverse of glycolysis (i.e. 2PGA  2PGAL 
hexose diphosphate  hexose phosphate polymerization
(3)
starch )
Formation of lipids
(3)
PGA enters into the glycolytic pathway and is converted
to an acetyl group which is added to coenzyme A to form
acetyl coenzyme A. This is converted to fatty acids in
both cytoplasm and chloroplast. Glycerol is made from
triose phosphate. Glycerol and fatty acids combine to
form lipids.
Formation of proteins
PGA is first converted to one of the carboxylic acids of
the Krebs cycle via acetyl coenzyme A/via other suitable
pathways. Subsequently amino acids are formed by
amination. (N, S and occasionally P are needed for protein
formation and are obtained from the soil water as
inorganic salts.) and/or transamination. Proteins are
formed by polymerization reaction.
(3)
2007 Autotrophic Nutrition
P.24
V.
Fates of photosynthetic products
hexose, sucrose, lipids and amino acids can ultimately be formed from the TP (PGA), depending on the
requirements of the plant at the time.
1.
Synthesis of carbohydrates
- triose phosphate (TP) can be synthesized into glucose and other hexose such as fructose
- excess triose phosphate can be used for either starch synthesis in chloroplast (for storage) or sucrose
synthesis in cytosol
- sucrose which is the main form in which carbohydrate is transported throughout dicots in phloem
- hexose may alternatively be polymerized into other polysaccharides such as cellulose (the main
component of plant cell wall)
2.
Synthesis of lipids and amino acids
carbohydrates produced and the intermediates of the Calvin cycle can be converted into various lipids and
amino acids via other metabolic pathways in the plant.
2007 Autotrophic Nutrition
P.25
GP / PGA
Relationship between photosynthesis and synthesis of food in
plants. Some intermediate steps are omitted.
2007 Autotrophic Nutrition
P.26
[HKALE 86 I]
6.
Using ONLY the following words :
fatty acid
glycerol
hexose
Krebs cycle
pyruvic acid
triose phosphate
phosphoglyceric acid
ribulose diphosphate
draw a flowchart to show how the carbon atom of a carbon dioxide molecule in a plant can be
incorporated into
(a)
(b)
(c)
a polysaccharide,
an amino acid, and
a fat molecule.
Total : 5 marks
6.
CO2
RDP
PGA
TP
Krebs
cycle
pyruvic
acid
amino
acid
fatty acid
hexose
polysaccharide
glycerol
fat
(5)
2007 Autotrophic Nutrition
P.27
E. Factors Affecting Photosynthesis
(a) Principle of Limiting Factors
When a chemical process is affected by more than one factor, its rate is limited by the factor which
is nearest its minimum value (i.e. limiting factor); it is that factor which directly affects a process if
its quantity is changed.
[HKALE 93 II]
3. (c) What is meant by “limiting factor”?
At any given moment, a biochemical process (e.g. photosynthesis) which involves a series of
reactions, will be limited by the slowest reaction in the series. This slowest reaction is determined by
the factor which is in shortest supply / which is nearest its minimum value. This factor is called the
limiting factor.
2007 Autotrophic Nutrition
(b)
P.28
Effects of Environmental Factors on the Rate of Photosynthesis
1.
[CO2]
Rate of photosynthesis

[CO2] in air
(provided if light intensity is not limiting & optimum temp.)
[CO2] in air ranges from 0.03-0.04%.
The optimum is about 0.1 % in general (i.e. normal [CO2] is often a major limiting factor of
photosynthesis on a sunny day).
an increase in carbon dioxide concentration at the chloroplasts favours the carboxylation
reaction in the competition for enzyme between oxygen ( in photorespiration) and carbon
dioxide (in CO2 fixation)
This has led to some greenhouse crops, such as tomatoes, being grown in
CO2-enriched atmospheres, by raising the carbon dioxide concentration to 0.1 percent from the
normal air concentration of 0.035 percent, to prevent photorespiration.
2007 Autotrophic Nutrition
2.
P.29
Light Intensity and Quality
- Light Quality (wavelength / frequency):
the absorption spectrum and action spectrum of chlorophyll show that red and blue lights are
more effective for photosynthesis than green light
-
Light intensity:
zero light intensity:
no photosynthesis (net release of CO2 due to respiration)
low light intensities: photosynthetic rate increases linearly with increasing light intensity (net
release of CO2 reduce, and equal zero at compensation point, then net
take up of CO2 increase proportional to increase in light intensity)
high light intensities: increase in photosynthetic rate reduced gradually to a constant level (i.e.
light-saturated) as the other factors become limiting
Therefore in a clear summer day, normally light intensity is not a major limiting factor,
especially in shade plants.
(* a very high light intensity may inhibitory to photosynthesis due to the destructive effect on the photosynthetic
pigments)
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P.30
3. Temperature:
- Photosynthesis is enzyme-controlled and therefore temperature-sensitive
 Q10  2 (for enzymatic reactions)
(i.e. the rate of reaction will double for an increase of 10oC .)
-
The rate doubles for every 10oC within a temperature range below the optimum temperature.
-
Further increase in temperature beyound the optimum temperauture leads to thermal denaturation
of the enzymes and a level off and then a quick drop in the rate of photosynthesis.
4. Water
in the situation of shortage of water or excessive loss of water (e.g. strong wind) , the closure of
stomatal pores reduced the supply of carbon dioxide and hence the photosynthetic rate
also a decrease in the hydration of the chloroplasts and other parts of the protoplasm
Q:
Could you conclude the principle for maximizing plant growth in greenhouse by the control of light,
temperature and carbon dioxide concentration? Try to apply the concept of limiting factors in design of a
greenhouse to increase the yield of plants.
[HKALE 84 II-2d]
(d)
List THREE environmental factors affecting photosynthesis in plants and briefly
explain how they operate.
answer:
Temperature:
Photosynthesis has a Q10 of about 2 (i.e. increase in 10oC will lead to doubling the rate
of photosynthesis).
Light:
The intensity of light affects directly the rate of photosynthesis (if the supply of CO2 is
not limiting).
The quality of light (i.e. differences in wavelength) also affects photosynthesis.
Carbon dioxide:
The rate of photosynthesis is directly proportional to CO2 concentration in air (if light
intensity is not limiting).
Oxygen:
Photosynthetic rates of most plants are inhibited by O2.
2 for
each
(any 3)
[HKALE 85 II]
8. Discuss photosynthesis with reference to
(a) the environmental factors affecting its rate;
(5 marks)
(a) Environmental factors :
1. Light intensity - usually an increase in light intensity increases the rate, when light
intensity is the limiting factor.
1
2. Light quality - red and blue lights form the optimal wavelenghts for photosynthesis,
the rate is fastest under these wavelengths. Green light is least effective.
3. Temperature - below 30C (usually the optimal temperature), generally an increase 1
in temperature increases the rate, when temperature is the limiting factor.
1
4. CO2 concentration - an increase in CO2 concentration generally increases the
rate, when CO2 is the limiting factor.
1
5. Humidity/water supply - its effect on photosynthetic rate.
1
The three factors (light intensity, temperature and CO2 concentration) interact to control
the rate of photosynthesis. The rate is primarily limited by “the factor” which is nearest
1
its minimum value (law of the limiting factor).
(5)
Bonus mark -
2007 Autotrophic Nutrition
F.
P.31
Mineral Nutrients Required by Photosynthetic Plants
Autotrophic nutrition involves not only photosynthesis, but also the subsequent use of minerals from soil
like nitrates, sulphates and phosphates to make other organic requirements, such as proteins, nucleic
acids and so on.
Essential elements include:
- sixteen elements have been demonstrated to be essential for plant growth.
- nine of these are required in fairly large quantities and are therefore known as macronutrients
(major element)
-- except potassium, their major functions in plant cells are structural
-- potassium has a very important role in maintaining the turgidity of cells because it is osmotically
active
- the remaining seven are needed in trace amounts for normal plant growth and development and are
known as micronutrients (trace elements)
-- they are usually acts as cofactors for activation of many enzymes
-
three of the 16 elements can obtained from water or gases in the atmosphere via photosynthesis: carbon, oxygen, hydrogen
-
the 13 remainder are obtained from the soil as dissolved mineral ions
-ultimate source is the parent rock from which the soil was formed
-the loss via leaching or exhaustion by plants can be compensated by fertilizer.
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P.32
(for reference)
Macronutrients
Structural:
(carbon, oxygen, hydrogen)
nitrogen
phosphorous
sulphur
calcium
magnesium
maintain turgidity:
potassium
Element
Magnesium
Nitrogen
Phosphorus
Micronutrients
usually act as cofactors:
iron
manganese
boron
copper
zinc
moybdenum
chlorine
Roles
Deficiency symptom
Required nonspecifically Stunted growth; chlorosis
by a large number of
萎黃病
enzymes involved in
phosphate transfer. A
constituent of the
chlorophyll molecule.
Formation of amino acids,
proteins, nucleotides,
nucleic acids, chlorophyll,
etc.
Production of sugar
phosphates, nucleotides,
nuclei acids, ATP and
membrane phospholipids
(has a key role in
reactions in which ATP is
involved)
Stunted growth; chlorosis
Stunted growth,
particularly of roots
Suggested Activities:
Find out the ingredients of the fertilizers available in supermarkets. Compare the chemical
ingredients of a fertilizer that claims to promote flowering. Try to figure out the reasoning behind
their claims. Then design and perform experiments to test the validity of these claims.
2007 Autotrophic Nutrition
P.33
Hydroponics for growing plants
✞ Hydroponics is an alternative way of growing plants (an application of scientific knowledge to agriculture
and horticulture)
✞ Is a technology for growing plants in nutrient solutions (water and fertilizers) with (e.g., sand, gravel,
vermiculite, rockwool, peat, coir, sawdust but not soil) or without the use of artificial medium to provide
mechanical support.
✞ All hydroponic systems in temperate regions of the world are enclosed in greenhouse-type structures to
provide temperature control, reduce evaporative water loss, and to reduce disease and pest infestations.
Advantages of hydroponics
1. The principal advantages of hydroponic controlled environment agriculture (CEA) include
-
high-density maximum crop yield,
-
crop production where no suitable soil exists,
-
a virtual indifference to ambient temperature and seasonality,
-
more efficient use of water and fertilizers
pollution problems are minimized),
-
minimal use of land area, and
-
suitability for mechanization, disease and pest control.
-
with specific treatments (e.g. DNA recombination), hydroponically grown crops can be made more
attractive and tastier.
(leaching of nutrients is prevented and wastage as well as
The major advantage of hydroponic (CEA) compared to field grown produce is the isolation of the crop from
the soil, which often has problems of diseases, pests, salinity, poor structure and/or drainage.
Disadvantages of hydroponics
The high costs of capital and energy inputs, and the high degree of management skills required for successful
production. Capital costs may be especially excessive if the structures are artificially heated and cooled. This is
why appropriate crops are limited to those with high economic value such as tomatoes.
Reference:
http://ag.arizona.edu/hydroponictomatoes/overview.htm
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P.34
The hydroponic technique was developed in the 1930s, mainly for studying the nutritional needs of plants at
that time.