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Chapter 14
Autotrophic Nutrition
Autotrophic organisms use an inorganic
form of carbon, e.g. carbon dioxide, to
make up complex organic compounds,
with energy from two sources:
(1) light and
(2) chemicals.
When using light, the process is
photosynthesis, as practised by all
green plants.
When using chemicals, the process is
chemosynthesis, as practised by certain
bacteria.
Photosynthesis is more common and
important because:
1. It is the means by which the sun's
energy is captured by plants for
use by all organisms.
2. It provides a source of complex
organic
molecules
for
heterotrophic organisms.
3. It releases oxygen for use by
aerobic organisms.
14.1 Leaf structure
 Equation for photosynthesis:
6CO2 + 6H2O  C6H12O6 + 6O2
chlorophyll
Adaptations
of
the
leaf
photosynthesis:
1 To obtain light (sunlight)
2 To obtain & remove gases
(carbon dioxide & oxygen)
3 To obtain & remove liquids
(water & sugar solution)
for
14.1.1 Adaptations for obtaining
energy (sunlight)
To ensure plants are efficient to absorb
sunlight, a leaf shows many
adaptations:
1. Phototropism causes shoots to grow
towards the light to allow the leaves to
obtain maximum illumination
2. Etiolation causes rapid elongation of
shoots which are in the dark, to ensure
that the leaves are brought up into the
light as soon as possible
3. Leaves arrange themselves into a
mosaic to minimize overlapping
4. Leaves have a large surface area to
capture as much light as possible
5. Leaves are thin to reduce filtration of
light into the lower layers
6. Cuticle and epidermis are transparent
to
allow
light
through
the
photosynthetic mesophyll beneath
7. The palisade mesophyll are packed
with chloroplasts and arranged with
their long axes perpendicular to the
surface to trap most light
8. Chloroplast within the cells can
move –
This allows them to arrange
themselves into the best positions
within a cell for efficient absorption of
light
9. The chloroplasts hold the chlorphyll
in a structured way –
The chlorophyll is contained within
the grana on the sides of a series of
unit membranes.
This presents the maximum amount
of light and close proximity to other
pigments.
14.1.2 Adaptations for obtaining and
removing gases
To ensure rapid diffusion of gases:
1 Numerous stomata are present in the
epidermis of leaves.
2 Stomata can be opened and closed by
differential expansion of the cell walls of
the guard cells surrounding the stoma
3 Spongy mesophyll possesses many
airspaces to provide uninterrupted diffusion
of gases between the atmosphere and the
palisade mesophyll
14.1.3 Adaptations for obtaining and
removing liquids
1 A large central midrib containing a large
comprising xylem and phloem tissue.
Xylem transports water and minerals to the
while phloem conducts away food, usually in
the form of sucrose.
2 A network of small veins to ensure a
constant supply of water and removing the
sugars.
Its sclerenchyma associated provides
a frame work of support to the leaves to
present maximum surface area to the light.
14.2 Mechanism of light absorption
14.2.1 The nature of light
There are 3 features of light which make it
biologically important:
1 spectral quality (colour)
2 intensity (brightness)
3 duration (time)
The visible section of the electromagnetic spectrum
The visible section of the electromagnetic spectrum
14.2.2 The
photosynthetic
pigments
Most important are
chlorophylls a and b
which absorb light
in the blue and the
red regions of the
visible spectrum.
Green is reflected thus
gives chlorophyll its
characteristic colour.
Structure of chlorophyll:
a porphyrin ring
(hydrophilic) lies on the
thylakoid membrane surface,
a long hydrocarbon tail
(hydrophobic) embedded in
thylakoid membrane
carotinoids
Other pigments:
carotenoids – carotenes
xanthophylls
- colour ranges from yellow,
through orange to red,
- depends on number of
double bonds (deeper colour
with more double bonds)
- colour usually masked by
chlorophylls but
apparent when chlorophylls
break down in autumn,
OR in many flowers and fruits
-they absorb lights in the blue-violet spectrum
--carotene as orange colour in carrots &
a good source of vitamin A
14.2.3
Absorption
and
Action Spectra
for common
plant pigments
14.2.3 Absorption and action spectra
An absorption spectrum is the degree of
absorption at each wavelength by a pigment
An action spectrum is the effectiveness of
different wavelengths of light in bringing
about photosynthesis
Results show that the action spectrum for
photosynthesis is closely related to the
absorption spectra for chlorophylls a and b
and carotenoids.
This suggests that these pigments are those
responsible for absorbing the light used in
photosynthesis.
The nature of photosynthesis
 Raw materials:
carbon dioxide and water
 Main product:
carbohydrates;
 By-product:
oxygen
 Light energy is changed into chemical energy
trapped in the carbohydrate formed
The nature of photosynthesis
 Photosynthesis: an anabolic process
 It takes place in chloroplasts of green plants
 Chlorophyll (a green pigment) in chloroplasts
absorbs light as energy to drive the reactions of
photosynthesis
The process of photosynthesis:
 Light reaction (in light only) &
 Dark reaction (in light or darkness)
Light Reaction: water is split by light into
hydrogen & oxygen (gas)
Water
sunlight
 hydrogen + oxygen
chlorophyll
The process of photosynthesis:
Dark Reaction:
 Hydrogen from light reaction combines with
carbon dioxide to form carbohydrates
(glucose)
 Water is produced as a by-product
carbon dioxide + hydrogen
 carbohydrate (glucose) + water
14.3 Mechanism of photosynthesis
Overall equation
6CO2 + 6H2O  C6H12O6 + 6O2
Experiments
showed
that
rate
of
photosynthesis is affected by both light
intensity and temperature.
As temperature does not affect processes such
as the action of light on chlorophyll, thus
temperature only affects a purely chemical
stage.
Photosynthesis is a process of energy transduction.
Light energy is firstly converted into electrical
energy and finally into chemical energy.
It has three main phases:
1. Light harvesting in which light is captured by the
plant using a mixture of pigments including
chlorophyll.
2. The light dependent stage (photolysis) in which a
flow of electrons results from the effect of light on
chlorophyll and so causes the splitting of water into
hydrogen ions and oxygen.
3. The light independent (dark) stage during which
these hydrogen ions are used in the reduction of
carbon dioxide and hence the manufacture of
sugars.
14.3.2 Light stage (photolysis)
- occurs in the grana of the chloroplast
- Photolysis means the splitting of water by
light
- Photophosphorylation means light is
involved in the addition of phosphorus
(phosphorylation)
Process of photolysis:
1. Light energy is trapped in pigment
system II and boost electrons to a higher
energy level.
Process of photolysis:
1. Light energy is trapped in pigment
system II and boost electrons to a higher
energy level.
2. The electrons are received by an electron
acceptor.
Process of photolysis:
1. Light energy is trapped in pigment
system II and boost electrons to a higher
energy level.
2. The electrons are received by an electron
acceptor.
3. The electrons are passed from the
electron acceptor along a series of
electrons carriers to pigment system I
which is at a lower energy level.
Process of photolysis:
1. Light energy is trapped in pigment
system II and boost electrons to a higher
energy level.
2. The electrons are received by an electron
acceptor.
3. The electrons are passed from the
electron acceptor along a series of
electrons carriers to pigment system I
which is at a lower energy level.
The energy lost by the electrons is
captured by converting ADP to ATP.
Energy has thereby been converted to
chemical energy.
4.Light energy absorbed by pigment system I
boosts the electrons to an even higher
energy level.
4.Light energy absorbed by pigment system I
boosts the electrons to an even higher
energy level.
5.The electrons are received by another
electron acceptor.
4.Light energy absorbed by pigment system I
boosts the electrons to an even higher
energy level.
5.The electrons are received by another
electron acceptor.
6.The electrons which have been removed
from the chlorophyll are replaced by
pulling in other electrons from a water
molecule.
4.Light energy absorbed by pigment system I
boosts the electrons to an even higher
energy level.
5.The electrons are received by another
electron acceptor.
6.The electrons which have been removed
from the chlorophyll are replaced by
pulling in other electrons from a water
molecule.
7. The loss of electrons from the water
molecule causes it to dissociate into
oxygen gas and protons.
8. The protons from the water molecule
combine with the electrons from the second
electron acceptor and these reduce NADP+.
8. The protons from the water molecule
combine with the electrons from the second
electron acceptor and these reduce NADP+.
9. Some electrons from the second acceptor
may pass back to the chlorophyll molecule
by the electron carrier system, yielding ATP
as they do so. This process is called cyclic
photophosphorylation.
8. The protons from the water molecule
combine with the electrons from the second
electron acceptor and these reduce NADP.
9. Some electrons from the second acceptor
may pass back to the chlorophyll molecule
by the electron carrier system, yielding ATP
as they do so. This process is called cyclic
photophosphorylation.
10. Non-cyclic photophosphorylation:
Electrons from chlorophyll are passed into
the dark reaction via NADP + H+. These
electrons are replaced from the water
molecules, without recycling back into the
chlorophyll.
Non-cyclic
photophorylation
14.3.3 The dark stage (light independent
stage)
 - occurs in the stroma of the chloroplasts
 - light independent because it takes place
whether or not light is present
The Dark
Stage
 Overall process:
Reduction of CO2 using
the reduced NADPH + H+ and ATP
1.CO2 diffuses into stroma of chloroplast
The Dark
Stage
 Overall process:
Reduction of CO2 using
the reduced NADPH + H+ and ATP
1.CO2 diffuses into stroma of chloroplast
2. CO2 combines with ribulose bisphosphate
(5-C) to form an unstable 6-C intermediate
The Dark
Stage
6-C
compound
 Overall process:
Reduction of CO2 using
the reduced NADPH + H+ and ATP
1.CO2 diffuses into stroma of chloroplast
2. CO2 combines with ribulose bisphosphate
(5-C) to form an unstable 6-C intermediate
3. 6-C breaks down into 2 molecules of
glycerate 3-phosphate (GP)
The Dark
Stage
 Overall process:
Reduction of CO2 using
the reduced NADPH + H+ and ATP
1.CO2 diffuses into stroma of chloroplast
2. CO2 combines with ribulose bisphosphate
(5-C) to form an unstable 6-C intermediate
3. 6-C breaks down into 2 molecules of
glycerate 3-phosphate (GP)
4. ATP from light stage helps to convert GP
into triose phosphate (GALP) or
glyceraldehyde 3-phosphate.
The Dark
Stage
Glyceraldehyde 3-phosphate
5. NADPH + H+ donates its H atoms to
reduce GP to triose phosphate, NADP+
goes back to the light stage to accept more
H.
The Dark
Stage
5. NADPH + H+ donates its H atoms to
reduce GP to triose phosphate, NADP+
goes back to the light stage to accept more
H.
6. Pairs of triose phosphate molecules are
combined to produce an intermediate
hexose sugar.
The Dark
Stage
5. NADPH + H+ donates its H atoms to
reduce GP to triose phosphate, NADP+
goes back to the light stage to accept more
H.
6. Pairs of triose phosphate molecules are
combined to produce an intermediate
hexose sugar.
7. Hexose sugar is polymerized to form
starch which is stored by the plant.
The Dark
Stage
5. NADPH + H+ donates its H atoms to reduce
GP to triose phosphate, NADP+ goes back to the
light stage to accept more H.
6. Pairs of triose phosphate molecules are
combined to produce an intermediate hexose
sugar.
7. Hexose sugar is polymerized to form starch
which is stored by the plant.
8. Some triose phosphate is used to regenerate
ribulose bisphosphate to accept CO2, with energy
supplied by ATP from the light reaction.
9. 5 triose phosphate
 3 ribulose bisphosphate
The Dark
Stage
14.3.4 Fate of photosynthetic products
From the products of photosynthesis a totally
autotrophic plant must synthesize all
organic molecules necessary for its survival:
Synthesis of other carbohydrates
1 glucose and fructose combine to form
sucrose
2 glucose polymerizes to form starch
3 fructose polymerizes to form inulin
4 glucose polymerizes to form cellulose to
form cell walls
Synthesis of lipids
glycerate 3-phosphate (GP)
 acetyl coenzyme A
 fatty acids
triose phosphate (GALP)
 glycerol
lipid
Functions of lipids:
1 As important storage substance
2 Major constituent of cell membranes &
waxy cuticle
3 Fatty acids provide some flower scent to
attract insects
 Synthesis of proteins
 glycerate 3-phosphate
  acetyl coenzyme A
  amino acids through transamination
reactions
 The nitrogen source is obtained from
nitrates in soil, with amino acids polymerize
into proteins
Functions of proteins:
1 essential for growth and development
2 structural component of cell membrane
3 as enzymes for metabolism
4 storage material
14.4 Factors affecting photosynthesis
14.4.1 Concept of limiting factors:
At any given moment, the rate of a
physiological process is limited by one
factor which is in shortest supply, and by
the factor alone.
It is the factor which is nearest its minimum
value which determines the rate of a
reaction.
Any change in the level of this factor (the
limiting factor) will affect the rate of the
reaction, e.g. photosynthesis and light
intensity
Limited by light intensity
14.4.2 Effect of light intensity on the rate of
photosynthesis
Compensation point
14.4.2 Effect of light intensity on the rate of
photosynthesis
Compensation point is the light intensity at
which the rate of photosynthesis equals to
that of respiration.
Light saturation is is the point at which
increase in light intensity has no effect on
the rate of photosynthesis.
14.4.3 Effect of CO2 concentration on the rate
of photosynthesis
Normal CO2 concentration of about 0.04% is
a major limiting factor in the natural habitat.
Farmers could cultivate greater yields in green
houses with enriched CO2 environment.
14.4.4 Effect of inorganic ions on the rate of
photosynthesis
Light stage is unaffected by temperature while
the dark stage is temperature dependent.
Why?
Because the dark stage is controlled by
enzymes while the light stage is a totally
photochemical reaction.
Rate of photosynthesis is proportional to
temperature. Rate doubles for every 10°C
rise in temperature until optimum which
varies from species to species.
Above the optimum temperature, rate levels
off and then drops down because of
denaturation at high temperatures.
14.4.5 Effect of inorganic ions on the rate of
photosynthesis
In the absence of some minerals, e.g. iron,
nitrogen & magnesium, leaves become
yellow (chlorosis) and therefore rate of
photosynthesis also much reduced.
14.4.6 Other factors affecting the rate of
photosynthesis
Water is very important for photosynthesis,
but its effect is difficult to determine
because water has too many functions to be
responsible.
Chemical like cyanides, sulphur dioxide, etc.
all reduce photosynthesis as air pollutants.
14.5 Chemosynthesis
- By autotrophic bacteria, with energy derived
from inorganic chemicals
Function in helping to recycle valuable
minerals in the nitrogen cycle
Chemoautotrophs: organisms using the
oxidation of chemicals as a source of energy
Photoautrtrophs: organisms using light …..