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Chapter 14 Autotrophic Nutrition

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Chapter 14 Autotrophic Nutrition Powered By Docstoc
					     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   for
  photosynthesis:
1 To obtain light (sunlight)
2 To obtain & remove gases
  (carbon dioxide & oxygen)
3 To obtain & remove liquids
  (water & sugar solution)
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
              Other pigments:
              carotenoids – carotenes
                            xanthophylls
carotinoids
              - 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)

          sunlight
Water    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)      lipid
  glycerol
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 …..

				
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