Chapter 14 Autotrophic Nutrition Autotrophic organisms use an inorganic form of carbon, e.g. carbon dioxide, to make up complex organic compounds, with.

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

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)

6CO2 + 6H2O C6H12O6 + 6O2chlorophyll

Equation for photosynthesis:

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- 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,

carotinoids

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 spectraAn absorption spectrum is the degree of

absorption at each wavelength by a pigmentAn 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.

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 nature 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 hydrogen + oxygensunlight

chlorophyll

Dark Reaction: Hydrogen from light reaction combines

with carbon dioxide to form carbohydrates (glucose)

Water is produced as a by-product

The process of photosynthesis:

carbon dioxide + hydrogen

carbohydrate (glucose) + water

14.3 Mechanism of photosynthesis

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.

6CO2 + 6H2O C6H12O6 + 6O2

Overall equation

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

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.

14.4.2 Effect of light intensity 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 …..