Photosynthesis
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Photosynthesis is the production of organic compounds from inorganic molecules using light
energy trapped by chlorophyll.
chlorophyll
Carbon Dioxide + Water + Light
>> Glucose + Oxygen
chlorophyll
6CO2 + 6H2O + Light
>> C6H12O6 + 6O2
Location of Chlorophyll in Photosynthetic Plant Cells
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Chlorophyll is present in the chloroplasts.
Chlorophyll occurs in the internal green membranes of the
chloroplast.
Chloroplasts are ‘the Little Green Slaves of Photosynthetic Plant
Cells’.
double outer membrane – inner membrane is smooth, not folded
green internal membranes
Light Stage in the green internal membranes
Dark Stage in the liquid portion of the chloroplast
Source of Light in Leaf Cells
1. The natural source of light for photosynthesis is the sun.
2. Artificial light can also be used if it contains red and/or blue wavelengths of visible light.
Source of Carbon Dioxide in Leaf Cells
1. Carbon dioxide in air is the major source.
Ms Cooney
Biology Notes
2. Carbon dioxide produced by the mitochondria during aerobic respiration is a minor source.
3. Carbon dioxide dissolved in water is the major source for aquatic plants.
Source of Water in Leaf Cells
1. Water from the soil is the major source. Delivered to the leaves in xylem tissue.
2. Some water is also produced by the mitochondria of leaf cells during aerobic respiration.
Role of Photosynthesis in the Biosphere
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Source of food for plants.
Source of oxygen for aerobic respiration of plants, animals and aerobic micro-organisms.
Direct source of food for herbivores and omnivores.
Indirect food source for carnivores and decomposers.
Original organic source from which fossil fuels formed.
Oxygen and Photosynthesis
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In normal conditions photosynthesis is much faster than aerobic respiration.
Oxygen formation by photosynthesis is far greater than that used by respiration.
The excess oxygen is released from the plant to the atmosphere.
Oxygen produced by photosynthesis = oxygen used in respiration + oxygen released into air.
General Outline of Photosynthesis
Light Stage
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Light energy used to make ATP.
Light energy used to produce NADPH from NADP+.
Oxygen gas as a by-product.
Half of the water used as a hydrogen source is recycled
Dark Stage
1. Carbon dioxide and hydrogen are used to make carbohydrate.
2. The energy to drive this process comes from ATP.
An outline of the Biochemistry of Photosynthesis
Ms Cooney
Biology Notes
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Light energy is absorbed by chlorophyll.
Absorbed light energy causes the emission of energised electrons.
Some of these electrons can release their energy in a controlled fashion synthesising ATP.
Other electrons are carried into the Dark Stage.
The absorbed light energy is also responsible for the break up of water.
The break up of water releases oxygen, electrons and protons (H+, hydrogen ions).
The oxygen passes from the chloroplast, some will be used by the mitochondria for aerobic
respiration and the remainder will be passed into the atmosphere.
8. The electrons from water run to chlorophyll releasing energy forming ATP.
9. The protons are released into the ‘pool of protons’ in the chloroplast.
10. In the Dark Stage electrons from chlorophyll, protons from the pool and carbon dioxide react
together forming carbohydrate
Detailed Description of Photosynthesis
The Light Phase or Light-Dependent Stage (only takes place in the presence of light)
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Cyclic Electron Transport – Pathway 1.
Non-cyclic Electron Transport – Pathway 2.
Cyclic Electron Transport
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Light energy is absorbed by chlorophyll in the green internal membranes of the chloroplast.
An ‘excitable’ electron in chlorophyll absorbs light energy.
This electron is elevated to a higher energy level.
The energised electron escapes the chlorophyll molecule.
An electron acceptor molecule picks up this energised electron.
The electron is passed along an ‘electron carrier system’ where its ‘excess’ energy is
released.
7. The excess energy is used to produce ATP by the phosphorylation of ADP.
8. The electron then returns to chlorophyll after all the excess energy has been given off.
Non-cyclic Electron Transport
Ms Cooney
Biology Notes
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Light energy is absorbed by chlorophyll in the green internal membranes of the chloroplast.
An ‘excitable’ electron in chlorophyll absorbs light energy.
This electron is elevated to a higher energy level.
The energised excited electron escapes form the chlorophyll molecule.
An electron acceptor picks up this energised electron.
The electron acceptor passes this electron to NADP+ becoming NADP (neutral).
NADP then receives another electron becoming NADP-.
NADP- now attracts a hydrogen ion (H+) or proton from the proton pool in the chloroplast.
NADP- now becomes NADPH.
The loss of electrons from chlorophyll causes the break up of water.
Water breaks up into oxygen, protons (hydrogen ions) and electrons.
The oxygen passes out of the chloroplast by diffusion.
The protons pass into the general pool of protons in the chloroplast.
The electrons pass to chlorophyll as replacement for those lost to NADP-.
The electrons from water pass through an electron transport chain resulting in ATP
formation.
The Dark Stage (The Light Independent Phase )
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Does not require light.
Occurs in light and also in darkness as long as the products of the light phase are still
available.
The dark stage runs if ATP, NADPH and carbon dioxide are present.
Takes place in the liquid portion of the chloroplast.
Carbohydrate is the crucial product of the dark stage.
ADP, P and NADP+ are recycled for use in the light stage.
Details of Pathway
1. CO2 diffuses into the chloroplast from the mitochondria or from the external environment.
2. NADPH delivers electrons and a proton to CO2 (NADP ? NADP+ + 2e- + H+)
3. Electrons, protons and carbon dioxide combine to form carbohydrate [Cx (H2O)y] e.g.
glucose.
4. The energy needed for this reaction is supplied by the break up of ATP.
5. NADP+, ADP and P pass back to the light stage.
6. NADP+: nicotinamide adenine dinucleotide phosphate
7. Function: to transfer energy for the formation of complex organic compounds.
8. The energy is in the form of energy-rich electrons.
9. In photosynthesis these electrons came from light activated chlorophyll.
10. NADP+ becomes NADPH in the light stage.
11. NADPH transfer electrons and protons to carbon dioxide reducing it to carbohydrate.
12. NADP- is regenerated when NADPH passes on the electrons and proton.
Ms Cooney
Biology Notes
Promoting Crop Growth in Greenhouses
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Increase the light intensity to increase the rate of photosynthesis
Increase the light duration to increase the total amount of photosynthesis.
Increase Carbon Dioxide Concentration:
Raises the rate of photosynthesis so more food produced.
Carbon dioxide enrichment is achieved by gas cylinders or kerosene burners.
Mandatory Activity
To Investigate the Influence of Light Intensity on the Rate of Photosynthesis
1. Place a funnel over Elodea,
pondweed, in a beaker of pond water
at 25°C.
2. The funnel is raised off the bottom
on pieces of blue-tack. This allows
continuous free diffusion of CO2 to
Elodea.
3. Invert a test tube full of water over
the stem of the funnel to collect any gas from the Elodea.
4. Place the beaker on a hot plate at 25°C.
5. Maintain and monitor the temperature of the water with a thermometer.
6. Excess sodium bicarbonate is placed in the water to give a constant saturated solution of
CO2.
7. Place the lamp (the only light source) at a predetermined distance from the plant.
8. Use a light meter to measure the light intensity at this distance. Record the light intensity.
9. Allow the plant five minutes to adjust to the new conditions.
10. Count the number of oxygen bubbles given off by the plant in a five-minute period.
11. Repeat the count twice more and calculate the average of the three readings. This is the rate
of photosynthesis at that particular light intensity.
12. The gas should be checked to prove that it is indeed oxygen — it relights a glowing splint.
13. Repeat at different light intensities by moving the lamp to different distances.
14. Run a control: identical set up but at a constant light intensity.
15. Result: no change in the rate of photosynthesis.
16. Conclusion: change in light intensity causes a change in the rate of photosynthesis.
17. Graph the results placing light intensity on the x-axis.
Ms Cooney
Biology Notes
Note: make sure you know the shape of this graph and are able to interpret it.
Ms Cooney
Biology Notes