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PhotosynthesisPhotosynthesis
IntroductionIntroduction
Photosynthesis transfers electrons from water to Photosynthesis transfers electrons from water to energy-poor CO2 molecules, forming energy-rich energy-poor CO2 molecules, forming energy-rich sugar molecules. sugar molecules.
This electron transfer is an example of an oxidation-This electron transfer is an example of an oxidation-reduction process: the water is oxidized (loses reduction process: the water is oxidized (loses electrons) and the CO2 is reduced (gains electrons).electrons) and the CO2 is reduced (gains electrons).
Photosynthesis uses light energy to drive the Photosynthesis uses light energy to drive the electrons from water to their more energetic states in electrons from water to their more energetic states in the sugar products, thus converting solar energy into the sugar products, thus converting solar energy into chemical energy.chemical energy.
ChlorophyllChlorophyll
There are a number of chlorophyll pigments in There are a number of chlorophyll pigments in chloroplasts and they all absorb different chloroplasts and they all absorb different wavelengths of light.wavelengths of light.
This allows the plant to absorb the most This allows the plant to absorb the most energy from the light. energy from the light.
There is chlorophyll a, chlorophyll b and There is chlorophyll a, chlorophyll b and carotenoids.carotenoids.
Reaction centersReaction centers
Chlorophyll pigments are arranged in functional sets Chlorophyll pigments are arranged in functional sets or clusters on the thylakoid membranes . or clusters on the thylakoid membranes .
These clusters are called These clusters are called photosystemsphotosystems. . For example: in spinach chloroplasts, these For example: in spinach chloroplasts, these
photosystems contain about 200 chlorophyll photosystems contain about 200 chlorophyll molecules and about 50 carotinoids, arranged in what molecules and about 50 carotinoids, arranged in what are called are called light harvesting antennalight harvesting antenna. .
These photosystems can absorb light over the entire These photosystems can absorb light over the entire visible spectrum but especially well between 400 to visible spectrum but especially well between 400 to 500 nm and 600 to 700 nm.500 nm and 600 to 700 nm.
When a chlorophyll molecule in the thylakoid When a chlorophyll molecule in the thylakoid membrane is excited by light, the energy level membrane is excited by light, the energy level of an electron in its structure is of an electron in its structure is boostedboosted by an by an amount equivalent to the energy of the amount equivalent to the energy of the absorbed light and the chlorophyll becomes absorbed light and the chlorophyll becomes excited. excited.
The packet of excitation energy now migrates The packet of excitation energy now migrates rapidly through the light harvesting pigment rapidly through the light harvesting pigment molecules to the molecules to the reaction centre of the reaction centre of the photosystemphotosystem where it causes an electron to where it causes an electron to acquire the large amount of energy.acquire the large amount of energy.
The Light ReactionThe Light Reaction
The energy trapped can be used in three ways:The energy trapped can be used in three ways: 1) 1) To build the chemiosmotic or proton gradientTo build the chemiosmotic or proton gradient.. 2) 2) Generate ATPGenerate ATP.. 3) 3) Reduce NADP+Reduce NADP+ to NADPH.to NADPH.
There are There are two waystwo ways to to generate ATPgenerate ATP 1) Non cyclic photophosphorylation.1) Non cyclic photophosphorylation. 2) Cyclic photophosphorylation.2) Cyclic photophosphorylation.
These two systems differ in the route taken by the These two systems differ in the route taken by the "light activated" electrons and in some of the "light activated" electrons and in some of the products formed.products formed.
Photosystems I and IIPhotosystems I and II
The thylakoid membranes of plant chloroplasts The thylakoid membranes of plant chloroplasts have two different kinds of photosystems each have two different kinds of photosystems each with its own set of light harvesting chlorophyll with its own set of light harvesting chlorophyll and carotenoid molecules and the photochemical and carotenoid molecules and the photochemical reaction centre.reaction centre.
Photosystem IPhotosystem I - is maximally excited by light at - is maximally excited by light at longer wavelengths. (P700)longer wavelengths. (P700)
Photosystems IIPhotosystems II - is maximally excited by - is maximally excited by shorter wavelengths. (Less than 680). (P680)shorter wavelengths. (Less than 680). (P680)
Non Cyclic PhotophosphorylationNon Cyclic Photophosphorylation
photons are absorbed in photosystem II and the photons are absorbed in photosystem II and the chlorophyll a of this photoactivation centre passes the chlorophyll a of this photoactivation centre passes the energy rich electron on into the energy rich electron on into the electron transport electron transport systemsystem. .
Once in the electron transport system of the thylakoid Once in the electron transport system of the thylakoid membrane the electron is passed from electron carrier membrane the electron is passed from electron carrier to electron carrier eventually entering photosystem I. to electron carrier eventually entering photosystem I.
In the "tumbling" down of the electron transport In the "tumbling" down of the electron transport chain the electron gradually loses energy. chain the electron gradually loses energy.
Some of that energy will be used to "pump protons" Some of that energy will be used to "pump protons" across the thylakoid membrane into the "lumen" of across the thylakoid membrane into the "lumen" of the thylakoid.the thylakoid.
NBNB: Each light activated electron allows the : Each light activated electron allows the "proton pump" to pump "proton pump" to pump oneone proton across the proton across the membrane. membrane.
These These partly spentpartly spent electrons then pass into electrons then pass into photosystem I P700 where they receive photosystem I P700 where they receive another "boost" to their highest energy level. another "boost" to their highest energy level.
But these electrons do not pump protons they But these electrons do not pump protons they use their energy to reduce NADP+ to NADPH.use their energy to reduce NADP+ to NADPH.
Cyclic PhotophosphorylationCyclic Photophosphorylation
If NADP is not available then a cyclic event If NADP is not available then a cyclic event takes over.takes over.
The electrons from Photosystem I are given an The electrons from Photosystem I are given an energy boost but they follow a different route.energy boost but they follow a different route.
Photosystem I becomes the donor and the Photosystem I becomes the donor and the acceptor of electrons.acceptor of electrons.
Hydrogen ions still build up and ATP is still Hydrogen ions still build up and ATP is still synthesised. synthesised.
Replacing electron to PS IIReplacing electron to PS II
The electron for PS I comes either from PS II The electron for PS I comes either from PS II or from itself.or from itself.
To replace the electron lost by PS II water is To replace the electron lost by PS II water is split.split.
This involves light and is called the photolytic This involves light and is called the photolytic splitting of water.splitting of water.
It splits it into 4 electrons, 4 hydrogen ions and It splits it into 4 electrons, 4 hydrogen ions and a molecule of oxygen.a molecule of oxygen.
Chemiosmotic PhosphorylationChemiosmotic Phosphorylation
cyclic and non-cyclic photophosphorylation cyclic and non-cyclic photophosphorylation serve to pump protons into the lumen of the serve to pump protons into the lumen of the thylakoids.thylakoids.
Just as in the mitochondria outer compartment Just as in the mitochondria outer compartment the protons are allowed to flow back into the protons are allowed to flow back into the the stroma.stroma.
The controlled flow of H+ down this The controlled flow of H+ down this concentration gradient occurs through protein concentration gradient occurs through protein channels which contain ATP synthase.channels which contain ATP synthase.
Products of the light reactionProducts of the light reaction
These systems therefore contribute ATP and These systems therefore contribute ATP and high-energy NADPH to the next series of high-energy NADPH to the next series of reactions in photosynthesis. reactions in photosynthesis.
Light Independent StageLight Independent StageCalvin CycleCalvin Cycle
One molecule of CO2 combines with one molecule of RuBP One molecule of CO2 combines with one molecule of RuBP with the aid of the enzyme rubisco (ribulose bisphosphate with the aid of the enzyme rubisco (ribulose bisphosphate carboxylase). (Carbon fixation)carboxylase). (Carbon fixation)
Resulting complexes split into 2 GP molecules (Glycerate-3-Resulting complexes split into 2 GP molecules (Glycerate-3-phosphate) phosphate)
GP is then reduced using energy and electrons from NADPH GP is then reduced using energy and electrons from NADPH and ATP to form GALP (glyceraldehyde-3-phosphate). and ATP to form GALP (glyceraldehyde-3-phosphate). (Reduction)(Reduction)
Two out of 12 GALP molecules form a 6C sugar and are Two out of 12 GALP molecules form a 6C sugar and are changed into starch for storage.changed into starch for storage.
Ten out of twelve GALP molecules are restructured into six Ten out of twelve GALP molecules are restructured into six RuBP molecules (Regeneration)RuBP molecules (Regeneration)
This is the C3 pathway and is typical of most This is the C3 pathway and is typical of most plants.plants.
The other type is called C4 or CAM plantsThe other type is called C4 or CAM plants
Comparing photosynthesis and Comparing photosynthesis and respirationrespiration
Leaf Structure and functionLeaf Structure and function
The leaf is a sandwich of photosynthetic The leaf is a sandwich of photosynthetic parenchyma between two layers of epidermis.parenchyma between two layers of epidermis.
Stomata are often confined to the lower Stomata are often confined to the lower epidermisepidermis
the parenchyma is divided into palisade cells the parenchyma is divided into palisade cells and spongey mesophyll. and spongey mesophyll.
The extra thickness of sun leaves is made up The extra thickness of sun leaves is made up of extra layers of palisade cells by comparison of extra layers of palisade cells by comparison with shade leaves of the same plant. with shade leaves of the same plant.
The mesophyll is The mesophyll is honeycombed with air honeycombed with air space, allowing access of space, allowing access of CO2 to the individual cells. CO2 to the individual cells.
Vascular bundles also run Vascular bundles also run through it, in close parallel through it, in close parallel lines in many monocots or lines in many monocots or branching extensively in branching extensively in dicots. dicots.
Individual mesophyll cells Individual mesophyll cells are never far from a xylem are never far from a xylem vessel for water supply or a vessel for water supply or a sieve tube for export of sieve tube for export of sugar. sugar.
Limiting FactorsLimiting Factors The maximum rate The maximum rate
of photosynthesis is of photosynthesis is controlled by the controlled by the limiting factor.limiting factor.
Light intensityLight intensity: : more light intensity more light intensity results in swifter results in swifter photosynthesis. photosynthesis.
Carbon dioxide Carbon dioxide concentrationconcentration: higher : higher levels result in a bigger levels result in a bigger rate. This is usually the rate. This is usually the rate-limiting step in rate-limiting step in natural settings. natural settings.
TemperatureTemperature: lower : lower temperatures slow down temperatures slow down the rate and higher the rate and higher temperatures denature temperatures denature the enzymes responsible the enzymes responsible for photosynthesis. for photosynthesis.
Availability of water:Availability of water: water is needed as a water is needed as a raw material for photosynthesis, and if it is raw material for photosynthesis, and if it is short, it will cause the plant to wilt and thereby short, it will cause the plant to wilt and thereby lose its ability to capture sunlight.lose its ability to capture sunlight.
Availability of nutrients:Availability of nutrients: On a deeper level, On a deeper level, other factors like amount of chlorophyll or other factors like amount of chlorophyll or availability of nutrients (e.g. Mg is needed for availability of nutrients (e.g. Mg is needed for chlorophyll synthesis) will also affect the rate chlorophyll synthesis) will also affect the rate of photosynthesis. of photosynthesis.
Carbon dioxideCarbon dioxide
As carbon dioxide concentrations rise, the rate at As carbon dioxide concentrations rise, the rate at which sugars are made by the light-independent which sugars are made by the light-independent reactions increases until limited by other factors. reactions increases until limited by other factors.
RuBisCO, the enzyme that captures carbon dioxide in RuBisCO, the enzyme that captures carbon dioxide in the light-independent reactions, has a binding affinity the light-independent reactions, has a binding affinity for both carbon dioxide and oxygen. for both carbon dioxide and oxygen.
When the concentration of carbon dioxide is high, When the concentration of carbon dioxide is high, RuBisCO will fix carbon dioxide. RuBisCO will fix carbon dioxide.
However, if the carbon dioxide concentration is low, However, if the carbon dioxide concentration is low, RuBisCO will bind oxygen instead of carbon dioxide.RuBisCO will bind oxygen instead of carbon dioxide.
Effect of light intensityEffect of light intensity
Move the lamp to Move the lamp to increase or decease the increase or decease the light intensity.light intensity.
Count number of Count number of bubbles per minute to bubbles per minute to see rate of see rate of photosynthesis.photosynthesis.
Could adapt practical to Could adapt practical to look at temperature and look at temperature and carbon dioxide.carbon dioxide.
