Photosynthesis Flashcards
What is photosynthesis?
Photosynthesis is the production of carbon compounds in cells using light energy. (Light energy is converted into chemical energy)
Living organisms require complex carbon compounds to build the structure of the cells and carry out life processes. The carbon compounds produced include carbohydrates, proteins and lipids.
photosystems and their function (path of electrons)
Photosynthesising organisms use a range of pigments called photosystems, but the main photosynthetic pigment is chlorophyll, which raises the energy level of electrons — photoactivation/excitation.
Such electrons are then passed from pigment to pigment (e.g. Photosystem II and I) until they reach a special chlorophyl molecule at the reaction centre of the photosystem. Only this chlorophyll can pass/donate two excited electrons away to the electron acceptors (plastoquinone) in the thylakoid membrane. It collects two excited electrons front Photosystem II and then moves away to another position in the membrane. Plastoquinone is hydrophobic, so although it is not in a fixed position, it remains within the membrane. Absorption of two photons of light causes the production of one reduced plastoquinone, with one of the chlorophylls at the reaction centre having lost two electrons to a plastoquinone molecule.
Reduced plastoquinone is needed, carrying the pair of excited electron from the reaction centre of Photosystem II to the start of the chain of electron carriers, which then are used to create a proton gradient.
Photolysis, which takes place in the fluid inside the thylakoids, also contributes to the proton gradient.
Light absorption by chlorophyll
Chlorophyll absorbs a red and blue lights most efficiently and reflects green light more than other colors. Photosynthesizing organisms use a range of pigments, but the main photosynthetic pigment chlorophyll. Their various forms of chlorophyll but they all appear green to us.
absorption spectra
A spectrum is a range of wavelengths of electromagnetic radiation.
Visible light has a range of wavelengths with violets the shortest wavelength (400nm - high energy) and red the longest (700nm - low energy but warmer).
This spectrum is absorbed by photosynthetic pigments in plants, mainly chlorophyll.
Oxygen production and photosynthesis
Oxygen is produced in photosynthesis from photolysis of water. The function of photolysis is to split water molecules to release electrons needed in the reaction centers of photosystems. The electrons are exicted and then used for later stages in photosynthesis. Oxygen is a waste product and diffuses away.
Effects of long-term photosynthesis on the earth
- The rise in the oxygen concentration
- The first glaciation, presumably due to the reduction in the greenhouse effect
- Abundance of steel: the increase in oxygen concentration in the oceans between 2,400 - 2,200 million years ago caused the oxidation of dissolved iron in the water, causing it to precipitates onto the seabed. A distinct rock from formation was produced called the banded iron formation, with layers of iron oxide alternating with other minerals.
Production of carbohydrates
Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide. Plants convert carbon dioxide and water into carbohydrates by photosynthesis. Carbon dioxide + water –> carbohydrate + oxygen
This chemical reaction requires energy (endothermic), which is obtained by absorbing light.
What is a limiting factor?
The one factor of three (temperature, light intensity, carbon dioxide concentration) that is furthest from its optimum, therefore limiting the rate of photosynthesis. If this limiting factor is changed to make it closer to the optimum, the rate of photosynthesis increases, but changing the other factors will have no effect, as they are not the limiting factor.
The products and location light dependent reactions
- Reduced NADP and ATP
- Takes place in the intermembrane space of thylakoids.
Photoactivation
Absorption of light by photosystems generates excited electrons.
Photophosphorylation: Photosystem II and ATP production (+function of plastocyanin)
A pair of excited electrons from the reaction centre of Photosystem II is passed to a chain of carriers. The electrons give up energy as they pass from one carrier to the next, enough to create a proton gradient by pumping protons across the thylakoid membrane from the stroma into the thylakoid space (inside the thylakoid).
At the end of the chain of carriers the electrons are passed to plastocyanin, a water-soluble electron acceptor in the fluid inside the thylakoids. Reduced plastocyanin is needed in the next stage of photosynthesis: Photosystem I.
Production of ATP in chloroplast is called photophosphorylation:
ATP synthase, also located in the thylakoid membranes, allows the hydrogen ions to diffuse back across the membrane to the stroma and uses the diffusion energy to produce ATP — chemiosmosis.
Photolysis (function and products)
Photolysis of water and generates electrons for the use in the light-dependent reactions. Once the plastoquinone becomes reduced, the chlorophyll in the reaction centre is then a powerful oxidizing agent and causes the water molecules nearest to it to split and give up electrons, to replace those that it has lost.
Oxygen is a waste product and diffuses away. The useful product of Photosystem II is the reduced plastoquinone, which not only carries a pair of electrons, but also much of the energy absorbed from light. This energy drives all the subsequent reactions of photosynthesis.
The electron transport chain
Transfer of excited electrons occurs between carriers in thylakoid membranes. The production of ATP, using energy derived from light is called photophosphorylation. It is carried out by the thylakoids. These are regular “stacks” of membranes, with very small fluid-filled spaces inside. The thytakoid membranes contain the following structures • Photosystem II • ATP synthase • a chain of electron carriers • Photosystem I. Reduced plastoquinone is needed, carrying the pair of excited electron, from the reaction centre of Photosystem 11. Plastoquinone carries the electrons to the start of the chain of electron carriers, which then are used to create a proton gradient because as the electrons pass, energy is released, which is used to pump protons across the thylakoid membrane, into the space inside the thylakoids. Photolysis, which takes place in the fluid inside the thylakoids, also contributes to the proton gradient.
Photosystem I and reduction of NADP
A pair of excited electrons is emitted from the reaction centre of Photosystem I and passes along a short chain of electron acceptors. At the end of this chain the electrons are passed to NADP in the stroma. NADP is converted to reduced NADP by accepting two electrons emitted by Photosystem I plus two protons from the stroma. Reduced NADP is needed in the light-independent reactions.
The electrons given away by Photosystem I are replaced by electrons that were emitted by Photosystem II and passed along the chain of electron carriers. Photosystem I can then absorb more photons of light to produce more excited electrons.
function of reduced NADP
Excited electrons from photosystem I are used to reduce NADP, which carries a pair of electrons that can be used to carry a reduction reactions.
Reduced NADP has a similar function as reduced NAD in cell respiration: it carriers a pair of electron that can be used to carry out reduction reactions. Chlorophyll molecules within Photosystem I absorb light energy and pass it to the special two chlorophyll molecules in the reaction centre. This raises an electron in one of the chlorophylls to a high energy level. As with Photosystem II, this is called photoactivation. The excited electron passes along a chain of carriers in Photosystem I, at the end of which it is passed to ferredoxin, a protein in the fluid outside the thylakoid. Two molecules of reduced ferredoxin are then used to reduce NADP, to form reduced NADP.
The electron that Photosystem I donated to the chain of electron carriers is replaced by an electron carried by plastocyanin. Photosystems I and II are therefore linked: electrons excited in Photosystem II are passed along the chain of carriers to plastocyanin, which transfers them to Photosystem I. The electrons are re-excited with light energy and are eventually used to reduce NADP.
chloroplast structure and function
Production of carbohydrates (formula of input and output, functions of triose phosphate)
Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide. Plants convert carbon dioxide and water into carbohydrates by photosynthesis:
Carbon dioxide + water —light—> carbohydrate + oxygen
This chemical reaction requires endothermic energy, which is obtained by absorbing light, specifically:
Glycerate 3-phosphate, formed in the carbon fixation reaction, is an organic acid and converted into a carbohydrate by a reduction reaction. The hydrogen needed to reduce is supplied by reduced NADP. Required energy is supplied by ATP; both of which are produced in light-dependent reactions.
The production of the reduction of glycerate 3-phosphate is a three-carbon sugar, triose phosphate. Triose phosphate can be converted into a variety of carbohydrates (e.g. glucose, starch, etc.)
Triose phosphate is also used to regenerate RuBP. As RuBP is both consumed and produced in the light independent reactions, these reactions form the Calvin cycle. Five molecules of triose phosphate are converted by a series of reactions into three molecules of RuBP. ATP is needed.
- For every six molecules of triose phosphate formed in the light-independent reactions, five must be converted to RuBP, leaving one RuBP for the production of vital carbohydrates like glucose.
Calvin cycle
The first reaction involves ribulose bisphosphate (RuBP, a sugar), which is regenerated by the light-independent reactions.
Carboxylation of RuBP — carbon fixation
Carbon dioxide enters the chloroplast by diffusion. In the stroma, carbon dioxide combines with RuBP in a carboxylation reaction. The reaction is catalysed by the enzyme rubisco (ribulose-1,5-bisphosphate carboxylase oxygenase).
The product of the carboxylation of RuBP is an unstable six-carbon compound, which immediately splits to form two molecules of glycerate 3-phosphate — the first product of carbon fixation ; the conversion of carbon dioxide into organic compounds.
For every six molecules of triose phosphate formed in the light-independent reactions, five must be converted to RuBP, leaving one RuBP for the production of vital carbohydrates like glucose.
Calvin’s experiment: set-up
A suspension of Chlorella (unicellular algae) was placed in a thin glass vessel (called the lollipop vessel) and was brightly illuminated. The Chlorella was supplied with both carbon dioxide and hydrogen carbonate. Before the start of the experiment the carbon in both of these carbon sources was 12C, but at the start of the experiment this was replaced with 14C.
Melvin Calvin and Andrew Benson, in the 1950s, took samples of the algae at very short time intervals and immediately killed and fixed them with hot methanol. They extracted the carbon compounds, separated them by double-way paper chromatography and then found which carbon compounds in the algae contained radioactive 14C by autoradiography.
Calvin’s experiment: results
The autoradiogram from samples of Chlorella exposed to radioactive carbon dioxide and hydrogen for 5 seconds shows that there was more labelled glycerate 3-phosphate than any other compound, indicating that it is the first product of carbon fixation. The autoradiogram for 30 seconds shows that by then many carbon compounds were labelled. The amount of radioactivity in the different compounds was measured. Changes in the amounts are shown in the graph below. Again, there is evidence for glycerate 3-phosphate as the first production with triose phosphate formed next.
Location of the light-independent reactions
They take place in the stroma, which is the space enclosed by the inner membrane of the chloroplast.
Experiments to investigate limiting factors
Principles of the experiments:
- Only one limiting factor should be investiaged at a time - the independent variable.
- A suitable range for the indepdent variable shoud be chosen, from the lowest possible level, to a level at which the factor is no longer limiting.
- The depedent variable is the rate of photsynthesis, usally in form of oxygen production per unit time.
- All other factors should be constant - the control variables.
