Photosynthesis
Describe Carbon Fixation.
A molecule of CO2 is fixed to a 5-carbon compound called RuBP via the enzyme Rubisco. This generates a 6 carbon molecule that quicly breaks down to two 3 carbon molecules called PGA.
What is a photosystem?
A photosystem is a unit of pigment molecules that are embedded in the thylakoid membrane.
Light can be characterized as a wave, but it actually consists of two waves. What are these two waves?
An electric component and a magnetic component that are at 90 degree angles to each other.
How was the "Hill Reaction" later further confirmed?
An experiment labeled H2O with a heavy isotope of Oxygen (18O2), which showed up in the Oxygen gas.
The enzyme Rubisco evolved before O2 was readily present in the environment. Why can this sometimes be a problem for plants?
Because of this, Rubisco can sometimes utilize oxygen instead of CO2. The causes photorespiration to occur, which causes the plant to lose carbon. It allows only one PGA molecule to form instead of two.
Photosynthesis converts light energy into chemical energy that living organisms can use. What is the general equation?
CO2 + H2O + light -> (CH2O) + O2
What are the three steps of the calvin cycle?
Carbon Fixation Carbon Reduction Regeneration of substrate (RuBP)
Describe the two types of Carotenoids and what they're used for.
Carotenes are accessory pigments in photosynthesis. Xanthophylls are used for photoprotection.
What are the three main classes of photosynthetic pigments?
Chlorophyll (a,b,c) Carotenoids Phycobilins
Describe the three types of chlorophyll pigments and where they are found.
Chlorophyll A is found in all photosynthetic eukaryotes, as well as in cyanobacteria. Chlorophyll B is an accessory pigment in plants, it's found in green algae and euglena. An accessory pigment is not directly involved in photosynthesis but it serves to broaden the range of light that can be used in photosynthesis. Chlorophyll C is sometimes found instead of Chlorophyll B in some groups of algae.
There are two photosystems involved in the Light Reactions. They are linked by an _______________.
Electron Transport Chain
Describe Photosystem II.
Incoming photons of light excite accessory pigments within antenna complexes that will transfer energy via resonance energy transfer until it reaches a electron acceptor, whose now energized electron is transferred down an electron transport chain. The electron lost here is replaced by the splitting of water, which utilizes manganese to oxidize it - this is called photolysis. Its hydrogen atoms get put into the lumen too. In the electron transport chain is a protein complex called the Cytochrome b/f complex, which pumps hydrogen ions into the lumen. This electron then reaches Photosystem I.
Describe Carbon Reduction.
Next, 1 ATP and 1 NADPH are used to reduce each PGA to G3P or PGAL, which is very high in energy.
Describe Regeneration.
Next, these G3P's are mostly recycled to regenerate RuBP, but some will be exported to create sugars. Three turns of the Calvin Cycle (3 CO2) gives you 1 usable G3P molecule. This requires 9 ATP and 6 NADPH.
Light can also be characterized as a particle, particularly a _______.
Photon; this particle can excite electrons in metals and other molecules.
Describe where Phycobilins are found and what they do.
Phycobilins are accessory pigments found in cyanobacteria and red algae.
Photosystems have many ANTENNA pigments that send energy, via resonance transfer, to a central pigment molecules called the __________.
Reaction Center. This resonance energy raises the reaction centers electron up high enough for the ELECTRON ACCEPTOR to accept.
What is resonance transfer?
Resonance transfer involves the transfer of energy (NOT ELECTRONS) from one molecule to another.
What are the two sets of reactions involved in photosynthesis?
The Light reactions and the Calvin Cycle.
Describe Photosystem I.
The electron from PS II goes to the electron acceptor of PS I. Photons excite this is electron in the same manner until it is energized enough to enter another electron transport chain. This is transported eventually to an enzyme that reduces NADP+ to NADPH. It's synthesized in the stroma.
Describe what major events happen in the Light Reactions and the Calvin Cycle.
The light reactions convert sunlight into ATP and NADPH. It splits H2O and releases O2. The Calvin Cycle reduces CO2 to carbohydrates.
Where within the chloroplast to the Light Reactions and Calvin Cycle take place?
The light reactions involve the thylakoid membrane. The Calvin Cycle takes place in the stroma of the chloroplast.
What are the main products of photosynthesis? Draw the better, balanced equation.
The main products are a high energy triose phosphate, water, and oxygen.
Describe how wavelength and energy are related. What kind of energy does a small wavelength have? Vice versa?
They are inversely related. A small wavelength will be very high in energy (Think X-rays...UV Light) A large wavelength has low energy (Radio waves)
Pigment molecules within a plant absorb light energy at different wavelengths. When light of a particular wavelength strikes a pigment molecules at its particular wavelength, an electron is excited. What are the three ways an excited electron can release this energy?
They can give it off as heat, light, or via resonance transfer.
Describe Cyclic photophosphorylation
This is the formation of ATP without making NADP via PS I. Electrons excited in PS I go through an electron transport chain that run through the Cytochrome b/f complex, which pumps in H ions to allow ATP synthase to function. The electron then returns back to the electron acceptor of PS I. Therefore, O2 is not produced.
Describe how Van Niel's and Robin Hill's work proved that water is the source of oxygen in photosynthesis.
Van Niel worked with purple sulfur bacteria, where he discovered that they use H2S as a substrate which is split and used to reduce CO2. Robin Hill is known for the "Hill Reaction", where he found that chloroplasts were able to produce O2 in the absence of CO2.
What is the source of Oxygen in photosynthesis?
Water
How is ATP formed from the Light Reactions?
When the H atoms that result from the splitting of water and the H atoms that get pumped into the lumen via Cytochrome b/f, a gradient is created. The high concentration of H in the lumen runs through a complex called ATP synthase, which phosphorylates ADP to form ATP.
