Photosynthesis Flashcards
Plancks’ law
E = (h x c )/ wavelength
h = Plancks’ constant (6.6626 x 10-34 J s)
c = speed of light
Energy and wavelength
Energy of photons is inversely proportional to its wavelength
Longer the wavelength = lower the energy
Main photosynthetic pigments
Chlorophyll A. Main pigment (violet-blue and red, 430 & 662 nm)
Chlorophyll B. Accessory pigment (blue and orange, 453 & 642 nm)
Carotenoids. Accessory pigments (blue-green range, 460-550 nm)
Chlorophyll a
primary pigment, Chlorophyll a has two main peaks of light absorption, one at a wavelength of 430 nm and thus in the violet-blue range of light and another at 662 nm and thus in the red range.
Chlorophyll b
two peaks of absorbance. One at 453 nm in the blue range and one at 642 nm in the orange range. Plants usually contain about half the amount of chlorophyll b compared to chlorophyll a.
Carotenoids
absorb light between 460-550 nm and therefore in the blue and green range. This is why in autumn, when some leaves have lost all their chlorophyll, they appear red, orange and yellow in color, because is the range of light not absorb by the carotenoids and therefore reflected.
Where does photosynthesis take place
Mesophyll by chloroplasts
Thylakoid membranes
thylakoid membranes are made of lipids, although these lipid contain the sugar galactose (making them galactolipids) or sulfate (making them sulfatelipids) as the head group, instead of phosphate as in phospholipids.
The use of galactolipids instead of phospholipids in photosynthetic membranes reduced very significantly the amount of phosphorus required by plants and allows photosynthesis to continue under low phosphorus availability.
The photosynthetic pigments are all hydrophobic to some degree, and therefore are embedded in the lipid bylayer of the tylakoid membranes
Photosynthetic reactions
Energy transduction or light reactions: light is used to form ATP and NADPH. Thylakoids.
Carbon fixation reactions: the energy of ATP and NADPH is used to fix CO2 and produce sugars. Stroma
Energy transduction reactions
chlorophyll molecules (and other pigments) are embedded in the thylakoids in discrete units called photosystems. Each photosystem includes 250 – 400 pigment molecules organised in an antenna complex, collecting light and transferring the energy to the reaction center complex, where chemical oxidation and reduction reactions takes place.
Photo system
consists of about 300 molecules of chlorophyll a and 50 molecules of carotenoids and chlorophyll b. The energy flows from pigment molecules to pigment molecules in the antenna complex until it flows to a special chlorophyll a molecule in the reaction center of the complex, where chemical oxidation and reduction reactions takes place.
PSII
P680: absorbs red light (~680nm)
Operate in grana
PSI
P700: absorbs far-red (>680nm)
Operate in stroma
Z scheme of electron transfer
The light is absorb in the antenna complex until it reaches the reaction center, where P680 gets into an excited stage with high energy. This increase energy causes electrons to move from P680 through a series of electron carriers until they reach P700 in PSI. P700 in turn, receives light energy from its own antenna complex, and the reenergised electrons pass through another series of electrons carriers, finally reducing NADP+ to NADPH.
As the electrons move through the electron transport chain, they lose energy, which is used to pump hydrogen ion (protons) into the lumen of the thylakoid membrane
The electrons lost by P680 are replaced with electrons from water, by a process that generates protons and oxygen. Two molecules of water are split to release four electrons, fourprotons H+, and an atom of oxygen. This is the reaction that causes plants to be the net producers of oxygen for the biosphere.
Electron transport chain
All these protons that are building up in the thylakoid lumen create a low pH, and are able to flow back into the stroma through a protein complex called ATP synthase, which collects the energy generated by the flow of hydrogn ions to synthesise ATP by adding an inorganic phosphate group to ADP.
The NADPH and ATP generated during the light reactions accumulates in the stroma, where they can be used to synthesise carbohydrats during the carbon fixation stage.
These reactions are called light reactions or photochemical reactions of photosynthesis because they use light energy to create the chemical energy of ATP and NADPH and therefore only take place in the light.
Who produces most of the air’s oxygen, phytoplankton or plants?
Phytoplankton (between 50-85%)
Carbon fixation reactions
the ATP and NADPH generated by the light reactions are used to fix and reduce carbon and to synthesize sugars (Calvin cycle or C3 pathway) from ribulose 1,5-biphosphate (RuBP).
3 stages of Calvin cycle
C fixation
Reduction
Regeneration of RuBP
Carbon fixation
carbon from CO2 is added to ribulose biphsophate, a 5-carbon sugar, producing a 6-carbon molecule that is not stable and immediately splits into two molecules of 3-phosphoglycerate (PGA), the first product of photosynthesis. Hence, this is called the C3 pathway.
The reaction is catalysed by the enzyme Ribulos biphosphate carboxylase/oxygenase or Rubisco
Reduction
reduction of PGA into glyceraldehyde 3-phosphate. Reduction is the gain of electrons as opposed to oxidation which is the loss of electrons. So PGA gain an electron from ATP, which dissociates into ADP and Pi and one from NADPH.
Regeneration of RuBP
the ribulose 1,5-biphopshate must be regenrated. It takes 5 molecules of PGAL take electrons from ATP to regnerate the initial ribulose1,5 bisphosphate, and one molecule of PGAL is used to make glucose, releasing inorganic phosphate and the glucose is then used to produce starch, because starch is not osmotically active (it will not attract water).
Antenna complex
Group of photosynthetic pigments
Enzymes involved in Calvin cycle
RuBisCo
3- phosphoglycerate kinase
Glyceraldehyde 3-phosphate dehydrogenase
