Unit 2: Phototrophy Flashcards
What is the overall net reaction of photosynthesis, and what types of bonds are in the reactants and products? How do electrons move as these bonds are formed and reformed? How does this effect the thermodynamics of the reaction?
The overall net reaction of photosynthesis is 6CO2 + 6H20 -> C6H12O6 + 6CO2, which is the reverse of cellular respiration.
The reactants have polar bonds, because electrons are pulled more closely to oxygen then the carbon or hydrogen molecules. The products then have non-polar bonds since electrons are held evenly between carbon-carbon and C-H bonds, as well as between O=O bonds.
So as the reactants are broken and products formed, the electrons move away from their lower energy state closer to the electronegative atom, and instead reside directly between two positive nuclei, which is extremely unstable and hence has a lot of potential energy. This is good for the plant because it has now produced high energy bonds that can be broken down and used to do cellular respiration. However to get to this state, energy had to be added, because the change in free energy of this reaction is positive, and therefore it is endergonic, which will not occur spontaneously.
The chloroplast is a __________ ___________ organelle.
Describe the chloroplast and what membranes it is made of, and what the spaces between each membrane are called.
What is a special property of the thylakoid?
The chloroplast is a triple membrane organelle.
The outer membrane doesn’t not really have any purpose, it is just evidence of endosymbiosis.
Between the outer and inner membranes is the inter membrane space, which again does not really have a purpose.
The inner membrane then follows the IMS.
The 3rd membrane is called the thylakoid membrane, and this membrane is what stores chlorophyll and does photosynthesis.
Between the inner and thylakoid membranes is the stroma, which is a viscous liquid similar to the cytoplasm of the cell.
Then past the thylakoid membrane is the thylakoid space or lumen.
The thylakoid membrane is bent and stacked, and therefore produces a lot of surface area for photosynthesis to occur.
Give a general overview of photosynthesis including the light dependent and light independent reactions and how they interact. Also state where each section occurs.
The light reactions occur in the chloroplast, specially on the thylakoid membranes, and this is where sunlight is turned into potential energy and reducing power.
They take in H2O and split it to produce O2 as a byproduct, because the oxygens are oxidized and therefore need to bond together to get those shared electrons.
The light independent reactions occur in the Calvin cycle, which is the part that takes in CO2, and produces G3P, which is then taken to the stroma to produce a carbohydrate.
The Calvin cycle takes the energy and reducing power produced by the light dependent reactions, (ATP and NADPH) and uses those electrons to form new high energy molecules. This then recycles the ADP and Pi and NADP+ to be used in the light dependent reactions.
CALVIN CYCLE OCCURS IN THE STROMA (BETWEEN THE INNER MEMBRANE AND THE THYLAKOID) .
LIGHT DEPENDENT REACTIONS OCCUR IN THE THYLAKOID MEMBRANES OF THE CHLOROPLASTS.
What part of the EM spectrum does life use, and what range of wavelengths is this?) What does the dual nature of light mean?
Shorter wavelength = ___________ frequency = ________ energy.
Longer wavelength = ___________frequency = _________ energy.
All life exists upon the visible light spectrum, which occurs from around 400-700nm.
The dual nature of light refers to the fact that light acts as a particle (called a photon) but it also moves like a wave.
Therefore, we can analyze its wavelength and therefore frequency to try and figure out how much energy that photon is moving with.
Shorter wavelength = higher frequency = high energy (purple)
Longer wavelength = lower frequency = lower energy (red)
Because energy E = hv or = hc/lambda (since c=wavelength x frequency).
What are the 3 options for light when a photon strikes and object?
What will allow it to be absorbed?
- The photon is reflected, so it changes direction and then moves with a constant velocity due to its elastic nature.
- The photon is transmitted, meaning that it passes straight through the object.
- The photon can be absorbed. This only occurs when the photons absorbed have the same energy as the energy difference between electron shells in that pigment’s atom. Meaning that it can excite electrons, causing them to jump up an energy level. But if it is slightly more or slightly less energy than that energy difference, the photons will NOT be absorbed.
What are pigments good at? When a pigment is only one colour, what does that mean in terms of reflecting and absorbing? Why do pigments only absorb certain wavelengths?
Pigments are good at absorbing photons of a specific wavelength. This is because the photons they absorb are solely based on the energy difference between energy levels. And the larger this energy difference, the higher energy photon and therefore higher frequency wavelength they will be able to absorb.
When a pigment is only one colour, that means that that colour (which corresponds to a specific wavelength) is the only wavelength that does not exactly correspond to the energy difference between levels, and therefore will not be absorbed. And whatever wavelength is reflected is what people will see.
In chlorophyll specifically, what wavelengths are absorbed and reflected, and why?
Well blue (450nm) is absorbed, because its energy directly corresponds to the energy difference between 2 electron energy levels, (and therefore those e- can jump up two levels).
Red (700nm) has less energy, but still exactly corresponds to the energy difference between one energy level. Therefore, e- can jump up one level and the photon is absorbed to do this.
But green is 550nm, and this is actually enough to jump up 1.5 energy levels. But because it is not exact, it will not jump up one OR two levels. Instead it will do nothing, and this wavelength will be reflected, producing the green colour that we see when we look at plants.
So then what are the yellows and oranges of trees in the fall if chlorophyll makes green? And why does this occur?
In the fall, as days get shorter and amount of sunlight present for the plants to absorb decreases, the plants notice this and stop producing chlorophyll.
This then means that red and purple will not be absorbed by this pigment, and other pigments present in the leaf are revealed, producing colours based on the wavelengths that they absorb or reflect.
Now, what are the 3 ways that the energy within that excited electron is released when it automatically drops back down to ground level?
- The electron returns to ground state by emitting a less energetic photon or releasing the energy as heat.
- As the e- returns to ground state, the energy that is released is transferred to an electron in a neighbouring pigment molecule, exciting that molecule and hence producing a chain reaction.
- The high energy electron is transferred to another molecule called an electron acceptor.
2 is what occurs in the photosystems of the thylakoid membrane.
What is the main and accessory pigments in the ________ membrane?
What the accessory pigments used for?
What are their two main purposes?
The main pigment is chlorophyll, as this is what absorbs the most amount of light possible from the sun.
Accessory pigments are called carotenoids, which absorb not as large a range of light, but add on to what chlorophyll can already absorb.
So in the fall, these pigments are revealed, and do some minor photosynthesis to keep the leaves alive until the tree is ready to go dormant.
These pigments absorb green but do not absorb reds, yellows and oranges, which are the colours we see in the fall.
The two main purposes of these pigments are:
1. To extend the range of wavelengths used for photosynthesis to absorbed more parts of the spectrum and hence have more energy.
2. To protect the tissue from too much light exposure, as that light can be absorbed in many places, rather then all being directed to one spot.
How are pigment molecules organized?
What are antenna pigments, and what are they grouped around?
Pigment molecules are organized into photosystems, which are light harvesting complexes.
These photosystems are made up of hundreds of antenna pigments which are grouped around a special molecule, called a reaction centre pigment. The reaction centre pigment donates electrons to the primary electron centre, using the energy that was given to it!
What is inductive resonance? How is it used for photosynthesis? What is the point of it? (Why isn’t all energy directed towards the reaction centre?
What is an example of this not using light?
Inductive resonance is the transfer of energy from one electron to another, as one electron falls down from an excited energy state and releases a photon. That photon is then absorbed by the neighbouring molecule which excites it to another energy level, until it falls back down.
This occurs between all the antenna pigments until it reaches the reaction centre, and this is the mechanism that is used to transfer light energy to where it is needed in the plant. This occurs because if all energy just went to the reaction centre it would damage DNA and membranes (just like cancer causes mutations in DNA). Therefore, these antenna pigments spread that energy out, saving the plant. This also allows much larger ranges of energy to be absorbed, getting as much energy as possible from the sun rays.
In photosystem II, how is this transfer of energy by the antenna pigments utilized?
Where does that energy end up and what does it do? What is the main purpose of PII?
How does it affect the next part of the process?
How are the electrons rejuvenated?
Photosystems II uses this transfer of energy to bring it all to the reaction centre pigment. Up until this point, only electrons have been excited and then fall back down, but no electrons have moved.
Once the reaction centre pigment is excited though, an electron will have enough energy to go up to the primary electron acceptor.
The reaction centre pigment is called P680, and once that electron is donated it’s called P680+. That excited electron is then donated to more electronegative PQ, which releases energy as it moves to a more stable electron state. Before PQ moves, it picks up an H+ ion from the stroma and brings it with it as it moves down to B6F (cytochrome complex). It then releases an electron to this complex, and releases its extra H+ ion to the thylakoid lumen.
SO ONCE AGAIN ,THE ENERY RELEASED BY THE TRANSFER OF ELECTRONS TO INCREASINGLY ELECTRONEGATIVE PROTEIN COMPLEXES RELEASES ENERGY WHICH IS UTILIZED TO BRING H+ IONS AGAINST THEIR CONCENTRATION GRADIENT.
Therefore, the main purpose of PII is to generate protein motive force!
And to recycle the electrons and turn P680+ back into P680, H2O is split in the thylakoid lumen into 2H+ and 1/2H2O, releasing 2 e- (one form each hydrogen).
What type of taxi is PQ and how does this help it to do its job?
PQ is a hydrophobic electron taxi which transfers electrons to the cytochrome complex B6F and at the same time releases H+ to the thylakoid lumen. It can only do this because it can easily diffuse through the lipid tails and therefore it can carry charged ions (H+) through this hydrophobic environment.
What is the simplified cycle of how energy and electrons are transferred and recycled for PII?
P680 (reaction centre pigment) is excited by photons and becomes P680*.
It then releases that electron and becomes P680+, and the electron goes to the primary electron acceptor.
P680+ needs to be reduced to allow this cycle to occur again.
How? It splits water.
So H2O —> 2H+ + 1/2O2, and because 2 protons are produced, 2 electrons must be released. These electrons then go to P68O+ to make it back into P680, and the process can continue.
