C1.3 - Photosynthesis Flashcards
What is photosynthesis?
Creating carbon compounds using light energy and other organic substances - light energy converted into chemical energy
How do plants gain glucose/carbohydrates for photosynthesis?
- Carbon dioxide and water are combined to create glucose and oxygen.
- This is a reduction reaction. hydrogen needed for it comes from photolysis. Photolysis = hydrogen is split into protons using light energy
- Oxygen = waste product
What were the first organisms to perform photosynthesis?
Prokaryotes, 3500 million years ago. Later it begun to occur in chloroplasts.
Describe oxygen as a waste-product of photosynthesis? What causes it? What happens to it after?
Oxygen comes from the process of photolysis. Photolysis makes higher concentration of oxygen in the chloroplast, and so the oxygen diffuses out of the chloroplasts, and eventually through stromata on the leaf.
Briefly describe the experimental technique used to separate photosynthetic pigments?
- Chromatography!
- Within chloroplasts, there are different pigments asides from chlorophyll, called accessory pigments. These pigments are seperated using thin layer chromatography.
- There’s a strip coated with porous material. A spot of leaf tissue is placed on one end of the strip. The strip is placed in a bit of solvent at the bottom. The solvent goes up the strip and seperates the pigments.
How to calculate Rf values from the chromatography procedure?
Rf = distance run moved by the pigment / distance run moved by the solvent
How do plants absorb light for the beginning stage of photosynthesis?
Pigments in the plants absorb light and reflect colours to us. White substances are all of the wavelengths being reflected, and transparent are all of the wavelengths being passed through. Black substances are all of the wavelengths being absorbed. When plants don’t absorb some wavelengths of light, those are the colours we see.
What is a photon?
A unit of light. The longer the wavelength of light, the less energy a photon is.
What is the relationship of photons and pigments?
Pigments will absorb photons, but only if the photon’s energy makes one of the pigment molecule’s electron get excited (go into a higher energy level) - needs specific wavelength
What is the main photosynthesis pigment?
Chlorophyll - appears green to us, this is why most ecosystems are majorly green. Chlorophyll looks green because the green wavelength does not excite the photons, so it is not absorbed and is reflected.
What is an absorption spectrum?
A graph that shows the percentage of light that is absorbed by pigments at each different wavelength
What is an action spectrum?
A graph that shows the rate of photosynthesis at each different wavelength
What are some differences and similarities between absorption and action spectrum?
Absorption :
- shows percentage of light absorbed by pigments at different wavelengths
- x-axis shows wavelengths of light
- y-axis shows absorption of light
Action :
- shows rate of photosynthesis at different wavelengths
- x-axis shows wavelengths of light
- y-axis shows measure of amount of photosynthesis
What is a limiting factor?
A process that depends on multiple factors will have a factor that limits the other ones
What are some factors that affect rate of photosynthesis?
Amount of water, amount of sunlight, amount of co2
What are FACE experiments?
- Free Air Carbon dioxide Experiments
- Increasing CO2 concentration levels above normal atmospheric levels increases natural rates of photosynthesis. Recently, CO2 concentrations in the atmosphere have increased, so FACE experiments test that impact on photosynthesis.
Where are FACE experiments taking place?
The first happened in agricultural environments, and the second is occurring in a woodland environment.
How do FACE experiments work?
CO2 is released from towers that are formed in a circle, and then the plant growth is observed
What is the atmospheric concentration of CO2 in 21st century vs 1780s/before Industrial Revolution?
550 ppm (now) vs 270 ppm (then)
What are photosystems?
Pigment-protein complexes that are found in thylakoid membranes
There’s usually 100 chlorophyll molecules and 30 accessory pigment molecules
What are examples of accessory pigments?
Carotene and xanthophyll
Describe basic structure? of photosystem structure (this is kinda a segway for next part!)
Each photosystem has a core complex which is connected to light-activated antenna complexes.
How do regular (NOT LIGHT HARVESTING ANTENNA) pigments molecules absorb light?
Once they absorb light energy from the photons, they absorb the light, and an electron gets excited and moves to a higher energy level.
What’s the relationship between wavelength and energy absorbed by the pigment molecules?
The amount of energy decreases as the wavelength increases.
What is fluorescence?
When the electron goes back down to it’s original energy level, the light energy the pigment molecule absorbes is re-emitted
What is excitation energy transfer?
In light-harvesting antenna complexes, when the pigment molecules’ electron goes down to it’s original energy level, the light energy that it releases goes to the next pigment molecule beside it, which makes that electron excited. This repeats. Light energy keeps getting transferred until it reaches the reaction centre in the core complex.
How long does excitation energy transfer take?
Very fast - few femtoseconds
What is a structural thing that’s needed for excitation energy transfer?
The pigment molecules have to be positioned in a specific way - protein subunits do this
What happens after excitation energy transfer reaches the reaction centre in the core complex?
Light energy goes into the core complex, reaching a pair of electrons that are in the core. This pair gives the light energy to electron acceptors. This energy is carried away from the photosystem to keep helping in photosynthesis.
What is the difference in efficiency of electron excitation energy transfer with low light and high light intensities?
Low light intensity = very efficient process, many photons are absorbed by the pigment molecules to carry out the process
High light intensity = less efficient process, other factors affect it, some light energy is lost to fluorescence
How many types of photosystems are there?
There are two photosystems - photosystem I and photosystem II in the chloroplast
Compare photosystem I and photosystem II?
Photosystem I:
1. Located in thylakoid membranes between grana (stroma lamellae)
2. Primary electron ‘donor’ / electron pairs is P700, which is a pair of chlorophyll electrons w/peak light absorbance 700nm
3. Excited electrons / light energy at end are transferred to enzyme NADP reductase which uses it to reduce NADP
4. Electrons are replaced with plastocyanin
Photosystem II:
1. Located in thylakoid membranes within grana (cylindrical stacks of thylakoids)
2. Primary electron ‘donor’ / electron pairs is P680, which is a pair of chlorophyll electrons w/peak light absorbance 680nm
3. Excited electrons / light energy at end are transferred to plastoquinone which gives it to electron carriers
4. Electrons are replaced with photolysis of water
What are advantages of having the structured pigment molecules in the photosystems?
- Photons of light are scattered as all the pigment molecules are in an array -> more absorbed at a time
- Different types of pigment molecules in one array -> different wavelengths of light can be absorbed, not just one
- Since the pigments are structured close together, light energy is less frequently lost to fluorescence during excitation energy transfers
What are some unanswered aspects or confusion about photosystems?
The mechanisms that happen within photosystems are more biophysics - does not involve enzyme-substrate collisions or molecular motion, and instead uses more of quantum mechanic principles.
What does absorption of photons of light in photosystem II do?
Causes the pair of chlorophyll electrons (P680) to become oxidized. P680 is a reducing agent - it can regain electrons from water.
How does P680 regain electrons from water?
Through the oxygen-evolving complex (OEC) in photosystem II.
What is in the oxygen-evolving complex (OEC)?
Manganese, calcium, oxygen…it’s in the core complex of the photosystem, next to the thylakoid space
How does the oxygen-evolving complex (OEC) work?
The OEC binds two water molecules together and then splits them apart to release 4 protons and 4 electrons.
The remaining 2 oxygens bind together to form O2.
This is called photolysis because it only happens in the light when P680 is oxidized.
Regains electrons for photosystem II.
What happens after this photolysis process in the OEC?
P680’s electrons are replaced, protons are released in thylakoid to make thylakoid membrane.
How did the production of oxygen via this photolysis process contribute to development of life on early Earth?
The production of oxygen led to oxidation of iron and other elements. This then caused oxygen to accumulate in the atmosphere and make life evolve.
What is the role of plastoquinone in the production of ATP using chemiosmosis? (step 1)
The excited electrons are passed onto plastoquinone. Plastoquinone takes these two electrons and two protons to become plastoquinol. This process happens at binding sites at the reaction centre of the photosystem.
What is the role of the cytochrome b6f complex in the production of ATP using chemiosmosis? (step 2)
Electron transport chains that move to photosystem I
What is the role of the electrons in the production of ATP using chemiosmosis? (step 3)
The electrons have less energy when they reach photosystem I. Because energy from these electrons was used to pump protons into the thylakoid space from the stroma - making a proton gradient. Photolysis also does this.
What is the role of ATP synthase in the production of ATP using chemiosmosis? (step 4)
ATP synthase generates ATP using the energy from the proton gradient. Protons travel across the gradient by passing through ATP synthase, which phosphorylates ATP from ADP -> chemiosmosis
What is NADP?
A molecule that supplies electrons for producing glucose from photosynthesis. Can exist in an oxidized or reduced state. Has one extra phosphate group compared to NAD.
How is reduced NADP produced?
Through adding 2 electrons. When P700 gets excited, it moves electron carriers to NADP reducatse, which reduces NADP.
How are photosystem I and photosystem II linked?
The electrons from photosystem I that reduce NADP are then replaced by the electrons that plastocyanin carries in. Electrons that are excited in photosystem II are passed to plastocyanin and that plastocyanin moves them to photosystem I.
What is cyclic photophosphorylation?
Excited electrons from P1 are given to plastoquinone and go back to P1 which makes proton gradient.
What is a thylakoid?
A sac-like vessel that does the light-dependent reactions of photosynthesis - absorbing light energy, splitting water via photolysis, reducing NADP, and producing ATP
What do thylakoids look like in cyanobacteria?
Varied shapes, attached to plasma membrane
What are the two kinds of thylakoids in algae and plants?
- Disc-shaped thylakoids in stacks called grana
- Unstacked thylakoids called stroma lamellae which connect the different stacks together
Why is a thylakoid considered a system?
The components won’t work individually, they interact.
Examples:
1. Thylakoid membranes seperate the fluid in the lumen and the fluid in the stroma to make a proton gradient
2. ATP synthase uses the proton gradient to make ATP
3. The OEC of photosystem II does photolysis in the lumen to supply electrons
4. Photosystem II absorbs light and excites electrons, which go to plastoquinone, and plastoquinone and the cytochrome complex b6f uses the electrons’ energy to pump protons
All that shows interacting elements
Are components of the thylakoid system evenly distributed between the stroma lamellae and grana? Why or why not?
No
- Grana has mostly photosystem II and cytochrome complex b6f
- Stroma lamellae mostly has photosystem I and ATP synthases (ATP synthase is here so it’s more exposed to stroma)
Why do we have to prevent CO2 from escaping?
CO2 dissolves readily and diffuses readily, but it’s the carbon source for photosynthesis, so we really need it. We prevent it from leaving through carbon fixation.
What happens in a carbon fixation reaction? Where does it occur?
CO2 is changed into a more complex carbon compound -> 3 carbon compound called glycerate 3-phosphate (G3P). This reaction occurs in the stroma.
What does carbon react with to form glycerate 3-phosphate (G3P)? What enzyme catalyses this reaction?
It reacts with a 5-carbon compound called ribulose biphosphate (RuBP), and makes 2 molecules of glycerate 3-phosphate. Rubisco catalyzes this reaction.
What is the equation of carbon fixation / formation of G3P?
ribulose biphosphate (RuBP) + carbon dioxide (CO2) -> (rubisco) -> 2 glycerate 3-phosphate (G3P) molecules
What is the efficiency of rubisco? What combats this?
Rubisco only carbon fixates 3 CO2 molecules per second - slower than most enzymes - inefficient. To combat this, there’s lots of rubisco in the stroma.
How much rubisco is on Earth?
0.7 Gt / 700 billion kgs
Ratio of hydrogen to oxygen in most carbohydrates?
2x more hydrogens than oxygens
What is the significance of hydrogen and oxygen ratios in this matter to RuBP and the synthesis of g3p (glycerate-3-phosphate) molecules?
When RuBP is converted to glycerate-3-phosphate molecules, the oxygen is used but the hydrogen isn’t, which ruins the ratio. So hydrogen has to be added to the g3p
How is triose phosphate made?
Adding hydrogen to G3P
Where does conversion of g3p to triose phosphate happen?
In the stroma of the chloroplast
Why is conversion of g3p considered light-independent?
Light is not directly used - however, in darkness, ATP / reduced NADP quickly run out so in darkness the reaction only happens in a short time.
What is the Calvin Cycle?
The light-independent reactions of photosynthesis, where RuBP is produced and consumed in a cycle
Why does the Calvin Cycle occur?
To keep regenerating RuBP.
Describe what to do to make the Calvin cycle continue indefinitely?
Make as much RuBP as we use.
Summarize Calvin’s Cycle?
3 CO2 add to 3 RuBP = 6 glycerate phosphates.
2 ATP and 2 reduced NADP make glycerate phosphates into 5 triose phosphates. 1 is removed.
Other 5 regenerate 3 RuBP.
How many turns of the Calvin’s Cycle is needed for glucose production?
Six turns of the Calvin cycle are needed to produce one molecule of glucose; each turn of the cycle contributes one of the fixed carbon atoms in glucose.
What is glucose used for?
cell respiration and making cellulose
What does glucose synthesize after photosynthesis?
sucrose or starch
What does triose phosphate synthesize after photosynthesis?
fatty acids
How are other carbon compounds produced in photosynthesizing cells, and what additional elements and nutrients are required?
Other carbon compounds can be made - need mineral nutrients. All 20 amino acids are made
Compare and contrast light-dependent and light-independent reactions of photosynthesis? (comparison from benabo slides)
Light dependent:
- Occur in thylakoids
- Uses light energy to form ATP, reduced NADP
- Uses electron transport chains and the 2 photosystems
Light independent:
- Occur in stroma
- Uses ATP, reduced NADP to make triose phosphate
- Uses the Calvin Cycle
Examples of the light-dependent and the light-independent reactions of photosynthesis? (comparison from oxford textbook)
Light dependent:
- Uses photolysis
- ATP synthesis by chemosmosis
- Light energy absorption through excited electrons
Light independent:
- Uses carbon fixation
- Synthesis of triose phosphate and carbon compounds
- Regeneration of RuBP
Explain the interdependence of light-dependent and light-independent reactions?
Light independent reactions can only go on for a few seconds in darkness because they need the substances made by the light dependent reactions which quickly run out. Light-dependent reactions also can’t keep going without some substances made by the light-independent reactions
How does LOW light intensity affect photosynthesis?
The production of ATP and reduced NADP is restricted.
Rate limiting step = conversion of glycerate 3-phosphate
How does HIGH light intensity affect photosynthesis?
Use of reduced NADP is restricted
Rate limiting step = carbon fixation
Some photons of light absorbed by photosystems re-emitted -> fluorescence
