5.6 - Photosynthesis (full) Flashcards
1
Q
What is the overall chemical equation of photosynthesis?
A
6CO2 + 6H2O –> C6H12O6 + 6O2
2
Q
Is photosynthesis endothermic or exothermic? What does that mean?
A
Endothermic - overall reaction takes in energy
3
Q
What is chemiosmosis?
A
Movement (diffusion) of protons across ATP synthase down the electrochemical gradient to generate ATP
4
Q
In chemiosmosis, where does the energy to pump protons come from?
A
Energy given off by an electron passing down the first electron transport chain in the light-dependent reaction
5
Q
Chemiosmosis allows the transformation of ADP into ATP. This process in photosynthesis is known as?
A
Photophosphorylation
6
Q
During photosynthesis, what is pumped across the membrane and into the thylakoid space of the chloroplast?
A
Hydrogen ions
7
Q
What pumps the hydrogen ions into the thylakoid space?
A
A cytochrome complex (electron carrier)
8
Q
Where does chemiosmosis occur?
A
Along the membrane
9
Q
What is grana?
A
Stacks of thylakoids that contain photosynthetic pigments
10
Q
What is the stroma and its function?
A
The fluid filled space surrounding the thylakoids. Site of light-independent stage (Calvin cycle)
11
Q
What is the structure that connects grana together?
A
Intergranal lamella
12
Q
What is the structure of a photosystem?
A
A light-harvesting complex that channels light towards a reaction centre (containing a primary pigment called chlorophyll a); accessory pigments channel photons towards the reaction centre
13
Q
What is one primary pigment and 3 accessory pigments?
A
Primary - chlorophyll A; accessory - xanthophylls, carotenoids, chlorophyll b
14
Q
What are the differences between photosystem I and photosystem II?
A
PSI contains chlorophyll a that absorbs light at a peak of 700nm (P700). PSII contains chlorophyll a that absorbs light at a peak of 680 nm. Electrons are excited to a higher energy level in PSI than PSII
15
Q
What the light absorption of carotenoids, xanthophylls and chlorophylls?
A
Carotenoids absorb blue light; xanthophylls absorb blue and green light; chlorophylls absorb blue and red light
16
Q
In which part of the chloroplast does the light-dependent reaction take place?
A
Thylakoid membrane
17
Q
In which part of the chloroplast does the light-independent reaction take place?
A
Stroma
18
Q
Why do the Rf values of some pigments show a range rather than a single figure?
A
The different types of these pigments have slightly different molecular masses/sizes of molecules/solubilities in the solvent
19
Q
Rf values for the different pigments differ according to the solvent used. Why this is the case?
A
The molecules of pigment will have different solubilities in different solvents
20
Q
What 4 processes occur in the light-dependent reaction?
A
Light harvesting at the photosystems; photolysis of water; photophosphorylation (production of ATP in the presence of light); formation of reduced NADP
21
Q
What is the role of water in photosynthesis?
A
- Electron donor
- source of protons/hydrogen ions
- source of the waste product product, oxygen
- keeps cells turgid so they can function (all metabolic reactions need to be in solution)
22
Q
Why might a lack of iron in soil reduce growth in plants?
A
Iron is needed for electron carriers in the chain (on thylakoid membranes) and for ferredoxin. A reduction of electron carriers could reduce rate of photosynthesis
23
Q
What is photolysis and where exactly does it take place?
A
The enzyme-catalysed splitting of water molecules, in the presence of light. It takes place in PSII on thylakoid membranes of chloroplasts
24
Q
What are the differences between non-cyclic and cyclic photophosphorylation?
A
NC - involves PSI and PSII; produces ATP, oxygen and reduced NADP (NADP is final electron acceptor). C - involves PSI only; produces ATP only as electrons are recycled back to PSI and NADP is not reduced
25
How does a proton gradient develop between the thylakoid lumen and the stroma?
Energy from electrons moving through chain of electron carriers is used to actively pump protons across the thylakoid membrane into the thylakoid lumen
26
What is the purpose of the proton gradient?
Protons can diffuse down their gradient through ATP synthase channels (using proton motive force), causing ADP to combine with inorganic phosphate to form ATP
27
What happens to protons that have moved through ATP synthase?
They are accepted along with electrons from the electron transport chain, by NADP. This is facilitated by NADP reductase (NADP has been reduced to NADPH)
28
How does an electron move through the electron transport chain?
A photon excites an electron from PSII, the electron rises to a higher energy level and gets accepted by an electron acceptor. This energy level is unstable so the electron moves through a chain of electron carrier proteins, losing energy as it moves (this energy is transferred to pump protons). It is accepted by chlorophyll a in PSI and re-excited by another photon of light to a higher energy level than before
29
Why do chloroplasts in guard cells only contain PSI?
Guard cells only contain ps1 because they produce only atp which actively brings potassium ions into the cells lowering water potential so that water follows by osmosis
This causes guard cells to swell and opens the stomata
30
What are the differences in the fate of electrons released from chlorophyll in both non-cyclic and cyclic photophosphorylation?
Cyclic - pass via electron carriers back to PSI (little ATP generated); Non-cyclic - from PSII, pass via electron transport chain to PSI, PSI electrons accepted by NADP
31
What is RuBisCo and its function?
Ribulose bisphosphate carboxylase/oxidase; an enzyme found in the stroma that catalyses carbon fixation (CO2 + RuBP --> GP)
32
Why is there always only low levels of RuBP in the stroma of chloroplasts?
It is being continually regenerated and then combined with CO2
33
What is the role of carbon dioxide in the Calvin cycle?
Combines with 5C-RuBP to form a 6C intermediate. This is unstable and so breaks into 2 x 3C-GP molecules. This reaction is catalysed by RuBisCO
34
What is the conversion of GP into TP in the Calvin cycle?
3C-GP is converted into 3C-TP using ATP and the H atom from reduced NADP
35
How is RuBP regenerated in the Calvin cycle?
5/6 of TP is converted back into RuBP through reshuffling of the arrangement of carbon atoms. This conversion also uses ATP (RuP --> RuBP)
36
What is the fate of triose phosphate?
1/6 leaves the Calvin cycle and is converted into sucrose, starch, cellulose, amino acids, glycerol, fatty acids, some is respired in glycolysis. The other 5/6 is used to regenerate RuBP
37
What are the 2 products of the light dependent reaction used in the Calvin cycle?
ATP and reduced NADP
38
How do plants respond to water stress and how does this affect their ability to photosynthesise?
Leaves lose more water by transpiration than can be replaced by uptake at roots. Cells lose water/become plasmolysed. Tissues become flaccid. Leaves wilt. Roots are unable to take in water and secrete abscisic acid, which travels in xylem to leaves causing stomata to close to prevent further water vapour loss. Closed stomata reduce gaseous exchange so not enough carbon dioxide for photosynthesis. Plasmolysed cells cannot function as enzymes cannot work. Reduced water availability for photosynthesis – as source of electrons and protons
39
What are the effects of reducing the light intensity on the Calvin cycle?
Light dependent reaction cannot occur, so less ATP/NADPH; less GP --> TP; TP levels fall and GP accumulates; if TP levels fall then RuBP cannot be regenerated
40
What are the effects of reducing carbon dioxide levels in the Calvin cycle?
RuBP cannot accept it and RuBP accumulates; GP cannot be made; TP cannot be made
41
What are the names of all three enzymes involved in photosynthesis and their functions?
ATP synthase (allows chemiosmosis to occur to join ADP and Pi together to make ATP); NADP reductase (joins NADP with H+ and e- to make reduced NADP); RuBisCo (catalyses carbon fixation - CO2 + RuBP --> GP)
42
What is meant by the term compensation point?
When the rate of photosynthesis is equal to the rate of respiration. This is when there is no net gain or loss of carbohydrate
43
What is a 'limiting factor'?
A factor that limits the rate of reaction when in short supply
44
What are three factors that affect photosynthetic rate?
Temperature; Carbon dioxide conc; Light intensity
45
Why does increasing light intensity also increases photosynthetic rate?
More energy absorbed by photosystems to excite electrons, so more electrons undergo ETC to generate proton gradient for more ATP production and reduced NADP production
46
Why does more carbon dioxide increases photosynthetic rate?
Higher rate of carbon fixation at Calvin cycle for more GP and TP production
47
How does temperature affects photosynthetic rate?
At lower temp, enzymes are inactive hence low PS rate. As temp increases, enzymes and substrates gain KE and hence higher rate of ESC formation, so higher PS rate. At higher temp, enzymes become denatured hence PS rate decreases
48
What is the equation for the light dependent reaction?
Water & ADP + Pi + NADP -> O2 & ATP + NADH
49
What is the equation for the light independent reaction?
CO2 & ATP + NADPH -> C6H12O6 + ADP + Pi & Ox NADP
50
What are the products of the light independent & light dependent reaction?
Light dependent reaction - O2 & ATP + NADH
| Light independent reaction - C6H12O6, ADP & Pi + Ox NADP
51
What are the reactants of the light independent & light dependent reaction?
Light dependent reaction - H20 & ADP + Pi + NADP
| Light independent reaction - CO2 & ATP + NADPH
52
What happens to glucose after it is made in the light independent reaction?
- Respired
- Stored as starch
- Converted into amino acids (Needing nitrogen in nitrates)
- Converted into lipids
53
What are the features of chloroplast?
- 2-10 micrometres
- Endosymbiotic theory:
- Double membrane
- DNA + Ribosomes (70s)
- Lipid droplets & starch grains
54
Why do chloroplasts have 70s ribosomes?
Chloroplasts have bacterial (70s) ribosomes as the endosymbiotic theory states that they have evolved form bacteria that had been engulfed
55
What is the structure of chloroplast?
- Outer membrane
- Inter membrane space
- Inner membrane
- Chloroplast envelope
- Thylakoid stacked up into stacks of Granum
- Intergranal lamellae (Between granum)
- Stroma (With lipid droplets and starch grains)
56
Where does the light independent & light dependent reaction take place?
Thylakoid - light dependent reaction
| Stroma - light independent
57
What are the structural adaptations of the chloroplast?
- Thylakoids have a large SA and are densely packed with Photosystems for the light dependent reaction in photosynthesis
- Stroma surrounds thylakoids, so that the products from the light dependent reaction can diffuse into the stroma and be reactants for the light independent reaction
58
What is a photosystem and what is their function?
Photosystems are proteins containing lots of pigments that are bound to the prosthetic groups. Photosystems carry out the absorption of light and are embedded in the thylakoid membrane
59
What are the pigments inside of photosystems?
- Accessory pigments - Allow the photosystem to catch a greater range of wave lengths of light
- Reaction centred pigments (Chlorophyll A) [P700 or P680] {P = Wavelength}
60
What is the function of the large antenna complex?
It increases the area over which photons can be harvested. (Photons are particles/packets of light energy)
61
What are the names of the accessory pigments?
- Chlorophyll A
- Chlorophyll B
- Xanthophyll
- Carotene
62
What is the absorption of light in general from plants?
Plants absorb all light other than green light which is reflected. The absorbance each accessory pigments has been researched through thin layer chromatography
63
what is the equation to work out Rf value?
Rf value = Distance of spot / distance of solvent front
64
What is the light dependant reaction?
- Non cyclic photophosphorylation - Z scheme that shows the energy levels of the electrons
- Cyclic photophosphorylation - The cycle that shows how electrons go up the etc and back down while releasing energy to make ATP
- Photolysis - The splitting of water
65
What is the process of Non cyclic photophosphorylation?
1) Photons of light energy hits photosystem 2 and the reaction centre pigment absorbs the light at p680
[This excites two electrons]
2) The electrons are accepted by the electron acceptor making Ps 2 electron deficient
3) The electrons flow down the electron transport chain, moving from acceptor to acceptor losing energy
[The lost energy is used to pump protons across the thylakoid membrane to create a proton gradient and make ATP]
4) The electrons end up at photosystem 1, from here photons of light excite two more electrons causing them to go up to another electron acceptor
5) The electrons go from one electron acceptor to the final electron acceptor where the energy is released and the electrons are used to reduce NADP
[2NADP + 2E- + 2H+ (From photolysis) -> NADPH, the NADPH goes to the Calvin cycle]
6) Protons diffuse down their concentration gradient through channels associated with ATP synthase
[4 protons must flow through ATP synthase in order to produce 1]
66
What is the process of Cyclic photophosphorylation?
[Electrons start in photosystem 1]
1) The electrons go up to the electron acceptor
2) The electrons are passed down the electron transport chain where they release energy
[The energy that is released is used to phosphorylate ADP + Pi into ATP by pumping protons across the thylakoid membrane to create a proton gradient and make ATP]
3) The electrons then go back down the electron transport chain to Photosystem 1 again and the process starts again
67
What is photolysis?
[Occurs only at Ps2 in the thylakoid lumen]
| 1) Light energy splits the water molecule into 2 H+, 2 E- & 1/2 of the molecule of oxygen)
68
What is the difference between cyclic and non cyclic?
| Table
¦ Cyclic ¦ Non-cyclic
-------------------------------------------------------------
Photosystems ¦ Ps1 ¦ Ps1 + Ps2
-------------------------------------------------------------
Products ¦ ATP ¦ ATP + NADPH
69
What is the difference between cyclic and non cyclic?
Cyclic: Uses Ps1
Product is ATP
Photolysis doesn't occur
Non-cyclic: Uses Ps1 and Ps2
Products are ATP and NADPH
Photolysis does occur
70
What is the whole process of Photophosphorylation?
| Light dependant stage
1) Light is absorbed at Ps2
2) The electrons get passed to proteins on the thylakoid membrane and they move through the electron carriers down the electron transport chain
[The energy created from this is used to pump H+ ions into the thylakoid space]
3) A gradient is created as H+ ions were taken away from the stroma but pumped into the thylakoid
[This causes the PH to increase outside, in the stroma]
4) This then allows H+ ions to flow down ATP synthase causing it to spin and produce ATP via chemiosmosis
[Using H+ gradient to spin ATP synthase and produce ATP]
71
What is the process of the light independent reaction? (The Calvin cycle)
{GANT TAR}
1) 2x GP (3C)
[2x ATP is hydrolysed into 2 ADP + Pi]
[2x NADPH is oxidised into 2x NADP]
2) 2x Triose phosphate (3C)
[Some of the Tp (2/12) is taken off to make glucose]
[Some the Tp (10/12) remains in the cycle]
3) Triose phosphate (3C)
[ATP is hydrolysed into ADP + Pi]
[The phosphate that is produced then joins to make the RuBP]
4) RuBP (5C)
[CO2 (1C) is added to make a 6C molecule that is then split to start the cycle again]
72
What enzyme catalyses the Calvin cycle? Why is it bad?
- Rubisco
- Even though it is very abundant it is not efficient as it can waste carbon by binding to O2 (Photorespiration) at higher temperatures
- Some plants have evolved other mechanisms that reduce the likelihood of photorespiration occurring
73
Where is the 2x ATP and 2x NADPH from that is needed in the Calvin cycle?
The light dependant reaction
74
What happens if light becomes unavailable in the Calvin cycle?
- Gp increases, as it can't be converted into triose phosphate
- RuBP decreases as it is converted in to GP
- Triose phosphate decreases as it is converted in to RuBP that is then converted straight into GP
75
What happens if CO2 concentration decreases in the Calvin cycle?
- RuBP increases
- GP decreases
- Triose phosphate decreases as it is converted in to RuBP
76
What are the factors that can slow and limit the rate of photosynthesis?
- CO2
- Light intensity
- Temperature
77
How is the rate of photosynthesis measured?
Using a photosynthetometer:
1) Bubbles of gas is produced from photosynthesis
2) The bubbles of gas collect in a funnel (The funnel is already filled with water) then are drawn up the capillary tube
3) This is repeated so the oxygen is being removed but the water remains
4) Time this process and measure how far the bubble moves (In 5 mins the bubble moved 30mm)
5) Volume of oxygen produced = Radius of the tube squared x Pi x the length of the capillary tube
78
What is the light compensation point?
When the net production of oxygen and glucose is zero as the production and the uptake of oxygen is the same
79
Exemplar for non cyclic photo-phosphorylation
1. When a photon of light strikes PS2 (p680) it’s energy is channelled to the primary pigment reaction centre
2. The light energy excites a pair of electrons inside the chlorophyll molecule
3. The energised electrons escape from the chlorophyll molecule and are captured by an electron carrier which is a protein with iron at its centre embedded in the thylakoid membrane
4. These electrons are replaced by electrons derived from photolysis
5. When this iron iron combines with an electron it becomes reduced to fe 2+. It can then donate the electron, becoming reoxidised to fe3+, to the next electron carrier in the chain
6. As electrons are passed along a chain of electron carriers embedded in the thylakoid membrane, at each step some energy associated with the electrons is released
7. This energy is used to pump protons across the thylakoid membrane into the thylakoid space
8. Eventually the electrons are captured by another molecule of chlorophyll a in PS1. These electrons replace those lost from ps1 due to excitation of light energy
9. A protein iron sulfur complex called ferredoxin accepts the electrons from ps1 and passes them to nadp in the stroma
10. As protons accumulate in the thylakoid space a proton gradient forms across the membrane
11. Protons diffuse down their concentration gradient through special channels in the membrane associated with atp synthase enzymes and as they do so the flow of protons causes adp and inorganic phosphate to join and form atp
12. As the protons pass through the channel they are accepted, along with electrons, by nadp which becomes reduced. The reduction of nadp is catalysed by the enzyme nadp reductase
80
What is endosymbiotic theory?
- The theory that mitochondria and chloroplasts have evolved from a bacterial cell that was engulfed from a eukaryotic cell making it a phagocytotic vesicle
- This is shown as chloroplast has a double membrane, its own DNA and ribosomes (70S ribosomes)
81
What happens to the products of photolysis?
- 2H+ ions are used for the reduction of NADP along with two more electrons from PS1 and 2NADP+
- The O2 is a waste product that is released for humans to breathe in
- The two electrons are used in non-cyclic photophosphorylation
