10 Photosynthesis Flashcards
What term describes an organism that makes its own food?
Autotroph
What term describes an organism that makes its own food using sunlight?
Photoautotroph
What term describes an organism that gains energy from eating other organisms?
Heterotroph
What is the structure of a chloroplast?
It has a double membrane with disks named thylakoids that are in stacks named grana.
The thylakoids are hollow, forming a hollow interior named the thylakoid.
Where does the oxygen in CO2 end up after photosynthesis?
In glucose and water
Where does the oxygen in H2O end up after photosynthesis?
Oxygen (not water)
Is photosynthesis a redox reaction?
Yes…
CO2 is reduced to form glucose
Water is oxidised to form oxygen
How do the light reactions extract energy?
By forming NADPH and by chemiosmosis in which ATP is generated, in a process known as photophosphorylation
What are inputs/outputs of the light reaction?
Input: NADP+, ADP and P, Water.
Outputs: ATP, NADPH, Oxygen
What are the inputs/outputs of the calvin cycle?
Input: CO2, ATP, NADPH, sugar in the form of CH2O
What is the incorporation of carbon into and organism named?
Carbon fixation
With what is the ability of a pigment to absorb light measured?
A spectrophotometer which measures the rate of reflection under different conditions.
What represents the how much of each wavelength a pigment can absorb?
An absorption spectrum
What represents the rate of a light dependent reaction across various wavelengths?
An action spectrum
What is the wavelength of purple?
400-450
What is the wavelength of blue?
450-500
What is the wavelength of green?
500-575
What is the wavelength of yellow/orange?
575 to 650
What is the wavelength of red?
650 to 750+
What are the common forms of chlorophyll?
Chlorophyll a and chlorophyll b
What are the common photopigments common in plants?
Carotenoids, Chlorophyll a and chlorophyll b
What is the absorption spectrum of chlorophyll?
High at blue/purple, drops at green and peaks again at orange/red.
How do the absorption spectra chlorophyll a, chlorophyll b and carotenoid differ?
On a graph from 400-700nm:
Chlorophyll-a peaks first, then chlorophyll b then carotenoids.
Only the two forms of chlorophyll peak gain after green with chlorophyll b peaking first.
What is the structure of chlorophyll?
It has a hydrocarbon head on a porphyrin ring as a head which has a magnesium atom at the centre.
How does chlorophyll a differ from chlorophyll b?
chlorophyll a has a CH3 in a specific place of the porphyrin whereas chlorophyll b has a CHO group at that location
Why are carotenoids found in plants?
They offer photoprotection (protection from too bright light) by absorbing some light wavelengths that would otherwise damage chlorophyll.
Where do the light reactions take place?
In the membrane of the thylakoid so that H+ ions are pumped out into the stroma and diffuse back to generate ATP.
What are the structures of the light reaction?
There are two photosystems (PSII) and (PS I). Between them,and after PS I, is an electron transport chain.
What are the photosystems of photosynthesis?
In order: Photosystem II (PS II) then Photosystem I (PS I)
How do the photosystems collect light energy?
The photons increase the energy levels of the electrons.
This energy is then collected as the electrons drop down to their original.
What is the structure of a photosystem?
It has a reaction-center complex that is surrounded by the light harvesting complex.
Where is chlorophyll found specifically?
In the light-harvesting complexes of both PS I and PS II
In what ways do the photosystems package light energy?
In the forms of ATP and NADPH
Also during step 3 (splitting H2O) H+ is formed in the stroma as a form of potential energy.
Which electron transporter does photosynthesis use?
NADP+
What is the reduced form of NAD?
NADH
How does photosystem II differ internally from photosystem I?
Photosystem II uses the P680 form of chlorophyll whereas photosystem I uses P700 as the electron donor
How many steps are there of photophosphorylation?
8
What is step 1 of photophosphorylation?
A photon hits a chlorophyll molecule in the light harvesting complex of PS II causing one of its electrons to jump to a higher energy state. As it falls back down it excites another electron in a chain that continues until a P680 in the reaction complex is excited.
What is step 2 of photophosphorylation?
An electron is transferred from P680, the electron donor, to the ‘primary electron acceptor’ of the reaction complex.
What are P680 and P700?
Special forms of chlorophyll that act as electron donors.
What is the electron donor of photophosphorylation?
P680 (PS II) and P700 (PS I)
What is step 3 of photophosphorylation?
An enzyme catalyses the splitting of H2O into O2 and H+. This provides electrons to replace that lost by P680 (it is highly electronegative so grabs the electron)
What does P680+ refer to?
A P680 molecule which has donated its electron and thus has a + 1 charge.
What is step 4 of photophosphorylation?
The ‘photoexcited’ high energy electrons enter the electron transport chain between PS II and PS I
What does the first electron transport chain of photophosphorylation include?
Pq (plastoquinone), Cytochrome complex, and Pc (plastocyanin)
What is step 5 of photophosphorylation?
As the electrons travel through the electron transport chain they drop back down to a lower energy level. This provides the energy for the cytochrome complex to synthesis ATP.
Where is ATP formed during photophosphorylation?
In the cytochrome complex of the first electron transport chain.
Where is H2O used in photophosphylation and why?
Only by PS II to regenerate the electrons donated by P680.
PS I does not need these electrons as it replaces P700’s donated electrons using those from the electron transport chain.
What is step 6 of photophosphorylation?
A photon hits a pigment in the light harvesting complex of PS I and through a chain of excited electrons dropping back and exciting others, P700 is excited causing it to donate a high energy electron to the primary receptor.
How is P700+’s donated electron replaced?
Using electrons from the first electron transport chain.
What is step 7 of photophosphorylation?
The photoexcited electrons travels from the primary acceptor to the second electron transport chain.
What is does the second electron transport chain include?
Fd (ferredoxin) and NADP+ reductase
What is step 8 of photophosphorylation?
Electrons of the second electron transport chain pass through Fd (ferredoxin) and then, using NADP+ reductase reduce NADP+ to NADPH to be used as energy
What occurs after photophosphorylation?
Chemiosmosis
Describe chemiosmosis in photosynthesis.
Electrons send into the storm during photophosphorylation reenter the thylakoid space through ATP synthase and thus generate ATP
What is an alternative form of photophosphorylation?
Cyclic electron flow
Describe cyclic electron flow.
It uses only PS I.
Electrons move in the path of:
Primary acceptor → Fd → Cytochrome complex → Pc (plastocyanin) → Pigment → Primary acceptor again
How does cyclic electron flow differ from normal photophosphorylation in terms of input and output?
Cyclic electron flow does split water or reduce NADP+ to NADPH.
The cytochrome complex does still generate ATP.
What organism tend to use cyclic electron flow?
Many prokaryotes such as cyanobacteria perform it exclusively.
However most plants can do it as it appears to have a photo protective role.
What are the inputs/outputs of the light reactions?
In: Light, H2O, ADP, NADP+
Out: oxygen, H+, ATP, NADPH
What are the inputs/outputs of the dark reactions?
In: CO2, ATP and NADPH
Out: ADP, NADP+ and sugar (G3P)
What does [CH2O] refer to?
A sugar as this represents its ratio (1 carbon to 2 hydrogens to 1 oxygen)
What is the input to the Calvin Cycle?
3 Carbon dioxide molecules at a time.
What are the stages of the Calvin Cycle?
1: Carbon fixation
2: Reduction
3: Regenration of the CO2 acceptor (RuBP)
How many steps are there of the Calvin Cycle?
6
What is step 1 of the Calvin Cycle?
3 molecules of CO2 and 3 molecules of Ribulose biphosphate (RuBP) combine to form 3 molecules of a short live intermediate.
This is catalysed by the enzyme Rubiso
What is step 2 of the Calvin Cycle?
3 of Short lived intermediates → 6 of 3-phosphoglycerate
What is step 3 of the Calvin Cycle?
6 of 3-phosphoglyercate → 6 of 1,3-biphosphoglycerate using ATP
What is step 4 of the Calvin Cycle?
6 of 1,3-biphosphoglycerate → 6 of G3P using NADPH
What is step 5 of the Calvin Cycle?
One G3P is outputted resulting in 5 G3P remaining
What is step 6 of the Calvin Cycle?
An ATP is used to regenerate 5 G3P into 3 ribulose biphosphate
What does RuBP stand for?
Ribulose biphosphate
What does Rubisco catalyse?
Carbon fixation i.e. RuBP + CO2
What does G3P stand for?
Glyceraldehyde 3-phosphate
What is Glyceraldehyde 3-phosphate also known as?
G3P
What is ribulose biphosphate also known as?
RuBP
Which steps of the Calvin Cycle use ATP and how much?
Step 3 (3-phosphoglycerate → 1,3-biphosphoglycerate) uses 6 ATP molecules and Step 6 (G3P to RuBP) uses 3 ATP molecules
What step of the Calvin Cycle uses NADPH and how much?
Step 4 (1,3biphosphoglycerate → G3P) uses 6 molecules
How many carbon atoms does the short lived intermediate have?
6
How many carbon atoms does 3-phosphoglycerate have?
3
How many carbon atoms does 1,3 biphosphoglycerate have?
3
How many carbon atoms does G3P have?
3
How many carbon atoms does RuBP have?
6
Which reaction of the Calvin Cycle is a reduction?
Step 4 (1,3-biphosphoglycerate to G3P) as is demonstrated by it using NADPH
To synthesis one molecule of G3P how much ATP and NADPH is used?
9 ATP and 6 NADPH
How many CO2 molecules enter the Calvin Cycle per ‘turn’?
3
What is the output of the Calvin Cycle?
1 G3P which can be converted to Glucose etc. (2 G3P = 1 glucose)
What are the main types of photosynthesis?
C3, C4 and CAM.
What is the “normal” and most common method of photosynthesis?
C3
What does the 3 in C3 and the 4 in C4 refer to?
The number of carbon atoms in the substance which fixes CO2
What does CAM photosynthesis stand for?
crassulacean acid metabolism
What is a major inefficiency of photosynthesis?
Photorespiration.
What does C4 respiration attempt to overcome?
Photorespiration
What triggers photorespiration?
Hot, dry days as this triggers the stomata to close and thus leaves the plant with low carbon dioxide for photosynthesis
What causes photorespiration chemically?
Under low CO2 concentrations Rubicso binds O2 instead of CO2 to RuBP.
This result splits, forming a 2-carbon sugar which is broken down by peroxisomes and the mitochondria to form CO2
Why is photorespiration disadvantageous?
It does not release ATP but uses some i.e. active transport.
It also wastes products i.e RuBP that could otherwise be used productively.
Why can photorespiration be advantageous?
It is photo-protective
How do C4 plants differ structurally from C3 plants?
They have two distinct photosynthetic cells: Mesophyll cells and bundle-sheath cells.
Where are bundle sheet cells found?
C4 plants
How do C4 plants differ physiologically?
They fix carbon in the mesophyll cells so that CO2 is concentrated for use in the calvin cycle which occurs in the bundle sheath cells.
How many steps are there of CO2 fixation in C4 plants?
4
What is step 1 of carbon fixation in C4 plants?
CO2 enters the mesophyll cell where the enzyme PEP carboxylase adds it to PEP. This forms oxaloacetate
What is step 2 of carbon fixation in C4 plants?
The oxaloacetate is converted into malate which travels through plasmodesmata to the bundle sheath cells.
What is step 3 of carbon fixation in C4 plants?
The oxaloacetate in the bundle sheet cell is broken down into pyruvate and CO2 for use in the calvin cycle.
What is step 4 of carbon fixation in C4 plants?
The pyruvate reenters the mesophyll cell where ATP is added to it. This regenerates PEP
Which plant would benefit if the global CO2 concentration increased?
C3 plants as less photorespiration would occur.
Why is C4 carbon fixation not always the best?
It requires ATP to regenerate PEP so reduces the net output.
In what conditions would one typically find a C3 plant?
Normal i.e. not too hot, not too dry
In what conditions would one typically find a C4 plant?
Mildly hot and quite dry
In what conditions would one typically find a CAM plant?
Extremely hot and dry i.e. dessert where water loss must absolutely be minimised.
What is an example of a C4 plant?
Sugar cane, corn and some grasses
What is an example of a CAM plant?
Pineapple
What do CAM plants tend to be?
Succulent (water storing)
Describe CAM photosynthesis.
During the night CO2 is fixed into a four-carbon organic acid.
During the day this store CO2 is used for the Calvin Cycle.
Why does the Calvin cycle still occur during the day?
It requires the ATP and NADPH provide by the light-dependant reactions which can only occur during the day.
Which forms of photosynthesis use the Calvin Cycle?
All of them (C3, C4 and CAM)
What is the advantage of CAM photosynthesis?
The desert plants that use it down’t have to open their stomata for CO2 which would lead to excessive water loss.
Where do CAM plants store the acid?
In their vacuole.
