Photosynthesis Flashcards
(47 cards)
1
Q
What are the two sets of reactions involved in photosynthesis?
A
- Light dependent reactions: where light energy is necessary
- Light independent reactions: where light energy is not needed
2
Q
What is light energy needed for?
A
- Photolysis: splitting of water into hydrogen and oxygen (oxygen is a waste product)
- Provide chemical energy in the form of ATP: for the reaction of carbon dioxide to carbohydrate in the light independent reactions
3
Q
What are the different photosynthetic pigments?
A
- Primary pigments: two forms of chlorophyll, and these primary pigments act as reaction centres
- Accessory pigments: absorbs light energy that is passed to the primary pigments
4
Q
Where are the photosynthetic pigments arranged?
A
- In light harvesting clusters called photosystems
- There are two types PSI and PSII
5
Q
What are the light dependent reactions?
A
- Splitting of water by photolysis
- Synthesis of ATP in photophosphorylation
- The hydrogen ions combine with a carrier molecule NADP to make reduced NADP
- ATP and reduced NADP are passed from the light dependent to the light independent reaction
6
Q
What are the different types of photophosphorylation?
A
-Photophosphorylation of ADP to ATP can be cyclic of non-cyclic, depending on the pattern of electron flow in one or both types of photosystem
7
Q
Describe the key points of cyclic photophosphorylation
A
- Involves only PSI
- Light absorbed by PSI and passed to the primary pigment
- The electron in the chlorophyll molecule is excited to a higher energy level and is emitted from the chlorophyll molecule, this is called photo activation
- The excited electron is captured by an electron acceptor snd passed back to a chlorophyll molecule via a chain of electron carriers
- During this process enough energy is released to synthesis ATP from ADP and an inorganic phosphate group (Pi) by the process of chemiosmosis
- The ATP then passes to the light independent reactions
8
Q
Describe the key points of non-cyclic photophosphorylation
A
- Involves both PSI and PSII in the ‘Z-Scheme’ of electron flow
- Light is absorbed by both photosystems and excited electrons are emitted from the primary pigments of both reactions centres
- These electrons are absorbed by electron acceptors and pass along chains of electron carriers, leaving the photosystem positively charged
- The primary pigment of PSI absorbs electrons from PSII
- Its primary pigment receives replacement electrons from the splitting (photolysis) of water
- As in cyclic photophosphorylation, ATP is synthesised as the electrons lose energy while passing along the carrier chain
9
Q
What are the photosystems involved in cyclic and non-cyclic photophosphorylation?
A
- Cyclic: PSI
2. Non-Cyclic: PSII
10
Q
Is photolysis involved in cyclic and non-cyclic photophosphorylation?
A
- Cyclic: no
2. Non-Cyclic: yes
11
Q
What is the fate of electrons released in cyclic and non-cyclic photophosphorylation?
A
- Cyclic: Returned to electron transport chain and then back to PSI
- Non-Cyclic: electron lost from PSI replenished by one from PSII and
12
Q
What are the products in cyclic and non-cyclic photophosphorylation?
A
- Cyclic: ATP
2. Non-Cyclic: O2, NADPH and ATP
13
Q
Describe the photolysis of water
A
- PSII includes a water-splitting enzyme that catalysis the breakdown of water
- Oxygen is a waste product of this process
- The hydrogen ions combine with electrons from PSI and the carrier molecule NADP to give reduced NADP
- Reduced NADP passes to the light independent reaction and is used in the synthesis of carbohydrate
14
Q
What are examples of chlorophyll (primary pigments) and their colours?
A
- Chlorophyll a (yellow-green)
- Chlorophyll b (blue-green)
15
Q
What are examples of carotenoids (accessory pigments) and their colours?
A
- Beta carotene (orange)
- Xanthophyll (yellow)
16
Q
Why do plants look green?
A
- Chlorophylls absorb mainly in the red and blue -violet regions of the light spectrum
- They reflect green light which is why plants look green
- The carotenoids absorb mainly in the blue-violet region of the spectrum
17
Q
What is an absorption spectrum?
A
A graph of absorbance of different wavelengths of light by a pigment
18
Q
What is an action spectrum?
A
- A graph of the rate of photosynthesis at different wavelengths of light
- This shows the effectiveness of the different wavelengths, which is related to their absorption and to their energy content
- The shorter the wavelength, the greater the energy it contains
19
Q
What is carbon dioxide fixation?
A
- The fixation of carbon dioxide is a light independent process in which carbon dioxide combines with a five carbon sugar, ribulose bisphosphate (RuBP0
- This gives two molecules of a three-carbon compound, glycerate 3-phospgate (GP/PGA)
- This happens by using the enzyme rubisco
20
Q
What happens to GP?
A
- GP in the presence of ATP and reduced NADP from the light dependent stages, is reduced to triose phosphate (TP) (three-carbon sugar)
- This is the point at which the carbohydrate is produced in photosynthesis
21
Q
What happens to the triose phosphates?
A
- Most of the triose phosphates (5/6) are used to regenerate RuBP
- 1/6 are used to produce molecules needed by the plant
- Some of these TPs condense to become Hexose Phosphates, which are in turn used to produce starch for storage, sucrose for translocation around the plant or cellulose for making cell walls
- Others are converted to glycerol and fatty acids to produce lipids for cellular membranes or to acetyl coenzyme A for use in respiration or in the production of amino acids for protein synthesis
22
Q
What are autotrophs?
A
- Almost all the energy transferred to all the ATP molecules in all living organisms is derived from light energy used in photosynthesis by autotrophs
1. Such photoautorophs include green plants, the photosynthetic prokaryotes and both single-celled and many-celled protoctists (including the green, red and brown algae)
23
Q
What are chemoautotrophs?
A
- A few autotrophs do not depend on light energy, but use chemical energy sources
- These include the nitrifying bacteria that are in the nitrogen cycle
- Nitrifying bacteria obtain their energy from iodising ammonia (NH3) to nitrite (NO2-), or nitrite to nitrate (NO3-)
24
Q
What is photosynthesis?
A
The fixation of carbon dioxide and its subsequent reduction to carbohydrate, using hydrogen from water
25
Name two pigment molecules found in chloroplasts
Chlorophyll a/b, Carotenoids
26
State one role of pigment molecules in a photosystem
Absorb light, traduce light
27
Where are photosystems within the chloroplast?
Thylakoid membrane
28
What is the enzyme involved in the fixation of carbon dioxide?
Rubisco
29
What is one way that photosystem 1 differs from photosystem 2?
-Reaction centre chlorophyll is P700 (in 1) and P680 (in 2)
30
Explain the role of ATP in the light independent stage of photosynthesis
1. Increase potential energy during conversion of GP to TP through the hydrolysis of ATP
2. Photophosphorylation TP to form RuBP
31
Describe the structure of a chloroplast
- Each chloroplast is surrounded by an envelope of two phospholipid membranes
- A system of membranes also runs through the ground substance or stroma
32
What is the membrane system in a chloroplast the site of?
- The light dependent reactions of photosynthesis
- It consists of a series of flattened fluid-filled sacs, or thylakoids, which in places form stacks called grana that are joined to one another by membranes
1. The membranes of the grana provide a LSA which holds the pigments, enzymes and electron carriers needed for the light dependent reactions
2. The membranes make it possible for. large number of pigment molecules to be arranged so that they can absorb as much light as necessary
33
How are the pigment molecules arranged?
- The pigment molecules are arranged in particular light-harvesting clusters for efficient light absorption
- In each photosystem, the different pigments are arranged in the thylakoid in funnel like structures
1. Each pigment passes energy to the next members of the cluster, finally feeding it to the chlorophyll a reaction centre (primary pigment)
- The membranes of the franc hold ATP synthase and are the site of ATP synthesis by chemiosmosis
34
What are the stroma?
1. The stroma are the site of the light independent reactions
2. It contains the enzymes of the Calvin cycle sugars and organic acids
3. It bathes the membranes of the grana and so can receive the products of the light dependent reactions
- Also within the stroma are small (70S) ribosomes, a loop of DNA, lipid driblets and starch grains
- The loops of DNA codes for some of the chloroplast proteins, which are made by the chloroplasts ribosomes
- However other chloroplast proteins are coded for by the DNA in the plant cell nucleus
35
What are limiting factors?
When a process is controlled by a number of factors the factor in least supply will limit the rate of the process
36
What are C3 and C4 plants?
1. In the light independent stage carbon dioxide combines with RuBP to form a six carbon compound, which immediately splits to form two three carbon molecules (these plants that do this are called C3 plants)
2. However some other plants e.g. Maize, produce a compound with four carbon atoms (C4 plants)
37
What is the problem with rubisco?
- This enzyme catalyses the reaction of carbon dioxide with RuBP
- But it can also catalyse the reaction of oxygen with RuBP
- When this happens, less photosynthesis takes place because some of the RuBP is being 'wasted' and less is available to combine with carbon dioxide
- This unwanted reaction is known as photorepsiration and it happens most readily at high temperature and high light intensity
38
What are some of the methods evolved by tropical grasses and maize to avoid photorespiration?
1. They keep RuBP and rubisco away from high oxygen concentrations
2. The cells that contain RuBP and rubisco are arranged around the vascular bundles and are called bundle sheath cells
3. They have no direct contact with the air inside the lead
39
How is carbon dioxide absorbed?
1. Carbon dioxide is absorbed by mesophyll cells, which are in contact with the air
2. The mesophyll cells contain an enzyme called PEP carboxylase, which catalyses the combination of carbon dioxide from the air with a three-carbon substance called phosphoenolpyruvate or PEP
3. The compound formed form this reaction is oxaloacetate
40
What happens the the oxaloacetate?
1. Still inside the mesophyll cells, the oxaloacetate is converted to malate and this is passed on to the bundle sheath cells
2. Now the carbon dioxide is removed from the malate molecules and delivered to RuBP by rubisco in the normal way
- The light independent reactions then proceeds as normal
41
What is different about enzymes in C4 plants?
1. Enzymes in C4 plants generally have higher optimum temperatures that those in C3 plants
2. This is an adaptation to growing in a hot climate
42
What is the photolysis of water?
-Photosystem 2 includes a water splitting enzyme that catalyses the breakdown of water
H2O --> 2H+ + 2e- + 1/2O2
-Oxygen is a waste product of this process
-The hydrogen ions combine with electrons from photosystem 1 and the carrier molecules NADP to give reduced NADP
2H+ + 2e- + NADP --> reduced NADP
-Reduced NADP passes to the light independent reactions and is sued in the synthesis of carbohydrate
43
What is the role of reduced NADP in light independent stage?
1. Donates hydrogen
| 2. GP to TP
44
Explain how the physiology of the leaves of a C4 plant such as maize is adapted for efficient carbon fixation at high temperatures
1. In C3 plants at high temperature rubisco combined with oxygen
2. Less reubsco to coming with carbon dioxide
3. In C4 plants such as maize spatial separation of light dependent stage and carbon fixation
4. Rubisco/RuBP in bundle sheath cells
5. Kept away from oxygen/air
6. Mesophyll cells absorb CO2
7. CO2 released to combine with RuBP
8. Reduce photorespiration
9. High optimum temperature for enzymes involved
10. Calvin cycle can continue
(11. CO2 reacts with PEP and PEP carboxylase)
45
Describe how in photosynthesis, light energy is converted into chemical energy in the form of ATP
1. Light energy absorbed by chlorophyll
2. Electron excited to higher energy level
3. Electron emitted by chlorophyll
4. Passes to electron acceptor/carrier
5. Passed along chain of electron carrier/electron transfer chain
6. Energy released is used to pump protons
7. Into thylakoids space
8. Thylakoids membrane is imperials to protons
9. Proton gradient forms
10. Proton moves down gradient
11. Through ATP synthase
12. Enzyme rotates
13. ATP produced from ADP and Pi
46
What does RuBP stand for?
Ribulose Biphosphate
47
Where are RuBP and GP located?
Stroma
