Flashcards in Photosynthesis (Chapter 13) Deck (102):
What are photo autotrophs?
Green plants, photosynthetic prokaryotes and both single-called and many-called protoctists
They all depend on light energy to make glucose and ATP
What are chemoautotrophs?
Nitrifying bacteria which obtain energy from oxidising ammonia to nitrite (NO2-) or nitrite to nitrate (NO3-)
What is photosynthesis?
The fixation (trapping) of CO2 and its reduction to carbohydrate, using hydrogen from water
Where does photosynthesis take place?
Inside the chloroplasts
What is the overall equation of photosynthesis in green plants?
6CO2 + 6H2O --> C6H12O6 + 6O2
What is necessary for photosynthesis to take place?
Light energy and chlorophyll
What are the two reactions involved in photosynthesis?
Light dependent reaction
Light independent reaction
What is necessary for the light dependent reaction to take place?
1) suitable pigments that absorb certain wavelengths of light
2) light energy for photolysis of water into hydrogen + oxygen
What is oxygen in the photolysis of water?
A waste product
How is light energy indirectly needed in the light independent reaction?
To provide chemical energy, in the form of ATP, for the reduction of CO2 to carbohydrate
What are the two types of photosynthetic pigments involved in photosynthesis?
1) primary pigments
2) accessory pigments
How are photosynthetic pigments arranged?
What are photosystems?
Light harvesting clusters of pigments
How many types of photosystem are there?
2 (I + II)
Describe the structure of a photosystem
- several hundred accessory pigment molecules surround one primary pigment molecule
- each photosystem is arranged as a funnel-like structure
What happens to light energy in a photosystem?
It is absorbed by accessory pigments and passed onto the primary pigment
What are primary pigments?
Reaction centres that emit electrons
What are the two reactions that the light dependent reaction includes?
1) the splitting of H2O by photolysis to give H+
2) the synthesis of ATP in photophosphorylation
What happens in the light dependent reaction?
1) the H+ ions combine with NADP to make reduced NADP
2) ATP and reduced NADP are passed from the LDR to the LIR
- photophosphorylation of ADP to ATP can be cyclic or non-cyclic, depending on the pattern of electron flow in one or both types of photosystem
Which photosystem(s) does cyclic photophosphorylation involve?
Photosystem I (P1)
What happens during cyclic photophosphorylation?
1) light is absorbed by P1 and passed to the primary pigment (a chlorophyll molecule)
2) an electron in the chlorophyll molecule is excited to a higher energy level and emitted (photo activation)
3) the excited electron is captured by an electron acceptor and passed back to a chlorophyll molecule via a chain of electron acceptors
4) enough energy is released during this process to synthesise ATP from ADP + Pi by the process of chemiosmosis
5) the ATP is then passed onto the LIRs
Which photosystem(s) does non-cyclic photophosphorylation involve?
Both photosystems (P1 and P2) in the 'Z-scheme' of electron flow
What happens during non-cyclic photophosphorylation?
1) light is absorbed by P1 and P2 and excited electrons are emitted from the primary pigments of both reaction centres
2) these electrons are absorbed by electron acceptors and pass along chains of electron carriers, leaving the photosystems positively charged
3) the primary pigment of P1 absorbs the electrons from P2 and the primary pigment of P2 receives replacement electrons from the photolysis of water
4) ATP is synthesised as the electrons lose energy while passing along the carrier chain
Describe how the photolysis of water works
1) P2 contains a water-splitting enzyme that catalyses the breakdown of water: H2O --> 2H+ + 2e- + ½O2
2) the H+ ions combine with electrons from P1 and NADP to give reduced NADP: 2H+ + 2e- + NADP --> reduced NADP
3) reduced NADP passes to the LIRs and is used in the synthesis of carbohydrate
How can the photolysis of water be demonstrated?
By the Hill reaction
Describe the Hill reaction
1) Robert Hill showed that isolated chloroplasts had 'reducing power' and liberated O2 from H2O in the presence of an oxidising agent (electron acceptor) e.g. Fe3+ or DCPIP to replace NADP
2) the 'reducing power' was demonstrated as the redox agent changed colour on reduction e.g DCPIP goes from blue to colourless when reduced
3) oxidised DCPIP -- (H2O --> O2) --> reduced DCPIP
What is another name of the LIRs?
The Calvin cycle
What kind of process is he fixation of CO2
Light independent process
What happens during the Calvin cycle?
1) carbon fixation - CO2 combines with ribulose biphosphate, RuBP, (5C sugar) forming 2 molecules of glycerate-3-phosphate, GP, (3C sugar) in the presence of an enzyme called rubisco
2) GP, in the presence of ATP and reduced NADP from the LDR, is reduced to triose phosphate, TP, (3C sugar)
3) Most (5/6) of the TP is used to regenerate RuBP but 1/6 is used to produce other molecules e.g. hexose phosphates to starch/sucrose/cellulose OR glycerol+fatty acids/acetyl CoA/amino acids
How many carbons to the sugars involved in the LIRs contain?
RuBP - 5C
GP - 3C
TP - 3C
Where does the light independent reaction take place?
In the stroma
What is the stroma?
The ground substance (cytoplasm) of the chloroplast
Describe the system of inner membranes in the chloroplast
It consists of a series of flattened fluid-filled sacs (thylakoids), which in places form stacks (grana) that are joined to one another by membranes
Where does the light dependent reaction take place?
In the thylakoids
How are the membranes of the grana adapted for photosynthesis?
1) they provide a large SA, which holds the pigments, electron carriers and enzymes needed for the light dependent reaction
2) they make it possible for a large number of pigment molecules to be arranged so that they can absorb as much light as necessary in photosystems
3) they hold ATP synthase and are the site of ATP synthesis by chemiosomosis
What does the stroma contain? (6)
Enzymes of the Calvin cycle, sugars, organic acids, 70s ribosomes, loop of DNA (which codes for some chloroplast proteins) and lipid droplets
Why does the stroma bathe the grana membranes?
So it can receive the products of the LDR
What can the Hill reaction be used to investigate?
The effect of light intensity/wavelength on the rate of photosynthesis of a suspension of chloroplasts
What are the necessary factors for photosynthesis to take place? (4)
1) the presence of a suitable photosynthetic pigment
2) a supply of CO2
3) a supply of water
4) light energy
What are the main external factors affecting the rate of photosynthesis? (4)
1) light intensity
2) light wavelength
4) CO2 concentration
How does the rate of photosynthesis vary with light intensity/[CO2] at a constant temperature?
1) the rate initially increases as light intensity/[CO2] increases
2) however, at higher light intensities[CO2] the rate reaches a plateau
How does the rate of photosynthesis vary with temperature at high light intensity?
The rate increases as the temperature is increased over a limited range
How does the rate of photosynthesis vary with temperature at low light intensity?
Increasing the temperature has little effect on the rate (but still increases)
Are photochemical reactions generally affected by temperature?
What does an experiment of the effect of temperature on the rate of photosynthesis at different light intensity show?
1) there must be two sets of reactions in photosynthesis: a) light dependent photochemical stage b) light independent, temperature dependent stage
2) illustrates concept of limiting factors
What is the rate of any process limited by?
The slowest reaction
What happens if a process has more than one limiting factor?
The rate will be limited by the factor nearest its lowest value
What is the limiting factor at low light intensity?
What is the limiting factor at high light intensity?
How do you measure the rate of photosynthesis? (2 ways)
1) count the number of bubbles of gas (mostly oxygen) in a unit time from a cut stem of an aquatic plant e.g. Elodea
2) gas can be collected and the volume produced in a unit time measured
(rate of production of oxygen=rate of photosynthesis)
How do you alter light intensity?
Alter the distance of a small light source from a plant (light intensity is proportioned to 1/distance squared)
How do you alter the wavelength of light?
Use different colour filters (but same light intensity transmitted)
How do you alter [CO2]?
Add different quantities of sodium hydrogencarbonate (NaHCO3) to water surrounding a plant
How do you alter the temperature of water?
Use beakers to maintain chosen temperatures (thermostatically controlled water bath)
What is the aim of growing plants in protected environments?
To increase the yield of the crop concerned
How can greenhouses increase the yield of e.g. tomato plants?
1) sensors monitor light intensity/humidity of atmosphere/[CO2] around the plant
2) the plants grow hydroponically - with their roots in a nutrient solution whose nutrient content can be varied at different stages of the plants' growth
3) the factors are managed by a computer to maximise yield of crop
4) insect pests and fungal diseases are more easily controlled than is possible with field grown crops, further improving yield
What is different about C3 plants?
In their light independent stage of photosynthesis, CO2 combines with RuBP to form a 6C compound, which immediately splits into two 3C molecules
What are C4 plants? Give two examples of them
In the light independent stage of photosynthesis, the first compound produced is a 4C compound
e.g. maize and tropical grasses
What is the problem with rubisco?
- Rubisco also catalyses the reaction of RuBP and oxygen
- Therefore, when this happens less photosynthesis takes place because some of this RuBP is being 'wasted' in this unwanted reaction (photorespiration) and less is available to be combined with CO2
When does photorespiration occur?
Mostly at high temperature and high light intensity (i.e. low altitudes in the tropics)
How do C4 plants avoid photorespiration?
- They keep RuBP and rubisco away from high [O2]
- Cells containing RuBP and rubisco (bundle sheath cells) are arranged around the vascular bundles, therefore they have no direct contact with the air inside the leaf
- CO2 is absorbed by mesophyll cells which are in contact with air and contain PEP carboxylase enzyme
How are enzymes in C4 plants adapted to hot climates?
They generally have higher optimum temperatures than C3 plants
How to C4 produce sugars (/starch)?
1) PEP carboxylase catalyses the production of looooob in the nether regions to allow for more rapid thrusting.
How do C4 plants produce sugars (/starch)?
1) PEP carboxylase catalyses the reaction of CO2 from air with PEP (phosphoenolpyruvate) (3C) forming oxaloacetate (4C) in the mesophyll cells
2) still inside the mesophyll cells, the oxaloacetate is converted to malate (4C) and this is passed onto bundle sheath cells
3) here, the CO2 is removed from malate molecules and delivered to RuBP by rubisco in the normal way
4) the LIR proceeds as normal in the bundle sheath cells
What is the difference between different pigments in chloroplasts?
They absorb different wavelengths of light
What are the primary pigments in chloroplasts?
What are the accessory pigments in chloroplasts?
What are the two types of chlorophyll pigments?
What colour is chlorophyll a?
What colour is chlorophyll b?
What wavelengths of light do chlorophyll pigments absorb?
They absorb mainly in the red and blue-violet regions of the light spectrum and reflect green light (therefore plants look green)
What are the two types of carotenoid pigment?
What colour is beta carotene?
What colour is xanthophyll?
What wavelength of light do carotenoids absorb?
Mainly in the blue-violet region of the spectrum
What is an absorption spectrum?
A graph of the absorbante of different wavelengths of light
What is an action spectrum?
A graph of the rate of photosynthesis at different wavelengths of light
What does an action spectrum show?
The effectiveness of the different wavelengths, which is related to their absorbance and energy content
The shorter the wavelength, the greater the energy it contains
What happens if you illuminate a solution chlorophyll a/b with UV light?
1) the UV light is absorbed and electrons are excited
2) but, as the energy cannot be usefully passed on to do work, the electrons return to their unexcited state
3) the absorbed energy is passed transferred to the surroundings as thermal and light energy at a longer, less energetic wavelength than that which was absorbed and is seen as red fluorescence
- in functioning photosystems, it is this energy that drives photosynthesis
How can the different pigments present in a chloroplast be separated?
By paper chromatography
How do you calculate the Rf value?
Rf = distance travelled by pigment spot/distance travelled by solvent
In general, what are the Rf values of pigments?
Carotenoids ~ 1
Chlorophyll b=much lower
Chlorophyll a=in between carotenoids and chlorophyll b
How does paper chromatography work?
The solvent carries each pigment at a different speed, therefore they move different distances
Where is chlorophyll located?
In the thylakoid membranes
What are the uses of reduced NADP?
1) provides H+ to combine with CO2 to make carbohydrates
2) provides energy for the light independent reaction
What is the role of photolysis?
1) the H+ are used to make reduced NADP
2) electrons are used to replace the ones lost by PSII
What are electron carriers and what do they do?
- They are molecules found in the thylakoid membranes
- They can pick up an electron which has left chlorophyll and pass it to another molecule (electron carrier)
- When they pick up the electron they are reduced and when they lose it again they are oxidised
What is the role of accessory pigments?
To trap light energy
What are the products of the LDR?
1) oxygen (respired/diffused out of the plant)
2) ATP - used in LIR
3) reduced NADP - used in LIR
When does cyclic phosphorylation occur?
When there's a lack of ATP or too much reduced NADP
What is not produced from cyclic phosphorylation?
Reduced NADP, H+ or oxygen
Why are chloroplasts with only PS1 unable to form sugars?
1) Because PSII forms ATP ∴ no ATP is produced ∴ no hexose is produced
Why is there a need for cyclic photophosphoryalation?
Bc more ATP than reduced NADP is needed in the LIR
What happens to the level of GP in the dark?
Initially stays the same but then would increase because GP would not be changed into TP as there is no ATP or reduced NADP from the LDR
What happens to the level of TP in the dark?
Initially stays the same due to residual ATP/reduced NADP but then it would decrease
What happens to the level of RuBP in the dark?
Would eventually decrease as there is no ATP to turn TP back into RuBP
Why does the rate of photosynthesis increase as light intensity increases?
1) more energy for photoexcitation in PSI and II
2) ∴ more photophosphorylation will occur leading to greater formation of ATP and reduced NADP, used in the LIR
3) more photolysis (?)
Contrast the [CO2] in the air spaces of the leaf in night and day
- Night: high conc bc low rate of photosynthesis and high rate of respiration
- Day: low conc bc more photosynthesis taking place than respiration (which stays constant during night and day)
Adaptations of leaf for photosynthesis?
1) large SA:vol ratio ∴ light can penetrate
2) thin leaf and air spaces for efficient diffusion of CO2
3) transparent cuticle, allowing light to get to chloroplasts in upper epidermis
4) guard cells regulate the opening/closing of stomata
Describe the process of chemiosmosis in the electron carrier system in the LDR
1) In an electron carrier system, energy from electron transfer during redox reactions enables certain carriers to pump H+ from the stroma across the thylakoid membranes into the thylakoid space
2) As the [H+] increases on one side of the membrane, an electrochemical gradient called the proton motive force develops
3) H+ diffuse down the electrochemical gradient back across the membrane through an ATP synthase complex which provides energy for synthesising ATP from ADP+Pi
Why does the rate of photosynthesis decrease at high temperatures?
- At high temps, Rubisco denatures or reacts with oxygen instead of CO2 in photorespiration