F214 - Photosynthesis Flashcards
Heterotroph
Definition
Organisms that break down molecules from other organisms to release energy
They rely on other organisms
What is light energy used for in photosynthesis?
To produce complex organic molecules
Why does respiration in autotrophs and heterotrophs depend on photosynthesis?
Autotrophs release energy through respiration by breaking down complex organic molecules e.g. Glucose
Glucose has to be synthesised from simple molecules and only autotrophs can do this through photosynthesis
Where does photosynthesis take place?
Both stages of photosynthesis take place inside chloroplasts
Light Dependent takes place on the thylakoid membranes of chloroplasts in palisade mesophyll cells
Light Independent takes place in the stroma
Chloroplast Structure
Envelope
The double membrane which surrounds the chloroplast
There is an inter membrane space about 10-20nm
The outer membrane is permeable to many small ions
The inner membrane is less permeable and has transport proteins imbedded in it
Chloroplast Structure
Thylakoids
The inner membrane of the envelope is folded into thin plates called thylakoids
Chloroplast Structure
Granum
Thylakoids are stacked like pennies to form grana
Between the grana there are intergranal lamellae
Chloroplast Structure
Stroma
A fluid filled matrix containing the enzymes for light independent photosynthesis, starch grains, oil droplets, DNA and ribosomes
Adaptations of Chloroplasts
Many Grana
Large surface area of thylakoids membranes for photosynthetic pigments, electron carriers and ATP synthase enzymes
Adaptations of Chloroplasts
Photosystems
Photosynthetic pigments are arranged into photo systems to maximise absorption of light energy, they are held in place by proteins embedded in the grana
Adaptations of Chloroplasts
Stroma
Contains the enzymes needed to catalyse the light independent reactions
Surrounds the grana so that the products of the light dependent reactions can easily pass into the storms
Adaptations of Chloroplasts
DNA
Chloroplasts can make some of the proteins needed for photosynthesis using chloroplast DNA and the chloroplast ribosomes
Autotroph
Definition
Organisms that can synthesise complex organic molecules from simple inorganic molecules
Photosynthetic Pigment
Definition
Molecules that absorb light energy
Each pigment absorbs a range of wavelengths and has its own distinct peak of absorption
How is light harvested in the chloroplast membranes?
Pigments form structures called photo systems
Light it’s the photo system
Photons are absorbed by accessory pigments
This energy excites electrons in the pigment to a higher energy level
When the electrons return to their original state a photon is released
Energy is passed from one pigment to another towards the reaction centre
What is chlorophyll?
A mixture of pigments
Consists of a long phytol (hydrocarbon) chain and a porphyrin group
What happens when light hits chlorophyll?
A pair of electrons associated with the magnesium in the porphyrin group are excited
Chlorophyll in Photosystem I
Chlorophyll A - P700
Peak of absorption = 700nm
Chlorophyll in Photosystem II
Chlorophyll A - P680
Peak of absorption = 680nm
Accessory Pigments
Cartenoids reflect yellow and orange light and absorb blue light
They do not have a porphyrin group and are not directly involved in the light dependent reaction
They absorb wavelengths that are not well absorbed by chlorophyll a
Carotene (Orange) and xanthophyll (yellow) are the main cartenoid pigments
Non Cyclic Phosphorylation
Light energy excites a pair of electrons from photosystem II
They are accepted by an electron acceptor
They are passed along a series of electron carriers taking part in redox reactions ate decreasing energy levels
The energy release is used to bind ADP to a phosphate ion to form ATP
The energy the electrons have left excites a pair of electrons from PSI, they then take their place in Photosystem I
The two new electrons are passed along another redox chain to the enzyme NADP reductase
They are used with the 2H+ from photolysis to reduce NADP
Cyclic Phosphorylation
Two electrons are excited from PSI
They go to an electron acceptor
They are passed down a redox chain which releases energy
This energy is used to form ATP from ADP+P
The remaining energy excites a pair of electrons from PSI
The old electrons take the place if these newly excited electrons
The new electrons are passed back to the electron acceptor above PSII and then down the same redox chain again
What are the products of the light dependent reaction?
Reduced NADP
ATP
Oxygen (a waste product)
Light Independent Photosynthesis
The Calvin Cycle
Carbon fixation - carbon dioxide is combined with ribulose bisphosphate (RuBP) by the enzyme RUBISCO
This briefly forms a six carbon sugar which splits in to two molecules of glycerate phosphate (GP)
Reduced NADP and ATP are used to turn the GP into two triose phosphate molecules (TP)
This also released NADP and ADP+P which can be recycled in the light dependent stage
1/6 of the triose phosphate is used to make glucose
5/6 is used with ATP to regenerate RuBP
Photolysis
At Photosystem II some light energy is used to split water molecules
H2O -> 1/2O2 + 2e- + 2H+
The oxygen is released as a waste product
The hydrogen ions are used by the enzyme NADP reductase to reduce NADP
The two electrons replace the electrons that are excited form PSII
What is triose phosphate used to make?
1/6 of the TP goes towards synthesising:
Carbohydrates
Lipids
Amino acids
The other 5/6 is used to regenerate RuBP
What is chemiosmosis?
As electrons move along the electron transport chains, some of the energy released is use by coenzymes to pump protons (H+ ions) into the thylakoid space
This creates a proton concentration and electrochemical gradient and a pH gradient all of which causes protons to move back across the membrane through protein channels associated with ATP synthase
The flow of hydrogen ions is chemiosmosis and is necessary for the formation of ATP
How does carbon dioxide enter the leaf and reach the chloroplasts to take part in the Calvin Cycle?
Carbon dioxide from the air diffuses into the leaf through the open stomata
It then diffuses through the air spaces in the spongy mesophyll until it reaches the palisade mesophyll layer
It diffuses through the cellulose cell walls, the cell surface membrane, the cytoplasm and the chloroplast envelope into the stroma
What are the limiting factors of photosynthesis?
Temperature
Light Intensity
Carbon Dioxide Concentration
Water Availability
How does light intensity affect the rate of photosynthesis?
As light intensity increases the rate of photosynthesis increases
This is because light causes the stomata to open so that more carbon dioxide can diffuse into the leaf, it excites electrons and splits water molecules
How does temperature affect the rate of photosynthesis?
The photochemical reactions of the light dependent stage are not really affected by temperature
The enzyme catalysed reactions of the Calvin Cycle are effected
Between 0 and 25C photosynthesis rate approximately doubles for each 10C increase
Above 25C the rate falls as enzymes work less efficiently and oxygen can more successfully compete with carbon dioxide for the active site of RUBISCO
High temperatures also cause more water loss by transpiration causing the stomata to close in a stress response which limits carbon dioxide availability
How does carbon dioxide affect the rate of photosynthesis?
Growing forests can absorb carbon dioxide as their rate of photosynthesis is greater than their rate of respiration
Mature forests produce CO2 because the net respiration and decomposition is greater than the rate of photosynthesis
In greenhouses CO2 levels can be artificially increased by burning oil, enhanced levels of CO2 will increase the rate of photosynthesis providing there are no other limiting factors
Limiting Factor
Definition
A factor which prevents a metabolic process from taking l,ace at its maximum rate
Low Carbon Dioxide Levels
Affect on Concentrations of GP, TP, RuBP
There would be less CO2 to bind with RuBP so less GP would be formed, all the GP that was available would be turned in to TP
All the available TP would be used to make glucose or regenerate RuBP so it’s concentration would decrease as there would be no GP to make more from
RuBP would build up as there would be little/no CO2 for it to fix, concentration would increase until all of the TP had been used up then it would plateau
High Carbon Dioxide Levels
Affect on Concentrations of GP, TP, RuBP
GP concentration would increase, as the number of RuBP molecules combining with CO2 would increase, until all of the RUBISCO enzymes were working at a maximum rate then GP concentration would reach a maximum
TP concentration would increase as there would be more GP for it to be made from, it would increase until reduced NADP or ATP concentration became a limiting factor
More RuBP would be used to fix CO2 but more RuBP would be regenerated
Low Light Intensity
Affect on Concentration of GP, TP, RuBP
Rate of GP production would not be affected as the fixing of carbon by the enzyme RUBISCO is not light dependent
TP would decrease as there would be less reduced NADP an ATP to convert the GP into TP
GP would build up as a result
RuBP would decrease as once the available TP had been used up to regenerate it, the RuBP would continue to combine with CO2 but it wouldn’t be replaced
High Light Intensity
Affect on Concentration of GP, TP, RuBP
If light intensity was high then the availability of the products of light dependent photosynthesis would not be a problem
Providing there were no other limiting factors, photosynthesis would take place at a maximum rate
Methods for Measuring for the Rate of Photosynthesis
Volume of oxygen produced per unit time
Rate of uptake of carbon dioxide
Rate of increase in dry mass of plants
Respirometer
- one end of capillary tube stuck into test tube containing plant
- plant photosynthesises producing oxygen
- oxygen enters the capillary tube
- other end of capillary tube can be connected to a syringe, gas pressure pushes syringe and change in volume can be measured
- or distance moved by coloured dye in the tube can be measured
- volume of gas collected = length of bubble x πr²
What are the limitations of using a respirometer to measure the rate of photosynthesis of an aquatic plant?
- some of the oxygen produced in photosynthesis will be used by the plant in respiration
- there may be some dissolved nitrogen gas collected
- some oxygen may dissolve in the water so it won’t be measured
