Topic 5 A: Photosynthesis Flashcards
Simple definition of photosynthesis?
Process where energy from light is used to make glucose from water and carbon dioxide.
Why is photosynthesis considered a metabolic pathway?
Contains lots of small reactions controlled by enzymes.
Equation for photosynthesis?
6CO2 + 6H2O = C6H12O6 + 6O2
carbon dioxide + water = glucose + oxygen
Can plants simply use the glucose?
No, they must turn it into ATP first. Which acts as an immediate source of energy.
ATP and phosphorylation?
- adding a phosphate group
- ADP + Pi = ATP (condensation reaction)
- ATP synthase catalyses reaction
- energy stored in phosphate bond
ATP and hydrolysis?
- breaking down ATP
- one phosphate bond broken
- ATP hydrolase catalyses reaction
- small amount of energy released from bond
Properties of ATP? (5)
- releases a small amount of energy, so none wasted.
- easily broken down as one bond is broken.
- ATP can’t pass out of cells so they will always have energy.
- it can make other molecules more reactive by adding a phosphate.
- its small and soluble so easily transported around the cell.
Exam Question: Humans synthesise more than their body mass of ATP each day. Explain why it is necessary to synthesise such large amounts of ATP? (3)
- ATP cannot be stored/ immediate energy source
- ATP only releases small amount of energy
What is the thylakoid?
folded membranes provide large surface area.
photosynthetic proteins (chlorophyll) and electron carrier proteins are embedded.
What is the stroma?
fluid-filled matrix which is filled with enzymes.
What are photosystems?
Pigments that are attached to the proteins in the thylakoid membranes.
What are the two different photosystems?
- PSI - has primary pigment that absorbs light best at wavelength of 700nm.
- PSII - has primary pigment that absorbs light best at wavelength 680nm.
What kind of reaction occurs in photosynthesis?
Redox reactions.
OILRIG?
Oxidation is loss (of electrons)
Reduction is gain (of electrons)
All about oxidation?
- losing electrons and hydrogen
- gaining oxygen
- exothermic as energy is given out to surroundings
All about reduction?
- gaining electrons and hydrogen
- losing oxygen
- endothermic as energy is taken in from surroundings
4 steps of light dependent reaction?
- photoionisation
- photolysis
- ATP and reduced NAPD
- chemiosmosis
Photoionisation simple definition?
Electrons being lost from cholorphyll using light energy.
- Photoionisation process? Part one
- light energy is absorbed by chlorophyll in PSII - this excited electrons within chlorophyll.
- as excited electrons gain more energy, they leave the chlorophyll molecule - chlorophyll has been ionised.
- high energy electrons released and move down the electron transport chain to PSI by electron carriers.
- Photoionisation process? Part two
- electrons lose energy at each stage - electrons passed along a number of electron carriers - each one at a slightly lower energy level.
- electron carriers are reduced and chlorophyll is oxidised.
Light energy is used for?
- ATP
- Reduced NADP
- Splitting water
Photolysis simple definition?
Splitting water into H+ ions, electrons and oxygen.
- Photolysis process?
- any electrons that left PSII (due to too much energy) are replaced by electrons from the photolysis of water.
- H+ ions/protons are picked up by NADP (co-enzyme) which forms NADPH
- oxygen is a waste product - can be used in respiration or diffuses out of leaf through stomata.
What does PSII contain that aids the splitting of water?
Water splitting enzyme called oxygen evolving complex.
Splitting of water equation?
H20 = 1/2 O2 + 2e- + 2H+
- Reduced NADP simple definition and function?
Excited electrons and h+ ions combine with NADP.
- provides H/electrons for GP or reduces GP to TP
Equation for reduced NADP?
2H+ + 2e- + NADP = NADPH
Role of a co-enzyme?
Transfer a chemical group from one molecule to another.
E.g., transferring hydrogen to NADP to form NADPH.
- Chemiosmosis simple definition?
Electrons create proton gradient to drive ATP synthesis.
- Chemiosmosis process?
- electrons pass through transport chain and release energy
- active transport of H+ ions/protons from stroma to thylakoid lumen
- electrochemical gradient created
- facilitated diffusion of H+ ions/protons from thylakoid lumen to stroma
- happens through enzyme ATP synthase
- ATP synthase also helps catalysing ADP+Pi = ATP
- proton movement through ATP synthase causes it to change shape to catalyse reaction
Exam question: Crops use light energy to produce photosynthetic products. Describe how crop plants use light energy during the light-dependent reaction? (5)
- light energy excites electrons
- electrons move along electron transfer chain, releasing energy
- energy used to ADP+Pi=ATP
- photolysis of water produces protons, electrons and oxygen
- NADP reduced by electrons (NADPH)
What is non cyclic photophosphorylation?
- type one
- both photosystems used
- three products: ATP, reduced NADP, and oxygen
What is cyclic photophosphorylation?
- type two
- only involves photosystem one
- only product: ATP in small amounts
What do ATP and reduced NADP produce?
Complex organic substances e.g., starch, sugar, and cellulose.
What doesn’t the light independent reaction require?
Light
Where does the LIR occur?
Stroma
LIR steps?
- carbon fixation
- reduction of GP
- regeneration of RuBP
Carbon fixation process?
‘the removal of co2 from the environment to become part of the plant’
- co2 enters leafe through stomata and diffuses into stroma
-co2 reacts with ruBP (ribulose biphosphate) which is catalysed by rubisco
-this creates an unstable 6C compound so splits into 2 GP molecules (3C)
Reduction of GP (glycerate 3-phosphate) process?
‘the gaining of hydrogen by GP and the reoxidation of NADP’
- 2 ATP hydrolysed to provide energy for conversion of GP into TP (triose phosphate)
- conversion also requires H+ ions, hydrogen in 2 reduced NADP is picked up by GP molecule (reduced), and NADP is being re-oxidised
- 2 re-oxidised NADP goes back to LDR to be reduced again
Regeneration of RuBP process?
‘to ensure RuBP is recycled to be used for another cycle’
- one of the 6 carbon atoms ( from two 3C TP ) is removed from the cycle each time it happens. the cycle needs to turn 6 times to create a hexose sugar
(hexose sugar creates useful organis substances e.g, lipids and amino acids)
- the remaining 5c used to regenerate RuBP, also requires ATP energy.
The maths within the regeneration of RuBP?
- 3 turns of the cycle produces 6 molecules of TP because two molecules of TP are made for every one co2 molecule used.
- 6 turns of the cycle needs 18 ATP (3 for each cycle)
- 6 turns of the cycle needs 12 NADPH (2 in each cycle)
Overall products used in regeneration of RuBP?
1 co2
3 ATP
2 NADPH
2 TP
1/6 of hexose sugar
Why cant LIR happen in darkness? (2)
Requires ATP and reduced NADP from LDR, and inputs would run out without light
LDR products needed by LIR
ATP - hydrolysis provides energy for reaction
NADPH - donates a hydrogen atom to reduce GP
C02 - from atmosphere
What is LIR also known as?
The calvin cycle
Exam question: During the light-independent reaction of photosynthesis, carbon dioxide is converted into organic substances. Describe how. (6)
- co2 combines with RuBP
- produces two GP
- GP reduced to TP
- using reduced NADP
- using energy from ATP
- TP converted to glucose
Light intensity affecting rate of photosynthesis?
- at lower light intensities, the increase in the rate is linear.
- reaches saturation point and then another factor becomes the limiting factor
- after, the graph levels off and rate is constant
- greater light intensity = more energy supplied = faster LDR
- produces more ATP & reduced NADP for the calvin cycle, so faster LDR
Agricultural use of light?
- aim to prevent light becoming limiting factor
- e.g., using glass materials and lamps
Carbon dioxide affecting rate of photosynthesis?
- at lower concentrations the increase in rate is linear
- reaches saturation point and then another factor becomes limiting factor
- after, graph levels off and rate is constant
- CO2 is one of the reactants entering calvin cycle, the more CO2 present = faster photosynthesis
Agricultural use of co2?
- aim to ensure co2 isnt limiting factor
- e.g., burning prophane in generator produces more co2
Temperature affecting rate of photosynthesis?
- rate increases as temperature increases due to number of collisions between substrates and enzymes increasing
- reaches optimum temperature and then enzymes begin to denature and rate decreases
- calvin cycle affected by temperature as LIR is enzyme-controlled e.g., rubisco
- temperature has no significant effect on LDR as its driven by light energy.
Agriculture usage of temperature?
- aim to manipulate temperature
- e.g., heaters, coolers, and air circulation
