C1.3 photosynthesis Flashcards
word and chemical eqn for photosynthesis
Carbon dioxide + water > glucose + oxygen
(with light)
6CO2 + 6H2O + energy> C6H12O6 + 6O2
oxygen comes from the SPLITTING OF WATER in photosynthesis
Formula for calculating Rf values
Distance travelled by pigment/Distance travelled by solvent
How does light energy affect pigments
Photosynthetic pigments e.g. chlorophyll, absorb light energy. This excites the electrons within the pigment. The excited electron then leaves the pigment and provides energy for the synthesis of organic compounds from CO2 and H2O
Different pigments absorb different wavelengths.
Absorbtion spectrum:
- peak = absorb ___ wavelengths best
- trough = reflect ___ wavelengths
similarities and differences btwn graphs for absorbtion and action spectrum.
Similarities: Peaks are the same
Difference:
- rate of photosynthesis for green light in action spectrum is much higher than the absorbtion of green light in the absorbtion spectrum. This is due to accessory pigments still carrying out photosynthesis.
- action represents the rate of photosynthesis vs absorbtion represents the amount of light energy being absorbed by a photosynthetic pigment
Describe photosystems
- molecular arrays of chlorophyll and accessory pigments with a special chlorophyll as the reaction centre from which an excited electron is removed
- consists of a REACTION-CENTRE COMPLEX and LIGHT-HARVESTING COMPLEXES.
- the light-harvesting complexes, made of pigment molecules bound to proteins, transfer the energy of photons to the reaction centre
- a primary electron acceptor in the reaction centre accepts these excited electrons and is REDUCED as a result.
Photosystems are found in the:
- thylakoid membrane of photosynthetic eukaryotes
- cynobacteria
Where do light dependent/light independent reactions take place?
Light dependent - thylakoid membrane
Light independent - stroma
Advantages of photosystems as arrays of chlorophyll and accessory pigments
- single molecules of chlorophyll are not able to provide the excited electrons required for photosynthesis
- the array of different light-absorbing pigments maximises the use of light energy
- the photosystem array facilitates the transfer of energy to the central chlorophyll molecule, resulting in the emission of high energy electrons
Describe the light-dependent reactions
Non-cyclic photophosphorylation:
- photoactivation of photosystem II: light energy excites electrons, causing them to leave chlorophyll and enter the ETC
- the photolysis of water replaces the electrons lost by photoactivation
- the excited electrons released by PSII move through the ETC to PSI, the movement of electrons across the ETC provides energy to actively transport protons to the thyakoid space.
- ATP is produced by chemiosmosis.
- PSI is photoactivated and reduces NADP+ to form NADPH using the excited electrons.
- end product: NADPH, ATP
Cyclic photophosphorylation:
- Photoactivation of PSI: light energy excites the electrons, leaving the chlorophyll and entering the ETC
- the excited electrons released from PSI move through ETC and return to PSI. The movement of electrons through ETC provides the energy to actively transport protons into the thylakoid space.
- ATP is produced by chemiosmosis
- end product: ATP
Describe chemiosmosis
Protons build up within the thylakoid space (for photosynthesis)/intermembrane space (for aerobic respiration). Protons move through ATP synthase by chemiosmosis (facilitated diffusion - moving from a higher conc to longer conc), providing enough activation energy for ATP synthase to convert ADP and an inorganic phosphate to ATP.
Reactions occuring in/on the thylakoid + byproducts
Photolysis of water (occurs WITHIN THYLAKOID SPACE) - electrons of H hydrogen incorporated into sugar molecules, O2 released as byproduct
Non-cyclic photophosphorylation: requires photolysis of water, generates ATP from ADP and PSI reduces NADP+ to NADPH
Cyclic photophosphorylation: generates ATP from ADP
The photophosphorylation of ADP and the reduction of NADP+ occurs in the STROMA of the chloroplast
When does cyclic/non cyclic photophosphorylation occur?
non cyclic photophosphorylation:
- When ATP is produced using energy from excited electrons flowing from PSII to PSI to NADP+ to form NADPH
cyclic photophosphorylation:
- When light is bright and not a limiting factor
- As light-independent reactions may occur more slowly than light-dependent reactions, NADP+ may run out
- As there is no acceptor to take them, electrons from PS1 rejoin the ETC and are used to generate more ATP
State the stages of the Calvin cycle and its site
- Carboxylation of RuBP (carbon fixation)
- Reduction of glycerate-3-phosphate
- Synthesis of glucose
- Regeneration of RuBP
Occurs in the STROMA
Describe the Calvin cycle
Carbon fixation/carboxyltion of RuBP:
1. RuBP, a 5-carbon compound, reacts with CO2 from the air to form an unstable 6-carbon compound. This reaction is catalysed by RUBISCO.
2. the unstable 6-carbon compound breaks down into glycerate-3-phosphate
Reduction of glycerate-3 phosphate:
- glycerate-3-phosphate is reduced by gaining electrons and hydrogen from NADPH to form TRIOSE PHOSPHATE. The energy to do this is provided by ATP, which is converted to ADP and a phosphate.
- NADPH is oxidised to NADP+ and returns to the light dependent reactions
basically:
reactants: 2 ATP, 2 NADPH, 2 glycerate-3-phosphate
products: 2 ADP, 2 phosphate, 2 NADP, 2 triose phosphate
Regeneration of RuBP:
- 5 molecules of triose phosphate are converted to 3 molecules of RuBP, allowing the calvin cycle to continue. ATP provides energy, being converted to ADP and a phosphate.
- The other molecule of triose phosphate also know as GLYCERALDEHYDE-3-PHOSPHATE (G3P) is combined with other molecules of triose phosphate to make glucose = 6 turns of the calvin cycle gives 1 glucose + 6 RuBP
Properties of Rubisco
Rubisco is the most abundant enzyme on earth
- it is needed in large quantities in stroma of chloroplasts as it works relatively slowly
- is not effective in low CO2 conc
Describe the synthesis of other carbon compounds using the products of the Calvin Cycle
Carbohydrates (e.g. glucose/sucrose/starch/cellulose):
- Glucose is usualy converted to sucrose for transport across the plant
- If there is excess glucose, glucose is converted to starch and stored temporarily in the chloroplasts. At night, when photosynthesis stops, the starch is broken down and carbohydrate exported from the leaf.
- large quantities of glucose needed for cell respiration and synthesis of cellulose
Lipids:
- Chloroplasts can convert triose phosphate from the Calvin cycle to fatty acids. Enzymes in glycolysis and the link reaction used to produce AcetylCoenzymeA, and linking together 2-carbon acetyl groups
- Glycerol can also be made from triose phosphate and linked together with fatty acids to form triglycerides. Droplets of stored oil are often visible in chloroplasts
Mineral nutrients/amino acids
- many other carbon compounds can also be produced from glycerate-3-phosphate or triose phosphate.
- mineral nutrients such as phosphate and sulfate are also needed to make compounds containing elements under than carbon, hydrogen, or oxygen
- all 20 amino acids can also be synthesised in photosynthesising organisms using branching metabolic pathways.
