Photosynthesis Flashcards
How significant Is photosynthesis
Photosynthesis is the largest biosynthetic activity on the plant
160 billion t of carbohydrate / yr
- Equivalent to 60 trillion Campbell textbook copies
- Would be 17 stacks of books from Earth to the Sun
Other very significant outcomes:
- Produces (almost) all organic material required by consumers
- Produces oxygen (O2) required by animal life-forms
What is the general formula for photosynthesis
Water + Carbon dioxide + light → Sugar + oxygen
6H2O + 6CO2 + light → C6H12O6 + 6O2
what is the chemical basis of photosynthesis
energy source = sun provides electromagnetic energy
Energy is stored in chemical bonds within carbohydrates
Photosynthesis is the transfer of light energy into chemical energy
Is a redox reaction: Water is oxidized into Oxygen. Carbon dioxide is reduced into carbohydrate.
why is the photosynthesis formula expanded with more water
Photosynthesis involves splitting water, which is independent of CO2 fixation.
12H2O + 6CO2 → C6H12O6 + 6O2 + 6H2O
formula has to expanded with 6 more water molecules, as oxygen molecules from the CO2 goes into both the glucose and the water molecules.
What are the three steps of photosynthesis and where do they occur
Three steps of photosynthesis and subcellular location
- Photochemistry (light reaction)
Pigments transfer radiant energy to chemical bonds
- Production of NADPH and ATP (light reaction)
Enzymes (proteins transfer chemical energy)
- Incorporation of CO2 into carbohydrate CH2O (dark reaction/Calvin cycle)
Enzymes (proteins transfer chemical energy)
Describe light
Light is a form of electromagnetic radiation
Characterized by its energy (related to wave length)
Short wavelengths like Gamma rays and X-rays have high energy levels
Long wavelengths like Radio waves and Microwaves have low energy levels.
Between 380-750 nm is the visible light spectrum, and near this level is what is mostly used by organisms. For vision, photosynthesis, etc)
What are the light absorbing pigments used in photosynthesis
When light meets matter it can be reflected, transmitted, or absorbed
Light captured by plants:
Three classes of light-absorbing compounds: pigments
- Chlorophylls (chl): chl a and chl b
- Carotenoids
- Phycobilins (only found in algae and cyanobacteria) which have chl a and chl c but not chl b.
Pigments absorb certain wavelengths of the visible light (and reflect or transmit the rest)
Why is the world green
When light meets matter it can be reflected, transmitted, or absorbed
Light captured by plants:
Three classes of light-absorbing compounds: pigments
- Chlorophylls (chl): chl a and chl b
- Carotenoids
- Phycobilins (only found in algae and cyanobacteria) which have chl a and chl c but not chl b.
Pigments absorb certain wavelengths of the visible light (and reflect or transmit the rest)
What is the absorption and action spectrum
Absorption spectrum: absorbance response of a pigment exposed to a series of wavelengths of light
Action spectrum: physiological response of the organisms to specific wavelengths. Can be measured by Oxygen release to determine rate of photosynthesis.
what is the difference between chlorophyll a and b
Chlorophyll a have a CH3 group attached to a specific carbon, where chlorophyll b has a CHO group.
what is the Porphyrin ring
Porphyrin ring: Light absorbing “head” of molecule; with magnesium at centre.
Why does chlorophyll have a bunch of conjugated double bonds
Conjugated double bonds: Alternates single bond, double bond. Allows electrons to flow around ring.
what kind of tail does chlorophyll have
Chlorophyll has a Hydrocarbon tail that interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown.
what happens when a photon hits chlorophyll
A photon hitting a chlorophyll molecule causes an electron to enter an excited state. Then it will release heat and/or photon (fluorescence) returning to its ground state.
What happens to chlorophyll under UV light
Chlorophyll in a solution under white light is green. Under UV light it is red, since the UV light is a different wavelength and causes fluorescence to be a different wavelength.
describe photosystems
- The pigments are linked together in ‘light harvesting complexes’chl a, chl b, and carotenoids transfer energy (not electrons)!, embedded in protein complex (20+ subunits)
- These are connected to ‘reaction centres’transfer of electrons to primary electron acceptor
Light harvesting complexes + reaction centre = photosystem
How is excitation energy exploited in photosystems (photosystem 2)
How is this excitation energy exploited in photosystems?
- Light excites pigment in PSII → excited electron → as this electron falls to ground state another electron in nearby pigment is excited → finally electron in pigment P680 is excited.
- Transfer of electron from chlorophyll a pair (P680) to primary electron acceptor (now P680+ missing e-):
- e- hole in P680+ must be filled: P680+ is the strongest biological oxidizing agent known → it pulls e- from water.
What are the two photosystems in the thylakoid membrane and how are they connected
In thylakoid membrane: two photosystems:
PS I with P700 and PS II with P680!
Electron transport chain transfers from Photosystem II to Photosystem I.
Made from Pq - Cytochrome complex - Pc. ATP synthesized
What happens in photosystem 1
In PS I same process as in PS II happens, electron gets transferred to primary acceptor.
After that a second electron transport chain (Fd) moves to NADP+ reductase, where NADP+ + H+ → NADPH
What is the aim of the light reaction
- Conversion of light energy to H+ gradient
- H+ (pH) gradient used for ATP synthesis
What does the light reaction do
water is split in Thylakoid space. electrons transferred through photosystems I, Pq, Cytochrome complex, Pc, photosystem I, Rd, NADP+ reductase
Allows 4 H+ to enter thylakoid space, then it + 2H+ from water pass through ATP synthase across membrane to allow ADP + Pi → ATP
ATP and NADPH go into Calvin Cycle
What can a plant do when the lights turn off
What happens when the lights go out
- All light reactions stop (photochemistry, electron transport, splitting of water, O2 production, H+ gradient generation)
However: CO2 absorption and Sugar production continues as long as substrates are available. Called the dark reactions or Calvin Cycle
Incorporation of CO2, into carbohydrate → Enzymes (proteins) transfer chemical energy (using ATP and NADPH from light reactions)
What are the three steps of the dark reaction
Carbon fixation
Calvin Cycle
Regeneration
describe carbon fixation in the dark reaction
- Step: carbon fixation
3CO2 + 3 Ribulose bisphosphate (RuBP) → 3 C6 → 6 C3 (3-phosphoglycerate)
Uses Rubisco (Ribulose bis phosphate carboxylase/oxygenase)
Describe the Calvin Cycle
6 3-phosphoglycerate + 6ATP → 1,3-Bisphosphoglycerate (6 C3) + 6 ADP
1,3-Bisphosphoglycerate (6 C3) + 6 NADPH → Glyceraldehyde-3-phosphate (G3P) (6 C3) + 6NADP+ + 6Pi
Glyceraldehyde-3-phosphate (G3P) (6 C3) → G3 (output) + 5 * C3 (goes into regeneration)
Describe regeneration in the dark reaction
5 G3P + 3 ATP → 3 ADP + 3 Ribulose bisphosphate (RuBP) (3 C5) → back into carbon fixation
what is G3P
G3P is the started for all biological organic matter
Becoming proteins, lipids, carbohydrates, DNA/RNA, hormones, secondary metabolites, etc
Summarize photosynthesis
Light rxn with PS II and PS I and e- transport chanins
water + light + ADP + NADP → Oxygen + ATP + NADPH
Calvin reaction
ATP + NADPH + carbon dioxide → Sugar + ADP + NADP
