Chapter 8 Flashcards
Photosynthesis process
6 CO2 + 12 H2O + (Light energy) = C6H12O6 + 6 H20 + 6 O2
Oxygenic Photosynthesis occurs in
Most plants (chloroplasts), almost all algae, cyanobacteria
Thylakoid
small discs, Stacked grana
Thylakoid membrane
Internal membrane
Contains chlorophyll and other photosynthetic pigments
Pigments clustered into photosystems
Grana
Stacks of flattened sacs of thylakoid membrane
Stroma
Semiliquid surrounding thylakoid membranes
Stoma
gas exchange
Mesophyll
Where photosynthesis occurs
Chloroplast
Organelle holding thylakoid
Light dependent reactions VS Carbon fixation reactions (Light independent reactions)
Light - Requires light. Capture energy from sunlight and makes ATP and NADPH (reduced NADP+)
Start: Sunlight, Water, NADP+ END: O2, NADPH, ATP
Dark/Carbon fixation - Does not require light. Use ATP and NADPH to synthesize organic molecules from CO2
Start: NADPH, ATP, CO2. END: Organic molecules (particularly sugars)
Pigments
Molecules that absorb light energy in the visible light range
Photon and wavelength
Particle of light. Acts as discrete bundle of energy
Energy content inverse proportional to wavelength of light. More energy=smaller wavelengths
Less energy = larger wavelengths
Photoelectric effect
Removal of an electron from a molecule by light
Electro Magnetic Spectrum
Light is for of electromagnetic energy.
Visible light 400-700 (or 740) nm
Absorption spectrum
Range and efficiency of photons a molecule is capable of absorbing
When photon strikes a molecule its energy either:
1 lost as heat
2 Absorbed by the electrons of the molecule, boosts electrons into higher energy level (how photosynthesis begins)
Two general pigments used in green plant photosynthesis
Chlorophylls
Carotenoids
Chlorophyll a
Main pigment in plants and cyanobacteria. Only pigment that can act directly to convert light energy to chemical energy. Absorbs violet-blue to red light
Chlorophyll b
Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb. Blue to light blue and yellows and oranges
Structure of chlorophyll
Porphyrin ring (complex ring structure with alternating double and single bonds). Magnesium ion at the center of ring.
Photons excite electrons in the ring, shuttled away from the ring
Action Spectrum
Relative effectiveness of different wavelengths of light in promoting photosynthesis. Corresponds with absorption spectrum for chlorophylls
Carotenoids (tetraterpenoids)
Carbon rings linked to chains with alternation single and double bonds.
Can absorb photons with a wide range of energies. Also scavenge free radicals- antioxidants
Can capture UV and prevent damage.
Typically orange pigments that aren’t seen because of green
Phycobiloproteins
Important in low-light ocean areas-algae
In cyanobacteria, absorb green light not absorbed by green algae at surface
Antenna complex and Reaction center
Antenna complex - Hundreds of accessory pigment molecules (Carotenoids and Chlorophyll b). Gather photons and feed the captured light energy to the reaction center. Light harvesting complex
Reaction Center - 1 or more chlorophyll a molecules. Passes excited electrons out of the photosystem
Excitation energy
(In antenna complex) Energy is transferred from one molecule to another until it encounters the reaction center (chlorophyll a) Now electron transfer is initiated
Reaction Center
Transmembrane protein - pigment complex.
When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level. Light energized electron can be transferred to the primary electron acceptor (reducing it). Oxidized chlorophyll then fills its electron “hole” by receiving an electron from a donor molecule (Oxidizing the donor).
Light dependent reactions (4 stages)
Primary photoevent - photon of light is captured by a pigment molecule
Charge separation - Energy is transferred to the reaction center
Electron transport - electron move through carriers to reduce NADP+
Chemiosmosis - Produces ATP
Two types of light dependent reactions
Cyclic photophosphorylation - Utilizes one photosystem. Does not generate oxygen (Sulfur bacteria). Excited electrons goes to electron transport chain=ATP production
Non Cyclic photophosphorylation - Green plants produces oxygen. Uses two photosystems that work together (NOT a circle). Produces ATP and NADPH for use in carbon fixation
2 photosystems noncyclic
Photosystem I (P700) transfers electrons ultimately to NADP+, producing NADPH. Electrons lost from photosystem I are replaced by electrons from photosystem II
Photosystem II (P680) oxidizes water to replace the electrons transferred to photosystem I.
Connected by electron transport chain (cytochrome/ b6-f complex)
Ferredoxin
Fd = electron acceptor in step 3
Non cyclic photophosphorylation 4 stages
1 photosystem II absorbs photons, exciting electrons are passed to ETC. Electrons lost are replaced by the oxidation of water, producing O2 (waste)
2 ETC pumps protons into thylakoid creates gradient
3 PS I absorbs photons, excited electrons passed to NADP+ to make NADPH. Lost electrons replaced from ETC.
4 ATP Synthase uses the proton gradient to make ATP. Chloroplast has ATP synthase enzymes in the thylakoid membrane, allows protons back into stroma
Carbon fixation - Calvin Cycle (Dark reactions) [C3 photosynthesis]
Build carbohydrates in stroma cells use. 1 Energy (ATP) from light dependent reactions. Cyclic and noncyclic photophosphorylation. Drives endergonic reactions. 2 Reduction potential - NADPH from photosystem I, source of protons and energetic electrons
Key step is attachment of CO2 to RuBP (ribulose 1, 5 bisphosphate) to form PGA. Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco
3 phases of Calvin Cycle
1 Carbon fixation - RuBP + CO2 = PGA
2 Reduction - PGA is reduced to G3P (Glyceraldehyde 3 phosphate)
3 Regeneration of RuBP - PGA regenerates RuBP
Three turns incorporate enough carbon to produce a new G3P (3 carbons). Six turns incorporate enough carbon for 1 glucose (or 2 G3P)
Output: Glucose NOT a direct product of Calvin Cycle. G3P is direct output. Used to form sucrose and starch.
Photorespiration
Rubisco has 2 enzymatic activities - carboxylation (Addition of CO2 to RuBP) under normal conditions
Photorespiration - Oxidation of RuBP by the addition of O2. Favored when stoma are closed in hot conditions. Creates low CO2 and high O2.
