Chapter 4: Photosynthesis

Question 1

photosynthesis

Answer 1

6CO2 + 6H2O = C6H12O6 + 6O2

1. photosynthesis begins with light absorbing pigments in plant cells; able to absorb energy from light; cholorophyll a, b, and carotenoids ( red, orange, yellow).
2. light is incorporate into electrons–excited electrons are unstable and re-emit absorbed energy
3. energy is then reabsorbed by electrons of nearby pigment molecules
4. process ends when energy is absorbed by one of the two special chlorophyll a molecules (p680 or p 700)
5. p700 forms pigment cluster (PS1) and p 680 forms pigment cluster (PS2).
6. Antenna pigments (chlorophyll b, carotenoids, phycobilins [red algae pigment], xanthophylls) capture wavelengths that chlorophyll a does not, passes energy to chlorophyll a where direct light rxn occurs. Chlorophyll a has porphyrin ring (alternating double and single bonds, double bonds critical for light rxns) completed w/Mg atom inside.

Question 2

noncyclic photorespiration (ADP + Pi + light = ATP)

Answer 2

1. Photosystem 2= electrons trapped by p680 in PS2 are energized by light
2. primary electron acceptor: two excited e- passed to primary e-acceptor, primary because it is the first in chain of acceptors
3. ETC: consists of a plastoquinone complex (PS2) which contains proteins like cytochrome and cofactor Fe2+; analogous to oxidative phosphorylation
4. phosphorylation: 2e- move down the chain=> lose energy (energy used to phosphorylate about 1.5 ATP)
5. PS1: ETC terminates with PS1 (p700); they are again energized by sunlight and passed on to another primary electron acceptor. From this point forward it can go to cyclic or noncyclic pathway …
   IF NONCYCLIC
6. NADPH: 2 electrons then pass down a short ETC (w/proteins like ferrodoxin) to combine NADP+ + H+ + 2e- => NADPH (coenzyme)
7. splitting of water (photolysis): loss of 2e- from PS2 (initially) is replaced when H2O splits into 2e-, 2 H+, and 1/2O2 that contributes to release as oxygen gas. Occurs at PS2
   [H2) + ADP + Pi + NADP+ + light => ATP + NADPH + O2 + H+
   ***note on photosystems: few hundred in each thylakoid, have a rxn center containing chlorophyll a surrounded by antenna pigments that funnel energy to it

Question 3

cyclic photophosphorylation

Answer 3

1. this replenishes ATP when Calvin cycle consumes it
2. when excited 2e- from PS1 join with protein carriers in the first ETC and generate 1ATP as they pass through; these 2e- are recycled into PS1 and can take either cyclic or noncyclic path

Question 4

calvin cycle

Answer 4

fixes CO2, repeat 6 times, uses 6CO2 to produce C6H12O6 (glucose)… C3 (dark reactions – light independent reactions)
1. carboxylation: 6CO2 + 6RuBP=> 12 PGA, RuBisCo (most common protein in the world, aka RuBP carboxylase) catalyzes this reaction
2. Reduction: 12 ATP + 12NADPH converts 12PGA => 12G3P or 12PGAL; energy is incorporated; by-products (NADP+ and ADP) go into noncyclic photophosphorylation
3. Regeneration: 6ATP convert 10G3P=>6RuBP (allows cycle to repeat)
4. Carbohydrate synthesis: 2 remaining G3P are used to build glucose
6CO2 + 18ATP+ 12NADPH + H+ => 18ADP + 18 Pi + 12NADP+ + 1 glucose (2G3P)
5. This is the “dark reaction”, bit it cannot occur w/out light because it is dependent on the high energy molecules produced from the light rxn (ATP and NADPH)

Question 5

Chloroplast:

Answer 5

light-dependent and light-independent reactions (double membrane like mito and nucleus)

1. outer membrane: plasma membrane (phospholipid bilyaer)
2. intermembrane space
3. inner membrane: also phospholipid bilayer
4. stroma: fluid material that fills area inside inner membrane; calvin cycle occurs here (fixing CO2=> G3P)
5. Thylakoids: suspended within stroma (stacks); individual membrane layers are thylakoids; entire stack is granum membrane of thylakoids contain (PS1 and PS2), cytochromes, and other e- carriers. Phospholipid bilayer
6. Thylakoid lumen: interior of the thylakoid; H+ accumulated here
   * ***thylakoid membrane absorbs light in chloroplast!!!

Question 6

chemiosmosis in chloroplast

Answer 6

uses H+ gradient to generate ATP

1. H+ accumulates inside thylakoids: H+ are released into lumen when H2O is split by PS2. H+ is also carried into the lumen from stroma by cytochrome between PS2 and PS1
2. A pH and electrical gradient is created: about pH 5
3. ATP synthase generates ATP: phosphorylate ADP + Pi => ATP (3H+ required for 1 ATP)
4. Calvin cycle produces 2G3P using NADPH & CO2 & ATP: at end of ETC follwoing PS1, 2 e- produces NADPH

Question 7

photorespiration

Answer 7

1. fixation of oxygen by rubisco (can also fix CO2)=> produces not ATP or sugar
2. rubisco is not efficient or fast because it will fix both CO2 and oxygen at the same time if both are present. Probably arose because early earth atmosphere didn’t have much O2 so it didn’t matter
3. PEROXISOMES BREAKDOWN THE PRODUCTS OF THIS PROCESS

Question 8

C4 photosynthesis

Answer 8

1. evolved from C3 when CO2 enters leaf; absorved by mesophyll cells (then moved to bundle sheath cells); instead of being fixed by rubisco into PGA, CO2 combines with PEP to from OAA by PEP carboxylase (in mesophyll)
2. OAA has 4C=>c4 photosynthesis
3. OAA=> malate and then transported through plasmodesmata into bundle sheath cell
4. malate=> pyruvate+CO2 (co2 can be used in calvin cycle) (pyruvate moved back to mesophyll then =>PEP)
5. overall purpose is to move CO2 from mesophyll to bundle sheath cell (little O2 presence reduces competition while rubisco is fixing); minimize photorespiration and H20 loss from stomata (leaf pores); found in hot, dry climates (faster fixation speed and more efficient)
6. require one additional ATP (which becomes AMP). C3 typically occurs in mesophyll cells, but in C4 it occurs in bundel-sheath cells
7. corn, sugarcane

Question 9

CAM photosynthesis

Answer 9

another add-on to C3 (crassulacean acid metabolism); identical to C4

1. PEP carboxylase fixes CO2 + PEP to OAA; OAA => malic acid
2. Malic acid is shuttled into vacuole of cell
3. At night, stomata are open (opposite of normal), PEP carboxylase is active, malic acid accumulates in vacuole.
4. During day, stomata are closed. malic acid is out of vacuole and converted back to OAA (require 1 ATP), releasing CO2 (moved onto Calvin Cycle with rubisco) and PEP
   * *overall advantages are can proceed during the day while stomata are closed (reduce H2O loss). Cacti, crassulacea plants