Topic 8.2: Photosynthesis

Question 1

What are phtosystems?(2)

Answer 1

collections of chlorophyll molecules; act a light-gathering “antenna complex”

Question 2

Where are the photosystems located?

Answer 2

in the thylakoid membrane

Question 3

What are the characteristics of photosystem II? (2)

Answer 3

chlorophyll a is the most dominant pigment; P₆₈₀ absorbs 680 nm wavelength of red light

Question 4

What are the characteristics of photosystem I?(2)

Answer 4

chlorophyll b is the most dominant pigment; P₇₀₀ absorbs 700 nm wavelength of red light

Question 5

What is the electron transport chain of photosynthesis?(9)

Answer 5

uses light energy, ATP, and NADPH; NADPH (stored) energy goes to Calvin cycle; photosystem II absorbs light; excited electron passes from chlorophyll to “primary electron acceptor”; need to replace electron in chlorophyll; enzyme extracts electrons from H₂O and supplies them to chlorophyll; to do this the enzyme spilts the H₂O; oxygen combines with another oxygen to form O₂; O₂ is released

Question 6

How does the light reactions elevate electrons?(2)

Answer 6

photosystem II generates energy as ATP; photosystem I generates reducing powers as NADPH

Question 7

What is cyclic photophosphorylation?(3)

Answer 7

if photosystem I can’t pass electron to NADPH, it cycles back to photosystem II and makes more ATP, but no NADPH; coordinates light reactions to Calvin cycle; Calvin cycle uses more ATP than NADPH

Question 8

What is the Calcin cycle?(5)

Answer 8

occurs in the stroma of the chloroplast; needs ATP and NADPH; first phase is fixation where Rubisco (enzyme) fixes CO₂ to an existing 5-Carbon Sugar (RuBP), splitting it into 2 3-Carbon Sugars (PGA); the next phase is reduction where ATP donates a phosphate to PGA making it G3P and ATP becomes ADP, then NADPH trades that phosphate for an electron and the carbohydrate is reduced; the final phase is where only one PGAL leaves to be further modified and the rest are recharged with an additional phosphate and then rearranged into 3 RuBP

Question 9

What is the gas exchange in the stomates?(3)

Answer 9

CO₂ in for the Calvin Cycle; O₂ out from the light reactions; H₂O out for the light reactions

Question 10

How do stomates control water loss from leaves?(4)

Answer 10

stomates close to conserve water; guard cells; if gaining water the stomates open; if losing water the stomates close; adaptation to living on land

Question 11

What is the problem with closed stomates?(3)

Answer 11

O₂ builds up from light reaction; CO₂ is depleted in Calvin Cycle; causes problems in the Calvin cycle

Question 12

What is the problem in the Calvin Cycle when the stomates are closed?(2)

Answer 12

Rubisco normally binds carbon to RuBP which leads to the reduction of RuBP and the building of sugars in photosynthesis; when O2 concentration is high the Rubisco bonds oxygen to RuBP which leads to the oxidation of RuBP and the breakdown of sugars in photorespiration

Question 13

What is the impact of photorespiration?(7)

Answer 13

oxidation of RuBP; short circuit of Calvin cycle; loss of carbons to CO₂; decreases photosynthetic output by siphoning off carbons; no ATP (energy) produced; no C₆H₁₂O₆ (food) produced; if photosynthesis could be reduced, plant would become 50% more efficient; strong selection pressure

Question 14

How do you reduce photorespiration?

Answer 14

separate carbon fixation from the Calvin cycle

Question 15

What are the characteristics of C4 plants?(6)

Answer 15

physically separate carbon fixation from Calvin cycle; different enzyme to capture CO2 in PEP carboxylase which stores carbon in 4-Carbon compounds; different leaf structure; adaption to hot, dry climates; have to close stomates a lot; sugar cane, corn, other grasses

Question 16

What are the characteristics of CAM plants?(8)

Answer 16

separate carbon fixation from Calvin cycle by time of day; open stomates fix carbon (capture CO₂) during night; store carbon in organic acids such as malic and isocitric acid; perform Calvin cycle during day; different adaptation to hot, dry climates; succulents, some cacti, pineapple; in day close stomates and release CO₂ from storage compounds to Calvin cycle; increases concentration of CO₂ in cells

Question 17

What are the characteristics of PEP carboxylase?(3)

Answer 17

higher affinity for CO₂ than O₂ therefore better than Rubisco; fixes CO₂ in 4-Carbon compounds; regenerates CO₂ in inner cells for Rubisco

Question 18

What are the parts of the C3 plant cell?(7)

Answer 18

upper epidermis; palisade mesophyll cell; vein; bundle sheath cell; spongy mesophyll cell; lower epidermis; stroma

Question 19

What are the parts of a C4 plant cell?(6)

Answer 19

upper epidermis; mesophyll cell; vein; bundle sheath cell; lower epidermis; stroma

Question 20

What occurs in the outer cells during C4 photosynthesis?(5)

Answer 20

light reaction; carbon fixation; punps CO₂ to inner cells; keeps O₂ away from inner cells; keeps O₂ away from Rubisco

Question 21

What occurs in the inner cells of C4 photosynthesis?(2)

Answer 21

calvin cycle; glucose to veins

Question 22

Why is C3 photosynthesis a problem?(6)

Answer 22

possible evolutionary baggage; Rubisco evolved in high CO₂ atmosphere; there wasn’t strong selection against active site Rubisco accepting both CO₂ and O₂; today it makes a difference; 21% O₂ vs. 0.03% CO₂; photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day