biochemistry of photosynthesis 2

Question 1

RuBP

Answer 1

5C compound ribulose biphosphate

Question 2

first step of the calvin cycle

Answer 2

carbon dioxide from the air combined with RuBP in the chloroplasts

Question 3

carbon fixation

Answer 3

when carbon is attaches to RuBP it is fixed

Question 4

RUBISCO

Answer 4

ribulose biphosphate carboxylase oxygenase
-aids carbon dioxide from air combining with 5C RuBP
-RUBISCO is rate limiting enzyme in process of photosynthesis

Question 5

result of reaction between RuBP and CO2

Answer 5

in theory 6C compound

Question 6

why is the 6C compound produced in theory only

Answer 6

because it is highly unstable and no one has been able to isolate it

Question 7

what happens to the 6C compound produced in the reaction between RuBP and CO2

Answer 7

-it splits to give 2 molecules of GP 3C

Question 8

what happens to GP after being produced by splitting of 6C

Answer 8

it is reduced by addition of hydrogen to form glyceraldehyde 3-phosphate 3C

Question 9

glyceraldehyde 3-phosphate

Answer 9

GALP 3C sugar

Question 10

where does the hydrogen that reduced GP come from

Answer 10

from reduced NADP and the energy required comes from ATP made in light stage

Question 11

2 pathways for GALP

Answer 11

-much of it passes through series of steps to replace RuBP needed in first step of cycle
-some synthesised into 6C glucose or passed directly into glycolysis pathway where its used for synthesis of other molecules needed by plant

Question 12

features of reactions that take place in calvin cycle

Answer 12

take place in both dark and light
-reactions only stop in dark when products of light reaction run out leaving no reduced NADP or ATP available in the chloroplasts

Question 13

how is RUBISCO inefficient

Answer 13

the active site cannot distinguish between the carbon oxygen double bonds of CO2 molecules and the oxygen oxygen double bonds of 02
-as a result there is competitive inhibition between the 2

Question 14

RUBISCO as carboxylase

Answer 14

in high levels of CO2 low O2 RUBISCO binds to oxygen and combined it with RuBP giving two GP which feeds into calvin cycle

Question 15

RUBISCO as oxygenase

Answer 15

in low CO2 high O2 RUBISCO binds to oxygen and combined RuBP to form one molecule of GP and one glycolate 2-phosphate
-this is converted into GP in photorespiration

Question 16

photorespiration

Answer 16

-process by which glycolate 2-phosphate is converted to GP
-reaction that used products of calvin cycle and ATP and releases CO2
-photorespiration name because it uses CO2 and O2

Question 17

affinity of RUBISCO

Answer 17

affinity for CO2 80% higher than affinity for oxygen

Question 18

inefficiency of photorespiration

Answer 18

needs to convert GP2 to 3C GP so wastes carbon and energy
-thats why plants 25& less efficient than they should be

Question 19

why is RUBISCO so inefficient

Answer 19

RUBISCO evolved when atmosphere was high in co2 with little o2 so photorespiration never occurred and no selection pressure against i t
-even today high oxygen low co2 atmosphere photorespiration not problem for plants
-no selection pressure for enzyme to evolve to become more specific to CO2
-however more efficient RYBISCO useful for people because crop plants would be 25% more productive

Question 20

whats the end product of photosynthesis

Answer 20

GALP and key molecule for synthesis of everything else needed in plant

Question 21

different uses of GALP in plant

Answer 21

some GALP directly used in glycolysis so fed into krebs cycle
-some of GALP produced in calvin cycle used to produce glucose by gluconeogenesis

Question 22

gluconeogenesis

Answer 22

process by which some of GALP produced in calvin cycle if used to produce glucose

Question 23

what happens to glucose produced in gluconeogenesis

Answer 23

-glucose may be converted into disaccharides such as sucrose for transport round the plant and into polysaccharides such as starch for energy storage and cellulose for structural support

Question 24

function of GALP that feeds into glycolysis and krebs cycle

Answer 24

used as fuel to provide energy in form of ATP for functions of the cell

Question 25

use of compounds from glycolysis and krebs cycle

Answer 25

used as the building blocks of amino acids combining the molecules with nitrates from the soil

Question 26

use of GALP if it continues round the calvin cycle

Answer 26

it can be used for the production of nucleic acids with the addition of phosphates from the soil

Question 27

function of some of GALP taken in by glycolysis pathway

Answer 27

converted to acetyl coenzyme A which can be used to synthesise fatty aids needed for production of phospholipids for membranes and lipids needed for storage and other functions within the plant

Question 28

C3 plants

Answer 28

majority of plants which use this pathway where CO2 fixed directly into 3C compounds

Question 29

C4 plants

Answer 29

alternative route to C3 plants
-they produce compound 4C and use this to raise the CO2 levels in cells at all times
-this minimised photorespiration making photosynthesis more efficient

Question 30

CAM plants

Answer 30

have evolved in hot dry ecosystems
-adaptation enables them to keep their stomate closed during the day reducing water loss and still having plenty of carbon dioxide for efficient photosynthesis

Question 31

what does the amount of light affect in photosynthesis

Answer 31

the amount of chlorophyll which is excited and therefore the amount of reduced NADP and ATP produced in light dependent stage

Question 32

what happens if there is low light levels in photosynthesis

Answer 32

insufficient NADP and ATP will be produced to allow the reactions of the light independent stage to progress at their max rate
-limiting factor

Question 33

why are co2 levels important in photosynthesis

Answer 33

if there is not enough co2 available for fixing in the calvin cycle the reactions cannot proceed at max rate

Question 34

most common limiting factor

Answer 34

co2

Question 35

temperature in relation to calvin cycle

Answer 35

all of calvin cycle reactions and light dependent reactions controlled by enzymes and are sensitive to temperature

Question 36

photosynthesis and its limiting factors in the real world

Answer 36

-in wild plants ecosystems develop are governed by competition by plants
-eg co2 levels dont vary in air but plants compete for warmth and light
-complete for nutrients that are needed to convert carbs into proteins and fats
-growth in height, spread of leaves depends on plant adapted to get as much light