5.2.1 Photosynthesis

Question 1

thylakoid disc

Answer 1

site of light-dependent reaction
contains photosynthetic pigments embedded in the thylakoid membrane

Question 2

purpose of photosynthesis

Answer 2

conversion of light energy to chemical energy in organic molecules

Question 3

granum

Answer 3

stack of thylakoid discs

Question 4

stroma

Answer 4

fluid-filled space of chloroplasts surrounding the thylakoids
light-independent stage of photosynthesis takes place
contains enzymes involved in the light-independent reaction (Calvin cycle)

Question 5

intergranal lamellae

Answer 5

connects grana
allows the sharing of resources

Question 6

primary pigments

Answer 6

chlorophyll a
absorb protons and excite electrons

Question 7

structure of chloroplast

Answer 7

outer membrane
inner membrane
stroma
thylakoid discs - stack to form a grana
intergranal lamellae
70s ribosome
starch grains
circular DNA

Question 8

accessory pigments

Answer 8

chlorophyll b,c,d
channel light energy towards the primary pigment
increase the range of wavelengths of light that can be absorbed

Question 9

use of the products of photolysis

Answer 9

H+ forms proton gradient
e- replace lost electrons in photosystem 2
oxygen - respired aerobically or diffuses out of stomata

Question 10

cyclic phosphorylation

Answer 10

electrons excited from photosystem 1 are unable to be accepted by NADP

Question 11

outline the importance of photosynthetic pigments in photosynthesis

Answer 11

pigments absorb light/photons

electrons excited to a higher energy level

accessory pigments pass energy to reaction centres, channel light towards primary pigment, increase the wavelengths of light that can be absorbed

primary pigments absorb light and excite electrons which pass electrons to ETC for light-dependent reaction

Question 12

the precise location of photosynthetic pigments in a chloroplast

Answer 12

photosystems in thylakoid membranes

Question 13

explain why temperature has a greater effect on the rate of the light-independent stage

Answer 13

the light-independent stage is controlled by enzymes
a higher temperature will increase the kinetic energy of enzymes
more successful collisions /ESCs formed
enzymes may be denatured at high temperatures

Question 14

absorption spectra

Answer 14

show the range of wavelengths of light that can be absorbed by certain pigments

Question 15

action spectra

Answer 15

show the rate of photosynthesis at each wavelength of light

Question 16

photosystem

Answer 16

system of photosynthetic pigments found in thylakoids of chloroplasts; each photosystem contains numerous accessory pigments that trap photons and pass their energy to a molecule of chlorophyll a during the light-independent stage of photosynthesis

Question 17

light-dependent reaction

Answer 17

photons channeled through PS 2, towards reaction centre and hit chlorophyll a
water is split in photolysis by water-splitting enzyme
chlorophyll a excites e- which is accepted by e- acceptor protein
e- lose energy as they move through chain of e- carrier proteins
energy is transferred to H+ where H+ is pumped from stroma into thylakoid lumen, across thylakoid memb.
H+ flow through ATP synthase channels down E.C gradient - allows ADP + Pi = ATP
e- accepted by PS 1 and re-excited by photon
e- pass through chain of e- carrier proteins and lose energy - transferred to H+ pump
e- recombine with H+ = H
H + NADP = NADPH (NADP reductase req.)

Question 18

photolysis

Answer 18

enzyme-catalysed splitting of water into H+, electrons and oxygen in the presence of light

Question 19

ATP synthase

Answer 19

transmembrane protein
H+ flow through ATP synthase by chemiosmosis and proton motive force
energy is converted from kinetic to chemical

Question 20

cyclic phosphorylation

Answer 20

e- excited by PS1 but unable to be accepted by NADP
e- return to PS1 via chain of e- carrier proteins
ATP still made as steep H+ gradient is maintained

Question 21

light-independent reaction
Calvin cycle

Answer 21

does not require photons to occur
occurs in stroma
requires products of light-dependent
NADPH - provides reducing power
ATP - releases energy when hydrolysed to ADP + Pi

Question 22

first reaction of Calvin cycle

Answer 22

CO2 from atmosphere diffuses into plant via stomata
CO2 combines with RuBP (5C) catalysed by the enzyme RuBisCo
an unstable 6C intermediate is formed

Question 23

second reaction of Calvin cycle

Answer 23

the unstable 6C intermediate compound splits into 2 molecules of glycerate-3-phosphate (3C)

Question 24

third reaction of Calvin cycle

Answer 24

GP is reduced to form triose phosphate (3C)
reduced using H atoms from NADPH -> NADP
ATP hydrolysed to ADP + Pi (x2)

Question 25

TP leaving Calvin cycle

Answer 25

1/6 of TP leaves cycle can be converted into glucose, cellulose, starch, amino acids, glycerol- depends on metabolic requirements of the plant

Question 26

regeneration of RuBP

Answer 26

5/6 of TP used to regenerate RuBP ATP hydrolysed: ATP -> ADP +Pi

Question 27

describe the fate of triose phosphate

Answer 27

5/6 regenerated to form RuBP 1/6 exits cycle + converted to glucose, cellulose, starch

Question 28

explain the role of ATP in the light independent stage

Answer 28

ATP releases energy when its hydrolysed to ADP +Pi for GP -> TP, TP -> RuBP

Question 29

RuBisCo

Answer 29

enzyme that catalyses the fixation of carbon dioxide

Question 30

explain why the conc. of GP in chloroplasts increases as light intensity decreases (sunset)

Answer 30

fewer photons to excite electrons less ATP made, less NADPH made CO2 is still available so RuBP -> 2x GP GP cannot be converted to TP increased conc. of GP

Question 31

state and explain the effect of a decrease in CO2 conc. on conc. of RuBP, GP & TP in chloroplasts

Answer 31

less CO2 means less RuBP fixed to GP so less GP if GP conc. decreases, TP conc. decreases RuBP accumulates, conc. increases

Question 32

factors affecting photosynthesis

Answer 32

light intensity CO2 concentration temperature

Question 33

light intensity affecting photosynthesis

Answer 33

1. as light intensity increases, more photons = more e- excited more ATP is formed = more NADPH formed = increases rate of photosynthesis 2. other factors start to become limiting 3. light intensity is no longer limiting, CO2 conc. and temp is limiting

Question 34

CO2 conc. affecting photosynthesis

Answer 34

1. increasing CO2 conc. = greater rate of carbon fixation by RuBisCo ( RuBP -> GP) 2. other factors become limiting e.g. temp/light intensity

Question 35

temperature affecting photosynthesis

Answer 35

low temp: enzymes & substrates have low KE so are unlikely to form ESCs as temp increases: enzymes & substrates gain KE = frequency of successful collisions increases higher temp: active site starts to change shape - tertiary structure changes = denaturation

Question 36

water stress

Answer 36

stomatal closure not enough water in soil occurs when there's insufficient water in soil Abscisic acid (ABA) synthesised in roots & transported up xylem in transpiration stream

Question 37

ABA

Answer 37

abscisic acid acts on receptors on guard cell, plasma membranes ions are pumped (AT) put of guard cells water follows ions by osmosis = decrease water potential guard cells become flaccid!

Question 38

compensation points

Answer 38

differ for every plant species based on temp & light intensity rate of PS = rate of respiration at CP net CO2 uptake = net CO2 production net sugar production = net sugar use