topic 2.9/8.3- photosynthesis

Question 1

define photosynthesis

Answer 1

the production of carbon compounds in cells using light energy (light->chemical)

Question 2

how can photosynthetic pigments be separated?

Answer 2

by chromatography
- pigments absorb different wavelengths of light and so look a different colour to us
- plastic strip that has been coated with a thin layer of porous material
- spot containing pigments placed near one end
- solvent is allowed to run up the strip to separate the different pigments

Question 3

Rf =

Answer 3

distance run by the pigment/distance run by solvent

Question 4

shortest wavelength of visible light

Answer 4

violet

Question 5

longest wavelength of visible light

Answer 5

red

Question 6

middle wavelength of visible light

Answer 6

green (525-575nm)

Question 7

range of wavelengths of visible light

Answer 7

400-700nm

Question 8

chlorophyll absorbs —- and —- light most effectively and reflects —— light more than other colours

Answer 8

red; blue

green

Question 9

draw an absorption spectrum for chlorophyll and an action spectrum for photosynthesis

Question 10

define an absorption spectrum

Answer 10

a graph showing the percentage of light absorbed at each wavelength by a pigment or group of pigments

Question 11

define an action spectrum

Answer 11

a graph showing the rate of photosynthesis at each wavelength of light

Question 12

why are action and absorption spectra so similar?

Answer 12

photosynthesis can only occur in wavelengths of light that chlorophyll or other photosynthetic pigments can absorb

Question 13

effect of the start of photosynthesis on the earth’s atmosphere

Answer 13

For the first 2 billion years after the Earth was formed, its atmosphere was anoxic (oxygen-free)
The current concentration of oxygen gas within the atmosphere is approximately 20%

Question 14

effect of the start of photosynthesis on the earth’s oceans

Answer 14

• Earth’s oceans initially had high levels of dissolved iron (released from the crust by underwater volcanic vents)
• When iron reacts with oxygen gas it undergoes a chemical reaction to form an insoluble precipitate (iron oxide)
• When the iron in the ocean was completely consumed, oxygen gas started accumulating in the atmosphere

Question 15

effect of the start of photosynthesis on the earth’s rock deposition

Answer 15

• The reaction between dissolved iron and oxygen gas created oceanic deposits called banded iron formations (BIFs)
• when BIF deposition slowed in oceans, iron rich layers started to form on land due to the rise in atmospheric O2 levels

Question 16

what type of reaction is photosynthesis and what does this mean?

Answer 16

endothermic reaction; this means it requires energy

Question 17

state the 3 possible limiting factors on the rate of photosynthesis

Answer 17

• temperature
• light intensity
• carbon dioxide concentration

Question 18

give 3 ways in which experiments on the limiting factors of photosynthesis can be carried out

Answer 18

Measuring CO2 Uptake (adding sodium hydrogen carbonate raises CO2 conc and you can cause a change in pH)

Measuring O2 Production (gas syringe)

Measuring Biomass (Indirect) as glucose production takes place

Question 19

where do light-dependent reactions take place?

Answer 19

in the thylakoid space and across the thylakoid membranes

Question 20

where do light independent reactions take place?

Answer 20

in the stroma - a thick, protein-rich medium enclosed by the inner membrane of the chloroplast

Question 21

what are the steps and final products of light-dependent reactions ?

Answer 21

• photoactivation
• photolysis
• electron transport
• proton gradient
• chemiosmosis
• ATP synthesis
• reduction of NADP

so the final products are reduced NADP and ATP

Question 22

describe photosystems

Answer 22

• located in the thylakoid membranes
• consist of chlorophyll and accessory pigments being grouped together in large light-harvesting arrays that have reaction centres
• two types of arrays: Photosystems I and II

Question 23

photoactivation

Answer 23

1. chlorophyll molecules absorb light energy and pass it to two special chlorophyll molecules in the reaction centre of the photosystem
2. these absorb the energy from a photon of light, causing an electron within the molecule to become excited- the chlorophyll is then photoactivated

Question 24

what special property do the chlorophylls at the reaction centre have?

Answer 24

they are able to donate excited electrons to an electron acceptor

Question 25

which photosystem do light-dependent reactions begin in?

Answer 25

photosystem II

Question 26

describe what happens after photo activation in photosystem II

Answer 26

• plastoquinone (electron acceptor) collects two excited electrons from photosystem II and then moves to another position in the membrane
• this process can be repeated to produce a second reduced plastoquinone

Question 27

although it is not in a fixed position, plastoquinone remains in the membrane; why?

Answer 27

as it is hydrophobic

Question 28

photolysis

Answer 28

• once the plastoquinone becomes reduced, the chlorophyll in the reaction centre is then a powerful oxidising agent and causes the water molecules nearest to split and give up electrons to replace those it has lost:

2H2O –> O2 + 4H+ + 4e-

Question 29

why is the reduced plastoquinone so important

Answer 29

it not only carries a pair of electrons, but also much of the energy absorbed from light - this drives all the subsequent photosynthetic reactions

Question 30

list 4 structures contained in the thylakoid membrane

Answer 30

• photosystem II
• ATP synthase
• chain of electron carriers
• photosystem I

Question 31

electron transport chain and proton gradient

Answer 31

• reduced plastoquinone carries the electrons to the start of the chain of electron carriers
• transfers its electrons
• these are passed from carrier to carrier, releasing energy which is used to pump protons across the thylakoid membrane into the space inside the thylakoids
• a concentration gradient of protons develops across the membrane (storing potential energy)

Question 32

chemiosmosis

Answer 32

• the protons travel back across the membrane, down the conc gradient through the enzyme ATP synthase
• the energy released by this passage is used to make ATP from ADP + phosphate
• when the electrons reach the end of the chain of carriers they are passed to plastocyanin (an electron acceptor in the fluid of the thylakoid) which is reduced

Question 33

reduction of NAD

Answer 33

• chlorophyll molecules within photosystem I absorb light energy and pass it to the special two chlorophyll molecules in the reaction centre
• this excites an electron in one of the chlorophylls (photoactivtion)
• the electron passes along a chain of carriers in PI, at the end of which it is passed to ferredoxin, a protein in the fluid outside the thylakoid
• two molecules of reduced ferredoxin are then used to reduce NADP

Question 34

how are photosystem I and II linked?

Answer 34

electrons excited in photosystem II are passed along the chain of carriers to plastocyanin, which transfers them to photosystem I to replace the electron donated to the chain of electron carriers by photosystem I

Question 35

difference between cyclic and non-cyclic photophosphorylation

Answer 35

• NADP sometime runs out
• PSI absorbs light energy
• Electrons are excited and released from PSI
• Energy from electrons is used to create an H+ gradient
• H+ diffuses out of the stroma through ATP synthase, making ATP
• Electrons are taken back up by PSI (recycled).

Question 36

steps of light independent reactions

Answer 36

• carbon fixation
• carboxylation of RuBP
• production of triose phosphate
• ATP and NADPH as energy sources
• ATP used to regenerate RuBP
• ATP used to produce carbohydrates

Question 37

where does carbon fixation occur?

Answer 37

in the stroma (the fluid that surrounds the thylakoids in the chloroplasts)

Question 38

carbon fixation

Answer 38

carbon dioxide reacts with ribulose bisphosphate (RuBP), a 5 carbon compound to produce two molecules of glycerate 3-phosphate

ribulose bisphosphate carboxylase (usually abbreviated to rubisco) catalyses this reaction

Question 39

production of triose phosphate

Answer 39

hydrogen is added to glycerate 3-phosphate to produce triose phosphate, a carbohydrate.

• ATP produced by the light-dependent reactions provides energy needed to perform the reduction
• reduced NADP provides the hydrogen atoms

Question 40

the use of triose phosphate

Answer 40

• two triose phosphate molecules can be combined to form hexose phosphate, which can be combined by condensation reactions to form starch
• some triose phosphate molecules are converted into RuBP using enzymes and ATP

Question 41

draw the Calvin cycle

Question 42

draw a diagram of a chloroplast and label it, describing the structure-function relationships

Answer 42

Thylakoids- flattened discs have a small internal volume to maximise hydrogen gradient upon proton accumulation
Grana – thylakoids are arranged into stacks to increase SA:Vol ratio of the thylakoid membrane
Photosystems – pigments organised into photosystems in thylakoid membrane to maximise light absorption
Stroma – central cavity that contains appropriate enzymes and a suitable pH for the Calvin cycle to occur

Question 43

describe Calvin’s experiments and how they elucidated the carboxylation of RuBP

Answer 43

Radioactive carbon-14 is added to a ‘lollipop’ apparatus containing green algae (Chlorella)
Light is shone on the apparatus to induce photosynthesis (which will incorporate the carbon-14 into organic compounds)
After different periods of time, the algae is killed by running it into a solution of heated alcohol (stops cell metabolism)
Dead algal samples are analysed using 2D chromatography, which separates out the different carbon compounds
Any radioactive carbon compounds on the chromatogram were then identified using autoradiography (X-ray film exposure)
By comparing different periods of light exposure, the order by which carbon compounds are generated was determined = CC