B11. Photosynthesis (A2)

Question 1

Which organelle in the leaf does photosynthesis occur within?

Answer 1

Chloroplasts

Question 2

How are leaves adapted for photosynthesis?

Answer 2

Large surface area to absorb sunlight; Arrangement of leaves to minimise overlapping, shadowing; Thin, most light absorbed in first few micrometers, short diffusion pathway for gases; Transparent cuticle and epidermis, let light through photosynthetic mesophyll cells underneath; Long, narrow upper mesophyll cells packed with chloroplast that absorb sunlight; Stomata open and close in response to light intensity; Air spaces in lower mesophyll, allow for rapid diffusion in gas phase of CO2 & O2;

Question 3

What is the overall equation of photosynthesis?

Answer 3

6CO2 + 6H2O =light=> C6H12O6 + 6O2

Question 4

Describe the structure of chloroplasts.

Answer 4

Double membrane envelope,outer/inner membrane; Thylakoid; Granum / Grana (100 disc like structures); Intergranal lamellae; Stroma, starch grains, 70s ribosomes, loop of DNA

Question 5

What are the stages of photosynthesis?

Answer 5

Capturing light energy; Light dependant reaction, occurs in thylakoid membranes; Light independent reaction, occurs in stroma

Question 6

What is oxidation and reduction?

Answer 6

Oxidation is loss of electrons; Reduction is gain of electrons

Question 7

What is photoionisation?

Answer 7

Photosystem absorbs light energy; Exciting a pair of electrons, raising them to the next energy level; Excited electrons leaves photosystem; Photosystem is ionised as a result

Question 8

What are photosystems?

Answer 8

Funnel like structures found in the thylakoid membrane; Collection of photosynthetic pigments that absorb light

Question 9

Describe the differences between photosystem I and photosystem II.

Answer 9

Photosystem II absorbs light in 680 nm wavelength, P680, located at the start of the electron transport chain, photolysis occurs here; Photosystem I absorbs light in 700 nm wavelength, P700, located in middle of electron transport chain

Question 10

What is photolysis?

Answer 10

Light energy used to break down H2O; H2O => (2H+) + (2e-) + (1/2 O2)

Question 11

What is photophosphorylation?

Answer 11

Production of ATP via light dependant reaction

Question 12

Explain what occurs in non-cyclic photophosphorylation.

Answer 12

Light absorbed by PSII, P680; Photoionisation; 2e- taken by electron acceptor proteins, passed to electron carrier proteins, down electron transport chain; Chemiosmosis; Photoactivation of PS1, P700, results in release of 2e- reducing NADP+, NADPH; Photolysis replenishes 2e- from PSII, P680

Question 13

Explain what happens occurs in cyclic photophosphorylation.

Answer 13

Light absorbed by PSI, P700; Photoionisation; 2e- taken by electron acceptor proteins, passed to electron carrier proteins,down electron transport chain; Chemiosmosis; De-energised e- return to PS, restoring electron supply; NADP+ not reduced, H2O not needed to replenish electron supply

Question 14

What are the differences between cyclic and non-cyclic photophosphorylation?

Answer 14

Cyclic: PSI; P700 active reaction centre; e- revert to PSI; ATP produced; H2O non required; NADPH not synthesised; O2 not evolved; predominant in bacteria
Non-cyclic: PSII & PSI; P680 active reaction centre; e- from PSI are accepted by NADP+; NADPH & ATP produced; O2 evolved; Predominant in green plants

Question 15

Where does photolysis occur in non-cyclic photophosphorylation?

Answer 15

PSII, P680; Contains oxygen-evolving complex, catalyses photolysis
H2O => (2H+) + (2e-) + (1/2 O2)

Question 16

Explain what occurs in chemiosmosis.

Answer 16

Energy released from alternative redox of ETC is used to actively transport H+ ions from stroma into thylakoid lumen through proton pump; H+ gradient between stroma and thylakoid lumen created; H+ ions diffuse via facilitated diffusion through chemiosmotic channel; Activating ATP synthase, catalysing the reaction between ADP + Pi = ATP

Question 17

Why do electrons release energy as they move down the electron transport chain?

Answer 17

Electron carriers are alternatively reduced, as they gain e-, and oxidised, as they lose e-, gradually releasing energy

Question 18

What do photosystems consist of?

Answer 18

Primary pigments, chlorophyll; Accessory pigments, carotenoids; PSII, chlorophyll B; PSI, chlorophyll A

Question 19

Why do photosystems have different pigments?

Answer 19

Efficiently absorb light over a larger range of wavelengths

Question 20

What is chromatography and how does it work?

Answer 20

Experimental technique used to separate mixtures; Different compounds travel through material at different speeds, causing separation

Question 21

What is the equation for calculating Rf values?

Answer 21

Distance traveled by pigment / Distance traveled by solvent

Question 22

Describe the method used for chromatography of a leaf.

Answer 22

Draw line in pencil 1 cm above bottom of filter paper; Cut section of healthy leaf and place in mortar; Add 20 drops of acetone, grind leaf to release pigments; Extract pigments using capillary; Spot on pencil line; Suspend paper in solvent; Mixture dissolved in solvent (Mobile phase); Dissolved material passes through static material (Stationary phase)

Question 23

What are the limitations of paper chromatography of a leaf?

Answer 23

Not as specific as other chromatographic techniques; Doesn’t give data on amount of each pigment present or wavelengths they absorb

Question 24

Describe the light-independent reaction.

Answer 24

Occurs in stroma; Ribulose bisphosphate carboxylase catalyses the fixation of carbon dioxide to ribulose bisphosphate; Unstable 6 carbon molecule formed; Dissociates into 2 molecules of gylcerate3phosphate; GP reduced to 2 molecules of Triose phosphate using NADPH & ATP; Triose phosphate regenerated into Ribulose bisphosphate using ATP; 1 additional carbon left over

Question 25

What factors are needed for photosynthesis?

Answer 25

Presence of photosynthetic pigments; CO2; H2O; Light energy; Suitable temperature

Question 26

What are the main limiting factors for photosynthesis?

Answer 26

Light intensity, light dependant stage, more ATP and NADPH for calvin cycle; CO2, raw material, calvin cycle, carbon fixation; Temperature, enzyme optimum temperature