Photosynthesis

Question 1

What are autotrophs and photoautotrophs?

Answer 1

Autotrophs sustain themselves without eating anything derived from other organisms.
Photoautotrophs use the energy of sunlight to make organic molecules from H20 and CO2 (plants).

Question 2

What are heterotrophs?

Answer 2

Organisms that obtain their organic material from other organisms.

Question 3

What are stroma?

Answer 3

Stacks of thylakoids

Question 4

What are the light reactions and where do they occur?

Answer 4

The light reactions occur in the thylakoids and involve the splitting of H20, the release of O2, the reduction of NADP+ to NADPH, and the generation of ATP

Question 5

What colours does chlorophyll a suggest is best for photosynthesis?

Answer 5

Violet-blue and red light

Question 6

What are carotenoids and what do they do?

Answer 6

They are photosynthetic pigments that absorb excessive light that would otherwise damage the chlorophyll.

Question 7

What are the two components of a photosystem?

Answer 7

A reaction-centre complex and light-harvesting complexes.

Question 8

Where does an excited electron from chlorophyll a go to?

Answer 8

A primary electron acceptor in the reaction center.

Question 9

What is the reaction-centre chlorophyll a of PSII called?

Answer 9

P680, because it absorbs light best at a wavelength of 680nm.

Question 10

What is the reaction-centre chlorophyll a of PSI called?

Answer 10

P700, because it absorbs light best at a wavelength of 700nm.

Question 11

Which is the primary pathway: linear or cyclic?

Answer 11

Linear

Question 12

Describe the first stages of Linear Electron Transport involving PSII.

Answer 12

A photon hits a pigment in PSII and is passed among light harvesting complexes II pigment molecules until it excites P680 in reaction centre II. An excited electron from P680 is transferred to the primary electron acceptor. This forms P680+, which is a very strong oxidising agent. H20 is split, and the electrons are transferred from the hydrogen atoms to P680+, reducing it to P680. O2 is released.

Question 13

Describe the stages of Linear Electron Transport involving PSI.

Answer 13

Electrons fall in energy from PSII to PSI. Light energy (photon) excites P700, which loses an electron to an electron acceptor. P700+ accepts an electron passed down from PSII via the electron transport chain.

Question 14

Describe the stages of Linear Electron Transport involving ferredoxin (Fd).

Answer 14

Fd is a protein that accepts electrons from the primary electron acceptor. These electrons are transferred to NADP+ and reduce it to NADPH. These electrons are then available for the Calvin cycle.

Question 15

Describe the Cyclic electron flow.

Answer 15

Only uses PSI and produces ATP, not NADPH. Cyclic electron flow generates lots of ATO, satisfying the higher demand in the Calvin cycle.

Question 16

What are the three phases of the Calvin cycle?

Answer 16

1. Carbon fixation
2. Reduction
3. Regeneration of the CO2 acceptor (RuBP)

Question 17

Describe phase 1 of the Calvin cycle: carbon fixation.

Answer 17

CO2 is fixated by RuBP, which is a 5C molecule. A short lived 6C intermediate exists, which then quickly forms 2x3C molecules. The enzyme Rubisco is used for this carbon fixation.

Question 18

Describe phase 2 of the Calvin cycle: reduction.

Answer 18

6ATP are hydrolysed into ADP + Pi, and this phosphorylates the 3C molcules. The molecules are then reduced, as NADPH is oxidised to NADP+. G3P is formed, which is then used to form glucose and other organic compounds.

Question 19

Describe phase 3 of the Calvin cycle: regeneration of the CO2 acceptor (RuBP)

Answer 19

The remaining G3P is then regenerated to form RuBP. 3ATP is used to form RuBP.

Question 20

On hot dry days do plants open or close their stomata?

Answer 20

They close the stomata to preserve H20 but this limits photosynthesis and the leaf heats up.

Question 21

When stomata close, what process does this favour?

Answer 21

Photorespiration

Question 22

What are the differences between photorespiration and photosynthesis?

Answer 22

In photorespiration rubisco adds O2 instead of CO2 in the Calvin cycle. O2 is then consumed and CO2 is released without producing ATP or sugar