Test 3 Unit 3-Photosynthesis

Question 1

What is the difference between non-cyclic and cyclic photophosphorylation

Answer 1

• Non-cyclic (linear) is the pathway of electron flow from photosystem II through photosystem I to synthesize NADPH.
• Cyclic is electron flow from photosystem I to ferredoxin is not followed by electron donation to the NADP+ reductase complex. Instead, reduced ferredoxin donates electrons back to the plastoquinone pool. Only gives ATP.

Cyclic is only photosystem 1.

Question 2

Demonstrate the complementarity between the light dependent reactions and the dark reactions.

Answer 2

The light reactions provide the necessary energy for the dark reactions to operate. For the synthesis of 1 G3P, the Calvin cycle (dark) requires a total of 9 ATP and 6 NADPH, which are generated from ADP and NAD+ by the light reactions.

Question 3

What is the connection between cellular respiration and photosynthesis?

Answer 3

The overall reactions of photosynthesis and cellular respiration are basically the reverse of eachother.

• Products of Photosynthesis= Sugars (glucose) and O2= reactants of Cellular respiration
• Products of Cellular respiration= H2O and CO2= reactants of photosynthesis

Both have ETC, (oxidative) phosphorylation, chemiosmotic synthesis of ATP, G3P.

Question 4

What are the 3 phases of the Calvin cycle?

Answer 4

1. Fixation
2. Reduction
3. Regenaration

Question 5

Explain phase 1 of the Calvin cycle.

Answer 5

3 CO2 enter 1 at a time and are fixed by Rubisco to RuBP and form 2 molecules of 3-phosphoglycerate

Question 6

Explain phase 2 of the Calvin cycle.

Answer 6

Each molecule of 3-phosphoglycerate gets a phosphate (ATP→ADP) and then is reduced by electrons from NADPH to produce a molecule of G3P that leaves the cycle.

Question 7

Explain phase 3 of the Calvin cycle.

Answer 7

The 5 other G3P are converted to 3 RuBP using 3 ATPs.

Question 8

How many turns of the Calvin cycle are required for 1 G3P to be released?

Answer 8

3 turns (3 CO2).

Question 9

How many ATP and NADPH molecules are used by the Calvin cycle?

Answer 9

9 ATPS and 6 NADPH.

Question 10

What is the necessity of the C4 pathway in C4 and CAM plants?

Answer 10

Rubisco is really inefficient at fixing CO2 because it is not specific to CO2. The C4 pathway compensates for Rubisco’s poor activity. It recycles molecules from cellular respiration and uses the enzyme PEP carboxylase which is more efficient than Rubisco, but doesn’t add CO2 on the same molecule as Rubisco.

Question 11

What is the purpose of the C4 pathway?

Answer 11

To increase the concentration of CO2 in the cell so that Rubisco is less likely to do photorespiration. It helps the Calvin cycle be more efficient.

Question 12

What is photorespiration?

Answer 12

Wastefull process where O2 binds to the active site and react with RuBP. One of the products is a two-carbon compound that is exported from the chloroplast and actually requires the cell to consume ATP to convert it into CO2, which is simply lost.

Question 13

What is the difference between C4 and CAM plants?

Answer 13

C4 plants separate the C4 pathway (mesophyll cell) and the Calvin cycle (bundle sheath cell) spatially, so they happen simultaneously but in different locations.
CAM plants separate the C4 pathway (night) and the Calvin cycle (day) temporally.

Question 14

Compare the chemiosmotic processes in photosynthesis and cellular respiration.

Answer 14

SAME: just like respiratory electron transport, photosynthetic electron transport operates with electrons flowing spontaneously from molecules that are easily oxidized to molecules that are progressively more easily reduced.

DIFFERENT: unlike NADH, the chlorophyll molecules in the reaction centres of photosystem II and photosystem I are not easily oxidized. The electron needs to get to a higher state (the energy of light is used to excite the electron within P680 and P700)

Question 15

Why is photosynthesis an anabolic pathway

Answer 15

Because it builds complex molecules (glucose) from simpler ones (CO2) and needs an input of energy (light).

Question 16

Why is cyclic photophosphorylation an essential process during photosynthesis?

Answer 16

When the plant has enough reducing agent (NADPH), there is no need for the production of more NADPH that involve both photosystems (I and II). In cyclic photophosphorylation ONLY photosystem I is active. So, The cyclic one is needed at this time because it can generate ATP with less cost.
-Only gives ATP and only pumps p+ in the lumen.

Question 17

What is enzyme specificity?

Answer 17

It is the ability of an enzyme to choose the exact substrate from a group of similar molecules.

Question 18

Does Rubisco have enzyme specificity? Why?

Answer 18

No, because it s not specific to CO2!!! A molecule of O2 can also bind to the active site and react with RuBP. it can not differentiate between these 2 molecules. If O2 binds instead of CO2, more ATP is consumed and the process is less efficient, which is a problem!

Question 19

Write an overview of the number of molecules in the Calvin cycle.

Answer 19

For 3 turns:

• 3 CO2 (3 carbons) added to 3 RuBP (15 carbons) = 6 3-phosphoglycerate (18 carbons)
• 6 ATPs consumed to phosphorylate 6 3-phosphoglycerate into 6 1,3-biphosphoglycerate (18 carbons)
• 6 NADPHs consumed to convert 6 1,3-biphosphoglycerate into 6 G3P (18 carbons)
• 1 G3P exits and 5 G3P (15 carbons) remain to regenerate 3 RuBP (15 carbons) which requires 3 ATPs

Question 20

How can light energy be converted to chemical energy in chloroplasts.

Answer 20

The absorbed light by a group of pigment proteins (antenna) is converted to chemical energy when an excited electron from the chlorophyll a is transferred to a primary acceptor, also in the reaction centre.

Question 21

Explain how the concentration gradient is built across the thylakoid membrane in chloroplasts .

Answer 21

During the electron transport process, H+ is pumped across the thylakoid membrane, and resulting electrochemical proton gradient drives the synthesis of ATP in the stroma.

1. The protons are transported from stroma to the thykaloid lumen by the ETC where electrons are passed on
2. Enhanced by the splitting of H2o into 2 H+ and 1/2 O2
3. Enhanced by the removal of 1 proton in the stroma during the reduction of NADP+ to NADPH