Photosynthesis

Question 1

What is photosynthesis?

Answer 1

The conversion of light energy to chemical energy (carbohydrates) in plants.
6CO2 + 6H2O – Light –> C6H12O6 + 6O2

Question 2

Where does photosynthesis occur?

Answer 2

In the chloroplasts

Question 3

Describe the Light reaction stage of photosynthesis

Answer 3

occurs in the thylakoid membranes

light energy is captured by pigments and energy is transferred to a “reaction centre”

This flow generates NADPH

light energy also generates a proton gradient across the thylakoid membrane –generates ATP

ATP and NADPH needed to drive the synthesis of carbohydrates in stage 2, the carbon reaction.

Question 4

Describe the Carbon reaction stage of photosynthesis.

Answer 4

occurs in the stroma of the chloroplasts

CO2 is converted to carbohydrates using the ATP and NADPH generated in the light reactions

Question 5

What are the main pigments in plants and what wavelengths do they absorb?

Answer 5

Chlorophyll A (red - violet and orange light or 642 nm)
Chlorophyll B (blue and yellow or 372nm)

Question 6

What range of light energy can be absorbed by plants?

Answer 6

approx 400-700nm

Question 7

What does the antennae complex do?

Answer 7

Collects light and funnels the energy to a reaction centre

Question 8

What is the combination of antennae complex and a reaction centre called?

Answer 8

Photosystem

Question 9

Plants have 2 photosystems (PS1 and PS2), what is the main difference?

Answer 9

PS1 absorbs far-red light (P700)
PS2 absorbs red light (P680)

Question 10

What else occurs at Photosystem 2?

Answer 10

Water is oxidised to oxygen

Question 11

What are the 4 electron carriers from PS2 to PS1?

Answer 11

Pheophytin
Plastoquinone
Cytochrome
Plastocyanin

Question 12

What excites P700 to a higher energy state?

Answer 12

Direct absorption of far red light
Energy transfer from antennae complex
Electron transfer from PS2.

Question 13

What is photophosphorylation?

Answer 13

ATP is generated by an enzyme known as ATP synthase.

Question 14

What-is photoprotection in plants?

Answer 14

The protection of cells to harsh sunlight, this can include layers over them that act as a sunscreen, or the formation of detoxifying enzymes.

Question 15

What are the 3 stages in the carbon reaction of photosynthesis?

Answer 15

1. Carboxylation - forms 3-PGA from RuBP (catalysed by rubisco)
2. Reduction - forms G3P from previous product
3. Regeneration - forms RuBP from previous product

Question 16

What is photorespiration?

Answer 16

Occurs when there is more O2 available (from photosynthesis) to use over CO2, as Rubisco cannot identify between both.
This is a problem in hot, dry conditions as that forces the stomates to close to reduce water loss, which in turn, decreases the amount of oxygen available over that time.

Is wasteful, results in energy loss

Question 17

How do plants combat photorespiration? What are the three categories of plants according to this defense?

Answer 17

C3 plants are that vast majority of plants, these plants photorespire without any combat as they experience cooler temperatures. They do however, have a metabolic pathway to partially recover the carbon lost.

C4 plants can combat to photorespire less, as they can push the CO2 into the rubisco environment and have a separate site of O2 production.

CAM plants are those that generally experience very hot and dry conditions, they do not respire at all. They have a similar system to C4, although they are better at concentrating CO2 in the rubisco environment by storing in the vacuole when not using. The plant’s stomata also remain closed during the day and open at night to reduce water loss.

Question 18

What is the function of spongy mesophyll cells found below the palisade?

Answer 18

Their large pore spaces surrounding creates increased light scattering properties, which increases probability of absorption.

Question 19

How do chloroplast move in low vs high light conditions and why?

Answer 19

In low light they move to cell surfaces parallel to the leaf plane to increase absorption.

In high light conditions they move to surfaces perpendicular to leaf plane, this decreases absorption by 15% and protects cells.

Question 20

What are the ways in which manipulating photosynthesis could help increase yields?

Answer 20

1. Manipulating rubisco could increase catalytic rate or reduce wasteful oxygenase activites
2. Manipulating a more efficient pathway to recover the lost energy in oxygenation
3. Engineering CO2 concentrating mechanisms (such as those found in CAM) into C3 plants?
4. Broadening the wavelengths of usable light (such as some photosynthetic bacteria that can absorb up tp 1100nm