Photosynthesis

Question 1

List the need for energy produced from photosynthesis

Answer 1

• Active transport
• Anabolic reactions
• Movement

Question 2

What does the reaction of photosynthesis do?

Answer 2

• fixes inorganic carbon (co2) into an organic form (C6H1206) through reduction using H+ ions from water

Question 3

Name 4 different photosynthetic pigments

Answer 3

• chlorophyll a
• chlorophyll b
• beta carotene (carotenoid)
• xanthophylss

Question 4

Where are photosystems found and what do they do?

Answer 4

• along the thylakoid membrane
• molecule of chlorophyll a which is primary pigment (reaction centre) so when light travels from stroma through thylakoid membrane other pigment molecules e.g. chlorophyll b, carotenoids, accessory pigments and antenna complexes move down to be focussed at the reaction centre by accessory pigments

Question 5

What are the two types of photosystems and describe them?

Answer 5

PHOTOSYSTEM 1: (second in photophosphorylation) based around chlorophyll a molecule with a peak absorption at 680nm. Found mainly in the granal lamellae
PHOTOSYSTEM 2: (first in photophosphorylation) based around a chlorophyll a molecule with peak absorption at 700nm. Found mainly in the intergranal part of the lamellae

Question 6

Describe overall what happens in the light dependant reaction and where it occurs.

Answer 6

• Light and H20 convert to ATP, NADPH and O2

- Occurs on thylakoid membrane

Question 7

Describe overall what happens in the light INdependant reaction and where it occurs.

Answer 7

• CO2, ATP and NADPH convert to sugars (e.g. glucose)

- Occurs IN the stroma

Question 8

Describe the process of non-cyclic photophosphorylation

Answer 8

• Light absorbed by PS2 and 2 electrons (from photolysis) become ‘excited’ so leave reaction centre
• These travel along the ETC and ATP is generated by chemiosmosis (movement of H+ across proteins in membrane into stroma)
• The electrons leave the ETC and enter the reaction centre of PS1
• Light strikes PS1 then the 2 electrons become excited and leave reaction centre to join with 2H+ from photolysis and movement of H+ across channel protein to reduce NADP (NADPH)

Question 9

What does NADP stand for?

Answer 9

• nicotinamide adenine dinucleotide phosphate

Question 10

Describe the calvin cycle

Answer 10

• CO2 combines with the 5C compound ribulose bisphosphate (RuBP). This is catalysed by ribulose bishosphate carboxylase (rubisco)
• This creates an unstable 6c compound which splits to give 2x glucerate-3-phosphate/ 3-phosphoglycerate (GP)
• The GP is then reduced using the ATP and NADPH from the light dependant reaction to give 2 x triosephosphate (TP)
• 5/6 molecules of TP produced are used to regenerate RuBP. 1/6 is used to make hexose sugars and other molecules
• 6 turns of the cycle is needed to generate 12 TP, 2 of which will make 1 glucose

Question 11

What is the function of C4 and CAM pathways?

Answer 11

• an adaption to allow certain species to minimise photorespiration by ensuring rubisco always encounters high conc of CO2 so its unlikely it’ll bind to O2

Question 12

Outline the C4 pathway

Answer 12

• light dependant reactions and calvin cycle physically separated (dependant in mesophyll cells and independant in bundle-sheath cells around leaf veins)
• atmospheric CO2 is fixed to form oxaloacetate (4C) by PEP carboxylase (has no tendency to bind O2)
• Oxaloacetate converted to malate that travels out mesophyll cells to bundle sheet and malate breaks down releasing CO2
• CO2 then fixed by rubisco and made into sugars via calvin cycle
• Does use ATP to move 3C “ferry” compound from bundle sheath to mesophyll to pick up another CO2
• occurs in hot sunny climates as benefits of reduced photorespiration outweighs ATP use

Question 13

Outline what happens in CAM plants

Answer 13

• Separate processes in time (cAM - AM/PM)
• At night CAM plants open stomata to allow co2 to diffuse into leaves
• CO2 then fixed into oxaloacetate by PEP carboxylase (same as C4) which is converted to malate which is stored in vacuoles until the next day
• In daylight CAM plants don’t open stomata but still photosynthesis as malate is transported out vacuole and broken down to release CO2 which enters Calvin cycle
• not only limit photorespiration but water-efficient so dominant in dry areas e.g. deserts

Question 14

What happens in photorespiration?

Answer 14

• RuBP picks up O2 instead of CO2 producing 3-PGA (3C) and phosphoglycolate (2C)
• phosphoglycolate waste product as can’t be used in calvin cycle meaning 1/4 of carbon that enters this pathway is lost as CO2
• Net effect is that you lost 3 fixed C atoms instead of gaming 6 fixed C atoms in the normal calvin cycle
• so wastes energy and undoes the work of the calvin cycle

Question 15

How can photorespiration be said to have certain benefits?

Answer 15

• some evidence can have photo protective effects (preventing light-induced damage to molecules involved in photosynthesis)
• help maintain redox balance in cells
• support plant immune systems

Question 16

Define photosynthetic pigment

Answer 16

A photosynthetic pigment is a pigment that is present in chloroplasts or photosynthetic bacteria and captures the light energy necessary for photosynthesis.

Question 17

State the law of limiting factors

Answer 17

For any process controlled by more than one factor, the rate of that process is limited by one factor which is furthest from its optimal value, and by that factor alone

Question 18

State the overall effect of light intensity on rate of photosynthesis

Answer 18

o As light intensity increases, ATP and reduced NADP are produced at a higher rate until a point where CO2 conc or temperature becomes the limiting factor (directly proportional relationship)
o Not all wavelengths of light are equally effective in causing a plant to photosynthesise. Peaks occur in the blue (420-480nm) and red (640-700nm) regions of the spectrum. Green light is reflected rather than absorbed

Question 19

State the overall effect of CO2 conc on rate of photosynthesis

Answer 19

CO2 needed as source of C so if all other conditions met increasing CO2 conc increases rate of carbon fixation in Calvin cycle and rate of TP production. A point is reached where further CO2 conc no longer has an effect on rate of photosynthesisso light and temp is limiting

Question 20

State the overall effect of temperature on rate of photosynthesis

Answer 20

o Affects rate of enzyme-controlled reactions – as temp increases the rate of enzyme activity increases (doubles for each 10rise) until the point at which the proteins denature. The rate of photorespiration increases above 25 meaning high photosynthetic rates may not be seen at higher temperatures even if enzymes not denatured
o Purely photochemical reactions aren’t affected by temp whereas totally chemical ones are. Light dependant phase is photochemical and independent is chemical
o Rubisco at high temps catalyses combination of O2 with RuBP (photorespiration) causing problems for plants when light intensity and temperature both high

Question 21

State the overall effect of water on rate of photosynthesis

Answer 21

o Although water is required for photosynthesis it is never considered a limiting factor because for water potential to have become low enough to limit the rate of photosynthesis the plant will’ve closed its stomata

Question 22

State the effect of light intensity on the rate of photosynthesis in relation to the calvin cycle

Answer 22

o reducing light intensity reduces rate of light dependant stage of photosynthesis
o reducing the quantity of ATP and reduced NADP.
o ATP and reduced NADP are needed to convert GP to TP
o Conc of GP will increase
o Conc of TP will decrease
o Less TP to regenerate RuBP – conc of RuBP decrease
o Reverse occurs when light intensity increases

Question 23

State the effect of CO2 conc on the rate of photosynthesis in relation to the calvin cycle

Answer 23

o CO2 = vital substrate of Calvin cycle (nothing to combine with RuBP) so low conc leads to reduced conc of GP (less CO2 to be fixed) and TP
o The conc of RuBP will initially increase as its being formed from TP not being used to fix CO2
o When TP sypplies completely run out RuBP cannot be re-generated so its levels fall also

Question 24

State the effect of temperature on the rate of photosynthesis in relation to the calvin cycle

Answer 24

o All reactions in Calvin cycle catalysed by enzymes
o At lower temps enzyme and substrate molecules have less KE, meaning less successful collisions therefore reduced rate of reaction
o Same effect at high temps as enzymes denatured irreversibly