Photosynthesis

Question 1

What are catabolic pathways?

Answer 1

• Break down complex molecules into simpler compounds

- Releases energy

Question 2

Give 2 examples of catabolic pathways

Answer 2

• Glycolysis

- Krebs cycle

Question 3

What are anabolic pathways?

Answer 3

• Builds complicated molecules from simpler ones

- Consume energy

Question 4

Give an example of an anabolic pathway

Answer 4

Photosynthesis

Question 5

What is the balanced equation for photosynthesis?

Answer 5

6CO2 + 6H2O + ENERGY ——> C6H12O6 + 6CO2

Question 6

Where does photosynthesis occur?

Answer 6

In chloroplasts

Question 7

Where does the light dependent stage occur?

Answer 7

On the thylakoid membranes

Question 8

What are the organisms that can photosynthesise called?

Answer 8

Photoautotrophs

Question 9

Where are chloroplasts found in leaves?

Answer 9

In mesophyll cells

Question 10

How are electrons excited?

Answer 10

Through absorbing sunlight

Question 11

How is a proton gradient generated?

Answer 11

• As high energy electrons pass from carrier to carrier, energy is released
• This energy is used to move H+ across the thylakoid membrane, creating a gradient

Question 12

How is a proton gradient maintained?

Answer 12

As a result of the impermeability of the membrane to H+

Question 13

What is the only way that protons can move back through the membrane, down their concentration gradient?

Answer 13

• Through hydrophilic membrane channels ( facilitated diffusion)
• Linked to ATP synthase
• Flow of protons through channels provides energy to synthesise ATP (ADP + Pi)

Question 14

What is the primary pigment in photosynthesis?

Answer 14

Chlorophyll a

Question 15

What colours does chlorophyll absorb?

Answer 15

Red and blue light

Question 16

What colour does chlorophyll reflect?

Answer 16

Green light

Question 17

Name 2 accessory pigments

Answer 17

1- Chlorophyll b

2- Carotenoids

Question 18

Where are the accessory pigments located?

Answer 18

Thylakoid membrane

Question 19

What is the antennae complex?

Answer 19

• Light harvesting system

- Absorbs light energy of different wavelengths and transfers it efficiently to reaction centre

Question 20

Where is chlorophyll a located?

Answer 20

Reaction centre

Question 21

What are photosystems?

Answer 21

• Proteins with pigments attached
• In thylakoid membrane
• Made up of antennae complex and reaction centre

Question 22

What are the 2 forms of chlorophyll a?

Answer 22

1- P700 found in photo system 2 (peak absorption of light is at wavelength 700nm)
2- P680 found in photosystem 2 (peak absorption of light is at wavelength 680nm)

Question 23

What are the 2 stages of photosynthesis?

Answer 23

• Light dependent stage

- Light independent stage

Question 24

Where does the light independent stage happen?

Answer 24

In the stroma

Question 25

Describe non-cyclic photophosphorylation

Answer 25

- Photosystems arranged in pairs, PS2 first, then PS1 - Chl-a in PS2 absorbs light, excited e- to a higher energy level - Passed along ETC via a protein carrier, as they do they lose energy - Energy released as the e- pass along is used to phosphorylase ADP = photophosphorylation - The e- that left PS2 are replaced by splitting of nearby water molecules (photolysis) - PS1 is also absorbing light and e- are excited - e- pass along ETC via carriers - Ferredoxin (iron-protein complex) passes the e- and H+ from the water to NADP - Causes NADP to become reduced = NADPH

Question 26

What is photolysis?

Answer 26

Water molecules are split into hydrogen ion, electrons and oxygen molecules using energy from sun - The electrons released replace the electrons lost from the reaction centre of PS2 - O2 = “waste gas”, used in respiration

Question 27

What are the 3 products from the light dependent stage?

Answer 27

1- ATP 2- NADPH 3- O2

Question 28

Describe cyclic photophosphorylation in the light dependent stage

Answer 28

- Only involves PS1 - Chlorophyll in PS1 absorbs light energy, exciting e- so they rise to a higher energy level - High energy e- pass down ETC through protein carriers - As e- pass down, energy is released - H+ is pumped into thylakoids from storms - Proton gradient forms across thylakoid membrane - H+ diffuses down gradient through ATP synthase - This releases energy - Allows ADP + Pi —> ATP - Electrons are continuously recycled

Question 29

Describe the light independent stage (Calvin cycle)

Answer 29

- CO2 diffuses into the leaf via the stomata, across spaces of spongy mesophyll into mesophyll cells, then into stroma - CO2 is picked up by 5C acceptor molecule = RUBP - This is catalysed by the enzyme RuBisCo - An unstable 6C molecule us formed - Immediately splits into 2x GP - GP is converted into 2x TP - NADPH acts as a reducing agent, donating H - ATP acts as an energy source

Question 30

How are the useful products from the light dependent stage used in the Calvin cycle?

Answer 30

- NADPH acts as a reducing ages to, donating H | - ATP acts as an energy source

Question 31

What is TP and what is it used for?

Answer 31

- 3C sugar - 5/6 used to regenerate RUBP - 1/6 converted into sugars

Question 32

How is RUBP regenerated?

Answer 32

- For 1 glucose molecule to be produced, 6 CO2 molecules have to enter cycle = 6 turns of cycle - 12 TP molecules made - 2 removed to make glucose - 10 TP molecules are recycled to regenerate 6 RuBP - 10 x 3carbon TP = 30 carbons shuffled - 6 x 5carbon RuBP = 30 carbons - Energy is supplied by ATP for the reactions involved in regeneration of RuBP

Question 33

Why is the RuBisCo enzyme not very efficient?

Answer 33

- The reaction between RuBisCo and CO2 is competitively inhibited by O2 - RuBisCo can also react with O2 = photoprespiration - less PHS

Question 34

When is RuBisCo more likely to react with O2 instead of CO2?

Answer 34

Above 30 degrees, causing problems for plants living at high temps

Question 35

Why is it less likely that RuBisCo will react with O2 instead of CO2?

Answer 35

RuBisCo has a higher affinity for CO2 than O2

Question 36

What is the role of water in PHS?

Answer 36

- Photolysis in light dependent stage - Source of e- to replace those from PS2 - Source of H+, used in chemiosmosis to make ATP and join with NADP at the end - Source of O2, reused in plant cell respiration - Keeps plant cells turgid, which is necessary for cellular reactions to take place

Question 37

What is photophosphorylation?

Answer 37

- Synthesis of ATP - Addition of phosphate group to ADP - Use energy from light

Question 38

What is chemiosmosis?

Answer 38

- Protons diffuse back out of the thylakoids down their concentration gradients - Diffuse through the ATP synthase enzyme - H+ release energy during this, which allows ADP to join with Pi to form ATP

Question 39

What does chemiosmosis depend on?

Answer 39

- The creation of a proton concentration gradient | - The energy to do this comes from excited electrons