Week 2 - Photosynthesis

Question 1

Photosynthesis is what?

Answer 1

the process autotrophic organisms use to convert light energy to make sugar and oxygen from carbon dioxide and water

Question 2

PS occurs in what

Answer 2

in an organelle called a chloroplast

Question 3

Chloroplasts contain several pigments, why?

Answer 3

Chlorophyll b and carotenoids absorb slightly different wavelength to Chlorophyll a extending the range of wavelengths useful in PS

Question 4

Light reactions?

Answer 4

it happens in thylakoid membranes, where pigments abosrb light energy and is used to produce intermediary molecules like ATP and NADPH

Question 5

Dark reactions?

Answer 5

happens in the stroma, where the ATP and NADPH from light reactions are used to produce sugars from Co2 (Calvin cycle)

Question 6

Steps of the light reaction

Answer 6

1. light absorbed by chlorophyll
   2.chlorophyll transmit energy and causes water to break apart
2. oxygen is released into the air
3. hydrogen remains in chloroplasts attached to NADPH
4. ATP is generated

Question 7

What are the photosystems?

Answer 7

photosystem 1 is the light harvesting complex and contains CP a and b as well as caroteniods
photosystem 2 is the reaction centre containing CP a

Question 8

Which pigment has the reaction centre?

Answer 8

Only chlorophyll A has the reaction centre to carry out the initial photochemical reaction, all other pigments absorb light and pass energy to the reaction centre

Question 9

What is the reaction centre?

Answer 9

its a transmembrane protein pigment complex

Question 10

What happens in the reaction centre

Answer 10

a chlorophyll absorbs a photon of light an electron is excited to a higher energy level, that electron can be transferred to the primary election acceptor reducing it

Question 11

the results after the reaction centre

Answer 11

the excited electrons are passed to primary electron acceptor to the electron transport chains, their energy ends up in ATP and NADPH, 02 is released when water is broken down

Question 12

What is Chemiosmosis?

Answer 12

the flow of H ions through the membrane is harnessed by ATP synthase to make ATP, in the stroma the h ions combine with NADP+ to form NADPH

Question 13

what does ATP synthase do?

Answer 13

it takes advantage of the concentration gradient, and allows protons to pass from an area of high concentration to low, while doing this the proton imparts energy for ATP synthase to catalyse a reaction of ADP with and inorganic Phosphate to make ATP

Question 14

Steps of the dark reaction

Answer 14

CO2 is joined to H from a water molecule to form glucose using energy from ATP

Question 15

the summary of the Calvin cycle/c3 photosynthesis

Answer 15

1. Carboxylation - RuBP + CO2 = 3-phosphoglycerate rubisco catalyses the reaction
2. reduction - 6-c compound splits to 2 molecules of PGA or 3-phosphoglyceric acid, 3PG is reduced to glyceraldehyde-3-phosphate (G3P)
3. Regeneration - some of the G3P is used to regenerate RuBP the initial CO2 acceptor so that the cycle can continue

Question 16

Facts about Rubisco

Answer 16

most abundant protein on earth, occurs in photosynthetic bacteria nad all higher plants, in temperate plants 15-30% of total leaf n is in the enzyme rubisco, at current atmospheric 02 concentrations Rubisco is not efficient

Question 17

What is the Problem with Rubisco?

Answer 17

it can catalyse 2 reactions, 1 fixes it to a carbohydrate, the other lowers photosynthesis levels by combining reacting with oxygen and inhibiting its active site also called photorespiration

Question 18

How does photorespiration occur?

Answer 18

in high heat and intense light, the heat causes the plants stomata to close, as photosynthesis continues CO2 is used up and O2 is produced causing high concentrations inside and photorespiration occurs

Question 19

C4 photosynthesis pathway?

Answer 19

Enzyme PEPcase fixes CO2 to a 4C acid in mesophyll cells, 4C is transferred to bundle sheath where its broken down releasing CO2 and a 3C acid, CO2 refixed by Rubisco in the Calvin cycle, the 3C acid returned to the mesophyll and used to regenerate PEP

the initial CO2 fixation and Calvin cycle are separated in space

Question 20

How does the C4 pathway limit photorespiration

Answer 20

Bundle sheath cells are far away from the surface meaning less O2 access, PEP carboxylase doesn’t have an affinity for O2 so allows plant to collect a lot of CO2 in Sheath cells where rubisco is.