photosynthesis

Question 1

process of the light dependent reaction

Answer 1

light energy absorbed by photosystem (chlorophyll)

electrons excited to higher energy level causing chlorophyll to become oxidised / lose electron - photoionisation

water broken down at photosystem 2 - photolysis of water

electrons from photolysis replace lost electrons

electrons from PS 2 passed to PS1 via ETC

excited electrons release energy down ETC - actively transports protons across thylakoid membrane

protons created gradient, protons return to stroma through ATP synthase - which adds Pi to ADP to form ATP - photophosphorylation

NADPH produced from NADP and proton from PS1

Question 2

less light causes - LIR

Answer 2

LIR required products of LDR

• energy from ATP is needed for regeneration of RuBP from TP
• protons from NADP needed to reduce glycerate 3-phosphate to triose phosphate

Question 3

water uses in LDR

Answer 3

photolysis - water splits by photosystem 2

electrons used to replace those lost by photosystem 2

protons used to be picked up by NADP - reduce to NADPH for LIR

Question 4

limiting factor

Answer 4

the factor that determines / limits the rate of photosynthesis

the factor that is in shortest supply

Question 5

why shine light on plant before experiment

Answer 5

allows photosynthesis to stabilise so any changes in rate are due to independent variable

Question 6

less light causes

Answer 6

rate of photosynthesis increases as light intensity increases then plateau

more energy supplied to plant; so faster light dependent reaction, so more oxygen evolved

plateau occurs when light intensity no longer a limiting factor

Question 7

problem with using algae

Answer 7

it is difficult to control the surface area of algae

so use immobilised algae beads / beads of jelly with known surface area and volume

Question 8

rubp and co2

Answer 8

RuBP combines with CO2 to produce 2 GP

Question 9

adaptations of chloroplasts

Answer 9

large surface area of grana - LDR

photosynthetic pigments arrange into specific structures in photosystems for maximum absorption of light

chloroplast contain own DNA and ribosomes for protein synthesis - needed to photosynthesis

combination of accessory pigments and primary pigments so can absorb lights of a range of wavelengths

Question 10

CO2 concentration and photosynthesis

Answer 10

directionally proportional at first and then plateaus

this is because CO2 is needed in the light independent stage of photosynthesis

more CO2 means more carbon fixation (more CO2 joined with RUBP)

plateaus when some other factor is limiting

(however CO2 is toxic at high concentrations)

Question 11

why do shade-tolerate plants have increased rate of photosynthesis

Answer 11

shade tolerant plants have more types of pigment in leaf

different pigments absorb different wavelengths of light

so shade tolerant leaf can absorb more wavelengths of light

so more light energy available for photosynthesis

Question 12

define photoionisation

Answer 12

light excites electrons in the plant’s chlorophyll

electrons are lost and the chlorophyll becomes positively charged