Module 5.6 - Photosynthesis

Question 1

What are pigments?

Answer 1

Absorb certain wavelengths of light
Reflect other wavelengths (colours seen)
Arranged in photosystems in thylakoid membranes

Question 2

Chlorophyll a

Answer 2

In the Primary Pigment Reaction Centre (PPRC)
Two forms: P700 (photosystem 1); P680 (photosystem 2)
Appears blue-green
Absorbs red light and some blue at 440nm
Contains a Mg atom so when light hits, pair of electrons become excited, oxidised by light

Question 3

What are the accessory pigments?

Answer 3

Chlorophyll b
Carotenoids
Xanthophylls

Question 4

Chlorophyll b

Answer 4

Absorbs light at 400-500nm and 640nm

Appears yellow-green

Question 5

Carotenoids

Answer 5

Absorbs blue light at 400-500nm
Reflects yellow and orange light
Absorbs light not normally absorbed by chlorophylls then passes energy on to chlorophyll a

Question 6

Xanthophylls

Answer 6

Absorb blue and green light at 375-455nm

Reflect yellow light

Question 7

What are photosystems?

Answer 7

Funnel-shaped light-harvesting cluster of photosynthetic pigments
Accessory pigments absorb different wavelengths of light to maximise sunlight utilised
Accessory pigments funnel energy associated with the light’s wavelength to the PPRC and chlorophyll a

Question 8

What is an absorbance spectrum?

Answer 8

A graph that shows which wavelengths of light are absorbed by a pigment

Question 9

What is an action spectrum?

Answer 9

A graph which shows the wavelengths of light that are actually used in photosynthesis

Question 10

Explain the link between the absorption and action spectra

Answer 10

They closely match each other, suggesting these absorbed wavelengths of lights are used in photosynthesis

Question 11

What processes does the LDS stage consist of?

Answer 11

Light harvesting at the photosystems
Photolysis of water
Photophosphorylation
Formation of reduced NADP

Question 12

What is photophosphorylation?

Answer 12

The generation of ATP from ADP and an inorganic phosphate ion in the presence of light

Question 13

What are the two types of photosystem?

Answer 13

PSI - P700 in PPRC

PSII - P680 in PPRC

Question 14

What is the role of water in the LDS?

Answer 14

An enzyme splits water, only in the presence of light, into 2 protons, 2 electrons and half an oxygen molecule
Source of protons used in photophosphorylation
Donates electrons to chlorophyll to those lost when light strikes chlorophyll
Source of by-product, oxygen
Keeps plants turgid

Question 15

What is non-cyclic photophosphorylation?

Answer 15

Involved PSI and PSII

Produces ATP, oxygen and reduced NADP

Question 16

What is cyclic photophosphorylation?

Answer 16

Involves only PSI

Produces ATP but in smaller quantities than by non-cyclic photophosphorylation

Question 17

5 stages of the Calvin Cycle

Answer 17

CO2 combines with RuBP, catalysed by rubisco
RuBP, by accepting the carboxyl group, becomes carboxylated, forming an unstable six-carbon compound that immediately breaks down
The product is two molecules of GP, a three-carbon compound (CO2 has now been fixed)
GP is reduced to TP using hydrogens from NADPH from the LDS
In 10 of every 12 TP molecules, the atoms are rearranged to regenerate six molecules of RuBP (requires phosphate groups)

Question 18

Stages of LDS (non-cyclic phosphorylation)

Answer 18

A photon strikes photosystem II
The light energy excites a pair of electrons inside chlorophyll
The energised electrons are captured by an electron carrier
The electrons are passed along the electron transport chain, releasing energy at each step
The energy released is used to create ATP
Eventually the electrons are captured by chlorophyll in PSI, replacing those lost from PSI
Electrons energised from PSI are captured by another electron carrier and pass along another electron transport chain
Energy released from the e.t.c. is used to pump protons across the thylakoid membrane into the thylakoid lumen
As protons accumulate in the lumen, a proton gradient forms across the membrane
Protons diffuse down their concentration gradient through ATP synthase by chemiosmosis, and the enzyme undergoes a mechanical change which causes ADP and Pi to join, forming ATP
Electrons and protons are accepted by NADP which becomes reduced

Question 19

Factors affecting the rate of photosynthesis

Answer 19

Light intensity
Carbon dioxide concentration
Temperature
Water availability