17.3 Light-dependent reaction

Question 1

What is the equation for photosynthesis?

Answer 1

6CO2 + 6H2O -> C6H12O6 +6O2

Question 2

What adaptations do chloroplasts have?

Answer 2

• Network of membranes present within chloroplasts provides a large surface area to maximise absorption of light.
• contain DNA - code for enzyme (ATP synthase needed in LDR or rubisco in LIDR)
• ribosome needed for protein synthesis , for example sythesis of pigments, photosystems.
• high surface area of thylakoids and densely packed with photosystems to maximise light absorbed for photosynthesis in Light dependent reaction

Question 3

What are thylakoids?

Answer 3

Flattened sacs of membranes which are stacked to form grana.

Question 4

What are lamelle?

Answer 4

These are membraneous channels which join the grana.

Question 5

What is stroma?

Answer 5

The fluid enclosed in the chloroplast

Question 6

What is the primary pigment in photosynthesis?

Answer 6

Chlorophyll A

It absorbs red and blue light and reflects green light

Question 7

What pigments are found in the thylakoid membrane?

Answer 7

Chlorophyll B, xanthophylls and carotenoids are embedded in the thylakoid membrane

Question 8

What is the light harvesting system?

Answer 8

When chlorophyll b, xanthophylls, carotenoids and other pigments form together (known as antennae complex) in order to absorb the light energy of different wavelengths and transfer the energy to the reaction centre quickly.

Question 9

Which pigment is located in the reaction centre?

Answer 9

Chlorophyll A

Question 10

What accessory pigments are responsible for the yellow/orange colours in leaves?

Answer 10

Carotenoids (carotene and xanthophyll)

Question 11

What pigment acts as a sunscreen for plants and how?

Answer 11

• Anthocynanin is a red/purple pigment (the colour is pH dependent)
• It is produced when the conc of sugars is high and help trees maximise production towards end of growing season.
• They act as sunscreen by absorbing blue-green and ultraviolet light, thereby inhibiting the destruction of chlorophyll.

Question 12

How do you calculate Rf value?

Answer 12

Rf= (distance traveled by component)/(distance travelled by solute)

Question 13

What is the mobile phase in TLC?

Answer 13

Solution containing a mixture of pigments

Question 14

What is the stationary phase in TLC?

Answer 14

A thin layer of silica gel applied to glass

Question 15

What are the two stages in photosynthesis?

Answer 15

1) Light dependent stage

2) Light independent stage

Question 16

What is the light dependent stage?

Answer 16

• When energy from the sunlight is absorbed by photosynthetic pigments and converted to chemical energy to form ATP. (By adding an organic phosphate group to ADP)
• Hydrogen from water is used to reduce the coenzyme NADP to REDUCED NADP.

Question 17

What is the light independent stage?

Answer 17

• Also known as the Calvin Cycle.
• When hydrogen from reduced NADP and carbon dioxide is used to build organic molecules, such as glucose. ATP supplies the required energy.

Question 18

Where does the light dependent reaction take place?

Answer 18

The light dependent reaction takes place in the thylakoid membrane of the chloroplast.

Question 19

Where does the light independent reaction take place?

Answer 19

The light independent reaction takes place in the stroma of the chloroplasts.

Question 20

What is the process called when ATP is made from ADP and an inorganic phosphate?

Answer 20

Photophosphorylation

Question 21

What is a coenzyme?

Answer 21

• A coenzyme is a molecule that aids the function of an enzyme.
• They usually work by transferring a chemical group from one molecule to another.

Question 22

What coenzyme is used in photosynthesis?

Answer 22

NADP.
- It is used to transfer hydrogen from one molecule to another, and this means it can reduce (give hydrogen to) or oxidise (take hydrogen from) a molecule.

Question 23

In the light dependent reaction, what is the light energy absorbed used for?

Answer 23

1) Making ATP from ADP and inorganic phosphate (Photophosphorylation)
2) Making reduced NADP from NADP
3) Splitting water into protons, electrons and oxygen (photolysis)

Question 24

What is photolysis?

Answer 24

The splitting of a molecule using light (photo) energy.

Question 25

What are two types of photophosphorylation in the light dependent reaction?

Answer 25

1) Non-cyclic photophosphorylation

2) Cyclic photophosphorylation

Question 26

How are the photosystems in the thylakoid membranes linked?

Answer 26

The photosystems are linked by electron carriers.

- Electron carriers are proteins that transfer electrons.

Question 27

What is electron transport chain?

Answer 27

A chain of proteins through which excited electrons flow.

Question 28

What wavelength can photosystem II absorb?

Answer 28

680nm

Question 29

What wavelength can photosystem I absorb?

Answer 29

700nm

Question 30

What are the processes in a non-cyclic photophosphoylation?

Answer 30

1) Light energy excites electrons in chlorophyll
2) Photolysis of water produces protons, electrons and oxygen.
3) Energy from the excited electrons makes ATP
4) Energy from the excited electrons generates reduced NADP

Question 31

Detail the process of photophosphorylaton.

Answer 31

1) Light energy is absorbed by PSII.
2) The light energy excites electrons in the chlorophyll and the electrons move to a higher energy level.
3) These high-energy electrons move along the electron transport chain to the PSI.
4) As the excited electrons from the chlorophyll leave PSII to move along the electron transport chain, they must be replaced.
5) Light energy splits water into protons, electrons and oxygen.
6) The excited electrons lose energy as they move along the electron transport chain.
7) This energy is used to transport protons into the thylakoid, via membrane proteins called proton pumps, so that the thylakoid has a higher concentration of protons than the stroma.
8) This forms a proton gradient across the membrane and protons move down their concentration gradient into the stroma via an enzyme called ATP synthase.
9) The energy from this movement combines ADP and an inorganic phosphate to form ATP.
10) Light energy is absorbed by PSI, which excites the electrons again to an even higher energy level.
11) Finally the electrons are transferred to NADP, along with a proton from the stroma, to form reduced NADP.

Question 32

What is cyclic photophosphorylation?

Answer 32

• Cyclic photophosphorylation only produces ATP and only uses PSI.
• It’s called “cyclic” because the electrons from the chlorophyll molecule aren’t passed onto NADP, but passed back to PSI via electron carriers.
• This means electrons are recycled and can repeatedly flow through PSI.