Process of Photosynthesis (HL)

Question 1

What are the 2 main stages of photosynthesis?

Answer 1

Light dependent reaction, which relies on light directly

The light-independent reaction, which does not use light directly

Question 2

Describe the structure of thylakoids and the thylakoid membrane

Answer 2

They are disc like structures that stack up to 100 to form grana, they also contain chlorophyll.

Question 3

Where do light dependent reactions take place?

Where do light independent reactions take place?

Answer 3

Light dependent: In thylakoid intermembrane space across thylakoid membranes

Light independent: In the stroma of chloroplasts

Question 4

What is an electron carrier used in photosynthesis? What is its regular and reduced form

Answer 4

NADP+

When NADP+ accepts protons it becomes reduced into NADPH

Question 5

Describe the process of light dependent reactions in detail

Answer 5

Light dependent reactions convert light energy into chemical energy in the form of ATP and reduced NADP

ATP and NADP are produced from the photolysis of water

When water undergoes photolysis, it is split into oxygen, electrons and protons

Protons are picked up by NADP, which reduces it to NADPH

ATP is formed from the phosphorylation of ADP

The useful products from the light dependent reactions are transferred to light independent reactions

Oxygen is given as a waste product

Question 6

What are photosystems and what do they contain?

Answer 6

Photosystems are structures where many photosynthetic pigments such as chloroplasts are grouped together.

They contain these pigments and a reaction center

Question 7

What are the types of photosystems and how do they differ?

Answer 7

Photosystem I and Photosystem II

Photosystem I contains the reaction center P700, which is activited by light of wavelength 700

Photosystem II contains the reaction center P680, which is activated by light of wavelength 680

Question 8

Describe what happens in Photosystem II in detail

Answer 8

Chlorophyll absorbs light energy in the form of photons and pass it to the reaction center P680

The chlorophylls in the reaction center are then said to be photoactivated

Electrons in the reaction center become “excited” to a higher energy level because of the photons of light

The excited electrons are now capable of being donated to an electron acceptor called plastoquinone in a reduction reaction

Plastoquinone accepts 2 electrons from Photosysetm II and is reduced, it then moves to another position in the thylakoid membrane

The process repeats with another plastoquinone molecule, and in total 2 plastoquinone molecules are reduced and 4 electrons are lost

Question 9

What is plastoquinone?

Answer 9

An electron acceptor in the light dependent reaction

Question 10

When does photolysis occur in light dependent reactions and what happens in it?

Answer 10

It occurs following the reduction of plastoquinone in Photosystem II

It produces oxygen protons and electrons:

The oxygen diffuses out of the leaf through stomata

The electrons are passed into the electron transport chain

The protons are picked up by the carrier molecules NADP forming reduced NADP

Photolysis is needed to replace electrons lost in photosystem II

Question 11

What is Photophosphorylation?

Answer 11

It is the overall term for the process of using light energy and the electron transport chain to generate ATP from ADP4

Question 12

Where does the electron transport chain occur in plants?

Answer 12

Thylakoid membranes

Question 13

Describe the electron transport chain in photosynthesis in detail?

Answer 13

Reduced Plastoquinone is carrying 2 excited electrons from the end of photosystem II

Plastoquinone carries the electrons to the start of a chain of electron carriers

The electron carries undergo constant redox reactions as they lose and gain electrons

As this happens the excited electrons gradually lose their energy which generates a proton gradient, their excitation falls until the electrons are picked up by the reaction center of photosystem I

NADP is then reduced by these electrons and passed to light independent reactions

The pathway of electrons is linear, photophosphorylation is referred to as non-cyclic photophosphorylation

ATP and reduced NADP are the main products of photophosphorylation and are passed to light independent reactions

Question 14

How is the proton gradient formed?

Answer 14

Electrons are passed from carrier to carrier, releasing energy gradually in the process, this energy powers the pumping of protons and forms a proton gradient because of protons accumulating in the intermembrane space

Question 15

Outline Chemiosmosis in photosynthesis

Answer 15

Protons travel down their concentration gradient through the membrane protien ATP synthase

ATP synthase synthesizes the phosphorylation of ADP to ATP

Question 16

Outline briefly the reduction of NADP

Answer 16

Excited photoactivated electrons are passed to a protein on the outside of the thylakoid membrane called ferredoxin and reduce it

The ferredoxin and protons from ATP synthase in chemiosmosis are used to reduce NADP

NADP+ + 2H+ + 2e- —> NADPH

Reduced NADP npw moves to light independent reactions

Question 17

What are the 3 main steps of light independent reactions in detail?

Answer 17

1. Carbon fixation:
   The enzyme rubisco catalyzes the fixation of carbon dioxide by combining it with a molecule of ribulose biphosphate (RuBP), a 5C compound.

This forms an unstable 6C compound that splits to two glycerate-3-phosphate (GP) molecules, a 3C molecule.

2.Reduction:
GP is reduced to triose phosphate (TP) in a reaction involving reduced NADP and ATP

3. Regeneration:
   RuBP is regenerated from TP in reactions that use ATP

During the cycle endergonic reactions take place that involve the hydrolysis of ATP and oxidation of reduced NADP

The light dependent reactions are sometimes referred to as the Calvin cycle

Question 18

What exactly is carbon fixation?

Answer 18

Carbon fixation involves carbon dioxide (1C) being removed from the external environment and becoming part of the plant, and is then said to be “fixed”

Question 19

Elaborate on the role of reduced NADP and ATP in light independent reactions

Answer 19

Energy from ATP and hydrogen from reduced NADP are used to reduce GP into a TP

Question 20

What is triose phosphate used for?

Why isn’t all of it used to make useful alternative compounds for the plant?

Answer 20

After reduction one sixth of the triose phosphate is converted into usable products for the plant such as:

1.Hexose phosphates that are used to produce carbs

2.glycerol and fatty acids

3.Production of amino acids for protein synthesis

*The other five sixths of triose phosphate are used in the regeneration of RuBP

Question 21

What is the size of a chloroplast?

Answer 21

2-10 micro meters