C1.3 Photosynthesis HL

Question 1

What are the light-dependent reactions?

Answer 1

The part of photosynthesis occurring in the grana of the chloroplasts where light is use to split water molecules (photolysis) , generating ATP (photophosphorylation) and NADPH.

Question 2

What are the light-independent reactions?

Answer 2

The part of photosynthesis occurring in the stroma of the chloroplasts, where the products of the light-dependent reactions are used to reduce (fix) carbon dioxide to glucose. But can only work until the NADPH+H^+ and ATP has run out

Question 3

What are photosystems? And where are they located? And what are they able to do?

Answer 3

Photosystems are always located on membranes and are molecular arrays of pigments found attached to the thylakoid membranes in plants. In cyanobacteria they are found attached to the membrane. They are able to generate and emit excited electrons.

Question 4

Why is it called the light-dependent reaction?

Answer 4

Because in the light-dependent stage, photons of light energy are trapped by chlorophyll molecules grouped together in photosystems.

Question 5

What is photoactivation? And why does it happen?

Answer 5

Photoactivation happens when photons of light strike the pigment molecules in the photosystem and excite their electrons, transferring them through the pigment array until they reach the reaction centre. Here a photochemical reaction occurs which results in the emission of an excited electron.

Question 6

What are the two photosystems? And what wavelengths activate their reaction centre?

Answer 6

Photosystem I is activated at 700nm and is also referred to as P700.
Photosystem II is activated at 680nm and is also referred to as P680.

Question 7

What are the steps in the light-dependent reaction?

Answer 7

1. A photon of light is absorbed by a pigment in photosystem II and is transferred to other pigment molecules until it reaches one of the chlorophyll a (P680) molecules in the reaction centre. The photon energy excites one of the chlorophyll a electrons to a higher energy state.
   
   2. This electron is captured by the electron acceptor of the reaction centre.
   3. Water is split by an enzyme to produce electrons, hydrogen ions and an oxygen atom (photolysis). These electrons are supplied one at a time to the molecules of chlorophyll a at the reaction centre and replace those lost to the electron acceptor.
   4. The excited electrons pass from the primary acceptor down an electron transport chain, losing energy at each exchange. The first of the electron carriers is plastoquinone (PQ). The middle carrier is a cytochrome complex.
   5. The energy lost from the electrons moving down the electron transport chain drives chemiosmosis and brings about phosphorylation of ADP to produce ATP.
   6. A photon of light is absorbed by a pigment in photosystem I. This energy is transferred through several accessory pigments until it is received by a chlorophyll a (P700) molecule. This results in an electron with a higher energy state being transferred to the electron acceptor. The electron that has lost its energy fills the void left by the newly energized electron.
   7. The electron with the higher energy state is passed down a second electron transport chain that involves the carrier ferredoxin.
   8. The enzyme NADP reductase catalyses the transfer of the electron from ferredoxin to the energy carrier NADP. Two electrons and one hydrogen ion from the stroma are required to reduce NADP to NADPH.

Question 8

Define the term photosystem.

Answer 8

They are large protein complexes found in the thylakoid membrane of the chloroplasts. They are involved in the light-dependent reactions of photosynthesis and contain pigments, including chlorophyll and accessory pigments, that capture light energy and initiate electron transport chain.

Question 9

Why is it important to have a large number of pigment molecules in a photosystem? And how does the arrangement of pigments allow for a wider range of wavelengths to be absorbed?

Answer 9

The large arrays of pigments allow them to absorb a wide range of wavelengths. It is just that those specific wavelengths are most effective at activating the chlorophyll and exciting the electrons in each reaction centre.

Question 10

Why is it necessary to funnel the absorbed energy to the reaction centre?

Answer 10

Funnelling the absorbed energy to the reaction centre ensures that the energy is harnessed from the light is efficiently converted into chemical energy through electron transfer, minimizing waste and maximising photosynthetic efficiency.

Question 11

Identify each of the components of the two photosystems and its role.

Answer 11

Question 12

Compare chemiosmosis in mitochondria and chloroplasts.

Answer 12

Question 13

Compare cyclic and non-cyclic phosphorylation.

Answer 13

Question 14

What happens in the light-independent reaction?

Answer 14

In the light-independent reactions of photosynthesis carbon dioxide is converted to carbohydrate. These reactions are also known as the Calvin cycle.

Question 15

If Calvin cycle is independent of light why does the light-dependent reaction need to occur?

Answer 15

The reactions of the Calvin cycle happen independently of light but they do need the products of the light-dependent reactions in order to proceed.

Question 16

What happens to the carbon dioxide in light-independent reaction?

Answer 16

Carbon dioxide is combined with an acceptor molecule in the presence of a special enzyme, ribulose bisphosphate carboxylase (RuBisCo).

Question 17

Where does the combination of carbon dioxide in photosynthesis occur?

Answer 17

Happen in the stroma.

Question 18

What is Rubisco know for?

Answer 18

Rubisco is probably the most abundant enzyme on Earth and is the bulk of the proteins found in green plants.

Question 19

Why are high concentrations of rubisco needed?

Answer 19

High concentrations of it are needed because it is quite slow-working and is not very effective at low CO2 concentrations.

Question 20

What is carbon fixation?

Answer 20

Carbon Fixation - ribulose bisphosphate is a 5-C sugar to which CO2 is added to produce a 6-C sugar which immediately splits into two 3-C molecules: glycerate 3-phosphate.
This is the first step in the fixation of carbon dioxide and is known as the fixation step.

Question 21

What occurs after the fixation step?

Answer 21

Glycerate 3-phosphate is immediately reduced to the 3-C sugar phosphate - triose phosphate. For this to happen reduced NADP (NADPH) supplies the hydrogen to reduce the glycerate 3-phosphate and ATP supplies the energy. This step is called the reduction step.

Question 22

What is the Calvin cycle?

Answer 22

A cycle of reactions in the stroma of the chloroplasts, light-independent reaction, in which CO2 is converted to carbohydrates.

Question 23

What is Rubisco(RuBisCo)?

Answer 23

Ribulose-1-5-biphosphate carboxylase is an enzyme involved in the 1st major step of carbon fixation; it is the control enzyme of photosynthesis.

Question 24

How does regeneration of RuBP?

Answer 24

5 molecules of triose phosphate (3C) are converted to 3 molecules of RuBP (5C) in the regeneration-of-acceptor step allowing the Calvin cycle to continue. The remaining triose phosphate is used to synthesise other products such as glucose. Each turn of the Calvin cycle produces two molecules of triose phosphate so three turns are needed to produce six molecules of triose phosphate. Five of these are used to regenerate RuBP and one is used for the production of glucose. The glucose can be used to produce carbohydrates such as sugars, sugar phosphates and starch, lipids, amino acids and organic compounds. This is called the product-synthesis step.

Question 25

What is the Calvin cycle diagram?

Answer 25