6 - Chloroplasts

Question 1

The light reaction

Answer 1

The light reaction converts solar energy to chemical energy
Light energy absorbed by chlorophyll in the thylakoid membranes is used
to transfer electrons and hydrogen to the electron carrier NADP+ so that it
becomes NADPH.
Water is split during this process and oxygen given off as a by product.
ATP is also generated from ADP and inorganic phosphate
The thylakoid membranes of the chloroplast are the site of the light
reactions

Question 2

How light energy is absorbed by chlorophyll

Answer 2

When pigments absorb light energy as photons an electron in the pigment is elevated to an orbital of higher energy.

Question 3

pigments in the thylakoid membranes

Answer 3

Chlorophyll a, Chlorophyll b and Carotenoids.
Only Chlorophyll a can participate directly in the light reactions but the other pigments absorb light energy at different wavelengths and transfer this energy to Chlorophyll a.
This broadens the range of wavelengths that can be used.

Question 4

What happens when pigments absorb light

Answer 4

electrons quickly drop down to their original energy losing their excess energy as heat.
In the thylakoid membrane a nearby molecule called a primary electron acceptor traps a high-energy electron that has absorbed the photon before it can drop back down.

Question 5

Reaction centre

Answer 5

the site where a pair of Chlorophyll a molecules can actually donate their
excited electrons to the primary electron acceptor

Question 6

2 photosystems in the light reaction

Answer 6

Photosystem I absorbs light energy best at 700nm so the reaction centre is called P700.
Photosystem II absorbs light energy best at 680nm so the reaction centre is called P680.

Question 7

Non-cyclic electron flow P700

Answer 7

Photosystem I and Photosystem II co-operate together to produce non-cyclic electron flow and to generate NADPH, ATP and split water.
Light excites electrons from P700 which pass to the electron acceptor but are now passed to NADP+ to produce NADPH. So P700 is now in a very oxidised state

Question 8

Non-cyclic electron flow P680

Answer 8

it also becomes excited and donates an electron to its primary acceptor. This electron then passes down an ETC until it is eventually accepted by the oxidised P700.
The P680 is now in an oxidised state and P680 gets its electrons from a
water molecule thus splitting it and generating oxygen.

Question 9

what is the energy from non-cyclic electric flow used for?

Answer 9

The energy generated by this process is used pump hydrogen ions out of thylakoid membrane and generate a hydrogen gradient which is used to ATP synthase to produce ATP

Question 10

Cyclic electron flow

Answer 10

Photosystem I can operate alone to produce ATP.
Electrons are passed from the primary electron acceptor through a chain which includes ferredoxin, the cytochromes and a copper containing protein called plastocyanin. The electron is eventually passed back to P700

Question 11

Cyclic electron flow

Answer 11

Photosystem I can operate alone to produce ATP.

Cyclic electron flow uses a single photosystem to make ATP and does not produce NADPH or oxygen

Question 12

plastocyanin

Answer 12

copper containing protein involved in cyclic electron flow

Question 13

What happens to electrons during cyclic electron flow?

Answer 13

Cyclic electron flow uses a single photosystem to make ATP and does not produce NADPH or oxygen
At each step the electron lose energy and the this energy is used to pump H+ ions across the thylakoid membrane.
This generates a H+ gradient across the membrane which is used to drive an ATP synthase molecule which generates ATP

Question 14

Calvin cycle

Answer 14

Carbon enters the cycle in the form of carbon dioxide and leaves in the form of sugar.
The cycle requires ATP and an energy source and consumes NADPH as a source of high energy electrons.

Question 15

where does the calvin cycle take place?

Answer 15

the stroma

Question 16

Chemiosmosis in chloroplasts and mitochondria

Answer 16

In both cases an ETC located in a membrane pumps H+ ions across the membrane.
The energy for this process is generated by passing electron through a series of progressively more electro-negative carriers.
The membrane also contains an ATP synthase complex which uses
gradient of H+ ions for the phosphorylation of ADP to ATP.
Some of the electron carriers including the quinones and cytochromes are
very similar in mitochondria and chloroplasts.

Question 17

differences in chemiosmosis in mitochondria and chloroplasts

Answer 17

In mitochondria the ETC pumps H+ ions out of the matrix.
In chloroplasts the ETC pumps into the thylakoid compartment
In mitochondria the high energy electrons fed into the ETC come from the oxidation of food molecules.
In chloroplasts the high energy electrons are generated in the photosystems by the absorption of photons.

Question 18

Envelope

Answer 18

The outer and inner membrane that surrounds the chloroplast

encloses the stroma

Question 19

Stroma

Answer 19

aqueous compartment in which starch granules are usually found
location of the calvin cycle which uses ATP NADPH and CO2 to produce sugar

Question 20

Thylakoid space

Answer 20

inner most compartment enclosed by the thylakoid membrane

Question 21

thylakoid membrane

Answer 21

Chlorophyll is present in the thylakoid membrane.

The thylakoid is responsible for the steps of photosynthesis which convert light energy into chemical energy.

Question 22

grana

Answer 22

dense stacks of thylakoid membrane