6 - Chloroplasts Flashcards
The light reaction
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
How light energy is absorbed by chlorophyll
When pigments absorb light energy as photons an electron in the pigment is elevated to an orbital of higher energy.
pigments in the thylakoid membranes
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.
What happens when pigments absorb light
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.
Reaction centre
the site where a pair of Chlorophyll a molecules can actually donate their
excited electrons to the primary electron acceptor
2 photosystems in the light reaction
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.
Non-cyclic electron flow P700
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
Non-cyclic electron flow P680
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.
what is the energy from non-cyclic electric flow used for?
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
Cyclic electron flow
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
Cyclic electron flow
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
plastocyanin
copper containing protein involved in cyclic electron flow
What happens to electrons during cyclic electron flow?
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
Calvin cycle
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.
where does the calvin cycle take place?
the stroma
Chemiosmosis in chloroplasts and mitochondria
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.
differences in chemiosmosis in mitochondria and chloroplasts
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.
Envelope
The outer and inner membrane that surrounds the chloroplast
encloses the stroma
Stroma
aqueous compartment in which starch granules are usually found
location of the calvin cycle which uses ATP NADPH and CO2 to produce sugar
Thylakoid space
inner most compartment enclosed by the thylakoid membrane
thylakoid membrane
Chlorophyll is present in the thylakoid membrane.
The thylakoid is responsible for the steps of photosynthesis which convert light energy into chemical energy.
grana
dense stacks of thylakoid membrane
