light-dependent reactions

Question 1

what do the light-dependent reactions involve? what is this used for?

Answer 1

the light dependent reactions involve the capture of light energy, used for two purposes:
- to excite electrons in the chlorophyll, which allows an inorganic phosphate molecule to be added to ADP, therefore making ADP (photophosphorylation).
- to split water into H+ ions (protons) and electrons (and oxygen). the splitting is caused by light therefore is known as photolysis.

Question 2

where does photophosphorylation occur?

Answer 2

photophosphorylation occurs in the thylakoid membranes of the chloroplasts, which contain tightly packed chlorophyll molecules

Question 3

where does photolysis occur?

Answer 3

photolysis occurs in the lumen of the thylakoid

Question 4

draw a diagram of the light-dependent reactions

Answer 4

see diagram

Question 5

explain the light dependent reactions (9)

Answer 5

• when light is absorbed by the chlorophyll molecules, a pair of electrons in the molecules gain energy (they become excited). the electrons become so energetic that they leave the chlorophyll molecule and are taken up by a molecule called an electron carrier.
• this loss of electrons by the chlorophyll molecule causes it to become oxidised/ionised. this is called photoionisation (as caused by the absorption of light).
• the electrons are now passed down a series of electron carriers in a series of REDOX reactions: each carrier becomes reduced when it receives an electron, then reverts to it’s oxidised state when the electron is passed on. the electron carrier molecules form an electron transfer chain (ETC) located in the membranes of the thylakoids.
• each new carrier is at a slightly lower energy level than the previous one so that the electrons lose the energy they gained from light at each stage.
• during photophosphorylation, this energy is ultimately used to combine an inorganic phosphate molecule with ADP to make ATP, via an H+ concentration gradient.
• at the same time, a molecule of water is split into hydrogen ions (protons), electrons and oxygen in photolysis.
• the electrons produced by photolysis are used to replace those lost from chlorophyll, allowing it to continually absorb light energy.
• the protons produced by photolysis react with oxidised coenzyme NADP, along with the electrons released at the end of the electron transfer chain to form reduced NADP.
• the oxygen produced by the photolysis of water is either used in respiration or diffuses out of the leaf as a waste product of photosynthesis.

Question 6

what is the equation for photolysis?

Answer 6

H2O → 2H+ + 2e- + 1/2O2
water → 2protons + 2electrons + 1/2oxygen

Question 7

what are the ATP and reduced NADP produced in the light dependent reactions used for?

Answer 7

the ATP and reduced NADP formed by the light dependent reactions provide the energy and hydrogen necessary to form carbohydrates in the light independent reactions. this energy has been transferred from the photon of light originally absorbed by the chlorophyll.

Question 8

what happens to the light energy captured by chlorophyll in the electron transport chain?

Answer 8

the light energy captured by chlorophyll and used to excite the pair of electrons is lost during the redox reactions in the electron transfer chain

Question 9

where is the electron transfer chain found?

Answer 9

the ETC is found embedded in the thylakoid membrane around the chlorophyll molecules

Question 10

what molecules make up the electron transfer chain and what are their roles?

Answer 10

they are a series of proteins with two roles:
- they can accept electrons from one molecule and donate them to another
- they can use energy from excited electrons to pump H+ (protons) from one side of the thylakoid membrane to the other

Question 11

what is associated with this chain of proteins?

Answer 11

a large protein complex which contains the enzyme ATP synthase that can attach inorganic phosphate to ADP

Question 12

explain what happens in the electron transfer chain?

Answer 12

• the electrons from chlorophyll are transferred to the first molecule (protein) in the electron transfer chain by a carrier molecule. they are then passed from one electron transfer protein to the next in a series of redox reactions: each carrier becomes reduced when it receives an electron, then reverts to its oxidised state when the electron is passed on. the electrons lose energy as they pass down the chain.
• this energy is used to actively pump protons through the thylakoid membrane from the stroma into the lumen of the thylakoid.
• the proteins accumulate in the lumen of the thylakoids so that a steep concentration gradient builds up between here and the stroma of the chloroplast.
• the protons can only diffuse back into the stroma through special protein complexes, which contain the enzyme ATP synthase.

Question 13

how is ATP released during the electron transfer chain?

Answer 13

as the protons pass through the enzyme complex, they cause a portion of it to revolve, transferring the energy from the proton gradient to allow the enzyme to attach inorganic phosphate groups to ADP molecules to form ATP in a process described as photophosphorylation. the use of a proton (H+) concentration gradient to synthesise ATP is known as chemiosmosis and occurs during photosynthesis and respiration.