Light-dependent Reaction

Which organelle does photosynthesis take place in?

The chloroplasts

What are the two stages of photosynthesis?

• The light-dependent reaction

• The light-independent reaction

Which two important molecules are produced by the light-dependent reaction?

ATP and reduced NADP (NADPH)

Where in the chloroplast does the light-dependent reaction take place?

The thylakoid membrane

What does the thylakoid membrane sit between?

The thylakoid space and the stroma

What is ATP synthase?

An enzyme found in the thylakoid membrane, responsible for the production of ATP

Which reaction does ATP synthase catalyse, and what is a key feature of this reaction?

ADP + Pi —> ATP

This reaction requires energy

What is the phrase used to describe a concentration gradient of H+ ions?

Proton gradient

Define chemiosmosis.

The movement of ions across a partially permeable membrane from an area of high concentration to an area of low concentration

What may a proton gradient also be referred to as?

An electrochemical gradient

What is an electrochemical gradient?

A concentration gradient involving a charged particle

Define oxidation in terms of hydrogen.

Loss of hydrogen

Define oxidation in terms of electrons.

Loss of electrons

Define reduction in terms of hydrogen.

Gain of hydrogen

Define reduction in terms of electrons.

Gain of electrons

What is NADP an example of?

A coenzyme

What is a coenzyme?

A molecule that helps an enzyme carry out its function

What is the first stage of the light-dependent reaction, and what happens during this stage?

Photoionisation- light is absorbed by chlorophyll and light energy is transferred to electrons within the chlorophyll. This causes the electrons to be in an ‘excited state’ and they leave the chlorophyll. The chlorophyll is left oxidised and with a positive charged.

What is the second stage of the light-dependent reaction (after photoionisation), and what happens during this stage?

Photolysis- light is used to split water into oxygen, protons and electrons in order to replace the electrons lost by the chlorophyll during photoionisation.

Where in the chloroplast does photolysis take place?

The thylakoid space

What is the reaction taking place during photolysis?

H₂O —> 2H+ + 2e- + 0.5O₂

What happens to the protons produced during photolysis?

They are used to help maintain a high concentration of protons in the thylakoid space

What happens to the oxygen produced during photolysis?

It diffuses out of the plant or is used in respiration

What is the third stage of the light-dependent reaction (after photolysis), and what happens during this stage?

The electron transport chain- the excited electrons which left the chlorophyll during photoionisation move along a set of protein carriers via a series of oxidation-reduction reactions. These reactions release energy. Electrons at the end of the electron transport chain react with NADP and a proton to form reduced NADP (NADPH)

What happens during the fourth stage of the light-dependent reaction, after the electron transport chain?

Energy released from the electron transport chain is used to actively transport protons from the stroma to the thylakoid space to maintain a proton gradient for chemiosmosis

What is the fifth stage of the light-dependent reaction (after protons have been actively transported from the stroma to the thylakoid space), and what happens during this stage?

Chemiosmosis- protons diffuse down a proton gradient from thylakoid space to the stroma via ATP synthase. This provides ATP synthase with energy

What is the sixth stage of the light-dependent reaction (after chemiosmosis), and what happens during this stage?

Photophosphorylation- energy from chemiosmosis is used by ATP synthase to catalyse the reaction ADP + Pi —> ATP

What are five adaptations chloroplasts have to ensure the light-dependent reaction is as efficient as possible, and why are they useful?

• The thylakoid membrane has a high surface area, maximising the amount of ATP and NADPH produced

• Chlorophyll is arranged so that the maximum amount of light is absorbed

• Thylakoid membranes contain ATP synthase for ATP production

• Thylakoid membranes are selectively permeable which allows a proton gradient to be established and maintained

• Chloroplasts contain DNA and ribosomes, so proteins involved in the light-dependent reaction are produced quickly and easily