Photosythesis Flashcards
Light dependent reaction (mark scheme answer) (5 marks)
- Light energy absorbed excites the electrons in chlorophyll.
- Photolysis (splitting of water) occurs to produce oxygen, electrons and hydrogen ions.
- Electrons used in the electron transport chain to replace those lost by chlorophyll.
- This generates ATP (photophosphorylation)
- This causes reduction (gain of electrons) of NADP, reducing it into NADPH.
Light dependent reaction 1-3
- Chlorophyll molecules in PSll absorb photons of light which excited the electrons (2 of them) in chlorophyll to a higher energy level.
- This causes electrons to move into an electron carrier.
- This causes the photolysis (splitting) of water molecules where light energy breaks the powerful bonds in water, producing hydrogen ions (protons), oxygen gas and electrons. 2H2O > O2 + 4H+ + 4e-
Oxygen is diffuses out of the chloroplast into the air. Hydrogen ions (protons) are released into the thylakoid membrane. The electrons replace the ones lost in the chlorophyll.
Photolysis equation
2H20 > O2 + 4H+ +4e-
What happens to each product of photolysis
Oxygen: diffuses out of chloroplast into the air
Hydrogen ions (protons): transported into thylakoid membrane
Electrons: replace those lost through excitation in the chlorophyll
Light dependent reaction 4-6
- The excited electrons pass along a chain of electron carriers (electron transport proteins) in the thylakoid membrane.
- Each new carrier is at a slightly lower energy level so the electrons lose energy at each stage.
- The energy in the electrons is used to pump protons from stroma to thylakoid lumen, creating a proton gradient across the thylakoid membrane.
Light dependent reaction 7-10
- Protons move down the concentration gradient via the enzyme ATP synthase. This causes ADP and inorganic phosphate to combine (photophosphorylation) creating ATP.
- In PSI more light energy is absorbed and passed to the electrons (causing further excitation)
- The electrons are eventually recombined with some of the protons to form hydrogen atoms These are taken up by electron carries called NADP so reducing it to NADPH:
NADP + (H+) + (e-) -> NADPH - The NADPH moves to the stroma for the light-independent reactions taking with it the electrons from the chlorophyll molecules.
What is the primary purpose of the light-dependent reaction?
To convert light energy into chemical energy (ATP)
Where in the chloroplast does the light dependent reaction take place?
The thylakoid membrane
What are the products of the light dependent reaction that are used in the Calvin cycle?
NADPH and ATP
What is the role of water in the light dependent reaction?
Water is split (photolysis) to provide electrons to chlorophyll, which helps to maintain the electron flow through the electron transport chain, and oxygen is released as a by-product.
Describe the role of NADP+ in the light-dependent reaction.
NADP+ accepts electrons and hydrogen ions to form NADPH for the Calvin cycle.
Outline the steps involved in the production of ATP during the light-dependent reaction
ATP is produced via photophosphorylation. Light energy excites electrons in chlorophyll, which pass through the electron transport chain, leading to the pumping of protons across the thylakoid membrane and the formation of a proton gradient. Protons flow back through ATP synthase, driving the conversion of ADP and inorganic phosphate into ATP.
Explain why the light-dependent reaction requires light.
Light provides the energy to excite electrons in chlorophyll, enabling the photolysis of water and the flow of electrons through the electron transport chain, all of which are essential for ATP and NADPH production.
