What Is Light Reaction?
The light reaction — also called the photochemical phase — is where radiant energy is first converted into chemical energy. In this quiz you will test your grasp of the machinery behind that conversion: the two discrete light harvesting complexes within Photosystem I and Photosystem II, the antenna pigments that gather light, and the single chlorophyll a molecule at each reaction centre.
Before you begin, recall these key points
- Light reactions include light absorption, water splitting, oxygen release, and formation of ATP and NADPH.
- Photosystems are named in the sequence of their discovery, not their function.
- All pigments except one chlorophyll a molecule form the antennae; that single molecule forms the reaction centre.
- PS I has P700 (peak at 700 nm); PS II has P680 (maxima at 680 nm).
Work carefully — several questions test the distinction between what a photosystem is named for and what it does.
Q1. The light reaction of photosynthesis is also known by which name?
The lesson states explicitly that light reactions, or the photochemical phase, include light absorption, water splitting, oxygen release and formation of ATP and NADPH.
Q2. Which set of events is included in the light reaction?
The photochemical phase is defined by these four interlocking events: light absorption, water splitting, oxygen release, and formation of the high-energy intermediates ATP and NADPH.
Q3. Within which two photosystems are the pigments organised into discrete light harvesting complexes?
The pigments are organised into two discrete photochemical light harvesting complexes within Photosystem I (PS I) and Photosystem II (PS II).
Q4. Photosystem I and Photosystem II are named according to:
The lesson stresses that the photosystems are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction.
Q5. A student assumes that PS I acts before PS II simply because of the numbering. Why is this reasoning faulty?
The names follow the sequence of discovery, not the sequence of function during the light reaction — a point the lesson says frequently confuses students.
Q6. The light harvesting complexes (LHC) are made up of:
The lesson states that the LHC are made up of hundreds of pigment molecules bound to proteins.
Q7. In each photosystem, which pigments form the light harvesting system called the antennae?
Each photosystem has all the pigments except one molecule of chlorophyll a forming the light harvesting system, also called antennae.
Q8. Which single molecule forms the reaction centre of a photosystem?
The lesson states clearly that the single chlorophyll a molecule forms the reaction centre.
Q9. How do the antenna pigments make photosynthesis more efficient?
These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light, widening the range of incoming light that can be utilised.
Q10. The reaction centre chlorophyll a of Photosystem I has an absorption peak at 700 nm and is therefore called:
In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called ##\ce{P700}##.
Q11. The reaction centre of Photosystem II has absorption maxima at 680 nm and is designated:
In PS II the reaction centre chlorophyll a has absorption maxima at 680 nm and is called ##\ce{P680}##.
Q12. Which statement about the reaction centres of the two photosystems is correct?
The lesson states that the reaction centre is different in both photosystems: ##\ce{P700}## in PS I and ##\ce{P680}## in PS II.
Q13. Which of the following is NOT listed in the lesson as an event of the light reaction?
The light reaction includes light absorption, water splitting, oxygen release and formation of ATP and NADPH. Regeneration of ##\ce{RuBP}## belongs to the biosynthetic phase, not the photochemical phase.
Q14. Which statement best describes the involvement of proteins in the light reaction?
The lesson states that several protein complexes are involved in the process, indicating a membrane-bound, multi-component system.
Q15. Which pairing of photosystem, reaction centre and absorption peak is correct?
In PS I the reaction centre chlorophyll a absorbs maximally at 700 nm and is called ##\ce{P700}##, while in PS II it peaks at 680 nm and is called ##\ce{P680}##.