11.2 - The Light-dependent Reaction Flashcards
The light-dependent reaction of photosynthesis involves the capture of light whose energy is used for two purposes…
- to add an inorganic phosphate (Pi) molecule to ADP, thereby making ATP
- to split water into H+ ions (protons) and OH- ions. As the splitting is caused by light, it is known as photolysis.
What is oxidation
- When a substance gains oxygen or loses hydrogen the process is called oxidation.
- The substance to which oxygen has been added or hydrogen has been lost is said to be oxidised.
What is reduction
When a substance loses oxygen, or gains hydrogen, the process is called reduction.
When a substance is oxidised, does it lose or gain electrons
Loses (therefore it gain electrons if reduction takes place)
Oxidation results in energy being ____ ___, whereas reduction results in it being ___ __
- given out
- taken in
Does oxidation and reduction take place together or separately
It always takes place together
Describe the process of photoionisation
- When a chlorophyll molecule absorbs light energy, it boosts the energy of a pair of electrons within this chlorophyll molecule
- this raises them to a higher energy level.
- These electrons are said to be in an excited state.
- In fact the electrons become so energetic that they leave the chlorophyll molecule altogether.
- As result the chlorophyll molecule becomes ionised and so the process is called photoionisation.
The electrons that leave the chlorophyll are taken up by a molecule called…
An electron carrier
Is the chlorophyll molecule oxidised or reduced as a result of photoionisation
- Having lost a pair of electrons, the chlorophyll molecule has been oxidised.
- The electron carrier, which has gained electrons, has been reduced,
How does the electrons that have left the chlorophyll molecule help create ATP
- The electrons are now passed along a number of electron carriers in a series of oxidation-reduction reactions.
- These electron carriers form a transfer chain that is located in the membranes of the thylakoids.
- Each new carrier is at a slightly lower energy level than the previous one in the chain, and so the electrons lose energy at each stage.
- Some of this energy is used to combine an inorganic phosphate molecule with an ADP molecule in order to make ATP.
The precise mechanism by which ATP is produced can be explained by which theory?
chemiosmotic theory
Describe how chemiosmotic theory works
- Each thylakoid is an enclosed chamber into which protons (H*) are pumped from the stroma using protein carriers in the thylakoid membrane called proton pumps.
- The energy to drive this process comes from electrons released when water molecules are split by light — photolysis of water
- The photolysis of water also produces protons which further increases their concentration inside the thylakoid space.
- Overall this creates and maintains a concentration gradient of protons across the thylakoid membrane with a high concentration inside the thylakoid space and a low concentration in the stroma.
- the protons can only cross the thylakoid membrane through ATP synthase channel proteins — the rest of the membrane is impermeable to protons. These channels form small granules on the membrane surface and so are known as stalked granules
- As the protons pass through these ATP synthase channels by facilitated diffusion they cause changes to the structure of the enzyme which then catalyses the combination of ADP with inorganic phosphate to form ATP.
Describe how photolysis of water works
- The loss of electrons when light strikes a chlorophyll molecule leaves it short of electrons.
- If the chlorophyll molecule is to continue absorbing light energy, these electrons must be replaced.
- The replacement electrons are provided from water molecules that are split using light energy.
- This photolysis of water also yields protons.
Describe the role of NADP in photolysis
- As protons pass out of the thylakoid space through the ATP synthase channels, they are taken up by an electron carrier called NADP.
- On taking up the protons the NADP becomes reduced.
- The reduced NADP is the main product of the light-dependent stage and it enters the light-independent reaction taking with it the electrons from the chlorophyll molecules.
- The reduced NADP is important because it is a further potential source of chemical energy to the plant.
- The oxygen by-product from the photolysis of water is either used in respiration or diffuses out of the leaf as a waste product of photosynthesis
What is the equation for photolysis
the light-dependent reaction of photosynthesis takes place in which part of the chloroplasts
The thylakoid
How are chloroplasts structurally adapted to their function of capturing sunlight and carrying out the light-dependent reaction of photosynthesis
- The thylakoid membranes provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes that carry out the light-dependent reaction.
- A network of proteins in the grana hold the chlorophyll in a very precise manner that allows maximum absorption of light.
- The granal membranes have ATP synthase channels within them, which catalyse the production of ATP. They are also selectively permeable which allows establishment of a proton gradient.
- Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins involved in the light-dependent reaction.
State precisely where within a plant cell the electron carriers involved in the light-dependent reaction are found.
onthe thylakoid membranes (of the grana in the chloroplast)
Describe what happens in the photolysis of water.
Water molecules are split to form electrons, protons and oxygen, as a result of light exciting electrons / raising the energy levels of electrons in chlorophyll molecules.
Ineach of the following, state whether the process involves oxidation or reduction of the molecule named:
A) An unsaturated fat molecule gains a hydrogen atom.
B) Oxygen is lost from a carbon dioxide molecule.
C) Light causes an electron to leave a chlorophyll molecule.
A) reduction
B) reduction
C) Oxidation
Name the parts labeled A,C,D
A) double membrane
C) granum
D) stroma
State in which of these labelled parts the lightdependent reaction takes place
C
Structure B is used for storage. Suggest the name of the substance likely to be stored in B.
Starch
ATP is produced in the light-dependent reaction of photosynthesis. Suggest three reasons why plants cannot use this as their only source of ATP.
1) the light-dependent reaction does not produce sufficient ATP for the plants’ needs
2) photosynthesis does not take place in the dark
3) cells without chlorophyll cannot produce ATP in this way and ATP cannot be transported around the plant.
