This document contains information from unit 5 Energy Transfers between and in organisms from AQA A-Level Biology. It is a summarised version for the stub topic photosynthesis with all that is needed to know.
PHOTOSYNTHESIS( TOPIC 5 - ENERGY TRANSFERS IN AND BETWEEN ORGANISMS )
LIGHT REACTIONS OF PHOTOSYNTHESIS
Photosynthesis takes place within the chloroplasts. There are two types of reactions to light that take place. They are
the light dependent reactions and the calvin cycle.
LIGHT DEPENDENT REACTIONS
The reaction occurs within the thylakoid and the excited electrons turn into energy to produce NADPH AND ATP
● PSII (ATP)
-PSII receives photons which are transferred to the chlorophyll reaction centre where the electrons
become excited
- The electrons escape PSII and move to the electron acceptor molecule
+¿¿
- A water molecule is split and releases 2 H , oxygen and 2 electrons to make up for the lost electrons
from PSII (process is called photolysis)
- The first set of electrons that moved from PSII moves down the ETC (electron transport chain)
releasing energy
- A hydrogen ion gradient is created because of the released energy
- A protein from the electron transport chain pumps ions into the thylakoid from the stroma
- There is a higher concentration of hydrogen ions in the thylakoid than the stroma
- The gradient produced contains a lot of E p (potential energy) which is used by ATP synthase.
+¿¿
- H flow down the concentration gradient through a channel in the ATP synthase which releases
energy in the process
- ATP synthase uses the energy to add Pi to ADP to form ATP
● PSI (NADPH)
- At PSI the electrons become energised when PSI absorbs more light energy
- The electrons leave PSI and move down the second ETC
- Electrons next to PSII move into PSI to replace the lost ones
- Photolysis takes place to replace the two electrons lost from PSII
¿
- The energised electrons at the end of the 2nd ETC and H ¿ are used to reduce NADP to NADPH
LIGHT INDEPENDENT REACTIONS - THE CALVIN CYCLE
The ATP and NADPH move to the stroma for the light independent reaction to take place (Calvin Cycle)#
- Carbon dioxide binds to the 5-carbon sugar ribulose bisphosphate (RuBP) to form 2 molecules of the 3- carbon
compound glycerate 3-phosphate (GP).
- This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase (rubisco) GP is reduced
and activated to form triose phosphate (TP)
- ATP and reduced NADP (NADPH) from the light-dependent reactions provide the energy for this step - one
molecule of each per GP
- The ADP, Pi and NADP return to the thylakoid membrane for recycling
- Most of the triose phosphate continues through a series of reactions to regenerate the RuBP and complete the
cycle. This requires ATP (1 per reformed RuBP)
- One new TP can be formed every 3 turns of the Calvin Cycle (since 3 CO2 molecules are ‘fixed’)
- This is used to synthesise useful organic substances
Making sugar
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