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Summary Topic 5_ Energy Transfers in and between Organisms.
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Life depends on continuous transfers of energy. In photosynthesis, light is absorbed by chlorophyll and this is linked to the production of ATP. In respiration, various substances are used as respiratory substrates. The hydrolysis of these respiratory substrates is linked to the production of ATP...

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  • February 1, 2025
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53.5 Energy transfers in and between organisms


Life depends on continuous transfers of energy.
In photosynthesis, light is absorbed by chlorophyll and this is linked to the production of
ATP.
In respiration, various substances are used as respiratory substrates. The hydrolysis of
these
respiratory substrates is linked to the production of ATP.
In both respiration and photosynthesis, ATP production occurs when protons diffuse down
an
electrochemical gradient through molecules of the enzyme ATP synthase, embedded in the
membranes of cellular organelles.
The process of photosynthesis is common in all photoautotrophic organisms and the
process of
respiration is common in all organisms, providing indirect evidence for evolution.
In communities, the biological molecules produced by photosynthesis are consumed by
other
organisms, including animals, bacteria and fungi. Some of these are used as respiratory
substrates
by these consumers.
Photosynthesis and respiration are not 100% efficient. The transfer of biomass and its
stored
chemical energy in a community from one organism to a consumer is also not 100%
efficient.



3.5.1 Photosynthesis

, The light-dependent reaction :

1. Photons of light hit chlorophyll molecules in PSII causing electrons to become
excited
○ photoionisation
○ charge separation from this drives the process of photolysis
2. Photolysis
○ splitting of water with light
○ One molecule of water requires 4 photons of light to split and produces
■ 1 molecule of oxygen
■ 4 protons
■ 4 electrons
○ Oxygen either naturally diffuses out through stomata or is used in respiration
○ Oxygen either naturally diffuses out through stomata or is used in respiration
○ 4 electrons replace those lost from chlorophyll
■ protons move into stroma creating proton gradient
3. Excited electron then moves down series of protein complexes
○ at one of complexes energy from electron used to pump 4 protons from
stroma to thylakoid space
4. Electron then moves down chain further to PSI
○ more photons of light absorbed causing electron to move back up to high
energy level
5. Electron then moves along chain to another complex
○ Electron combines with a proton to form hydrogen atom
■ used to reduce NADP
■ forming reduced NADP
6. Pumping of protons across membrane means there is now greater concentration of
protons in thylakoid space than stroma
○ proton gradient forms with high concentration in thylakoid space and low
concentration in stroma
■ protons move across membrane by diffusion through protein known as
stalked particle
■ movement of protons drives process of phosphorylation
■ enzyme ATP Synthase phosphorylates ATP from ADP and Pi
The hydrolysis of ATP, also from the light-dependent reaction, provides the additional
energy for this reaction.




The light-independent reaction in such detail as to show that:

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