● There is `color{violet}"one more process"` that creates an important difference between `C_3` and `C_4` plants – `color{Brown}"Photorespiration"`.
● To `color{violet}"understand photorespiration"` we have to know a little bit more about the first step of the `color{violet}"Calvin pathway"` – the first `CO_2` fixation step.
● This is the reaction where `color{violet}"RuBP combines"` with `CO_2` to form `color{violet}"2 molecules of 3PGA"`, that is catalysed by `color{violet}"RuBisCO"`.
● `color{violet}"RuBisCO"` that is the most abundant enzyme in the world is `color{violet}"characterised"` by the fact that its `color{violet}"active site"` can bind to both `CO_2` and `O_2` – hence the name.
● `color{violet}"RuBisCO"` has a much `color{violet}"greater affinity"` for `CO_2` than for `O_2`.
● This binding is `color{violet}"competitive"`.
● It is the `color{violet}"relative concentration"` of `O_2` and `CO_2` that determines which of the two will `color{violet}"bind to the enzyme"`.
● In `C_3` plants some `O_2` does `color{violet}"bind to RuBisCO"`, and hence `CO_2` `color{violet}"fixation is decreased"`.
● Here the `color{violet}"RuBP"` instead of being converted to `color{violet}"2 molecules of PGA"` binds with `O_2` to form one molecule and `color{violet}"phosphoglycolate"` in a pathway called `color{iBrown}"photorespiration"`.
● In the `color{violet}"photorespiratory pathway"`, there is neither synthesis of `color{violet}"sugars, nor of ATP. "`
● Rather it results in the `color{violet}"release"` of `CO_2` with the `color{violet}"utilisation of ATP."`
● In the `color{violet}"photorespiratory pathway"` there is no synthesis of `color{violet}"ATP or NADPH"`.
● Therefore, `color{violet}"photorespiration"` is a `color{violet}"wasteful process"`.
● In `C_4` plants `color{violet}"photorespiration does not"` occur.
● This is because they have a `color{violet}"mechanism"` that `color{violet}"increases the concentration"` of `CO_2` at the enzyme site.
● This takes place when the `C_4` acid from the `color{violet}"mesophyll is broken"` down in the `color{violet}"bundle cells to release"` `CO_2` – this results in increasing the `color{violet}"intracellular concentration"` of `CO_2`.
● In turn, this ensures that the `color{violet}"RuBisCO functions"` as a `color{violet}"carboxylase minimising"` the `color{violet}"oxygenase activity"`.
● In addition these `C_4` plants show `color{violet}"tolerance to higher temperatures"`.
● There is `color{violet}"one more process"` that creates an important difference between `C_3` and `C_4` plants – `color{Brown}"Photorespiration"`.
● To `color{violet}"understand photorespiration"` we have to know a little bit more about the first step of the `color{violet}"Calvin pathway"` – the first `CO_2` fixation step.
● This is the reaction where `color{violet}"RuBP combines"` with `CO_2` to form `color{violet}"2 molecules of 3PGA"`, that is catalysed by `color{violet}"RuBisCO"`.
● `color{violet}"RuBisCO"` that is the most abundant enzyme in the world is `color{violet}"characterised"` by the fact that its `color{violet}"active site"` can bind to both `CO_2` and `O_2` – hence the name.
● `color{violet}"RuBisCO"` has a much `color{violet}"greater affinity"` for `CO_2` than for `O_2`.
● This binding is `color{violet}"competitive"`.
● It is the `color{violet}"relative concentration"` of `O_2` and `CO_2` that determines which of the two will `color{violet}"bind to the enzyme"`.
● In `C_3` plants some `O_2` does `color{violet}"bind to RuBisCO"`, and hence `CO_2` `color{violet}"fixation is decreased"`.
● Here the `color{violet}"RuBP"` instead of being converted to `color{violet}"2 molecules of PGA"` binds with `O_2` to form one molecule and `color{violet}"phosphoglycolate"` in a pathway called `color{iBrown}"photorespiration"`.
● In the `color{violet}"photorespiratory pathway"`, there is neither synthesis of `color{violet}"sugars, nor of ATP. "`
● Rather it results in the `color{violet}"release"` of `CO_2` with the `color{violet}"utilisation of ATP."`
● In the `color{violet}"photorespiratory pathway"` there is no synthesis of `color{violet}"ATP or NADPH"`.
● Therefore, `color{violet}"photorespiration"` is a `color{violet}"wasteful process"`.
● In `C_4` plants `color{violet}"photorespiration does not"` occur.
● This is because they have a `color{violet}"mechanism"` that `color{violet}"increases the concentration"` of `CO_2` at the enzyme site.
● This takes place when the `C_4` acid from the `color{violet}"mesophyll is broken"` down in the `color{violet}"bundle cells to release"` `CO_2` – this results in increasing the `color{violet}"intracellular concentration"` of `CO_2`.
● In turn, this ensures that the `color{violet}"RuBisCO functions"` as a `color{violet}"carboxylase minimising"` the `color{violet}"oxygenase activity"`.
● In addition these `C_4` plants show `color{violet}"tolerance to higher temperatures"`.