● 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"`.

● An understanding of the `color{violet}"factors that affect photosynthesis"` is necessary.


● The `color{violet}"rate of photosynthesis"` is very important in determining the `color{violet}"yield of plants"` including crop plants.

● `color{violet}"Photosynthesis"` is under the influence of `color{violet}"several factors"`, both `color{Brown}"internal (plant) and external"`.

● The plant factors include the `color{violet}"number, size, age"` and `color{violet}"orientation of leaves"`, `color{violet}"mesophyll cells"` and `color{violet}"chloroplasts, internal"` `CO_2` `color{violet}"concentration"` and the `color{violet}"amount of chlorophyll"`.

● The `color{violet}"plant or internal factors"` are dependent on the `color{violet}"genetic predisposition"` and the `color{violet}"growth of the plant"`.

● The `color{violet}"external factors"` would include the availability of `color{violet}"sunlight, temperature"`, `CO_2` `color{violet}"concentration and water."`

● As a `color{violet}"plant photosynthesises"`, all these factors will `color{violet}"simultaneously affect"` its rate.

● Hence, though `color{violet}"several factors interact"` and simultaneously `color{violet}"affect photosynthesis"` or `CO_2` fixation, usually one factor is the `color{violet}"major cause"` or is the one that limits the rate.

● Hence, at `color{violet}"any point"` the rate will be determined by the `color{violet}"factor available"` at `color{violet}"sub-optimal levels."`

● When several factors affect `color{violet}"any [bio] chemical process"`, `color{violet}"Blackman’s (1905)"` `color{Brown}"Law of Limiting Factors'"` comes into effect.

● This `color{violet}"states the following"`:

If a `color{violet}"chemical process"` is affected by `color{violet}"more than one factor,"` then its rate will be `color{violet}"determined by the factor"` which is nearest to its `color{violet}"minimal value"`: it is the factor which `color{violet}"directly affects the process"` if its quantity is changed.

● For example, despite the `color{violet}"presence of a green leaf"` and `color{violet}"optimal light"` and `CO_2` conditions, the plant may not photosynthesise if the `color{violet}"temperature is very low"`.

● This leaf, if given the `color{violet}"optimal temperature"`, will start `color{violet}"photosynthesising"`.