● All living organisms `color{violet}"need energy"` for carrying out `color{violet}"daily life activities"`, be it absorption, transport, movement, reproduction or even breathing.

● In reality, the `color{violet}"process of breathing"` is very much connected to the process of `color{violet}"release of energy"` from food.

● All the energy required for `color{violet}"life’ processes"` is obtained by `color{violet}"oxidation of some macromolecules"` that we call ‘food’.

● Only `color{vBrown}"green plants and cyanobacteria"` can prepare their own food; by the `color{violet}"process of photosynthesis"`
they `color{violet}"trap light energy"` and convert it into `color{violet}"chemical energy"` that is stored in the `color{violet}"bonds of carbohydrates"` like glucose, sucrose and starch.

● We must remember that in `color{violet}"green plants"` too, not all `color{violet}"cells, tissues and organs"` photosynthesise; only `color{violet}"cells containing chloroplasts"`, that are most often located in the `color{violet}"superficial layers"`, carry out photosynthesis.

● Hence, even in green plants all other organs, tissues and cells that are `color{violet}"non-green"`, need `color{violet}"food for oxidation"`. Hence, food has to be translocated to all nongreen parts.

● `color{Brown}"Animals are heterotrophic"`, i.e., they obtain food from `color{violet}"plants directly"` (`color{Brown}"herbivores"`) or indirectly (`color{Brown}"carnivores"`).

● `color{Brown}"Saprophytes"` like fungi are dependent on `color{violet}"dead and decaying matter"`.

● These all Organisms carry out `color{Brown}"cellular respiration"` or the `color{violet}"mechanism of breakdown"` of food materials within the cell to `color{violet}"release energy"`, and the `color{violet}"trapping of this energy"` for synthesis of ATP.

● `color{Brown}"Photosynthesis"`, of course, takes place within the `color{violet}"chloroplasts"` (in the eukaryotes), whereas the `color{violet}"breakdown of complex molecules"` to yield energy takes place in the `color{violet}"cytoplasm"` and in the `color{Brown}"mitochondria"` (also only in eukaryotes).

● The `color{violet}"breaking of the C-C bonds"` of complex compounds through `color{violet}"oxidation"` within the cells, leading to release of `color{violet}"considerable amount of energy"` is called `color{Brown}"respiration"`.

● The `color{violet}"compounds"` that are `color{violet}"oxidized"` during this process are known as `color{Brown}"respiratory substrates"`.

● Usually `color{violet}"carbohydrates are oxidised"` to release energy, but `color{violet}"proteins, fats and even organic acids"` can be used as `color{violet}"respiratory substances"` in some plants, under certain conditions.

● During oxidation within a cell, all the `color{violet}"energy contained"` in `color{violet}"respiratory substrates"` is not released free into the cell, or in a `color{violet}"single step"`.

● It is released in a `color{violet}"series of"` `color{violet}"slow step-wise reactions"` controlled by `color{violet}"enzymes"`, and it is trapped as chemical energy in the form of `color{violet}"ATP."`

● Hence, it is important to understand that the `color{violet}"energy released by oxidation"` in respiration is not (or rather cannot be) `color{violet}"used directly"` but is used to `color{violet}"synthesise ATP,"` which is broken down whenever (and wherever) `color{violet}"energy needs"` to be utilised.

● Hence, `color{Brown}"ATP"` acts as the `color{violet}"energy currency"` of the cell.

● This `color{violet}"energy trapped in ATP"` is utilised in various `color{violet}"energy-requiring processes"` of the organisms, and the `color{violet}"carbon skeleton"` produced during respiration is used as `color{violet}"precursors for biosynthesis"` of other molecules in the cell.

● Plants require `O_2` for `color{violet}"respiration"` to occur and they also `color{violet}"give out"` `CO_2`.

● Hence, plants have `color{violet}"systems in place"` that ensure the `color{violet}"availability"` of `O_2`.

● `color{violet}"Plants, unlike animals"`, have no specialised organs for `color{violet}"gaseous exchange"` but they have `color{Brown}"stomata and lenticels"` for this purpose.

● There are `color{violet}"several reasons"` why plants can get along `color{violet}"without respiratory organs"`.

`star` First, `color{violet}"each plant part"` takes care of its own `color{violet}"gas-exchange needs"`.

`star` There is `color{violet}"very little transport of gases"` from one plant part to another.

`star` Also, plants `color{violet}"do not present great demands"` for gas exchange.

● `color{violet}"Roots, stems and leaves"` respire at rates far lower than `color{violet}"animals"` do.

● Only during photosynthesis are `color{violet}"large volumes of gases"` exchanged and, each `color{violet}"leaf is well adapted"` to take care of its own needs during these periods.

● When `color{violet}"cells photosynthesise"`, availability of `O_2` is not a problem in these cells since `O_2` is released within the cell.

● Third, the `color{violet}"distance"` that `color{violet}"gases must diffuse"` even in large, `color{violet}"bulky plants"` is not great.

● Each `color{violet}"living cell in a plant"` is located quite `color{violet}"close to the surface"` of the plant.

● In `color{Brown}"stems"`, the ‘living’ cells are organised in `color{violet}"thin layers inside"` and beneath the bark. They also have `color{violet}"openings"` called `color{Brown}"lenticels"`.

● The cells in the `color{violet}"interior are dead"` and provide only `color{violet}"mechanical support"`.

● Thus, most `color{violet}"cells of a plant"` have at least a `color{violet}"part of their surface"` in contact with air.

● This is also `color{violet}"facilitated"` by the `color{violet}"loose packing of parenchyma"` cells in leaves, stems and roots, which provide an `color{violet}"interconnected network"` of air spaces.