Lundegardh Hypothesis

Cytochrome Pump Theory:-

Ø Lundegardh and Burstrom (1933) believed that there was a definite correlation between respiration and anion absorption. Thus when a plant is transferred from water to a salt solu­tion the rate of respiration increases. This increase in rate of respiration over the normal res­piration has been called as anion respiration or salt respiration.

Ø The inhibition of salt respiration and the accompanying absorption of anions by CO and cyanides (which are known inhibitors of cytochrome oxidase of electron transport chain in mitochondria), later on led Lundegardh (1950, 54) to propose cytochrome pump theory. This is based on the following assumptions:-

i. The mechanism of anion and cation absorption is different.

ii. Anions are absorbed through cytochrome chain by an active process.

iii. Cations are absorbed passively.

Ø According to this theory:-

i. ehydrogenase reactions on inner side of the membrane give rise to protons (H⁺) and electrons (e^–).

ii. The electron travels over the cytochrome chain towards outside the membrane, so that the Fe of the cytochrome becomes reduced (Fe⁺⁺) on the outer surface and oxidized (Fe⁺⁺⁺) on the inner surface.

iii. On the outer surface, the reduced cytochrome is oxidized by oxygen releasing the electron (e^–) and taking an anion (A^–).

iv. The electron thus released unites with H⁺ and oxygen to form water.

v. The anion (A^–) travels over the cytochrome chain towards inside.

vi. On the inner surface the oxidized cytochrome becomes reduced by taking an elec­tron produced through the dehydrogenase reactions, and the anion (A^–) is released.

vii. As a result of anion absorption, a cation (M⁺) moves passively from outside to inside to balance the anion.

Ø Main defects of the above theory are:-

i. It envisages active absorption of only anions.

ii. It does not explain selective uptake of ions.

iii. It has been found that cations also stimulate respiration.