In my textbook it is given that photosynthesis is a physico-chemical process. How can it be a physical process?
Well, all processes are basically physical.
If you search "physicochemical processes" in NCBI Medical Subject Headings it displays the page on "Chemical Phenomena" which is defined as:
The composition, structure, conformation, and properties of atoms and molecules, and their reaction and interaction processes.
Also, this website (Reference.MD) defines it as:
Physical reactions involved in the formation of or changes in the structure of atoms and molecules and their interactions.
In any case, there are several "physical reactions" (reactions that do not involve making and breaking of chemical [covalent] bonds) happening during photosynthesis. These would include:
- Movement of protons and electrons
- The rotation of the ATP synthase
- Absorption of light by the photosystem and activation of the reaction center.
This question appears to be based on the false premise that the ‘physico’ component of the word ‘physico-chemical’ has that meaning of the word ‘physical’ that relates to bodily activity. It does not. It refers to ‘physics’ and physico-chemical has the meaning of ‘relating to physics and chemistry’.
I had pointed this out in a comment, suggesting that the question should therefore be closed. However this has not happened, my comment has been deleted, and one of the moderators has implied approval of the question by providing an answer. In these circumstances, and because I disagree with the answer from @WYSWYG I have felt obliged to provide an answer of my own.
What is the question?
The question in the body of the text relates clearly to the meaning of the word physico-chemical. It is necessary to define it first to point out the false premise on which the question is based. The title asks a slightly different question — “why photosynthesis is a physico-chemical process?” — which does not entail this premise, and which I shall attempt to answer.
Meaning of ‘physico-chemical’
“Relating to physics and chemistry or to physical chemistry.”
This, incidentally, as the definition Google uses (without attribution) when you search for the term.
“Of, concerned with, or relating to physical chemistry or both physics and chemistry”
And, although it is the poster — rather than the unnamed textbook — who uses the term ‘physical’, it is worth pointing out that the latter adjective can have at least three meanings in English, the third of which is given by the Oxford Dictionary as:
“Relating to physics or the operation of natural forces generally.”
So that the use of this word without further clarification is ambiguous.
Physics and Chemistry are regarded as different
Although there are interfaces between different scientific disciplines, I do not regard statements such as that made in another answer — “all processes are basically physical” — helpful in explaining the physico-chemical nature of photosynthesis. Indeed, the very use of the adjective implies that the two are generally regarded as different. Dictionary definitions of scientific concepts are not always correct, but the following seem reasonable:
“The branch of science concerned with the nature and properties of matter and energy.”
“the branch of science concerned with the substances of which matter is composed, the investigation of their properties and reactions, and the use of such reactions to form new substances.”
Why is photosynthesis a physico-chemical process?
To quote the title of the first section on photosynthesis in the 6th edition of Berg et al. Biochemistry:
“Photosynthesis converts light Energy into Chemical Energy”
The study of light belongs to the realm of physics — its wave and particle properties, its reflection, refraction, polarization, diffraction etc. Biological metabolic chemistry usually involves the interaction of molecules to form different molecules. Of course it has a physical (= related to physics) component in that it obeys the laws of thermodynamics, it involves the activation energy of molecules, and the flow of electrons. But how photosynthesis is different — why the author took the trouble to point out that it was a physico-chemical process — is because it is the energy of light — of a photon — that drives the thermodynamically unfavourable chemical reactions.
The details of photosynthesis are complex, but the following figure from Berg et. al. illustrates the essential concept:
[Fig. 19.2 Light is absorbed and the energy is used to drive electrons from water to generate NADPH and to drive protons across a membrane. These protons return through ATP synthase to make ATP.]
The energy of photons of light is being used to split water and reduce NAD+ to NADH. This is different from e.g. the normal cellular reactions that reduce NAD+.
Footnote: The terminology of other ‘physicalities’
Nomenclature in biological sciences is fluid, but it may be worth mentioning the current terms in use for other biochemical processes in which the term ‘physical’ has more to do with “Involving bodily contact or activity” than physics.
If the poster had thought of ‘physical’ in terms of things like muscle contraction, biomechanics is perhaps the term to use.
In his answer @WYSWYG seems to suggest that the rotation of the ATP synthase may be what the text meant by physico-chemical. The term normally used to describe ATP synthase is molecular motor. In any case the ATP synthase is not unique to photosynthesis.