I am curious how, food broken down into glucose powers the movement of proteins in our cells (e.g., in muscle fibers for example). Is the thermal energy converted to kinetic energy somehow? As a mechanical engineer, I am just trying to visualize how the energy is converted to mechanical motion at the cellular level.
Scope of this Answer
This answer attempts to summarize the principles involved in biological energetics, with only the most minimal reference to particular molecules or systems. It is directed at those approaching the subject for the first time, particularly from a physical science or engineering background.
Energetic processes in biology
Energetic processes in biology involve chemical energy — essentially the energy embodied in the chemical bonds between the atoms of molecules.
In considering biological processes involving energy transfer, it is usual to adopt a thermodynamic approach in which the change in (Gibbs) Free Energy is considered. Chemical reactions are energetically unfavourable if they involve an increase in free energy, but are energetically favourable if they involve a decrease in free energy. In reactions of the latter type occurring in isolation, the chemical bond energy is converted to thermal energy (heat). In biological chemistry, however, processes that involve an increase in free energy are made possible by ‘combining’ them with such energetically favourable processes so that, rather than being released (lost†) as heat, the free energy is used to achieve an overall negative free energy change.
Such a ‘combination’ of two reactions (generally termed ‘coupling’) is, in effect, a different reaction (or succession of reactions), although the overall free energy change is the same as the sum of the separate ones. The coupling requires specialized protein macromolecules (enzymes), which actually participate in the reaction (naïvely speaking, direct the flow of electrons from one bond to another) and help lower the activation energy of the reaction that prevents it occurring spontaneously at ambient temperature.
The process of acquiring energy from food consists of a series of energetically favourable oxidation reactions that eventually result in the bond energy of the initial food molecule (carbohydrate, fat etc.) being used to form bonds in a ‘form’ that is useful to the cell for driving energetic processes. This ‘form’ is predominantly the phosphodiester bonds of a molecule entitled ATP, which not only has a high negative free energy of hydrolysis (which breaks these bonds), but for which a plethora of enzymes (and other proteins) have evolved that can facilitate the transfer of this bond energy to form bonds in other molecules.
This explains in principle how the energy from oxidation of foodstuffs can be used to sythesize other complex molecules required by the cell — chemical energy being both donor and recipient. What about the conversion of chemical energy to mechanical energy?¶ In general in these cases the hydrolysis of ATP is coupled to a change in conformation of a protein and its points of interaction with other proteins, resulting in their relative movement, as in the case of the actin and myosin components of muscle fibres. Proteins are molecules with three-dimensional conformations determined by the most energetically favourable combination of many non-covalent chemical interactions (mainly hydrogen bonds and hydrophobic interactions). Often there exist alternative structures with only small difference in free energy, so that a change in structure (or the point of interaction with another protein) can be effected by coupling this (although energetically unfavourable) to the hydrolysis of ATP.
† Thermal energy generated by metabolic processes can not be used in the directed manner implied in the question. Its only use for the organism would be to maintain body temperature (and indeed the specialized brown fat cells exist for that purpose).
¶ The question refers to kinetic energy, a term that is not much used in biochemical energetics, except in relation to the random movement of molecules. Although a protein moving along a filament will, I suppose, possess kinetic energy (à la billiard ball) this is a consequence of the changes in interaction of molecules, rather than from a direct ‘cue stroke’ from ATP hydrolysis.
Beyond the Principles
To go beyond the principles, one must read the book, as it were, and get to grips with the molecules. In my original answer I gave links below to some relevant sections of Biochemistry by Berg et al. the fifth edition of which was then available on NCBI Bookshelf. This is no longer the case, presumably because the edition is 20 years old. However as a basic introduction for the cash-strapped student who is not going to specialize in biochemistry, a second-hand copy of this edition is generally to be had on ebay for the price of two cups of coffee. The relevant sections are:
- Free Energy, chemical reactions and ATP
- Thermodynamics of redox reactions
- Glycolysis, the Citric Acid Cycle and ATP synthesis
- Motor Proteins
I agree with a comment that your question is broad, but I think a good answer can be given anyway. That answer is essentially: ATP, Adenosine Triphosphate, also known as "the energy currency of the cell".
This small molecule has three phosphate groups, hence the name, and energy is released when one or two of those are separated from the molecule (creating ADP or AMP, Adenosine Diphosphate and Adenosine Monophosphate). This energy is what thermodynamically allows most endergonic reactions in the organism. The phosphate groups can then easily be added back, recycling the components back to ATP. This mediator, "currency" allows energy to be transferred from chemical reaction to chemical reaction: a reaction that "creates energy for the cell" really regenerates ATP, which goes wherever and is broken down in reactions that "use energy".
See for example this textbook describing how the breakdown of carbohydrates produces ATP: https://opentextbc.ca/anatomyandphysiology/chapter/24-2-carbohydrate-metabolism/
And I will recycle Bryan Krause's link in comments to a question about how ATP is used in muscle contractions, which I believe is the final step of answering your question: How is ATP involved in muscle contraction?