A single molecule is the one that is responsible for initiating a signal transduction pathway. The answer in this question is a single molecule. Molecule is defined as an electrically neutral group of two atoms or more held together by a chemical bonds.
Answer;
Light microscope
Explanation;
-A light microscope is an instrument that uses visible light and magnifying lenses to examine small objects not visible to the naked eye, or in finer detail than the naked eye allows.
-Light microscopes use a single lens to magnify an object and cannot reach high magnification. Illuminator is the light source for a microscope, typically located in the base of the microscope.
In cellular respiration, the cells take the carbohydrates in and through complex metabolic processes, they break them down and release the energy.
Pressure would be my best guess, since no choices were given.