The short intro I did in class is found here.
One of the main things a membrane has to be able to do is control what flows through it. Sometime, that's just opening a channel and letting the dissolved thing flow "down gradient." But, sometime the cell has to do active work to pump ions to one side or the other of a membrane.
Once that gradient is established, it can be used to do work…and in fact used in myriad ways you can hardly imagine. Since it is so important, we will spend some time on it.
By the way, the figure is a cartoon of a protein called "aquaporin." Guess what it transports. It's a fascinating protein. Think about how you can make a protein that allows water…and only water…to pass through.

The image above was lifted from an article in "The Daily Mail," found here.
It is a really bad representative cell, since it is a famous cancer cell line known as HeLa cells and it behaves in ways very much unlike good, normal cells. But, you can see a nucleus (stained for DNA in blue) and some fibers that are part of the "cytoskeleton." To see those, the researchers have used a technique to make them glow either red (microtubules) or green (actin fibers…or microfilaments.

This is a bit of a recap on cytoskeleton and junctions.

Enzymes really are at the heart of how our bodies work. The brief definition is a "bio-catalyst." These are almost always proteins. They lower activation energy of a particular reaction and as such speed it up, both in the forward and backward direction. Almost all enzymes will be able to work either "backwards" or "forwards." Some are effectively one-directional because of large negative delta G⁰, for example involving ATP hydrolysis. But, even most of these can be reversed.
Enzyme function requires particular structures. Structures of proteins can be altered by the binding of other proteins or other smaller molecules, or the addition of a phosphate to the enzyme at a specific point, or the local pH or charge distribution…so, every step of enzyme function can be regulated.