Cell size & efficency. Surface area/volume ratio.

[fallen_6666]

Hi, Can someone help me and explain why cells become less efficient as size increases. I know the SA:Volume decreases as it grows but what does that mean exactly.

[Skye]

A greater proportional surface area means that there can be faster intake of nutrients and removal of wastes. This increases the concentration of nutrients and decreases the concentration of waste products inside the cell. Because the rates of chemical reactions depend on the concentration of reactants, and inversely on the concentration of products, this means reaction rates are increased.

[Primarygun]

How does a nucleus control a cell membrane?

[Xavier]

A cells nucleus, or the DNA within, 'controls' the rest of the cell by creating the building blocks from which it is made along with the construction tools and the tools that will ultimately be used for its demolition. The control seems rather weak but lifetimes of molecules and structures on the cellular level can be very short indeed (milliseconds) and still be effective. The control exerted simply through the supply of the fixtures and fittings can be very precise but it would be enormously hard to consciously design as it is hugely chaotic with vast numbers of variables in play at once (There are thousands of different proteins that float around in cells that we have no idea of the function of, if any, not to mention the dozens of different ions that are actively or passively manipulated around a cell - Calcium ions are currently thought to be the most important of the 'second messengers' that take information that is passed through the cell membrane (in the form of a particular chemical) and passes through the nuclear membrane to signal the arrival.)) This could only be brought into existence by a massive sequence of trials and error. Fortunately we have one called evolution!

 

The way the system has evolved for earthbound life uses chemistry more the way we use electricity; almost instant and highly manipulable (is you have the appropriate setup) to exert effortless control over almost any aspect of the world. The scale makes it work as bulk transport/diffusion are far less limiting and there is none of this waiting for a reaction to reach equilibrium.

 

The insides of cells are landscaped with chemical gradients as reactions proceed at a rate of the same order as the time taken to diffuse across a portion of the cell so the ratio of product to reactant at any point tells how far it is from the site of release of that substrate.

 

Thats the trouble with 'How does it work' questions in biology; the answer is invariably either a) Its complicated; b) It simple...except for these several exceptions..or c) We don't know (but if you give us a million dollars we might be able to find out)

[Primarygun]

Thank you very much.