Biological cells

Biological cells come in many varieties. The simplest forms of life, such as bacteria, are composed of single cells. Animals and plants are made up of thousands, even hundreds of millions of cells.

Cells are rather more than simple building blocks from which larger multicellular can be constructed by stacking them on top of each other. They have complex internal structures, and internal dynamics. A typical cell is made up of a fluid inner core enveloped by an outer membrane. Within the inner core there are a number of cellular organs - called organelles -. The largest of these is the nucleus which contains DNA. The DNA carries sets of instructions which another kind of organelle - ribosomes - use to construct many different types of proteins. These proteins perform many tasks within the cell. Some of them are structural proteins which form the physical structure of the cell. Others are capable of performing a variety of tasks. Some can break down proteins and fats into small subcomponents for re-use. Others can assemble fats and sugars and protein components into larger assemblages - the ribosomes that make these various proteins are themselves made up of a number of proteins. Others, embedded in the cell outer membrane, are able to capture and induct various sorts of molecules into the cell. The cell membrane can also induct and fold around materials in the external environment, and induct them into the cell in membrane- bound packages called pinosomes, whose contents are then released into the cell after being broken down by digestive proteins. All the various components of cells have quite short lifetimes, and need to be broken down and have their components re-used to construct replacements. The interior of a cell is a hive of activity in which proteins and other components are continually being broken up and reconstructed: cells are continually remaking themselves. This industry requires raw materials and energy to drive it. Although cells recycle many of their components, growing cells have to induct extra raw materials. Virtually all the energy-using cell processes are powered by the energy released by combining glucose and oxygen. This is equivalent to burning sugar, but in cells another set of organelles - called mitochondria - perform this oxidization in stages, each releasing a small amount of energy into a number of energy-rich packets - called ATP. These packets, which might be thought of as tiny rechargeable batteries which deliver one shot of energy, are used to power almost all the cell's various industries, before being recycled round to the mitochondria to be recharged. To maintain the cell's activity, sugars - glucose among them -, and oxygen, and proteins, and any number of other materials, need to be inducted into the cell from the external environment. And at the same time, waste materials, such as the carbon dioxide released in oxidizing glucose, need to be expelled from the cell.