The cell is the structural and functional unit of all living organisms, and is sometimes called the "building block of life." Some organisms, such as bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 µm; a typical cell mass is 1 nanogram.) The largest known cell is an ostrich egg.

The cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells. All cells come from preexisting cells. Vital functions of an organism occur within cells, and all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

The word cell comes from the Latin cellula, a small room. The name was chosen by Robert Hooke when he compared the cork cells he saw to the small rooms monks lived in.

Properties of cells

Each cell is at least some what self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities.

All cells share several abilities:

• Reproduction by cell division (binary fission, mitosis or meiosis).
• Use of enzymes and other proteins coded for by DNA genes and made via messenger RNA intermediates and ribosomes.
• Metabolism, including taking in raw materials, building cell components, converting energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules.
• This energy is derived from metabolic pathways.
• Response to external and internal stimuli such as changes in temperature, pH or nutrient levels.
• Cell contents are contained within a cell surface membrane that contains proteins and a lipid bilayer.
• Some prokaryotic cells contain important internal membrane-bound compartments,but eukaryotic cells have a highly specialized endomembrane system characterized by regulated traffic and transport of vesicles.

There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually singletons, while eukaryotic cells are usually found in multicellular organisms.

Continued: Cell Biology Wikipedia

Scientists weigh a single cell BBC - April 26, 2007

Scientists have managed to measure the mass of single living cells to an unprecedented level of accuracy. Previously, such precise measurements of living cells were impossible because any sample would need to be dried - a process that kills the cells. Samples as light as one thousandth of a millionth of a millionth of a gram (one femtogram) can now be weighed while they remain in fluid. The work by US researchers is published in the journal Nature.

Thomas Burg, an author of the study from the department of biological engineering at the Massachusetts Institute of Technology (MIT), said: "The most precise mass measurements done today, down to the zeptogram (one thousandth of a billionth of a billionth of a gram) [in weight], require that objects are weighed in a vacuum. "But so far, we haven't been able to do this for living biological samples."

So the team, led by Dr Scott Manalis, an associate professor of biological and mechanical engineering at MIT, decided to re-examine these ultra-accurate systems - called micromechanical resonators - to see if they could be modified to measure materials in fluid. Traditional micromechanical resonators work by attaching a sample to a tiny solid silicon slab, called a resonator, within a vacuum and making it vibrate.

Because objects of different mass vibrate at different frequencies, their mass can then be calculated. But if fluid is then added to the vacuum, the sensitivity of the measurement is diminished, Dr Burg explained. "So, we turned the problem inside out," he said. "We decided to make a hollow resonator, within which you could put the fluid sample. The resonator is still surrounded by a vacuum, but the fluid is inside of it. So you 'ping' it, it vibrates, and you can then look at the frequency to determine the mass."

The researchers say this means that living cells and other samples that need to be kept in fluid can be measured to a much higher degree of accuracy than was possible before. They hope to develop the technique further so even lighter objects can be weighed, and in the future it may have a number of applications in cell biology, such as seeing how the mass of a cell changes as it goes through cell division.

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