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What is a Cell?

Paramecia are unicellular organisms.
An animal cell.
A plasma membrane keeps the cell intact.
Plant cells are larger than animals cells are are surrounded by a rigid cell wall.
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  • Written By: Michael Anissimov
  • Edited By: Bronwyn Harris
  • Last Modified Date: 03 October 2014
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A cell is the most fundamental unit of biological life. All known life, except for viruses, is made up of cells. Cells are also the smallest metabolically functional unit of life, meaning the smallest unit that can take in nutrients from the bloodstream, convert them into energy, perform useful functions, and excrete waste. There are two primary types of cells in the kingdom of life - prokaryotic cells, smaller bacterial cells without a nucleus, and eukaryotic cells, larger plant and animal cells with a true nucleus.

Cells are quite small. Prokaryotic cells are typically 1-10 µm (micrometers, or millions of a meter) across, while eukaryotic cells are 10-100 µm. Eggs are large single cells, and the largest known cell today is the egg of the ostrich, although prehistoric birds and some dinosaurs had eggs almost a foot in length. Every cell is produced from another cell, and each contains special genetic programming to manufacture proteins to replace things when they break down, divide, and perform the functions of life.

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An aggregation of cells is known as a multicellular organism, humans being one example. These cells are so tiny and numerous, and work together so smoothly and uniformly that it took until 1839 for us to figure out that all life is made of cells. This "cell theory" is attributed to Matthias Jakob Schleiden and Theodor Schwann, German botanists who observed cells under a microscope. Soon after, Robert Hooke, the English scientist, named these little structures cells, after the Latin cellula, meaning a small room.

Another difference between prokaryotic and eukaryotic cells is the presence of intracellular machinery, or organelles. Prokaryotic organelles are quite minimal, with a plasma membrane (phospholipid bilayer) that does most of the work done by specialized organelles in eukaryotes, such as serving as the power plant of the cell and packaging macromolecules synthesized by the ribosomes. Aside from the ribosomes, cytoplasm (cell fluid), and the plasma membrane, prokaryotic cells may have another additional organelle called the mesosomes, but recent research suggests that these may merely be artifacts formed during the process of chemical fixation for electron microscopy and thus not even natural.

For some organelles in the more complex eukaryotic cells, see the article "What are some Organelles in the Cell?"

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anon350802
Post 10

What does an animal cell look like?

allenJo
Post 9

@miriam98 - I think the cell organelles are the real machinery of life. They are the ones that seem to perform specific functions for the cells.

I’ve seen videos of these little wavy creatures floating about inside the cells, busily performing whatever tasks they are supposed to be doing. These are the real work horses in my opinion, not the cells themselves.

miriam98
Post 8

@Charred - I guess I can see the point he was making, but from a biological perspective there are levels of complexity as the article makes clear.

I once saw a computer animation called the game of life, which simulated the multiplication and division of cell growth. I don’t know which kinds of cells it was simulating, but it seemed realistic.

Creators of the program vouched that it was true to real life cell life reproduction, anyway. Cells multiplied and divided very quickly, creating new generations while old generations died out.

Charred
Post 7

I read a book by a molecular biologist once. He had studied the human cell and knew it inside and out.

In the book he explained that there really is no such thing as a “simple cell.” From a biological standpoint, those distinctions exist, and they’re important, but his point was that there was nothing simple about a cell.

It’s a miniature machine, processing a whole host of functions, just like the article says. To replicate the functionality of a single human cell using human technology would be a feat itself.

When you consider the vast number of cells that make up the human body – and their ability to work together so seamlessly as the article talks about – it’s even more amazing.

anon220453
Post 6

i need to know how bacteria cells excrete and secrete wastes. can anyone help me?

anon58973
Post 3

what is a description of a cell? i need help.

kennyli2
Post 2

it helped a lot.

anon3060
Post 1

what will be the damages of cell membrane if it will be damaged?

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