Cells

Functions and structure

• Can we see cells?• How many cells do you have in your

body?

• All living organisms on Earth are divided in pieces called cells. There are smaller pieces to cells that include proteins and organelles. There are also larger pieces called tissues and systems.

• A cell is a small, membrane enclosed structure filled with an aqueous solution where organelles and other subcellular structures are found.

• Cells are small compartments that hold all of the biological equipment necessary to keep an organism alive and successful on Earth.

Looking at cell functions

• A main purpose of a cell is to organize. Cells hold a variety of pieces but in the big picture, a cell's purpose is much more important than acting as small organizational pieces, there are several main functions that the cell has to carry out: molecule transport, reproduction, and energy conversion.

Cell types

• Living cells are divided into two types - procaryotic and eucaryotic (sometimes spelled prokaryotic and eukaryotic). This division is based on internal complexity.

PROKARYOTES - MISSING A NUCLEUS

• If you're looking to learn about cells with a nucleus, this is the wrong place. Prokaryotes do not have an organized nucleus. Their DNA is kind of floating around the cell. It's clumped up, but not inside of a nucleus.

Most prokaryotes are bacteria and bacteriacan do amazing things.Although they are verysimple organisms, they are found everywhere on the planet. Some scientists even think that they may be found on other planets (maybe even Mars). Some places you can find bacteria every day are in your intestines, a cup of natural yogurt, or a bakery. Prokaryotes are the simplest of simple organisms.

• Prokariotic main characteristics are:

(1) Prokaryotes have no organized nucleus. Like we said, the DNA is clumped in an area but there is no organized nucleus with a membrane.

(2) Prokaryotes do not usually have any organelles. They will probably have ribosomes inside of their cells, but ribosomes are not technically considered organelles. No chloroplasts. No mitochondria. No nucleus. Not much at all.

(3) Prokaryotes are very small. Because they don't have all of the normal cell machinery, they are limited in size. As always in biology, there are exceptions, but generally, prokaryotes are very small (compared to other cells). Mind you, compared to a virus they are big, but next to an amoeba, tiny.

Prokaryotic structure

Internal Structure External Structure

Internal Structure

Bacteria have a very simple internal structure, and no membrane-bound organelles.

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Nucleoid: DNA in the bacterial cell is generally confined to this central region. Though it isn't bounded by a membrane, it is visibly distinct (by transmission microscopy) from the rest of the cell interior.Back

Ribosomes: Ribosomes give the

cytoplasm of bacteria agranular appearance

in electron micrographs. Though smaller than the ribosomes in eukaryotic cells, these inclusions have a similar function in translating the genetic message in messenger RNA into the production of peptide sequences (proteins).

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Surface Structure

Beginning from theoutermost structureand moving inward, bacteria have some or all of the following structures:

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Capsule: This layer of polysaccharide (sometimes proteins) protects the bacterial cell and is often associated with pathogenic bacteria because it serves as a barrier against phagocytosis by white blood cells.

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• cell wall: Composed of peptidoglycan (polysaccharides + protein), the cell wall maintains the overall shape of a bacterial cell. The three primary shapes in bacteria are coccus (spherical), bacillus (rod-

shaped) and spirillum (spiral). Mycoplasma are bacteria that have no cell wall and

therefore have no definite shape.Back

• Plasma membrane: This is a lipid bilayer much like the cytoplasmic (plasma) membrane of other cells. There are numerous proteins moving within or upon this layer that are primarily responsible for transport of ions, nutrients and waste across the membrane.

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• Pili: These hollow, hairlike structures made of protein allow bacteria to attach to other cells. A specialized pilus, the sex pilus, allows the transfer of plasmid DNA from one bacterial cell to another. Pili (sing., pilus) are also called fimbriae (sing., fimbria).

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Flagella: The purpose of flagella (sing., flagellum) is motility. Flagella are long appendages which rotate by means of a "motor" located just under the cytoplasmic membrane. Bacteria may have one, a

few, or many flagella in different positions on

the cell.Back

QUIZ

• Which of these would not be found in a prokaryotic cell?A. ribosomesB. cell membraneC. nucleusD. DNA

• Prokaryotic cells have the following structures:A. Ribosomes, cell membrane, nucleus, cell

wallB. Capsule, cell wall, plasma membrane,

nucleoid , ribosomes

C. cell wall, plasma membrane, nucleoid, ribosomes and mitochondria

D. Capsule, cell wall, plasma membrane, nucleus , ribosomes

• http://www.cellsalive.com/cells/cell_model.htm

• http://www.biology4kids.com/files/cell2_main.html

• The cell song: http://www.youtube.com/watch?v=oNZsvG_ODG4&feature=related

• How the body works (video): http://www.neok12.com/php/watch.php?v=zX606f017b425c70037a435d&t=Cell-Structures

• Cell structure Video: http://www.mhln.com/ob/assets/00035501/htm/00035804/00035804.htm