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cell structures and how materials are moved

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The cell membrane is semi-permeable, allowing cer- tain molecules to enter into the cell freely, while oth- ers are prohibited from entering the cell. It is com- posed of phospholipids, which have a head consist- ing of a phosphate group and a tail of two fatty acid chains. The phosphate group is attracted to water

(hydrophilic) while the fatty acid chains are repulsed by water (hydrophobic). When in water, the properties of the phospholipids cause them to form two layers: The hy- drophobic tails face the inside of the double layer (away from the water), and the hy- drophilic heads face out (toward the water). Because two layers are formed, the mem- brane is made up of a phospholipid bilayer, as seen in the image.

The cell wall surrounds the cell membrane in plant cells, bacteria, and some fungi. In plant cells, the cell wall is composed of cellulose. In bacteria, the wall is made mostly of polypeptides (protein) or polysaccharides (carbohydrates). The cell wall provides support and protection and is responsible for giving plant cells their shape.

Another important structure found only in eukaryotic cells is the nucleus. This struc- ture contains the genetic information and is the control center of the cell. Protecting the nucleus is a double-membrane called the nuclear envelope, which, like the plasma membrane, is semi-permeable. It is important to note that although prokaryotes lack a nucleus, they still contain genetic information.

Within the nucleus is the nucleolus. This is the site where ribosomes are formed. Ri- bosomes function to assemble proteins. Many cells have multiple nucleoli, which con- tain concentrated areas of DNA and RNA.

Flagella (singular is flagellum) is Latin for whip. Flagella are whip-like projections of- ten found in prokaryotes, eukaryotic single-celled organisms, and some specific cells (like human sperm). These structures extend beyond the cell membrane and cell wall and are used for locomotion (movement). Although flagella are found in both eukary- otes and prokaryotes, the structure of the flagella is different for each cell type.

Cilia (singular is cilium) are structurally similar to eukaryotic flagella but are smaller and more hair-like. Cilia are found in some eukaryotic organisms. Some cilia are used for locomotion, as in the single-celled paramecium. In other organisms, the cilia act as