10/1/13 10:29 AMBIO156 – Lab 4
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Lab 4 Cell Structure, Osmosis, and Diffusion
Introduction: Connecting Your Learning
The basic building block of life is the cell. Each cell contains several structures, some of which are common to both eukaryotic and prokaryotic cells and some that are unique to specific cell types. This lab will discuss cell structures and how materials are moved in and out of the cell. Specifically, the principles of diffusion and osmosis will be demonstrated by performing a scientific investigation that studies the effect of salt concentration on potato cells.
Focusing Your Learning
In 1662, Robert Hooke investigated the properties of cork when he discovered cells. He named them after small rooms in a monastery because they reminded him of them. Years later, in 1837, Schleiden and Schwann were attributed with developing the cell theory. While their original theory was modified, the fundamental ideas be- hind the theory held true. Three general postulates are included in the cell theory: 1) All organisms are composed of cells. 2) The cell is the unit of life. 3) All cells arise from pre-existing cells.
Because a cell is the basic building block of living things, it is important to become familiar with its characteristics. Several structures comprise a cell. Many of these structures are visible with the use of a standard compound microscope. Below are pictures of idealized plant and animal cells, illustrating the important structures.
The cell membrane encloses all cells and is responsible for separating the internal en- vironment from the extracellular space (the space between cells). Because other struc- tures within the cell are also surrounded by a membrane, the outer membrane is of- ten called the plasma membrane.
10/1/13 10:29 AMBIO156 – Lab 4
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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.
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