A Tour of the Cell

A Tour of the Cell - A Tour of the Cell All organisms are...

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A Tour of the Cell All organisms are made of cells. ° Many organisms are single-celled. ° Even in multicellular organisms, the cell is the basic unit of structure and function. The cell is the simplest collection of matter that can live. All cells are related by their descent from earlier cells. A. A Panoramic View of the Cell 1. Prokaryotic and eukaryotic cells differ in size and complexity. All cells are surrounded by a plasma membrane. The semifluid substance within the membrane is the cytosol, containing the organelles. All cells contain chromosomes that have genes in the form of DNA. All cells also have ribosomes, tiny organelles that make proteins using the instructions contained in genes. A major difference between prokaryotic and eukaryotic cells is the location of chromosomes. In a eukaryotic cell, chromosomes are contained in a membrane-enclosed organelle, the nucleus. In a prokaryotic cell, the DNA is concentrated in the nucleoid without a membrane separating it from the rest of the cell. In eukaryote cells, the chromosomes are contained within a membranous nuclear envelope. The region between the nucleus and the plasma membrane is the cytoplasm. ° All the material within the plasma membrane of a prokaryotic cell is cytoplasm.
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Within the cytoplasm of a eukaryotic cell are a variety of membrane-bound organelles of specialized form and function. ° These membrane-bound organelles are absent in prokaryotes. Eukaryotic cells are generally much bigger than prokaryotic cells. The logistics of carrying out metabolism set limits on cell size. ° At the lower limit, the smallest bacteria, mycoplasmas, are between 0.1 to 1.0 micron. Metabolic requirements also set an upper limit to the size of a single cell. As a cell increases in size, its volume increases faster than its surface area. ° Smaller objects have a greater ratio of surface area to volume. The plasma membrane functions as a selective barrier that allows the passage of oxygen, nutrients, and wastes for the whole volume of the cell. Rates of chemical exchange across the plasma membrane may be inadequate to maintain a cell with a very large cytoplasm. The need for a surface sufficiently large to accommodate the volume explains the microscopic size of most cells. Larger organisms do not generally have larger cells than smaller organisms—simply more cells. Cells that exchange a lot of material with their surroundings, such as intestinal cells, may have long, thin projections from the cell surface called microvilli. Microvilli increase surface area without significantly increasing cell volume. 2. Internal membranes compartmentalize the functions of a eukaryotic cell. A eukaryotic cell has extensive and elaborate internal membranes, which partition the
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A Tour of the Cell - A Tour of the Cell All organisms are...

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