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TWO TYPES OF CELL
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 Prokaryotic and Eukaryotic Cell
At first sight cells exhibit a staggering diversity. Some lead a solitary existence, others live in communities; some have defined geometric shapes, others have flexible boundaries; some swim, some crawl, and some are sedentary. Many are green; others are red, blue, or purple or have no obvious coloration. Given these differences, it is perhaps surprising that there are only two types of cell.
Those of most organisms, from protozoa to mammals and from fungi to plants, are eukaryotic. They are generally 5-100 um across, although a few are large enough to be seen with the naked eye. Eukaryotic cells contain a variety of specialized structures including membrane-bound organelles, the largest of which, the nucleus, contains the genetic material. Bacteria are said to be prokaryotic. They are small (1-2 um across) and have very little internal organization so that, for instance, the genetic material is free within the cell.
The structure and function of organelles will be described in detail in subsequent chapters. Table 1 provides a brief glossary of the major intracellular structures and organelles and summarizes the differences between prokaryotic and eukaryotic cells. These differences are further illustrated in Fig. The entire insides of the cell, except the nucleus, is called the cytoplasm. Cytoplasm consists of all the bits-ribosomes, centrosomes, organelles-suspended in a watery medium called the cytosol. In fact despite all their complexity, organisms are mostly water.
Table 1. Differences between prokaryotic and eukaryotic cells
Prokaryotes Eukaryotes
Size 1-2 um 5-100 um
Nucleus Absent Present: bounded by nuclear envelope
DNA Single, circular Multiple, linear, associated with protein (chromatin)
Cell division Simple fission Mitosis or meiosis
Internal membranes Rare Complex (Golgi apparatus, ER, etc.)
Ribosomes 70S 80S (70S in organelles)
Cytoskeleton Absent Microtubules, microfilaments, intermediate filaments
Motility Flagella 9 + 2 Cilia and Flagella
First appeared 3.5 billion years ago 1.5 billion years ago
Special Properties of Plant Cells
Perhaps the most striking difference among eukaryotic cells is between those of animal and plants. Higher plants have evolved a sedentary life style and a mode of nutrition that demands that they hold up a leaf canopy. Unlike the flexible boundaries of animal cells, plant cells are enclosed within a rigid cell wall that gives shape to the cell and structural rigidity to the organism. Plant cells frequently contain one or more large bags called vacuoles that contain a simple, watery solution with a high concentration of sugars and other soluble compounds. Water enters the vacuole to dilute these sugars, generating hydrostatic pressure that is counterbalanced by the rigid wall. In this way the cells of the plant become stiff or turgid, in the same way that a balloon inflated inside a stocking becomes stiff. Vacuoles are frequently pigmented and the spectacular colors of petals and fruit reflect the presence of compounds such as anthocyanins (purple pigments) in the vacuole.
Cells of photosynthetic plant tissue contain a special organelle, the chloroplast, which houses the light-harvesting and carbohydrate-generating systems. Higher plant cells lack centrioles, which are present in most animal cells and also in some simple plants such as algae.
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Origin of Eukaryotic Cells
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