Table of Contents:
As its name implies, the cytoskeleton is the cell's internal supporting framework. Like the bony skeleton of the body the cytoskeleton made up of rather rigid, rodlike pieces that not only provide support but also allow movement. In this section, we will discuss the basic characteristics of the cell's skeleton, as well as several organelles associated with it.
FIGURE 3-7 The cytoskeleton. Interpretation of the cell's internal framework, Notice that the "free" ribosomes and other organelles are not really free at all.
Cell Fibers
FIGURE 3-8 Cell fibers. A, Microfilaments are thin, twisted strands of protein molecules. B, Intermediate filaments are thicker, twisted protein strands. C, Microtubules are hollow fibers made of a spiral arrangement of protein subunits.
- Until recently, cell fibers were poorly understood, but two new research methods have allowed scientists to gain insight into these structures.
- One of these methods uses fluorescent molecules, while the other involves stereomicroscopy, which provides three-dimensional images of whole, unsliced cells using high-voltage electron microscopes.
- Through these techniques, researchers have discovered intricate arrangements of fibers with varying widths. The smallest fibers, known as microfilaments, have a width of only 3 to 6 nanometers, less than one millionth of an inch.
- These fibers form a three-dimensional, irregularly shaped lattice within the cell, acting like a scaffolding that supports various cellular components. These include the endoplasmic reticulum, mitochondria, and "free ribosomes," which were previously thought to float freely in the cytoplasm.
- Microfilaments are the smallest type of cell fibers and often function as "cellular muscles." They are composed of thin, twisted strands of protein molecules and usually form bundles that lie parallel to the long axis of the cell.
- Different proteins within microfilaments can slide past each other, causing shortening of the cell. One prominent example of this is muscle cells, where many bundles of microfilaments work together to produce powerful contractions.
- Overall, these new research methods have shed light on the intricate and essential role that cell fibers play in supporting and enabling cellular functions.
Centrosome
- The centrosome is a non-membranous structure near the nucleus that coordinates the building and breaking of microtubules in the cell.
- It is sometimes called the microtubule-organizing center.
- The centrosome is important during cell division, where it plays a role in constructing a special "spindle" of microtubules to move chromosomes around the cell.
- Centrioles are cylindrical structures located in the centrosome that help identify its general location.
- Centrioles consist of nine bundles of microtubules, with three tubules in each bundle.
- The special arrangement of the two tubular-walled cylinders is at right angles to each other.
- Before separating, a daughter centriole is formed perpendicularly to each of the original pair, so that a complete pair may be distributed to each new cell.
- Centrioles are essential for the formation of microtubular cell extensions.
Cell Extensions
- Microvilli are finger-like projections that extend from the plasma membrane of certain cells, such as epithelial cells in the intestines.
- They increase the surface area of the cell, allowing for faster absorption.
- Microvilli are very small, measuring only 0.5 um long and 0.1 um or less across.
- Cilia and flagella are cell processes with a core made of microtubules arranged in a cylinder.
- The arrangement of microtubules in cilia and flagella is suited for movement.
- Cilia are shorter and more numerous than flagella, and look like tiny hairs.
- Cilia in the respiratory tract move contaminated mucus towards the throat, while cilia in the female reproductive tract move the ovum towards the uterus.
- Flagella are single, long structures found only in human sperm cells, allowing them to swim towards the female sex cell.
NUCLEUS
- The nucleus is a large cell structure that occupies the central portion of the cell.
- It varies in shape and number of nuclei present in a cell.
- The nucleus is enclosed by two membranes, known as the nuclear envelope, which are extensions of the endoplasmic reticulum.
- The nucleus contains DNA molecules that determine heredity and dictate the structure and function of cells.
- DNA molecules appear as granules or threads, named chromatin in nondividing cells and as tightly coiled chromosomes during cell division.
- Normal human cells (except mature sex cells) contain 46 chromosomes, each consisting of one DNA molecule plus some protein molecules.
- The nucleolus is a small nonmembrane body in the nucleus that synthesizes ribosomal RNA (rRNA) and combines it with protein to form ribosomes.
- The nucleolus consists chiefly of a nucleic acid called RNA (ribonucleic acid).
- The size of the nucleolus depends on the amount of protein a cell makes, with cells that make more protein having larger nucleoli.
QUICK CHECK
1. List at least three functions of the plasma membrane.
2. Define the term organelle.
3. Identify three organelles by name and give one function of each.
4. Distinguish between membranous and nonmembranous organelles.
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