NOTE: Lecture notes are intended to help the student organize their notes and facilitate assimilation of the material. They are in no way a substitute for the actual lectures; however, this material will be covered on exams!
The cell is the most basic living organism. Some organisms are one-celled organisms, like amoebas and bacteria. Other organisms, such as plants and animals, are described as MULTICELLULAR, because they consist of many cells. Humans are multi-cellular organisms and the human body is made up of about 100 trillion cells, divided into about 200 different types.
The CELL is defined as the basic living structural and functional unit of all living organisms. You can think of cells as the structural units of the body, as logs are the structural units of a log house. The cell is also the functional unit of the living organism. All functions that occur within tissues and organs take place at the cellular level.
Cells are microscopic in size and are measured in metric units called micrometers.
1 meter = 1,000 millimeters (mm)
1 mm = 1000 micrometers
Cells vary in size, but they are all extremely small. One of the smallest cells in the body are red blood cells, which are 8 micrometers across. One of the largest cells (the fertilized egg) is about 140 micrometers across and is still just a speck, nearly impossible to see with the naked eye. To observe a cell, you must use a light microscope, which magnifies objects up to 1000 times their size. Electron microscopes magnify objects up to 1 million times and are used to observe cellular structures. An electron micrograph is a photograph taken with an electron microscope. Several electron micrographs are in this textbook (Fig. 2.14b, for example).
There are a tremendous variety of cell shapes. Some cells are spherical (fat cells), some are disc-shaped (red blood cells), and some are long and cylindrical (muscle cells) (see FIG. 2.22). Some cells, such as sperm cells and nerve cells, have long extensions.
All cells are surrounded by some type of body fluid, called extracellular fluid. Extracellular fluids include interstitial fluid (found in tissues), plasma (in blood), lymph (in lymphatic vessels), and cerebrospinal fluid (found around the brain and spinal cord). Cells also contain fluid inside them, called intracellular fluid, also known as cytosol. See Fig. 2.3.
No cell is exactly like all others, but cells do have many common structural and functional features. The generalized animal cell gives us an idea of what most cells are like. See FIGURE 2.1 for the generalized animal cell. Keep in mind that not all cells contain all the structures we will discuss.
All cells have 3 major parts:
1. PLASMA MEMBRANE
2. CYTOPLASM
3. NUCLEUS
PLASMA MEMBRANE:
The PLASMA MEMBRANE (FIGS. 2.1 & 2.2) is the thin, outer membrane that separates the cell's interior from its external environment. The plasma membrane also separates each cell from its neighboring cells, so that each cell is an individual entity. It is very thin (about a billionth of a meter) and requires an electron microscope to see it.
The current model for plasma membrane structure is the FLUID MOSAIC MODEL. In this model, proteins float like iceburgs in a "sea" of phospholipids. The PHOSPHOLIPIDS are arranged in two parallel layers, forming what is called a PHOSPHOLIPID BILAYER. The phospholipid bilayer forms the basic framework of the plasma membrane. It is somewhat fluid and flexible, about the consistency of olive oil. It is also self-sealing. If you poke a needle in and back out, the hole in the membrane closes up.
CHOLESTEROL is another lipid present in the plasma membrane. Cholesterol makes the plasma membrane less fluid and reduces its permeability to water-soluble chemicals.
PROTEINS have a variety of functions in the plasma membrane. Some of the proteins act as CHANNELS through which chemicals (ions, water) can move into or out of the cell. Other plasma membrane proteins are TRANSPORT (CARRIER) PROTEINS that function in carrying molecules into or out of the cell across the cell membrane. Still other proteins act as RECEPTORS for chemical messengers such as hormones and neurotransmitters.
Also present in the plasma membrane are glycoproteins and glycolipids. GLYCOPROTEINS consist of carbohydrate (sugar) molecules attached to protein. GLYCOLIPIDS are carbohydrates attached to lipid (fat-like) molecules. The carbohydrate (sugar) portions of glycoproteins and glycolipids are arranged on the outside surface of the plasma membrane. Therefore, the surface of the cell is "sugar-coated". These sugars help cells stick together within tissues. Also, because every cell has a different pattern of sugars on its surface, the sugars act as distinctive CELL IDENTITY MARKERS (ANTIGENS) by which cells can recognize each other. For example, a sperm cell recognizes the egg by the egg's specific cell identity markers. Cell identity markers also allow your own body's cells to recognize invading cells that cause disease. Once your body recognizes these foreign invaders, it can mount an "immune response" and destroy the invaders.
CYTOPLASM:
The term CYTOPLASM refers to all of the cellular contents located between the plasma membrane and the nucleus. The semi-fluid portion of the cytoplasm is the CYTOSOL, which is also called INTRACELLULAR FLUID. The organelles are suspended in the cytosol. So, the cytoplasm includes the cytosol and all the organelles (except the nucleus).
The CYTOSOL is a thick, elastic, semitransparent fluid which contains mostly water (75-90%). It also contains carbohydrates, salts, amino acids, fatty acids, proteins, lipids, ATP, and other solutes (dissolved chemicals). Many chemical reactions take place in the cytosol. For example, enzymes in the cytosol begin the breakdown of nutrients to provide energy for cellular activities. Substances are also synthesized (built) in the cytosol.
Suspended in the cytosol are very small tubules and filaments that make up the CYTOSKELETON. The cytoskeleton consists of protein microtubules and microfilaments (FIG. 2.6) that provide support and shape to the cell, and are involved in cellular movement. You can think of the cytoskeleton as the bone and muscle of the cell. For example, the plasma membrane forms fingerlike projections called MICROVILLI (see FIGURE 2.1), which are formed by microfilaments of the cytoskeleton. These microvilli increase the surface area of the plasma membrane. Some of the cells that line your digestive tract have microvilli. The microvilli increase the surface area for nutrient absorption, so that you can absorb more nutrients from the foods you digest.
Some body cells have projections on the cell surface formed by cytoskeleton microtubules that are involved in cellular movement. These are called cilia and the flagellum. CILIA are hair-like cellular projections that occur in large numbers on the surface of certain cells (see FIG. 2.8, & Table 3.1D & E). The cilia move in a wave-like motion, creating a current that propels substances in one direction across the cell surface. For example, the ciliated cells that line the respiratory tract propel dust and bacteria trapped in mucus upward away from the lungs. The FLAGELLUM is long in proportion to the size of the cell and is used to move the entire cell. The only example of a cell with a flagellum in the human body is the sperm cell. Its flagellum is commonly called its "tail" (see the sperm in FIGURE 2.22).
ORGANELLES:
Suspended in the cytoplasm are the ORGANELLES. Organelles are specialized compartments within the cell that perform specific functions. Organelle means "little organs", because each organelle performs its own job to maintain the life of the cell, like organs do in our bodies. All organelles are surrounded by a phospholipid bilayer membrane, similar to the plasma membrane.
The NUCLEUS (FIGS. 2.1 & 2.15) contains the genetic material, or DNA, of the cell. It is the one organelle that is not part of the cytoplasm. It is a spherical- or oval-shaped organelle surrounded by a double-layered membrane called the nuclear envelope. Most cells contain one nucleus. However, there are exceptions to the rule. Mature red blood cells have no nucleus; skeletal muscle cells contain several nuclei. The nucleus contains DNA, the hereditary genetic material of the cell. FIGURE 2.16 illustrates how DNA is packaged into chromosomes. DNA controls the structure and activities of a cell by providing the instructions for protein synthesis. Proteins serve a myriad of purposes - they act as hormones, enzymes, pigments, and structural components of organelles, to name just a few functions.
RIBOSOMES (FIGS. 2.1, 2.9, 2.10, & 2.12) are tiny granular structures in the cytoplasm. Ribosomes are not true organelles because they do not have a membrane around them. Ribosomes receive genetic instructions from the nucleus (in the form of messenger RNA) to produce specific proteins. So, ribosomes are the sites of protein synthesis. Some of the ribosomes float free in the cytoplasm; these "free ribosomes" synthesize proteins that are to be used in the cytoplasm. Other ribosomes are attached to the outer membranes of the ROUGH ENDOPLASMIC RETICULUM (Fig. 2.10 & 2.12). The function of these ribosomes is described below.
The ENDOPLASMIC RETICULUM (FIGS. 2.1, 2.10, & 2.12) is an extensive system of interconnected, parallel membranes that enclose tubes or channels ("cisterns"). The endoplasmic reticulum membrane is found throughout the cytoplasm and is connected to the plasma membrane and the nuclear envelope of the nucleus. There are 2 types of endoplasmic reticulum (ER): rough ER and smooth ER. ROUGH ER consists of parallel channels with ribosomes embedded on its surface. The SMOOTH ER is smooth because it does not have ribosomes on its surface. Smooth ER also has a tubular appearance.
SMOOTH ER is involved in the production of LIPIDS (fat-like substances). For example, smooth ER produces CHOLESTEROL, which is part of the plasma membrane. Smooth ER in certain hormone-producing cells produces STEROID HORMONES such as the sex hormones (estrogen, progesterone, testosterone). The smooth ER is also involved in the DETOXIFICATION of substances. Liver cells have a lot of smooth ER, because the liver makes cholesterol and detoxifies drugs and alcohol.
Ribosomes are attached to the external surface of the ROUGH ER, giving the surface of the rough ER a granular appearance. As proteins are assembled on the ribosomes, the proteins make their way inside the rough ER channels. See FIG. 2.12. The rough ER packages these proteins into round membranous sacs called TRANSPORT VESICLES. These transport vesicles pinch off from the rough ER and make their way to the GOLGI COMPLEX.
The GOLGI COMPLEX (FIGS. 2.1, 2.11, & 2.12) looks like 4-8 flattened membrane sacs stacked like dishes, with tiny membranous sacs or VESICLES nearby. The vesicles that bud off from the rough ER migrate to the Golgi complex and fuse with the Golgi complex membranes (see FIG. 2.12). Inside the Golgi complex, the proteins that were made by the ribosomes of the rough ER are modified in some way. For example, carbohydrates might be attached to the protein, making it a glycoprotein. These modified proteins are then packaged into vesicles and sent to their destination. MEMBRANE VESICLES contain proteins or glycoproteins embedded in the membrane of the vesicle and are sent to fuse with the plasma membrane. Other vesicles contain proteins that are to be SECRETED or released from the cell. These SECRETORY VESICLES (FIG. 2.1 & 2.12) migrate to the plasma membrane and discharge their contents from the cell (secretion). Other vesicles produced by the Golgi complex are called TRANSPORT VESICLES, which transport proteins to other structures, such as LYSOSOMES (see next paragraph).
The LYSOSOMES (FIGS. 2.1, 2.12 & 2.13) are membrane-enclosed spheres that contain powerful digestive enzymes (made of protein) which are capable of digesting substances that are inside the cell (intracellular digestion). For example:
1. White blood cells, which ingest bacteria, contain large numbers of lysosomes. The digestive enzymes in the lysosome are used to digest and destroy the ingested bacteria.
2. Lysosomes can also engulf old, worn-out organelles and break them down with digestive enzymes. The digested components of the organelles are then returned to the cytoplasm for recycling into new organelles.
3. Lysosome digestive enzymes also degrade non-useful tissues, like the tail and webbing between the fingers and toes of the developing human embryo. This programmed cell death is important for embryological development.
The MITOCHONDRIA (FIGS. 2.1 & 2.14) are small, kidney bean-shaped organelles. Each mitochondrion has a smooth outer membrane, but the inner membrane is made up of a series of folds called cristae. Mitochondria are called the "powerhouses of the cell", because they produce chemical energy in the form of ATP by a process called aerobic cellular respiration. This process uses oxygen and glucose to produce molecules of ATP (adenosine triphosphate). ATP is a molecule that stores a great deal of energy. When the cell breaks down ATP, it releases energy that the cell can use for various processes, such as movement or nerve impulse transmission. ATP is like the gasoline that powers your car or the electricity that powers your lights. Active cells, such as muscle cells and sperm cells, have large numbers of mitochondria because they use a great deal of energy.
TABLE 2.2 on page 42 contains the summary of cell parts and their functions.
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