The Cell and Its Membrane
The cell is the basic functional unit of all living things. The plasma membrane (cell membrane) bounds the cell and encloses the nucleus (discussed presently) and cytoplasm. The cytoplasm consists of specialized bodies called organelles suspended in a fluid matrix, the cytosol, which consists of water and dissolved substances such as proteins and nutrients.
The plasma membrane
The plasma membrane separates internal metabolic events from the external environment and controls the movement of materials into and out of the cell. The plasma membrane is a double phospholipid membrane (lipid bilayer), with the nonpolar hydrophobic tails pointing toward the inside of the membrane and the polar hydrophilic heads forming the inner and outer faces of the membrane (Figure 1).
Proteins and cholesterol molecules are scattered throughout the flexible phospholipid membrane. Proteins may attach loosely to the inner or outer surface of the plasma membrane (peripheral proteins), or they may lie across the membrane, extending from inside to outside (integral proteins). The mosaic nature of scattered proteins within a flexible matrix of phospholipid molecules describes the fluid mosaic model of the cell membrane. Additional features of the plasma membrane follow:
- The phospholipid bilayer is semi‐permeable. Only small, uncharged, polar molecules, such as H 2O and CO 2, and hydrophobic molecules—nonpolar molecules such as O 2 and lipid soluble molecules such as hydrocarbons—can freely cross the membrane.
- Channel proteins provide passageways through the membrane for certain hydrophilic (water‐soluble) substances such as polar and charged molecules.
- Transport proteins spend energy (ATP) to transfer materials across the membrane. When energy is used to provide a passageway for materials, the process is called active transport.
- Recognition proteins (glycoproteins) distinguish the identity of neighboring cells. These proteins have oligosaccharide (short polysaccharide) chains extending from their cell surface.
- Adhesion proteins attach cells to neighboring cells or provide anchors for the internal filaments and tubules that give stability to the cell.
- Receptor proteins initiate specific cell responses once hormones or other trigger molecules bind to them.
- Electron transfer proteins are involved in moving electrons from one molecule to another during chemical reactions.
Figure 1. The phospholipid bilayer of the plasma membrane.
Organelles are bodies within the cytoplasm that serve to physically separate the various metabolic activities that occur within cells. They include the following (Figure 2):
- The nucleus is bounded by the nuclear envelope, a phospholipid bilayer similar to the plasma membrane. The nucleus contains DNA (deoxyribonucleic acid), the hereditary information of the cell. Normally, the DNA is spread out within the nucleus as a threadlike matrix called chromatin. When the cell begins to divide, the chromatin condenses into rod‐shaped bodies called chromosomes, each of which, before dividing, is made up of two long DNA molecules and various histone molecules. The histones serve to organize the lengthy DNA, coiling it into bundles called nucleosomes. Also visible within the nucleus are one or more nucleoli, each consisting of RNA that is involved in the process of manufacturing the components of ribosomes. The components of ribosomes move to the cytoplasm to form a complete ribosome. The ribosome will eventually assemble amino acids into proteins. The nucleus also serves as the site for the separation of chromosomes during cell division.
- The endoplasmic reticulum, or ER, consists of stacks of flattened sacs involved in the production of various materials. In cross‐section, they appear as a series of mazelike channels, often closely associated with the nucleus. When ribosomes are present, the ER (called rough ER) attaches polysaccharide groups to polypeptides as they are assembled by the ribosomes. Smooth ER, without ribosomes, is responsible for various activities, including the synthesis of lipids and hormones, especially in cells that produce these substances for export from the cell. In liver cells, smooth ER is involved in the breakdown of toxins, drugs, and toxic byproducts from cellular reactions.
- A Golgi apparatus ( Golgi complex or Golgi body) is a group of flattened sacs arranged like a stack of bowls. They function to modify and package proteins and lipids into vesicles, small, spherically shaped sacs that bud from the ends of a Golgi apparatus. Vesicles often migrate to and merge with the plasma membrane, releasing their contents outside of the cell.
- Lysosomes are vesicles from a Golgi apparatus that contain digestive enzymes. They break down food, cellular debris, and foreign invaders such as bacteria.
- Mitochondria carry out aerobic respiration, a process in which energy (in the form of ATP) is obtained from carbohydrates. The mitochondria can also produce energy from noncarbohydrate sources such as fats.
- Ribosomes carry out the process of producing protein.
- Vaults are one of the newest organelles discovered. It appears they function to transport messenger RNA through the cytosol to the ribosomes. They seem to also be involved in developing drug resistance.
- Microtubules, intermediate filaments, and microfilaments are three protein fibers of decreasing diameter, respectively. All are involved in establishing the shape or movements of the cytoskeleton, the internal structure of the cell.
- Microtubules are made of the protein tubulin and provide support and mobility for cellular activities. They are found in the spindle apparatus (which guides the movement of chromosomes during cell division) and in flagella and cilia (described later in this list), which project from the plasma membrane to provide motility to the cell.
- Intermediate filaments help support the shape of the cell.
- Microfilaments are made of the protein actin and are involved in cell motility. They are found in almost every cell, but are predominant in muscle cells and in cells that move by changing shape, such as phagocytes (white blood cells that scour the body for bacteria and other foreign invaders)
- Flagella and cilia protrude from the cell membrane and make wavelike movements. Flagella and cilia are classified by their lengths and by their number per cell: Flagella are long and few; cilia are short and many. A single flagellum propels sperm, while the numerous cilia that line the respiratory tract sweep away debris. Structurally, both flagella and cilia consist of microtubules arranged in a “9 + 2” array—that is, nine pairs (doublets) of microtubules arranged in a circle surrounding a pair of microtubules (Figure 3).
- Centrioles and basal bodies act as microtubule organizing centers (MTOCs). A pair of centrioles (enclosed in a centrosome) located outside the nuclear envelope gives rise to the microtubules that make up the spindle apparatus used during cell division. Basal bodies are at the base of each flagellum and cilium and appear to organize their development. Both centrioles and basal bodies are made up of nine triplets arranged in a circle (Figure 3).
- Peroxisomes are organelles common in liver and kidney cells that break down potentially harmful substances. Some chemical reactions in the body produce a byproduct called hydrogen peroxide. Peroxisomes can convert hydrogen peroxide (a toxin made of H 2O 2) to water and oxygen.
Figure 2. The general organization of a typical cell.