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Transcript
1.2 Cells: The Basic Units of Life Cells are the basic structural and functional units of life. There are many different kinds of cells, which are specialized to carry out particular functions. In animals, muscle cells move limbs by contracting, bone cells produce the hard material that makes bones rigid, brain cells conduct electrical signals, and skin cells form waterproof sheets that cover your body. In plants, root cells absorb water from the environment, and green cells in leaves produce food. Although they perform different functions, cells have many common features. General Structure of a Cell A cell can be viewed as a factory with different departments, each carrying out specialized tasks that help keep the cell alive. Whether they are nerve cells in the brain of an animal, or epidermal cells in the leaf of a plant, cells are bathed in an aqueous (water-based) solution called extracellular fluid. On the inside, a cell contains an aqueous solution called cytosol in which a number of structures called cell organelles (little organs) are suspended (Figure 1). A special organelle called the nucleus acts as the control centre of the cell, issuing orders that determine how the other organelles do their work. Cytoplasm is the name given to the cytosol and cell organelles outside of the nucleus. The contents of an animal cell (cytosol and cell organelles) are separated from the extracellular fluid by a thin, flexible film called the cell membrane or plasma membrane. In addition to separating the interior of the cell from the external environment, the cell membrane holds the contents of the cell together and controls the movement of materials into and out of the cell. Plant cells have an additional outer covering called the cell wall (Figure 2). Cell walls are firm yet porous structures that give plants their rigidity while allowing water and dissolved materials to pass through easily. Wood is composed of the cell walls of dead plant cells. Figure 3 shows a typical plant cell and a typical animal cell. Many of the smaller organelles identified in these diagrams cannot be seen with a compound light microscope, but instead, require the magnification and resolving power of a transmission electron microscope. PRINT PAGE 11 Membranes In addition to the membrane that surrounds the cell (the cell membrane), plant and animal cells have a number of organelles that are surrounded by membranes such as the nucleus, mitochondria, and chloroplasts. A membrane is composed of a bilayer (double layer) of fat (lipid) molecules, called phospholipids, and other compounds, called proteins and carbohydrates (Figure 4). The parts of a membrane are always moving, and the membrane is said to be "fluid." That is why if a membrane is punctured with an ultra-fine needle it will not leak. Instead, the phospholipids and proteins will move in to fill the gap as the needle is removed. The membrane is more like a strong soap bubble than a rubber balloon. Nucleus As mentioned, the nucleus is the control centre of the cell and directs all of the cell's activities. The nucleus is separated from the cytoplasm by a porous double membrane called the nuclear envelope, and is filled with nucleoplasm. Nucleoplasm is a mixture of chemicals that stores information used by other cell organelles in carrying out their functions. Nucleoplasm is rich in compounds called nucleic acids. These include ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Ribosomes Ribosomes are organelles used by the cell to produce proteins (protein synthesis). Ribosomes are either floating in the cytoplasm or attached to membranes. In general, free-floating ribosomes produce proteins that are used inside the cell, and membrane-attached ribosomes manufacture proteins for use outside the cell. Ribosomes are so small (approximately 30 nm in diameter) that even when viewed with a transmission electron microscope they appear as small fuzzy dots (Figure 5). Endoplasmic Reticulum The endoplasmic reticulum (ER) is a complicated system of membranous tubes and canals that connect with the nuclear envelope (Figure 6, on the next page). The endoplasmic reticulum is like a complex subway system with crisscrossing tunnels and stations. As you can see in Figure 6, there are two types of endoplasmic reticulum: rough endoplasmic reticulum (RER), containing attached ribosomes, and smooth endoplasmic reticulum (SER), with no ribosomes. PRINT PAGE 12 Because the rough endoplasmic reticulum contains ribosomes, many proteins are manufactured in it. The smooth endoplasmic reticulum is thought to be a place where molecules of fat are produced. The products of the endoplasmic reticulum become enclosed in membrane-bound structures called vesicles. Vesicles form when the membrane of the endoplasmic reticulum pinches off at the ends. These vesicles then travel toward another membranous organelle called a Golgi apparatus. Golgi Apparatus The Golgi apparatus is composed of several membranous tubes that, under the microscope, usually look like a stack of flattened balloons (Figure 7). It is named after Camillo Golgi, the medical doctor who first identified it in 1898. The Golgi apparatus chemically changes the fats and proteins produced in the endoplasmic reticulum and then packages them in vesicles. In many cases, the vesicles move through the cytoplasm, attach to the cell membrane, and release their contents into the extracellular fluid. Lysosomes (found only in animal cells) Some of the vesicles formed by the Golgi apparatus are called lysosomes. Lysosomes are cell organelles containing proteins that can break down the molecules that cells are made of into their individual chemical components. Lysosomes are found only in animal cells, and may be used to digest food particles brought into the cell from the extracellular fluid. They are also used to destroy potentially dangerous microorganisms, such as bacteria and viruses, that may force themselves into the cell (Figure 8). When an animal cell gets old, lysosomes break open and decompose the entire cell. This process, in which the cell could be said to "commit suicide," is called apoptosis. The organism then uses the resulting compounds to build new cells. Hence, lysosomes are sometimes referred to as "suicide sacs." PRINT PAGE 13 Mitochondria Mitochondria (singular: mitochondrion) are circular or rodshaped membranous organelles that float freely in the cytosol (Figure 9). Many important chemical reactions occur in mitochondria. These reactions contribute to cellular respiration, a series of chemical changes that produces compounds that cells use as a source of energy. Like the nucleus, the mitochondrion contains two membranes: a smooth outer membrane and a highly folded inner membrane. The folds of the inner membrane are called cristae (singular: crista). Cristae contain compounds that help carry out the reactions of cellular respiration, and provide a large surface area on which these reactions occur. Cells that require large amounts of energy, such as muscle cells in animals and root tip cells in plants, usually contain large numbers of mitochondria, each with many cristae. Cells that do not require large amounts of energy, such as most fat cells in animals and leaf cells in plants, have smaller numbers of mitochondria with fewer cristae. The interior space of a mitochondrion is filled with a protein-rich fluid called matrix. Many chemical reactions of cellular respiration also occur in the matrix. PRINT PAGE 14 Plastids (found only in plant cells) Plastids are free-floating membranous organelles found only in the cytosol of plant cells. They are usually large enough to be seen with a compound light microscope. Plastids store materials that are useful to the plant cell and perform other important functions that keep the cell alive. One of the most important plastids is the chloroplast. Chloroplasts contain all of the chemicals necessary to perform photosynthesis, a set of chemical reactions that converts carbon dioxide and water (from the extracellular fluid) into molecules that plant cells use as food. Like mitochondria, chloroplasts possess two membranes, an outer and an inner membrane. However, in chloroplasts, other membranes are arranged into a system of interconnected compartments called thylakoids, and thylakoids are usually stacked on top of one another forming structures called grana (singular: granum) (Figure 10). The membranes of thylakoids contain molecules of green pigment called chlorophyll. Chlorophyll is responsible for the green colour of plants and is also responsible for the start of photosynthesis. The space between thylakoid membranes and the inner chloroplast membrane is filled with a protein-rich fluid called stroma. Stroma contains compounds needed for photosynthesis and DNA that allows chloroplasts to reproduce. Other plastids in plant cells include amyloplasts and chromoplasts. Amyloplasts are white or colourless plastids that store the energy-rich products of photosynthesis in the form of starch. Chromoplasts are colourful plastids containing the red, orange, and yellow pigments commonly found in flowers, fruits, and vegetables. Vacuoles Vacuoles are usually large, membrane-bound sacs filled with a watery solution containing dissolved sugars, minerals, and proteins. They are common in plant cells, where the pressure inside one or two large vacuoles helps keep the cell membrane pressed firmly against the cell wall (Figure 11). This pressure, called turgor pressure, is responsible for the firm texture of fresh vegetables such as celery stalks and carrot roots, and of the stems and leaves of all plants. When plant cells lose water, the vacuoles shrink, and the decrease in turgor pressure causes structures such as stems and roots to become limp and wilted.