chapter 4,5,6,7,8,12,12,35

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University of Toronto Scarborough
Biological Sciences
Mark Fitzpatrick

Chapter 4 The use of carbon dioxide in photosynthesis is a hallmark of life and leaves a unique chemical signaturea specific ratio of isotopes of carbon (C13:C12) in the resulting carbohydrates. Each cell is surrounded by a membrane that separates it from its environment, creating a segregated (but not isolated) compartment. This plasma membrane is composed of a phospholipid bilayer, with the hydrophilic heads of the lipids facing the cells aqueous interior on one side of the membrane and the extracellular environment on the other (see Figure 3.20). Proteins and other molecules are embedded in the lipids. * The plasma membrane allows the cell to maintain a more or less constant internal environment. A self-maintaining, constant internal environment (the state known as homeostasis) is a key characteristic of life that will be discussed in detail in Chapter 40. * The plasma membrane acts as a selectively permeable barrier, preventing some substances from crossing it while permitting other substances to enter and leave the cell. * As the cells boundary with the outside environment, the plasma membrane is important in communicating with adjacent cells and receiving signals from the environment. We will describe this function in Chapter 15. * The plasma membrane often has proteins protruding from it that are responsible for binding and adhering to adjacent cells. Biologists classify all living things into three domains: Archaea, Bacteria, and Eukarya. The organisms in Archaea and Bacteria are collectively called prokaryotes because they have in common a prokaryotic cell organization. Prokaryotic cells do not typically have membrane-enclosed internal compartments. The first cells were probably similar in organization to modern prokaryotes. Eukaryotic cell organization, on the other hand, is particular to members of the domain Eukarya (eukaryotes), which includes the protists, plants, fungi, and animals. The genetic material (DNA) of eukaryotic cells is contained in a special membrane-enclosed compartment called the nucleus. Eukaryotic cells also contain other membrane-enclosed compartments in which specific chemical reactions take place. Prokaryotes can live on more different and diverse energy sources than any other living organisms, and they can inhabit greater environmental extremes, such as very hot springs and very salty water. www.notesolution.comProkaryotic cells are generally smaller than eukaryotic cells, ranging from 0.25 1.2 m to 1.5 4 m. Each individual prokaryote is a single cell, but many types of prokaryotes are usually seen in chains, small clusters, or even clusters containing hundreds of individuals. # The plasma membrane encloses the cell, regulating the traffic of materials into and out of the cell and separating it from its environment. # The nucleoid contains the hereditary material (DNA) of the cell The rest of the material enclosed in the plasma membrane is called the cytoplasm. The cytoplasm is composed of two components: the more fluid cytosol and insoluble suspended particles, including ribosomes. * The cytosol consists mostly of water that contains dissolved ions, small molecules, and soluble macromolecules such as proteins. * Ribosomes are complexes of RNA and proteins about 25 nm in diameter. They are the sites of protein synthesis. CELL WALLS Most prokaryotes have a cell wall located outside the plasma membrane. The rigidity of the cell wall supports the cell and determines its shape. The cell walls of most bacteria, but not archaea, contain peptidoglycan, a polymer of amino sugars, cross- linked by covalent bonds to form a single giant molecule around the entire cell. In some bacteria, another layerthe outer membrane (a polysaccharide-rich phospholipid membrane)encloses the peptidoglycan layer. Unlike the plasma membrane, this outer membrane is not a major permeability barrier. Enclosing the cell wall in some bacteria is a layer of slime, composed mostly of polysaccharides and referred to as a capsule. The capsules of some bacteria may protect them from attack by white blood cells in the animals they infect. The capsule helps keep the cell from drying out, and sometimes it helps the bacterium attach to other cells. Many prokaryotes produce no capsule, and those that do have capsules can survive even if they lose them, so the capsule is not essential to prokaryotic life. We know that the flagella cause the motion of the cell because if they are removed, the cell does not move. Pili project from the surfaces of some groups of bacteria. Shorter than flagella, these hairlike structures help bacteria adhere to one another when they exchange genetic material, as well as to animal cells for protection and food. Eukaryotic cells generally have dimensions up to 10 times greater than those of prokaryotes The nucleus contains most of the cells genetic material (DNA). The replication of the genetic material and the first steps in decoding genetic information take place in the nucleus. # The mitochondrion is a power plant and industrial park, where energy stored in the bonds of carbohydrates and fatty acids is converted into a form more useful to the cell (ATP). # The endoplasmic reticulum and Golgi apparatus are compartments in which some proteins synthesized by the ribosomes are packaged and sent to appropriate locations in the cell. # Lysosomes and vacuoles are cellular digestive systems in which large molecules are hydrolyzed into usable monomers. # Chloroplasts (found in only some cells) perform photosynthesis. In the cell, information is stored in the sequence of nucleotides in DNA molecules. Most of the DNA in eukaryotic cells resides in the nucleus. Information is translated from the language of DNA into the language of proteins at the ribosomes. # A region within the nucleus, the nucleolus, begins the assembly of ribosomes from RNA and specific proteins. The nucleus is surrounded by two membranes, which together form the nuclear envelope. Inside the nucleus, DNA combines with proteins to form a fibrous complex called chromatin. Chromatin consists of exceedingly long, thin threads. Prior to cell division, the chromatin aggregates to form discrete, readily visible structures called chromosomes RIBOSOMES In prokaryotic cells, ribosomes float freely in the cytoplasm. In eukaryotic cells they are found in two places: in the cytoplasm, where they may be free or attached to the surface of the endoplasmic reticulum; and inside mitochondria and chloroplasts. In each of these locations, the ribosomes are the sites where proteins are synthesized under the direction of nucleic acids. Although they seem small in comparison to the cell in which they are contained, ribosomes are huge machines made up of several dozen kinds of molecules. Chemically, ribosomes consist of a special type of RNA called ribosomal RNA (rRNA), to which more than 50 different protein molecules are noncovalently bound. Much of the volume of some eukaryotic cells is taken up by an extensive endomembrane system. This system includes two main components, the endoplasmic reticulum and the Golgi apparatus. Continuities between the nuclear envelope and the endomembrane system are visible under the electron microscope. Tiny, membrane-surrounded droplets called
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