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Chapter 3

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Western University
Biology 1225
Michael Butler

Chapter 3: How Cells are Put Together 3.1 What is “A Cell?” -cell theory: 1. every organism consists of one or more cells 2. a cell is the smallest unit with the properties of life; it has the capacity for metabolism, controlled responses to the environment, growth and reproduction 3. only living cells give rise to new cells -cells differ in size, shape, and activities yet similar in three respects: all start out life with a plasma membrane, a region of DNA, and cytoplasm -plasma membrane: thin most outermost membrane separates each cell from its outside environment (house with many windows and doors that do not open just for anyone) -nucleus or nucleoid: in every eukaryotic cell, DNA occupies a membrane-enclosed sac called the nucleus. Inside prokaryotic cells, DNA occupies a region with no membrane, this region is called the nucleoid. -cytoplasm: everything in between the plasma membrane and the region of DNA. It consists of a semifluid matrix and other components like ribosomes (structures on which proteins are built) -bacteria and archaea are prokaryotic cells. Unlike eukaryotic cells, they do not have an abundance of organelles, and they do not have a nucleus -all other cells are eukaryotic, they have internal membranes that form organelles, particularly a nucleus 3.2 Most Cells are Really Small -a physical relationship called the surface-to-volume ration constrains increases in cell size. By this relationship, any object’s volume increases with the cube of its diameter, but the surface area increases only with the square. -if a cell expands in diameter during growth, then its volume will increase faster than its surface area will 3.3 The Structure of Cell Membranes -when you immerse many phospholipid molecules in water, they interact with water molecules and with one another until they spontaneously cluster in a sheet or film at the water’s surface. Some of them will line up in two layers, with the fatty acid tails sandwhiched between the outward-facing hydrophilic heads. This arrangement, called lipid bilayer, is the structural basis of every cell membrane -fluid mosaic model: organization of membranes (a cell membrane is a mixed composition- a mosaic- of phospholipids, glycolipids, sterols, and proteins -all cell membranes have protein receptors, transporters and enzymes. The plasma membrane also incorporates adhesion, communication and recognition proteins. 3.4 A Closer Look at Prokaryotic Cells -word means before the nucleus (reminds us that bacteria and archaea originated before cells with a nucleus evolved) -sticky polysaccarides often surround the prokaryotic cell wall- help the cell attach to surfaces -projecting from many of these cells are one or more flagella (rotate like propellers-used to move through enviro) -other surface projections include pili which help attach to surfaces and to one another -simplest cells 3.5 A Closer Look at Eukaryotic Cells -interior divided into organelles, each responsible for a specialized function -ribosomes, a cytoskeleton, and other structures help support these functions *table 3.1 -organelles physically separate chemical reactions. Organelles assemble, store and move substances to and from the plasma membrane, or to a specific destinations in the cytoplasm The nucleus -chromosome: one double-stranded DNA molecule togertther with many attached protein molecules -chromatin: name for all the DNA and associated proteins in a nucleus -two functions of nucleus: isolates a cell’s DNA from potentially damaging reactions in the cytoplasm, controls access access to DNA through the receptors, transport proteins, and pores at its surface -nuclear envelope: two lipid bilayers studded with pores and transport proteins The Endomembrane system -many new polypeptide chains enter the flattened sacs and tubes of the endomembrane system: the ER, Golgi bodies, and vesicles -endoplasmic reticulum: dynamic channel of flattened membrane sacs and tubes that starts at the nuclear envelope and extends through the cytoplasm -rough
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