Chapter 5 Cells

Chapter 5 Cells - The sizes of things Surface...

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The sizes of things
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Surface area-to-volume ratio Cells are small because smallness confers a high surface-are-to volume ration. The volume of a cell determines the amount of chemical activity, but the surface area determines the amount of substances (e.g., oxygen, nutrients) the cell can take in from the outside environment and the amount of waste products (e.g., carbon dioxide, heat) it can release into the environment.
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The plasma membrane The plasma membrane is a phospholipid bilayer . Oil-like substances can easily cross, but hydrophilic substances depend on pathways provided by proteins that span the membrane. As a result, the membrane may be impemeable or selectively permeable to ions, sugars, amino acids, etc. The membrane thus enables the cell to maintain a fairly constant internal environment ( cytoplasm ) and to control changes. The membrane also may be involved in communication with neighboring cells.
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Prokaryotic cells Prokaryotic cell organization is characteristic of the kingdoms Eubacteria and Archaebacteria. All prokaryotic cells have a selectively permeable plasma membrane , a nucleoid region with DNA, and a cytoplasm containing (bacterial) ribosomes (where proteins are synthesized), and cytosol (dissolved enzymes , water , ions and small molecules). Some prokaryotes have a rigid cell wall containing peptidoglycan, an outer membrane , a mucus-like capsule composed of polysaccharide, photosynthetic membranes , pili , and flagella . Prokaryotic cells lack internal compartments.
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Flagella and pili Pili provide a path for exchange of DNA between bacteria (sex), for adhering to things, and for entry of bacterial viruses. Bacterial flagella enable prokaryotes to swim. Their structure is quite wonderful. Pili
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Bacterial flagellum The flagellum rotates, driven by a motor, and is one of the earliest “nanomachines”. Its structure is NOT like that of eukaryotic cilia or flagella.
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Eukaryotic animal cells Eukaryotic cells also have a plasma membrane , cytoplasm , and (eukaryotic) ribosomes . However, eukaryotic cells are larger ( 10 μ m vs. 1 μ m diam ), have a nucleus, many membrane enclosed organelles , and an elaborate cytoskeleton .
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Eukaryotic plant cells Add chloroplasts, a cell wall, a vacuole, and plasmo- desmata.
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Cell fractionation To study the different structures in a cell, investigators can break open the cell (with, for example, a blender or sonicator) and then centrifuge the mixture, causing separation of components based on size or density.
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DNA resides in the nucleus , ~5 μ m diam, that is surrounded by two membranes, the nuclear envelope . Nuclear pores have complex structures governing what leaves (messenger RNA) and enters (proteins) the nucleus. Some proteins have a certain amino acid sequence -- a nuclear localization signal ( NLS ) -- to cross the nuclear envelope. The
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Chapter 5 Cells - The sizes of things Surface...

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