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Chapter 2

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Biology 1201A
Richard Gardiner

Chapter 2: The Cell An Overview -in the 1820s, thanks to improvements in microscopes, Robert Brown noticed a discrete spherical body inside some cells, which he called a nucleus -Theodore Schwann proposed that all animals and plants consist of cells that contain a nucleus -Rudolf Virchow propthed that cells arise only from pre-existing cells by a process of division -by the end of the 19 century, the cell theory had been formed: 1. All organisms are composed o one or more cells 2. The cell is the basic structural and functional unit of all living organisms 3. Cells arise only from the division of pre-existing cells 2.1 Basic Features of Cell Structure and Function -cells carry out the essential processes of life contain highly organized systems of molecules, including the nucleic acids DNA and RNA o carry hereditary information and direct the manufacture of cellular molecules use chemical molecules or light as energy sources for their activities -cells also respond to changes in their external environment by altering their internal reactions -cells duplicate and pass on their hereditary information as part of cellular reproduction -life as we know it does not exist in units simpler than individual cells 2.1a Cells Are Small and Are Visualized Using a Microscope -all forms of life are grouped into one of three domains the Bacteria the Archaea the Eukarya -cells assume a wide variety of forms, but are all organized according to the same basic plan, and have structures that perform similar activities -to see cells and the structures within them we use microscopy a technique for producing visual images of objects that are too small to be seen by the human eye -the instrument of microscopy is the microscope light microscopes use light to illuminate the specimen electron microscopes use electrons to illuminate the specimen magnification is the ratio of the object as viewed to its real size resolution is the minimum distance by which two points on the specimen can be separated and still be seen as two points depends primarily on the wavelength of light or electrons used to illuminate the specimen o the shorter the wavelength, the better the resolution -volume of a cell determines the amount of chemical activity that can take place within it, while surface area determines the amount of substances that can be exchanged between the inside of the cell and the outside environment some cells increase their ability to exchange materials with their surroundings by flattening or by developing surface folds or extensions that increase their surface area 2.1b Cells Have a DNA-Containing Central Region That Is Surrounded by Cytoplasm -all cells are bounded by the plasma membrane, a bilayer made of lipids with embedded protein molecules -the lipid bilayer is a hydrophobic barrier to the passage of water-soluble substances, but selected water-soluble substances can penetrate cell membranes through transport protein channels selective movement of ions and water soluble molecules through the transport proteins maintains the specialized internal ionic and molecular environments -the central region of all cells contains DNA molecules, which store hereditary information organized in the form of genes central region also contains proteins that help maintain the DNA structure and enzymes that duplicate DNA and copy its information into RNA -all parts of the cell between the plasma membrane and the central region make up the cytoplasm contains the organelles o little organs, small, organized structures important for cell function contains the cytosol, an aqueous solution containing ions and various organic molecules contains the cytoskeleton, a protein-based framework of filamentous structures that, among other things, helps maintain proper cell shape and plays key roles in cell division and chromosome segregation -many of the cells vital activities occur in the cytoplasm synthesis and assembly of most of the molecules required for growth and reproduction conversion of chemical and light energy into forms that can be used conducts stimulatory signals from the outside into the cell interior and carries out chemical reactions that respond to these signals 2.1c Cells Occur in Prokaryotic and Eukaryotic Forms, Each with Distinctive Structures and Organization -prokaryotic cell lacking a nucleus; the DNA-containing central region of the cell (the nucleoid), has no boundary membrane separating it from the cytoplasm -eukaryotes DNA is contained within a membrane-bound compartment called the nucleus the cytoplasm typically contains extensive membrane systems that form organelles with their own distinct environments and specialized functions 2.2 Prokaryotic Cells -three shapes most common among prokaryotes spherical rodlike spiral -genetic material of archaea and bacteria is located in the nucleoid -for most species, the DNA is a single, circular molecule that unfolds when released from the cell prokaryotic chromosome -individual genes in the DNA molecule encode the information required to make proteins information is copied onto a type of RNA molecule called messenger RNA (mRNA) the ribosomes use the information in the mRNA to assemble amino acids into proteins o a prokaryotic ribosome consists of a large and a small sub-unit, each formed from a combination of ribosomal RNA (rRNA) and protein molecules each prokaryotic ribosome contains three types of rRNA molecules and more than 50 molecules -the plasma membrane is surrounded by the cell wall, which ranges in thickness from 15-100nm or more cell wall provides rigidity to prokaryotic cells and with the capsule protects the cell from physical damage coated with an external layer of polysaccharides called the glycolax when the glycolax is diffuse and loosely associated, it is a slime layer; when it is gelatinous and more firmly attached , it is a capsule -the plasma membrane performs several vital functions transporting materials into and out of cells contains most of the molecular systems that metabolize food molecules into the chemical energy of ATP -many prokaryotic species contain few if any internal membranes most cellular functions occur either on the plasma membrane or in the cytoplasm -prokaryotic cells have filamentous cytoskeletal structures with functions similar to those in eukaryotes important role in creating and maintaining the proper shape of cells, in cell division and for certain bacteria, in determining the polarity of cells -long, threadlike protein fibres called flagella allow the cell to move through liquids and across wet surfaces extend from the cell surface the bacterial flagellum, which is helically shaped, rotates in a socket in th
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