Ch 3 The Cellular Level of Organization A cell is the basic unit of all living things Prokaryotic cells are simple cells pro before karyon nucleus they have no nucleus most are unicellular bacteria Eukaryotic cells complex cells w nucleus and sub cellular structures organelles containing all fungi plants and such A Generalized Cell Plasma Membrane outer cells boundary selectively permeable regulates the flow of materials into and out of cell role in cellular communication among cells and between cells and their external environment separates internal environment everything inside the cell from external environment everything outside the cell Cytoplasm all of the other cellular contents between plasma membrane and nucleus has 2 components cytosol and organelles o Cytosol fluid portion of cytoplasm contains mostly water dissolved solutes suspended particles o Organelles within the cytosol each type of organelle has a characteristic shape and specific functions eg Cytoskeleton ribosome s endoplasmic reticulum Golgi complex lysosomes peroxisomes and mitochondria Nucleus contains the genetic library large organelle that contains DNA in molecules called chromosomes each chromosome consists of a single molecule of DNA and associated packaging proteins a chromosome contains thousands of hereditary units called genes that control most aspects of cellular structure and function The Plasma Membrane Much more than just a fence flexible yet sturdy intelligent semi permeable regulator that covers protects cell controls what goes in comes out links to other cells flies certain flags to tell other cells who it is best described by using a structural model called the fluid mosaic model Fluid Mosaic Model describes arrangement of molecules within the membrane resembles a continually moving sea of fluid lipids with protein icebergs floating in it o Lipids act as a barrier to certain polar substances proteins act as gatekeepers allowing passage of specific molecules ions The basic structural framework of the plasma membrane is the lipid bilayer two back to back layers made up of 3 types of lipid molecules phospholipids cholesterol glycolipids Phospholipids about 75 of membrane lipids are lipids that contain phosphorus form lipid bilayer cholesterol about 20 a steroid with an attached OH group and glycolipids about 5 sugar lipids attached carbohydrate groups also contribute Integral Proteins extend into or through the bilayer among the fatty acid tails and are firmly embedded in it most integral proteins are transmembrane proteins o Transmembrane proteins span entire lipid bilayer and protrude into both the cytosol and extracellular fluid Peripheral proteins not as firmly embedded in the membrane attached to the polar heads of membrane lipids or to integral proteins at the inner or outer surface of the membrane Glycoproteins many integral proteins membrane proteins with a carbohydrate group attached that protrude into the extracellular fluid the carbohydrates are oligosaccharides chains of 2 to 60 monosaccharides that may be straight or branched Glycocalyx the entire sugary coating surrounding the membrane made up of the carbohydrate portions of the glycolipids and glycoproteins Functions of Membrane Proteins Some integral proteins form ion channels pores or holes that specific ions such as potassium ions K can flow through to get into or out of the cell most ion channels are selective they allow only a single type of ion to pass through Other integral proteins act as carriers selectively moving a polar substance or ion from one side of the membrane to the other also known as transporters Integral proteins called receptors serve as cellular recognition sites each type of receptor recognized and binds a specific type of molecule ex insulin receptors bind the hormone insulin a specific molecule that binds to a receptor is called a ligand of that receptor Some integral proteins are enzymes catalyze specific chemical reactions at the inside or outside surface of the cell Integral proteins may also serve as linkers that anchor proteins in the plasma membranes of neighboring cells to one another or to protein filaments inside and outside the cell peripheral proteins may also serve as enzymes and linkers Membrane glycoprotein s and glycolipids often serve as cell identity markers may enable a cell to 1 recognize other cells of the same kind during tissue formation or 2 recognize and respond to potentially dangerous foreign cells ex ABO blood type markers Peripheral proteins help support the plasma membrane anchor integral proteins and participate in mechanical activities such as moving materials and organelles within cells changing cell shape in dividing and muscle cells and attaching cells to one another Figure 3 3 page 67 Membrane Permeability Because of the distribution of lipids and the proteins embedded in it membrane allows some substances to cross but not others this is called selective permeability Rule of thumb small neutrally charged lipid soluble substances can freely pass Water is a special case it is highly polar yet still freely permeable For those substances that are needed by the cell but for which the membrane is impenetrable impermeable transmembrane proteins act as channels transporters They assist the entrance of certain substances that either can t pass at all Glucose or for which the cell needs to hasten passage Transport Processes Passive Processes involve substances moving across the cell membranes without the input of any energy said to move with or down concentration gradient ex diffusion Active Processes involve the use of energy primarily from the breakdown of ATP to move a substance uphill against its concentration gradient ex active transport Another way that some substances may enter and leave cells is an active process in which tiny speherical membrane sacs referred to as vesicles are used Examples endocytosis vesicles detach from plasma membrane while bringing materials into a cell and exocytosis the merging of vesicles with the plasma membrane to release materials from the cell Passive Processes Diffusion of solutes diffusion of water osmosis facilitated diffusion requires specific channel or carrier molecule but no energy is used Diffusion is the passive spread of particles through random motion from areas of high concentration to areas of low concentration Several factors influence the diffusion rate of substances across plasma membranes o Steepness of the concentration
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