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Cellular Form and Function

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Northeastern University
BIOL 1117
Christopher Richardson

L8- Cellular Form and Function 9/19/13 • Plasma Membrane o Many physical processes, such as immune responses, binding of sperm with an egg, cell signaling, detection of smell or taste occur at the surface of the cell o Defines cell boundaries o Governs interactions with other cells o Controls passage of materials in and out of cell o Intracellular face: side that faces cytoplasm o Extracellular face: side that faces outward o Membrane is selectively permeable; allowing some substances through while preventing others from entering and leaving the cell o **Creates internal and controlled space (due to plasma membrane being selectively permeable) • Membrane Lipids o 98% of plasma membrane made of lipids o Phospholipids  75% of membrane lipids are phospholipids  Hydrophilic phosphate heads face water on each side of membrane  Hydrophobic tails are directed toward the center, avoiding water o Cholesterol  20% of the membrane lipids  Holds phospholipids still and can stiffen membrane o Glycolipids  5% of the membrane lipids  Phospholipids with short carbohydrate chains on extracellular face  Contributes to glycocalyx: carbohydrate coating on the cell’s surface • Membrane Proteins o Membrane proteins  2% of the molecules in plasma membrane  50% of its mass o Transmembrane proteins  *hydrophilic and hydrophobic regions  Hydrophilic regions in contact with cytoplasm and extracellular fluid  Hydrophobic regions pass through the lipids of the membrane  Most are glycoproteins  Proteins which are conjugated with short lengths of saccharides that face the extracellular side  *Some can drift about freely in phospholipid layer while others are anchored to cytoskeleton o Peripheral proteins  Adhere to one face of the membrane  Usually tethered to the cytoskeleton  Most peripheral proteins are typically associated with some transmembrane protein • Membrane Transport o Materials pass through the plasma membrane via passive or active mechanisms by carrier-mediated proteins or protein channels. o Passive transport mechanisms requires noATP  Filtration, diffusion, osmosis o Active transport mechanisms usesATP  Active transport and vesicular transport o Carrier-mediated mechanisms use a membrane protein to transport or carry substances from one side of the membrane to the other  Can either be passive or active • Filtration o Filtration: process in which particles are driven through a selectively permeable membrane by hydrostatic pressure (force exerted on a membrane by water) o The energy comes from hydrostatic pressure o Examples  Filtration of nutrients through gaps in blood capillary walls into tissue fluids  Blood pressure is behind how the kidneys filter waste materials out of the blood • Simple Diffusion o Simple Diffusion: the net movement of particles from area of high concentration to area of low concentration o This is movement down the concentration gradient: concentration of a substance differs from one point to another o Random molecular motion of particles has the necessary energy as they move from high concentration to low concentration  Because the original gradient had potential energy o Diffusion readily occurs in both air and in water • Membrane Permeability o If a membrane is present, a substance will move from one side of the membrane to the other, down or with the concentration gradient if the membrane is permeable to that substance o Diffusion through lipid bilayer  Nonpolar, hydrophobic, lipid-soluble substances diffuse through lipid layer o Diffusion through channel proteins  Charged, hydrophilic solutes or polar substances only diffuse through channel proteins in membrane  Cannot pass through hydrophobic inner layer o Water to an extent can go through the membrane o Significant amounts of water can enter many cells through channel proteins called aquaporins. • Diffusion Rates o Factors affecting diffusion rate through a membrane  Temperature: temp., motion of particles  Molecular weight: larger molecules move slower  Steepness of concentration gradient: difference, rate • The degree of difference in the concentration across the membrane  Membrane surface area: area, rate  Membrane permeability: permeability, rate  Cells control and adjust permeability to different materials by regulating the number of protein channels in membrane or by opening and closing of gates to channels • Osmosis o Osmosis is the net diffusion of water down a concentration gradient from side with higher water concentration to the side with lower water concentratio
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