Lesson 2 The Cell and the Plasma Membrane An Introduction to Cells Cell Theory o Cells are the building blocks of all plants and animals o All cells come from the division of preexisting cells o Cells are the smallest units that perform all vital physiological functions o Each cell maintains homeostasis at the cellular level Developed from Robert Hooke s research he viewed slices of cork under a light microscope Cytology The study of cells Somatic Cells All body cells except sex cells o Soma body Sex Cells Germ cells or reproductive cells o Male sperm o Female oocyte a cell that becomes an egg 1 2 Plasma Membrane Extracellular Fluid Interstitial Fluid A watery medium that surrounds a cell Plasma membrane separates the inside of the cell cytoplasm from the extracellular fluid Cytoplasm Region of cell found between the plasma membrane and the nuclear membrane Cytosol Liquid component of the cytoplasm Four 4 General Functions of the Plasma Membrane Intracellular structures organelles o 1 Physical isolation o 2 Regulation of exchange with environment environment Can regulate Physical barrier separating the inside of the cell from the outside Entry of ions nutrients Release of wastes cellular products by exocytosis o 3 Sensitivity to the environment Extracellular fluid composition can alter plasma membrane Can detect chemical signals through plasma receptors proteins o 4 Structural support Anchors cells and tissues to each other or to the extracellular matrix Provides stability to cells Plasma membrane is composed of o Membrane Lipids Largest component 3 Phospholipid Bilayer Two layers of phospholipids Hydrophilic heads toward watery environments on both sides Hydrophobic fatty acid tails inside membrane away from water Bilayer forms a barrier to ions water soluble compounds o Can t move past the hydrophobic lipid tails o Allows for interior of cell to have a different composition than the extracellular environment o Membrane Proteins Second largest component 55 of the weight of the membrane Integral Membrane Proteins Embedded within the membrane More numerous Transmembrane Protein Traverse the width of the plasma Peripheral Membrane Proteins Bound to inner or outer surface of the membrane plasma membrane Receptor Proteins Bind and respond to ligands ions hormones Binding of the ligand can cause a change within the cell Carrier Proteins Transport specific solutes through membrane May may not require energy ATP Ex Glucose transport doesn t require energy but Na and Ca2 transport does Channels Integral proteins with a central pore Regulate water flow solutes through membrane Many channels are highly specific for a certain ion o Important in nerve impulse conduction muscle Anchoring Proteins Stabilize the cell contraction Attach to inside cytoskeletal or outside extracellular protein fibers or other cells structures Recognition Proteins Label cells as normal or abnormal Prevents immune system from reacting to the cell Many recognition proteins glycoproteins Enzymes Catalyze reactions inside or outside the cell May be integral or peripheral proteins Catalyze reactions inside or outside the cell 4 Diffusion and Osmosis Membrane transport o Plasma membrane a barrier o Nutrients must get in and products wastes must get out o Permeability determines what moves in and out of a cell Impermeable Lets nothing in or out Freely Permeable Lets anything pass Selectively Permeable Restricts movement Plasma membrane selectively permeable o Allows some materials to move freely restricts others o Restricts material based on Size of the material Electrical charge Molecular shape Lipid solubility Correlation between drug potency and lipid solubility Transport through a plasma membrane can be o Active requires energy ATP o Passive no energy needed Types of transport o Diffusion Passive All molecules are constantly in motion Molecules in solution move randomly Random motion causes mixing Factors influencing diffusion Distance the particle has to move Molecule size o Smaller faster 5 Temperature o More heat faster motion Concentration gradient o The difference between high and low concentrations o Greater the difference the faster the diffusion Electrical forces o Opposites attract like charges repel Simple Diffusion When materials diffuse through the plasma membrane Lipid soluble compounds alcohols fatty acids steroids Dissolved gases oxygen carbon dioxide Channel Mediated Diffusion Diffusion through a channel in a transmembrane protein Water soluble compounds ions Factors size charge interaction with channel Leak Passive Channels Always open allow the passage of ions in either direction o Net flow of ions is from higher lower concentration Osmosis The diffusion of water across the cell membrane o For osmosis to occur Membrane must be freely permeable to water selectively permeable to solutes solutes Water molecules diffuse across membrane toward solution with more More solute molecules lower concentration of H2O molecules As H2O moves to that side the volume increases on the side with more solutes to dilute it to the same concentration as the other side o Hydrostatic pressure vs osmotic pressure o Osmosis can be prevented by resisting the change in volume o Osmolarity Total solute concentration 6 o Tonicity Effect of the osmotic solution on cells o Two fluids may have equal osmolarity but different tonicity o Isotonic A solution that does not cause osmotic flow of water in or out of a cell o Hypotonic Has less solutes and loses water through osmosis A cell in a hypotonic solution gains water and will rupture if left unchecked Ex Hemolysis of red blood cells o Hypertonic Has more solutes and gains water by osmosis A cell in a hypertonic solution loses water and will shrink Ex Crenation of red blood cells o Carrier Mediated Transport Moves ions organic substrates Passive or active Characteristics 1 Specificity o One transport protein one set of substrates 2 Saturation limits 3 Regulation o Rate depends on transport proteins o Cofactors such as hormones Types out diffusion Cotransport Symport Two substances move in the same direction at the same time Countertransport One substance moves in while another moves Carrier mediated transport without the use of energy facilitated Transports molecules too large to fit through channel proteins glucose amino acids o Molecule binds to receptor site on carrier protein o Receptor site specific to certain molecules o Protein changes shape to move molecules across the
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