Chapter 6 Interactions Between Cells and the Extracellular Environment 6 1 Extracellular Environment Body Fluids The intracellular compartment contains 67 total body water the extracellular compartment contains the remaining 33 total body water o Blood plasma is composed of about 20 of this extracellular fluid contained in the cardiovascular system Blood transports o Oxygen from lungs to body cells and carbon dioxide from body cells to lungs o Nutrients from food to body cells other nutrients between organs o Metabolic wastes to the kidneys and liver for elimination o Hormones from endocrine glands to target organs Interstitial fluid extracellular tissue fluid is composed of remaining 80 of this extracellular fluid and contained in the extracellular matrix o Oxygen nutrients and regulatory molecules in blood must pass through interstitial fluid to reach body cells and waste products and hormone secretions must pass through interstitial fluid to reach blood plasma Extracellular Matrix The extracellular matrix consists of collagen and elastin provide structural strength to connective tissues and gel like ground substance composed of glycoproteins and proteoglycans o Example The basal lamina is a collagen that bonds the epithelium to its underlying connective tissues Integrins are glycoproteins in the cell that extend from the cytoskeleton through the plasma membrane into the extracellular matrix and serve as an adhesion molecules which bind intracellular and extracellular compartments o The actions of integrins help impart polarity to the cell affect cell adhesion in a tissue motility and the ability of cells to proliferate in their tissues Categories of Transport Across the Plasma Membrane The plasma membrane exhibits selective permeability to certain molecules o It is not generally permeable to large molecules like proteins nucleic acids and other molecules needed for structure and function o It is generally permeable to molecules that facilitate nutrient and waste movement for metabolism and ions used for production of impulses in nerve muscle cells Carrier mediated transport requires membrane protein carriers non carrier mediated transport allows free diffusion through the membrane o Passive transport is the net movement of molecules and ions across a membrane from higher to lower concentration o Active transport is the net movement of molecules and ions across a membrane from lower to higher concentration requires ATP energy input ATP powers pumps or specific carrier proteins 6 2 Diffusion and Osmosis Diffusion is the random motion of molecules that eliminates concentration differences between two regions of a solution separated by a permeable membrane Net diffusion is the movement of molecules from the region of higher to lower concentrations until equilibrium is established Mean diffusion time increases with square distance the molecules must travel Diffusion Through the Plasma Membrane Nonpolar lipid soluble or uncharged molecules pass easily through the membrane o Gas exchange occurs by diffusion between cells and their extracellular environments because there is a low concentration of O2 inside the cell and high concentration of CO2 produced inside the cell the concentration gradient for O2 produces net diffusion from air to blood and the concentration gradient for CO2 produces net diffusion from blood to air Despite not being lipid soluble water molecules are small and lack charge so they may pass through the plasma membrane to a limited extent but is aided by water channels called aquaporins in most membranes Ion channels permit passage of ions like Na and K through the membrane some are always open for ongoing diffusion while others are gated such as specific channels for passage of Na and K which respond to changes in voltage The speed of diffusion depends on the magnitude of the concentration gradient permeability of the membrane temperature of solution and membrane surface area o The plasma membrane is 20 times more permeable to K than Na therefore K diffuses much more rapidly however changes in protein structure of membrane channels may change this rate i e during production of a nerve impulse which stimulates faster Na diffusion o In body areas specialized for rapid diffusion the surface area of membranes may be increased by folds or microvilli may aid molecule transport Rate of Diffusion Osmosis different solute concentrations movement Osmosis is the net diffusion of water across a selectively permeable membrane which separates o Osmotically active solutes are those that cannot pass through the membrane and promote o There is net movement from higher to lower water concentration Osmotic Pressure is the pressure needed to stop osmosis o The greater the concentration of a solute the greater its osmotic pressure Molarity and Molality A mole always contains 6 02x10 23 molecules A one molar solution is composed of 1 mole solute dissolved in water to make 1 liter of solution Molality in a 1 molal solution 1 mole of solute is dissolved in 1 kilogram water Osmolality Osmolality Osm expresses the total molality of a solution o Osmotic pressure does not depend on the chemical nature of the solute but ratio of solute to solvent for example a solution of 1 0 m glucose and 1 0 m fructose has an osmolality total molality of 2 0 osmol L 2 0 Osm o A 1 0 m solution of NaCl has a total concentration of 2 0 Osm Measurement of Osmolality Freezing point depression is a good measure of osmolality since it depends on total concentration of the solution and not on chemical nature of the solute One mole of solute per liter depresses the freezing point of water by 1 86 C o A 1 0 m glucose solution freezes at 1 86 C and a 1 0 m NaCl solution freezes at 2x 1 86 3 72 C Tonicity Tonicity describes the effect of a solution on the osmotic movement of water o Isotonic solutions contain the same amount of dissolved substance o Hypo osmotic hypertonic solutions have lower concentrations relative to solvent hypertonic solutions have higher concentrations relative to solvent Crenation will occur when water moves out of the cell into a region of higher solute concentration Regulation of Blood Osmolality A fairly constant osmolality of extracellular fluid must be maintained in order to prevent swelling or shrinkage of the brain o When dehydration occurs blood conc increases and volume reduces Increased plasma osmolality and osmotic pressure stimulate osmoreceptors neurons located in the hypothalamus
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