Cell Transport - General Biology PDF

1  Introduction  Why there are differences in concentration of some substances between ECF and ICF? All cells are generally separated from their surrounding environment by plasma membrane. The cell membrane is semipermeable, some substances can pass through it freely, while others canno...

1  Introduction  Why there are differences in concentration of some substances between ECF and ICF? All cells are generally separated from their surrounding environment by plasma membrane. The cell membrane is semipermeable, some substances can pass through it freely, while others cannot. The movement of substances that can pass freely through the membrane depends only on the concentration gradient for that substance.  Introduction All cells have a cell membrane with Functions: 1) Controls what enters and exits the cell to maintain an internal balance called homeostasis. 2) Provides protection and support for the cell  Two Types of Cellular Transport according to the needed for energy 1. Passive Transport 1. Does not use energy 2. Moves from a high concentration to a low concentration 2. Active Transport 1. Requires energy 2. Moves from a low concentration to a high concentration  Two Types of Cellular Transport according to the needed for energy 1. Passive Transport Types of Passive Transport A. Simple Diffusion B. Facilitated Diffusion C. Filtration D. Osmosis Types of Passive Transport Facilitated diffusion (Channel Protein) Diffusion (Lipid Bilayer)  Two Types of Cellular Transport according to the needed for energy conformational change low 2. Active Transport Types of Active Transport A. Protein Pumps ATP B. Endocytosis C. Exocytosis high  Two Types of Cellular Transport according to the Mediated transport Two types of transport process occur across the membrane. 1. Non-mediated transport Non-mediated transport occurs through the simple diffusion process and the driving force for the transport of a substance through a medium depends on its chemical potential gradient. 2. Mediated transport Mediated transport requires specific proteins.. cannels or carrier or pump. Mediated transport is classified into two categories 1.Passive-mediated transport or facilitated diffusion (cannels or carrier ) 2.Active-mediated transport (pump )  Diffusion equilibrium is reached when the concentration in the two compartments become equal.  Most of the molecule will diffuse across a protein-free lipid bilayer down its concentration gradient, if provided enough time.  Nonpolar molecules diffuse through the lipid portions of membrane much more rapidly than do polar or ionized molecule because nonpolar molecules can dissolved in lipids in the membrane.  The smaller the molecule and the more soluble in oil (the more hydrophobic or non-polar), the more rapidly it will diffuse across a cell membrane.  Carrier-mediated diffusion Changes in the conformation of transporter move the binding site to the opposite side of the membrane, where the larger molecules or solute dissociates from the protein. Channel-mediated diffusion provide water filled pores for charged ions to pass through  1. Primary active transport. Primary active transport, also called direct active transport, directly uses energy to transport molecules across a membrane.  Example: Na-K pump, which helps to maintain the cell potential. Used by animal cells to maintain a high internal concentration of K+ ions and a low internal concentration of Na+ ions. Maintains a concentration gradient that is used to power many other important physiological process  2. Secondary active transport: Secondary active transport or co-transport, also uses energy to transport molecules across a membrane. However, in contrast to primary active transport, there is no direct coupling of ATP; instead it uses the energy released when a molecules move by diffusion (electrochemical potential difference ) to supply energy to active transport of a different molecules.  e.g A symport is used Glucose-Na+ symport captures the energy from Na+ diffusion to move glucose against a concentration gradient  The three main forms of active transporters: Carrier proteins used in active transport include:  Uniporters – move one molecule at a time  Symporters – move two molecules in the same direction  Antiporters – move two molecules in opposite directions antiport symport ATP (a) Uniport: In uniport one molecule of ion or solutes are pumped in one direction across a membrane against the concentration gradient. (b) Antiport: In antiport two species of ion or solutes are pumped in opposite directions across a membrane. One of these species is allowed to flow from high to low concentration which yields the entropic energy to drive the transport of the other solute from a low concentration region to a high one. Example: the sodium-calcium exchanger or antiporter, which allows three sodium ions into the cell to transport one calcium out. c) Symport: Symport uses the downhill movement of one solute species from high to low concentration to move another molecule uphill from low concentration to high concentration (against its electrochemical gradient). Example: glucose symporter, which co-transports one glucose (or galactose) molecule into the cell for every two sodium ions it imports into the cell.  Filtration  It is the process of the movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the system.  Depending on the size of the membrane pores, only solutes of a certain size may pass through it.  The membrane pores of the Bowman's capsule in the kidneys are very small, and only albumins (smallest of the proteins) can filter through.  On the other hand, the membrane pores of liver cells are extremely large, to allow a variety of solutes to pass through and be metabolized.  movement of water toward an area of high solute concentration in osmosis, only water is able to pass through the membrane Osmotic Balance Cells crenate in hypertonic solutions Hypertonic solutions (have a higher relative solute concentration) Water moves from intracellular to extracellular. Osmotic Balance Cells lyse in hypotonic solutions Hypotonic solutions (have a lower relative solute concentration) Water moves from extracellular to intracellular. Osmotic Balance Cells are maintained in isotonic solutions Isotonic solutions (have equal relative solute concentrations) Water goes in both directions.